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A Percutaneous Transtubular Middle Fossa Approach for Intracanalicular Tumors
Original Article
A Percutaneous Transtubular Middle Fossa Approach for Intracanalicular Tumors Antonio Bernardo1, Alexander I. Evins1, Apostolos J. Tsiouris2, Philip E. Stieg1
OBJECTIVE: In cases of small intracanalicular tumors (£1.5 cm), the middle fossa approach (MFA) provides the ability for adequate tumor removal with preservation of existing auditory function. Application of a minimally invasive tubular retractor in this approach may help mitigate the risk of postoperative seizures, aphasia, and venous complications by minimizing intraoperative retraction of the temporal lobe. We propose a minimally invasive microscopic and/or endoscopic percutaneous transtubular MFA for the management of intracanalicular tumors.
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METHODS: Subtemporal keyhole craniectomies were performed on 5 preserved cadaveric heads (10 sides), with 6 sides previously injected with a synthetic tumor model. A ViewSite Brain Access System tubular retractor (Vycor Medical, Inc., Boca Raton, Florida, USA) was used to provide minimal temporal retraction and protection of the surrounding anatomy. An extradural dissection of the internal auditory canal was performed under microscopic and endoscopic visualization with a minimally invasive surgical drill and tube shaft instruments, the intracanalicular tumors were removed, and degree of resection was assessed.
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RESULTS: All 10 approaches were completed successfully through the tubular retractor with minimal retraction of the temporal lobe. Excellent visualization of the structures within the internal auditory canal was achieved with both the microscope and 3-dimensional endoscope. On the 6 synthetic intracanalicular tumors resected, 5 gross total (Grade I) and 1 near total (Grade II) resections were achieved.
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Key words Endoscopic - Internal auditory canal - Middle fossa - Minimally invasive - Transtubular - Tubular retractor -
Abbreviations and Acronyms CN: Cranial nerve CPA: Cerebellopontine angle CT: Computed tomography GSPN: Greater superior petrosal nerve IAC: Internal auditory canal ICA: Internal carotid artery MFA: Middle fossa approach
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CONCLUSION: A percutaneous transtubular MFA is a feasible minimally invasive option for resection of small intracanalicular tumors with potential preservation of auditory function, reduced temporal retraction, and enhanced protection of surrounding structures.
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INTRODUCTION
T
he middle fossa approach (MFA), first proposed in the earlier 20th century for the treatment of otosclerosis and later for the resection of acoustic neuromas by House and Shelton, is indicated for the management of small tumors (1.5 cm) of the internal auditory canal (IAC) with an extension of less than 1 cm into the cerebellopontine angle (CPA) in patients with no preoperative auditory deficits (2, 16, 17, 31). The MFA provides exposure of the entire length of the IAC and the facial (cranial nerve [CN] VII) and vestibular (CN VIII) nerves from the inner ear to the pons (2, 16, 17). Compared with other approaches used for the resection of intracanalicular vestibular tumors, the MFA provides the greatest preservation of auditory and facial nerve function (3, 25). However, the complex microsurgical anatomy of the petrous bone and the neurovascular structures it encases, and the difficulty of identifying landmarks on the petrous ridge, make this a technically challenging approach (Figure 1). The main drawback of the classic MFA is the risk of retraction injury to the temporal lobe (19, 31). Extensive temporal retraction can result in postoperative seizures, aphasia, or other focal neurologic deficits, and/ or venous complications from the compromise of the inferior anastomotic cerebral vein (vein of Labbé) (2, 3, 7, 9, 15). Stereotactic cylindrical retractors, first introduced by Kelly et al. (20) in 1988 for the excision of deep intraparenchymal lesions and MMA: Middle meningeal artery 3D: Three-dimensional VBAS: ViewSite Brain Access System From the 1Department of Neurological Surgery, Weill Cornell Medical College, New York Presbyterian Hospital, New York, New York, USA; and 2Department of Neuroradiology, Weill Cornell Medical College, New York Presbyterian Hospital, New York, New York, USA To whom correspondence should be addressed: Antonio Bernardo, M.D. [E-mail: [email protected]] Citation: World Neurosurg. (2015) 84, 1:132-146. http://dx.doi.org/10.1016/j.wneu.2015.02.042 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2015 Elsevier Inc. All rights reserved.
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ORIGINAL ARTICLE ANTONIO BERNARDO ET AL.
MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 1. Three-dimensional model of the neurovascular structures around and within the temporal bone, including the trigeminal nerve with the Gasserian ganglion and its ophthalmic, maxillary, and mandibular divisions; geniculate ganglion; greater superficial petrosal nerve; CN VII-VIII complex; chorda tympani; superior, lateral, and posterior semicircular canals; and cochlea. The intrapetrous internal carotid artery, inferior and superior petrosal sinuses, and the sigmoid sinus are shown in red and blue. The Eustachian tube, endolymphatic sac, and endolymphatic duct also are visible among other anatomical structures.
proposed for use in surgical approaches to deep-seated intraparenchymal and intraventricular lesions (30), may help mitigate the aforementioned postoperative complications. A transtubular approach to a skull base pathology that would allow for a smaller bone opening, minimal extradural cortical retraction, enhanced protection of surrounding tissues, and an improved cosmetic outcome has yet to be described. We propose and evaluate the feasibility of a minimally invasive microscopic and/ or endoscopic percutaneous transtubular MFA for the management of intracanalicular tumors. METHODS Microscopic (5 sides) and 3-dimensional (3D) endoscopic (5 sides) percutaneous MFAs were performed through a tubular retractor system on 5 preserved cadaveric heads (10 sides), all previously injected with colored latex—red for arteries, blue for veins (Figure 2). Six sides also were injected previously with a synthetic intracanalicular tumor model. Dissections were completed with a neurosurgical microscope (Zeiss OPMI Neuro/NC 4 System, Carl Zeiss Meditec AG; Jena, Germany) and 3D endoscope (VSIII; Visionsense Ltd.; New York, New York, USA) with 0 , 30 , and 90 angled optics. An Anspach eMax 2 Plus (Synthes Inc.; Palm Beach Gardens, Florida, USA) electric neurosurgical drill was used with a minimally invasive attachment. Synthetic Tumor Model Three specimens (6 sides) were injected preoperatively with synthetic tumor models (ST-540 Injection Resin, Strata-Tech; Des Moines, Iowa, USA) bilaterally using a Teflon integrated curved IV catheter through a retrosigmoid route to simulate small
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Figure 2. The transtubular middle fossa approach. Depiction of the percutaneous keyhole opening, surgical trajectory, and key anatomical landmarks.
intracanalicular tumors (1.5 cm in diameter) with minimal extension into the posterior fossa (4, 10, 12). The tumors were placed anterior (2 sides), posterior (2 sides), superior (1 side), and inferior (1 side) to the CN VIIeVIII complex. The synthetic tumor resin was mixed with a radiopaque solution (Omnipaque [iohexol] solution, GE Healthcare Inc.; Little Chalfont, United Kingdom) to appear hyperdense on computed tomography (CT) (Figure 3). Neuronavigation For image-guided neuronavigation, 6 skin markers (fiducials) that appear on CT scans were affixed to the cranium. One-millimeter spiral CT axial slices were obtained (Biograph TruePoint PETCT, Siemens AG; Munich, Germany) of each specimen and transferred to the neuronavigation workstation (Kolibri ImageGuided Surgery Platform, Brainlab AG; Feldkirchen, Germany) for spatial registration. Entry and Trajectory Planning Because of the small size of the bone opening and the rigid nature of the tubular retractor, determination of an optimal entry point is essential for ensuring an accurate trajectory to the petrous bone. To define an optimal entry zone, the lateral petroclival angle—the angle between the petrous bone and a sagittal line passing through the petroclival suture at the level of petrous ridge—and the distance from the petroclival suture to the porus acusticus
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On the basis of the calculated optimal entry zone, a 2.5- to 3-cm diameter burr hole was fashioned centered 4 cm posterior to the lateral orbital wall and 2 cm anterior to the external auditory meatus with its inferior margin placed as basal as possible and flush with the superior rim of the zygomatic arch (Figure 7). The dura was detached gently from the margins of the burr hole and raised from the floor of the middle fossa. Tubular Retractor System The tubular retractor system (17L [17 mm width 11 mm height 70 mm length] ViewSite Brain Access System [VBAS], Vycor Medical Inc.; Boca Raton, Florida, USA) consisted of an introducer inside of a working channel port. The working channel used herein was modified to provide a beveled tip that facilitated dural retraction when positioned against the angled petrous ridge (Figure 8). The tubular retractor was mounted onto an extension arm (Vycor Medical Inc.) attached to a self-retaining snake retractor (Mizuho America, Inc., Union City, California, USA) and inserted gently into the extradural space through the burr hole under direct microscopic visualization.
Figure 3. Synthetic tumor model. Computed tomography scan of a cadaveric specimen with a small (1. 5 cm in diameter) hyperdense contrast-enhancing synthetic intracanalicular tumor (yellow arrow) with extension into the cerebellopontine angle.
internus, the petroclival-acoustic distance, were first determined with the 5 obtained CT scans and an additional 20 normal adult head CT scans (50 sides) (Figure 4) (1). A trajectory, perpendicular to the petrous ridge, was then plotted and measured from the porus acusticus internus to an external point on the skull flush with the floor of the middle fossa. Mean lateral petroclival angles and corresponding distances were calculated to determine an optimal entry zone (Figure 5). The calculated optimal entry zone was correlated to external landmarks and then verified for target accuracy using the trajectory planning feature of the neuronavigation software on all ten cadaveric sides (Figure 6). Positioning, Incision, and Burr Hole Placement Three-point fixation was achieved using a Mayfield head holder and each head was positioned supine with 90 rotation to the contralateral side to provide an unobstructed view of the middle fossa floor and 20 lateroflection to facilitate gravitational retraction of the temporal lobe. A 4-cm vertical linear skin incision was placed beginning at the inferior rim of the zygomatic arch 4 cm anterior to the external auditory meatus and continued superiorly along the hairline. Care was taken to preserve the frontal branch of the facial nerve, and the fibers of the temporalis muscle were dissected and retracted bilaterally to expose the squamous temporal bone.
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Identification and Dissection of the IAC The microscope was angled so that visualization through the retractor could be achieved, the blunt end of the introducer was advanced into the opening, and the dura was gently peeled off of the floor of the middle fossa. The retractor could yaw slightly along the axial plane allowing for identification of the middle meningeal artery (MMA) and foramen spinosum. The greater superior petrosal nerve (GSPN) was identified at its exit from the facial hiatus and carefully dissected from the outer layer of temporal dura and middle fossa periosteum. The tubular retractor was yawed slightly to follow the GSPN posteriorly until the arcuate eminence was identified. The introducer was removed, the working channel was positioned up against the petrous ridge, and the arcuate eminence was fully exposed (Figure 9). Improper positioning of the retractor could result in obstruction of the surgical field by temporal dura. The tubular retractor was mounted onto a self-retaining snake retractor, via an extension arm, and locked in place. For the procedures being conducted endoscopically, the 3D endoscope was introduced into the tubular retractor at this point. Before proceeding with dissection of the IAC, correct surgical positioning was verified by confirming the location of anatomic structures in relation to the tubular retractor. The transtubular surgical field was divided into 4 quadrants: anterosuperior, anteroinferior, posterosuperior, and posteroinferior. The GSPN and facial hiatus were visualized and confirmed to be in the anterosuperior quadrant and the arcuate eminence in the posterosuperior quadrant (Figure 10). Because the arcuate eminence has a constant relationship to the location of the IAC, an imaginary line bisecting the angle between the arcuate eminence and the GSPN was located and corresponded to the location of the IAC. The location of the cochlea was identified at the vertex of the angle between the GSPN and the IAC, where the GSPN corresponds anatomically to the course of the horizontal portion of the intrapetrous internal carotid artery (ICA) (Figure 11). A minimally invasive surgical drill was used through the retractor to remove the thin layer of bone along this
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ORIGINAL ARTICLE ANTONIO BERNARDO ET AL.
MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 4. Entry point geometry. (A) A midsagittal line was drawn between the nasion and inion, and the distance, A, to a parallel sagittal line passing through the petroclival suture was measured. The lateral petroclival angle, a, was measured between the petrous bone and the sagittal line passing through the petroclival suture at the level of petrous ridge. The petroclival-acoustic distance, B, also was measured at the level of petrous ridge from the petroclival suture to the porus acusticus internus. A trajectory perpendicular to the petrous ridge, C, was then plotted and measured from the porus acusticus internus to an external point on the skull. (B) A representative computed tomography scan showing collection of the midsagittal-petroclival suture distance, lateral petroclival angle, petroclival-acoustic distance, and trajectory length. (Note: To calculate the true petroclival angle—the angle between the posterior surface of the clivus and the posterior surface of the petrous bone at the level of the petrous ridge—90 degrees should be added to the lateral petroclival angle).
bisecting line to expose the entire length of the IAC from the fundus to the porus acusticus (Figure 12). Drilling of the IAC was initiated medially at the intersection with the petrous ridge, where the bone is thickest, and continued laterally toward the fundus where Bill’s bar was exposed. To further confirm surgical accuracy, the superior semicircular canal was blue-lined in 4 cadaveric sides and a line 60 anterior to the blue-line was used to indicate the course of the IAC. With the high-speed drill, bone was removed along this imaginary line down to the porus acusticus. Bone removal extended from the premeatal triangle to the postmeatal triangle, where it was limited posteriorly to avoid damage to the superior semicircular canal. Care was taken to avoid damage to the cochlea, located in the vertex of the angle between the ICA and the most lateral aspect of the IAC (Figures 1 and 11). The IAC was unroofed by approximately 270 and exposed in its entire length from the
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lateral aspect of the fundus to the porus acusticus. The dura of the IAC was exposed and incised at the posterior aspect of the IAC toward the petrous ridge. After the dura was opened, the facial nerve, Bill’s bar, the superior vestibular nerve, the loop of the anterior inferior cerebellar artery, and the lateral surface of the pons were observed (Figure 13). Care was taken to preserve the internal auditory (labyrinthine) artery, which was identified between the facial and the cochlear nerves. In specimens containing a synthetic tumor model, resection was completed. After closure of the IAC, the retractor was removed, and the field was irrigated. Assessment of Tumor Resection, Exposure, and Maneuverability The degree of tumor resection was assessed quantitatively for all specimens with synthetic intracanalicular tumors (Table 1). Additionally, the degree of exposure of important surgical
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Figure 5. Optimal entry zone. (A) Three-dimensional graphical representation of the mean optimal entry zone, not to scale. The black dots indicate the calculated optimal entry trajectories from each of the 50 sides evaluated, and the green circle indicates the location of the optimal entry zone. All individual trajectories fell within the margins of the burr hole centered on the optimal entry zone. (B) Three-dimensional view of the optimal entry zone with the tubular retractor placed through the subtemporal burr hole.
landmarks was accomplished to evaluate surgical maneuverability on and around key structures (Table 2) (7, 30). Accordingly, a value of exposure less than 90 indicates that the structure can be exposed from a single angle, but circumferential control is absent and surgical maneuverability is not possible. A degree of exposure between 90 and 180 indicates that the structure can be exposed from different angles, but full circumferential control and surgical maneuverability are still somewhat difficult, particularly if the structure is completely encircled within the lesion. A degree of exposure greater than 180 indicates that the structure is fully exposed from different angles, control along its entire circumference is complete, and surgical maneuvers using a combination of microinstruments, suction, and/or bayoneted and tube shaft instruments through the tubular retractor are possible. RESULTS Surgical Opening and Trajectory The mean lateral petroclival angle was 41 5 degrees and mean petroclival-acoustic distance was 21 2 mm (Table 3). The corresponding mean optimal entry point was located 4 cm posterior to the lateral orbital wall, 2 cm anterior to the external auditory meatus, and flush with the superior rim of the zygomatic arch (Figure 5). This calculated optimal entry zone and trajectory was verified successfully for target accuracy on all 10 cadaveric sides via use of the trajectory planning feature of the neuronavigation software (Figure 6). Tubular Retractor All 10 approaches, microscopic and endoscopic, were successfully completed through the tubular retractor and minimally invasive burr hole with minimal retraction of the temporal lobe. The 17L VBAS provided sufficient space to allow for simultaneous
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placement of the endoscope and any combination of 2 microsuction aspirators, microinstruments, bayoneted instruments, and/or tube shaft instruments. Use of bayonetted and/or tube shaft instruments in the retractor did not obstruct the microscopic visual field. The conical shape of the retractor greatly facilitated its insertion and advancement, and its transparent walls provided excellent visualization of the peripheral anatomy—especially the key bony landmarks. Micro suction, microinstruments, and bayonetted and tube shaft instruments all were used through the retractor without difficulty. A neuronavigation pointer was also easily used through the retractor, and irrigation was applied through the retractor as needed without difficulty. Transtubular MFA The accurate placement of key visible anatomical structures in the correct transtubular visual quadrants helped ensure safe drilling of the petrous bone (Figure 10). The arcuate eminence and GSPN provided superficial landmarks to identify the deep intrapetrous locations of the superior semicircular canals and the intrapetrous ICA, respectively. The cochlea was identified in the anterosuperior quadrant, the tegmen tympani in the posterosuperior quadrant, and the intrapetrous ICA—running underneath the GSPN—in the anterosuperior quadrant. The medial portion of the IAC coursed within the anteroinferior quadrant. In 2 cases, dehiscent geniculate ganglia were observed and required additional care while drilling. After correct confirmation of surgical orientation, the cancellous bone covering the IAC was removed in the proximity of the petrous ridge using a cutting burr (Figure 12). As the bone became more compact and the dura of the IAC was approached, a diamond burr was used with copious irrigation to mitigate the risk of thermal injury to any neurovascular structures pushed up against the dura by mass effect from a tumor. In some cases,
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 6. Neuronavigation trajectory planning screenshot. The calculated optimal entry zone was verified for target accuracy using the trajectory planning feature of the neuronavigation software before each procedure.
a thin shell of bone was left over the dura and subsequently removed using a blunt dissector or fine Kerrison punch. A curved microsuction aspirator provided an unobstructed transtubular field and allowed for enhanced visualization of the surgical field. Drilling proceeded laterally using the diamond burr and bone was removed until the Bill’s bar was exposed at the fundus of the canal. At this step, extreme care was taken as the facial nerve becomes increasingly superficial as it nears the fundus and joins the geniculate ganglion—which in some cases may be dehiscent. Once the dura was properly identified, a cutting burr was again used to remove bone in the pre- and postmeatal triangles. The tubular retractor appeared to adequately limit the amount of exposure needed. The dura of the IAC was exposed and incised linearly at the posterior aspect of the IAC toward the petrous ridge. In most cases, sacrifice of the superior petrosal sinus was not necessary unless there is tumor extending medially and rostrally into the CPA. Clinically, proper intraoperative management of venous bleeding from venous plexuses and sinuses with
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hemostatic agents and cerebrospinal fluid leaks caused by opening of the tympanic cavity is essential (22). After the dura was opened, the intracanalicular segment of CN VII and the superior vestibular nerve were well observed. The inferior vestibular nerve was found inferior to the superior vestibular nerve and the cochlear nerve was observed underneath CN VII. The 90-degree medial rotation of the nerves at their entrance into the porus acusticus and the resulting anatomy, with the superior vestibular nerve posterior, the inferior vestibular nerve inferoposterior, and the cochlear nerve inferior relative to CN VII, was clearly visualized (7). Within the CPA, CN VII and the superior vestibular nerve were observed superior to the cochlear nerve, and the inferior vestibular nerve was observed anterior to CN VII and posterior to the superior vestibular nerve (Figure 13). At the fundus, Bill’s bar was observed clearly dividing the superior aspect of the IAC. Anterior to Bill’s bar, in the anterosuperior quadrant of the transtubular visual field, CN VII and the nervus intermedius were observed. The superior vestibular nerve was observed posteriorly.
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3D endoscope (Table 4). The magnification and angled view provided by the 3D endoscope allowed for enhanced anatomical exposure. DISCUSSION
Figure 7. Skin incision, burr hole placement, and tubular retractor insertion. A 4-cm vertical linear skin incision was placed beginning at the inferior rim of the zygomatic arch 4 cm anterior to the external auditory meatus and continued superiorly along the hairline. Based on the calculated optimal entry zone, a 2.5- to 3-cm burr hole was fashioned centered 4 cm posterior to the lateral orbital wall and 2 cm anterior to the external auditory meatus with its inferior margin placed as basal as possible and flush with the superior rim of the zygomatic arch. The tubular retractor was inserted gently into the extradural space through the burr hole and advanced into the field under direct microscopic visualization.
Tumor Resection Of the 6 synthetic intracanalicular tumors (1.5 cm in diameter) resected, gross total (Grade I) resections were achieved in 5 cases with greater than 98% of the tumor recovered as determined by weight and included the tumors placed anterior, posterior, and superior to the CN VIIeVIII complex. A near-total (Grade II) resection, with 96% of the tumor recovered, was achieved in the remaining case in which the tumor was placed inferior to the CN VIIeVIII complex. All minute portions of tumor that extended into the posterior fossa were resected without difficulty. Surgical Exposure Excellent visualization of the structures within the IAC was achieved through the tubular retractor with both the microscope and
Figure 8. The ViewSite Brain Access System (VBAS). The VBAS consists of an introducer inside of a working channel port. The working channel used here was modified to provide a beveled tip that facilitates dural retraction when positioned against the angled petrous ridge.
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The middle fossa, translabyrinthine, transcochlear, and retrosigmoid approaches are commonly used routes to access lesions of the IAC and CPA, including vestibular and facial schwannomas, intracanalicular meningiomas, and vascular pathologies. Determinants in selecting the most suitable surgical route to an intracanalicular lesion include pathologic characteristics and extension, size, status of auditory function, and the surgeon’s preferences. In specific cases, the MFA provides the possibility of preserving auditory and facial nerve function without the extensive cerebellar retraction required in the retrosigmoid approach. The classic microscopic MFA is generally considered for the management of small tumors (1.5 cm) of the IAC with an extension of less than 1 cm into the CPA in patients with serviceable hearing. Intraoperatively, the MFA provides exposure of the entire length of the IAC and its contents from the inner ear to the pons allowing the removal of both medially and laterally located tumors (2, 3, 16, 17). Compared with the other common approaches used for the resection of vestibular tumors, the MFA provides the greatest preservation of auditory and facial nerve function—despite the fact that CN VII often is located between the surgeon and the tumor and thus subject to surgical manipulation (3, 11, 25). However, the complex microsurgical anatomy of the petrous bone, and the neurovascular structures it encases, and the difficulty of identifying landmarks on the petrous ridge make this a technically challenging approach (Figures 1 and 11). The main drawback of the classic MFA is the risk of retraction injury to the temporal lobe as traditionally deep midline dissection of the skull base has necessitated a certain degree of temporal retraction to gain sufficient surgical exposure (3, 11, 19, 31). Extensive temporal retraction can result in postoperative seizures, aphasia, or other focal neurologic deficits, and/or venous complications (2, 3, 7, 9, 15, 31). Compromise of the inferior anastomotic vein (vein of Labbé) is a potentially serious complication that could lead to massive infarction and death in cases with a small or obstructed superior anastomotic vein (vein of Trolard) wherein the vein of Labbé may be the only site of venous drainage for the entire hemisphere (7, 9, 15). To mitigate this risk, keyhole and endoscopic subtemporal approaches to this region have been proposed (19, 21, 23, 24). However, fully endoscopic transcranial approaches carry the risk of iatrogenic injury to neurovascular structures from instruments behind the tip of the endoscope and not visible to the surgeon (21). Therefore, endoscope-assisted approaches through conventional craniotomy openings may be used but sacrifice any potential for minimally invasiveness. To overcome this, we propose the first transtubular approach to a skull base pathology and evaluate the feasibility of performing a minimally invasive percutaneous 3D endoscopic or microscopic MFA for the resection of intracanalicular lesions that provides minimal bone resection, minimal temporal retraction, protection of surrounding tissues from iatrogenic or thermal injury, and the potential for hearing preservation and an improved cosmetic outcome.
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 9. Intraoperative placement of the tubular retractor. (A) Two-dimensional and (B) 3-dimensional intraoperative images of the working channel positioned up against the right petrous ridge with exposure of the arcuate eminence in the posterosuperior quadrant (see Figure 10) after removal of the introducer.
Stereotactic cylindrical retractors, first introduced by Kelly et al. in 1988 for the excision of intraparenchymal lesions, have been since proposed for use in transcortical and transventricular surgical approaches (5, 8, 20, 29, 30). The use of frameless stereotactic techniques combined with minimally invasive tubular retractor systems have shown evidence of faster recovery times and lower morbidity rates (14). Tubular retraction in cranial surgery has been preliminarily shown to reduce pressure on cortical surfaces and, compared to brain spatulas, exerts reduced and conically distributed pressure as low as less than 10 mm Hg onto impacted surface areas, reducing the risk of neuronal damage (27). This is significant as an increase in cortical damage from retractors held in place for more than 15 minutes with 20 mm Hg of pressure has been observed in a rat model (28). Furthermore, systemic intraoperative factors, such as hypotension and blood loss, can increase vulnerability for retractor mediated cortical ischemia (27). Additionally, by limiting the range of instrument movement and protecting the surrounding tissues from instruments within the
working channel, the tubular retractor may reduce inadvertent iatrogenic instrumental injury and thermal injury from the endoscope light or electrocautery (30). Compared with freehand manipulation of the retractor, the use of a self-retaining snake retractor and extension arm may also decrease the risk of injury from torque effect (14). Postoperative magnetic resonance imaging data, following use of the tubular retractor, has shown minimal T2/fluid-attenuated inversion recovery changes along the surgical corridor indicating minimal tissue damage and ischemia (13, 14, 26). A 17L VBAS was found to be suitable for this approach. The tubular retractor was modified to provide a beveled tip that would contour to the slope of the petrous ridge and mitigate the ability for temporal dura to obscure the surgical field (Figure 8). The transparent construction allowed for visual monitoring of the retracted surfaces throughout the procedure and provided good visualization of the surrounding anatomy. There was sufficient space within the retractor to allow for simultaneous placement of the endoscope and any combination of 2 microsuction
Figure 10. Intraoperative transtubular surgical anatomy and segmentation. The key surgical landmarks, including the middle meningeal artery, greater superior petrosal nerve (GSPN), facial hiatus (FH), arcuate eminence (AE), mandibular nerve (V3), petrous ridge, and tegman tympani, as seen through the tubular retractor (top) and through the petrous bone (bottom). The transtubular surgical field was divided into 4 quadrants: anterosuperior, anteroinferior, posterosuperior, and posteroinferior. The GSPN and facial hiatus were visualized and confirmed to be in the anterosuperior quadrant and the arcuate eminence in the posterosuperior quadrant. The tip of the retractor could achieve 20 degrees of yaw and the opening of the retractor could achieve 10 degrees of negative pitch (downward in relation to the visual field, superior/rostral toward to the top of the head in relation to the anatomy) at full insertion.
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Figure 11. Three-dimensional temporal bone model. (ALC) A left temporal bone seen in increasing transparency to reveal the neurovascular structures within including the trigeminal nerve with the Gasserian ganglion and its ophthalmic, maxillary, and mandibular divisions; geniculate ganglion; greater superficial petrosal nerve; cranial nerve VII-VIII complex; chorda tympani; superior, lateral, and posterior semicircular canals, and cochlea. The intrapetrous internal carotid artery, inferior and superior petrosal sinuses, and the sigmoid sinus are shown in red and blue. The Eustachian tube, endolymphatic sac, and endolymphatic duct also are visible among other anatomical structures.
Figure 12. Drilling of the internal auditory canal (IAC). (A, C) Two-dimensional and (B, D) three-dimensional images showing the surgical drill and micro-suction aspirator in the transtubular surgical field (A, B) and the ensuing removal of bone overlying the IAC (C, D). Drilling was initiated medially at the intersection with the petrous ridge, where the roof of the IAC is thickest, and continued laterally toward the fundus.
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 13. Intradural exposure. (A, C) Two-dimensional and (B, D) three-dimensional images of the facial nerve, Bill’s bar, the superior vestibular nerve (cut to expose the inferior vestibular nerve), the loop of the anterior inferior cerebellar artery, the labyrinthine artery, and the lateral surface of the pons following dural opening. (E) Endoscopic view of the same anatomy with residual tumor seen in the cerebellopontine angle.
aspirators, microinstruments, bayoneted instruments, and/or tube shaft instruments without visually obstructing the surgical field. Instruments could be interchanged easily and quickly—with the surrounding tissues protected from accidental injury. If the use of a larger instrument or device is necessary, a retractor with a
larger distal port size may be used, but would necessitate a larger bone opening. Tube shaft, bayonetted, and long endoscopic instruments were most efficient geometrically and ergonomically, and provided more space for movement within the retractor.
Table 1. Synthetic Tumor Resection Scale Grade
Description
Percentage Removed*
I
Gross total removal of all visible macroscopic tumor model.
98100%
II
Near-total removal with only a small amount of inaccessible or unobservable tumor left in situ.
9197%
III
Subtotal removal with significant portions of inaccessible tumor left in situ.
5090%
IV
Partial removal with portions of visible tumor left in situ.
< 50%
V
Biopsy-like access with removal of only a small portion of tumor.
< 20%
*By weight.
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Table 2. Characteristics of the Different Degrees of Exposure (9) Degree of Exposure
Angles Exposed
Circumferential Control
Single
Absent
90180
Multiple
Difficult
> 180
All
Present
< 90
The rigid nature of the tubular retractor coupled with the use of a small minimally invasive bone opening required accurate determination of an optimal entry point that would provide a direct trajectory to the petrous bone and adequate control of the entire surgical field (Figures 2 and 4). The calculation of a mean lateral petroclival angle, petroclival-acoustic distance, and trajectory perpendicular to the petrous ridge at the porus acusticus from a small series of 25 CT scans (50 sides) enabled determination of an optimal entry zone—located 4 cm posterior to the lateral orbital wall, 2 cm anterior to the external auditory meatus, and flush with the superior rim of the zygomatic arch. Placement of the opening flush with the floor of the middle fossa was essential for achieving minimal temporal lobe retraction. The 2.5- to 3-cm diameter burr hole facilitated easy insertion of the retractor and allowed for the tip of the retractor to achieve 20 degrees of yaw and the opening of the retractor to achieve 10 degrees of negative pitch (downward in relation to the visual field, superior/rostral toward to the top of the head in relation to the anatomy) at full insertion (Figure 10). Insertion of the tubular retractor into the surgical field was performed from a lateral to medial direction with the tip aimed a few degrees anterior to the planned trajectory toward the MMA and V3. The MMA, a tethering point that hinders further elevation of the temporal lobe dura from the floor of middle fossa, was in most cases not incised as complete elevation of the temporal lobe dura was not necessary. Careful dissection of the layers of the temporal dura around foramen ovale and V3 coupled with minimal elevation of the temporal lobe can help to reduce epidural venous bleeding in clinical cases. The periosteal dura encasing GSPN was separated from the temporal dura to allow for the nerve to remain on the floor of the middle fossa while elevation of the temporal dura was continued posteriorly. The GSPN, identified medial to the MMA in its anterior course toward V3, was followed to its exit from the facial hiatus by yawing the tip of the retractor 10 degrees posteriorly. Slight elevation of the GSPN and periosteal fibers enabled visualization of the facial hiatus. The tip of the retractor was yawed a few additional degrees posteriorly to enable identification of the most lateral segment of the arcuate eminence. Bisecting the 120-degree angle between the GSPN, overlying the course of the intrapetrous ICA, and the arcuate eminence is an effective technique for locating the course of the IAC. A perpendicular surgical trajectory to the superior surface of the petrous bone between these 2 structures is thus essential for correctly identifying this angle and its bisecting line. In the approximately 15% of patients who lack an identifiable arcuate eminence (18), an imaginary line at a 60-degree angle medial to the GSPN may be used to locate the course of the IAC. Because of the complex microsurgical anatomy of this region, the transtubular surgical field was divided into 4 quadrants to ease
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surgical orientation, including anterosuperior, anteroinferior, posterosuperior, and posteroinferior. Before proceeding with dissection of the IAC, correct surgical positioning was verified by confirming the location of anatomic structures in relation to these quadrants (Figure 10). The anterosuperior quadrant contains the facial hiatus, proximal portion of the GSPN, initial horizontal portion of the intrapetrous ICA, and the cochlea. Of these, the facial hiatus and GSPN should be visualized in this quadrant. The posterosuperior quadrant contains the lateral segment of the arcuate eminence, posterior semicircular canal, a portion of the tegmen tympani, and the posterior portion of the fundus. Of these, the lateral segment of the arcuate eminence should be visualized in this quadrant. The anteroinferior quadrant contains the majority of the anterior aspect of the medial portion of the IAC and a part of the cancellous bone of the petrous apex. There are no visible landmarks in this quadrant. The posteroinferior quadrant contains the posterior aspect of the medial portion of the IAC and the bone of the postmeatal triangle. There are no visible landmarks in this quadrant, although in 2 specimens it was possible to visualize the superficial portion of the arcuate eminence here. Because of the 45-degree angle between the course of the IAC and the superior surface of the petrous bone, the tubular retractor was pitched 10 degrees superiorly to align the surgical trajectory as perpendicular as possible to the petrous bone and with the roof of the IAC while applying only minimal retraction of the temporal lobe (Figure 9). This maneuver allows for full exposure of the fundus of the IAC. Final placement of the tip of the retractor—with the facial hiatus in the anterosuperior visual quadrant and the lateral segment of the arcuate eminence in the posterosuperior visual quadrant—proved in all cases to be an accurate location for uncomplicated exposure of the IAC. Multiangled surgical exposure within the IAC was present and enabled sufficient intraoperative maneuverability (Table 4). After the dura of the IAC was opened, the facial and superior vestibular nerves were immediately exposed from the CPA up to their divergence at Bill’s bar and into the fundus of the canal. CN VII may be moved slightly anteriorly or, after the superior vestibular nerve is cut, slightly posteriorly to allow for improved access to the cochlear nerve. The 270-degree exposure of the IAC with removal of bone in both the pre- and postmeatal triangles provides the ability for full surgical control of the contents of the canal, especially in cases in which tumor displaces the anatomy anteriorly or posteriorly within the canal. The use of a synthetic tumor model provided a means for more closely simulating clinical conditions and provided an additional method of quantitatively evaluating the surgical applicability of this approach through the assessment of the
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Table 3. Petroclival Angles and Petroclival-Auditory Distances Midsagittal*-Petroclival Suture Distance, mm
Lateral Petroclival Angle*
Petroclival-Acoustic Distance, mm
Trajectory Length, mm
1
11
39
22
42
2
10
31
23
44
3
11
43
21
42
4
9
41
21
37
5
10
44
21
37
6
9
34
22
39
7
10
43
27
40
8
10
28
25
38
9
12
43
23
43
10
12
45
24
40
11
10
36
23
43
12
10
39
22
44
13
12
40
20
39
14
11
39
21
41
15
11
40
22
41
16
10
46
22
41
17
12
33
22
42
18
10
36
23
43
19
12
48
21
37
20
11
47
21
36
21
10
48
19
38
22
10
44
19
39
23
9
37
20
39
24
9
35
20
39
25
11
41
21
42
26
11
42
23
40
27
10
44
18
34
28
10
41
20
36
29
8
42
20
43
30
9
41
21
42
31
10
41
19
33
32
10
45
20
35
33
11
38
21
36
34
11
45
24
40
35
11
44
20
47
36
11
32
21
46
37
12
42
18
44
38
12
45
23
42
39
10
41
21
40
No.
Continues
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Table 3. Continued No.
Midsagittal*-Petroclival Suture Distance, mm
Lateral Petroclival Angle*
Petroclival-Acoustic Distance, mm
Trajectory Length, mm
40
10
37
21
38
41
11
42
19
39
42
11
45
20
40
43
11
37
21
39
44
10
36
21
38
45
8
33
20
38
46
8
32
20
40
47
12
46
22
47
48
11
42
23
47
49
11
49
24
37
12
48
24
39
Mean
10 1
41 5
21 2
40 3
Range
8e12
28e49
18e27
33e47
50 y
*The true petroclival angle can calculated by adding 90 degrees to the lateral petroclival angle. yThe midsagittal line was placed between the nasion and the inion.
degree of tumor resection (Figure 3 and Table 1). To more closely recreate lesions encountered clinically, tumors were placed anterior (2 sides), posterior (2 sides), superior (1 side), and inferior (1 side) to the CN VIIeVIII complex. In cases in which tumor was pushing the nerve complex toward the superior aspect of the dura of the IAC, the dural opening was fashioned near to the petrous ridge on the side of the postmeatal triangle where there is increased distance between the dura and the nerves. When attempting to expose tumors that follow CN VII in the space anterior to Bill’s bar, care should be taken to avoid damaging the cochlea, which is located immediately anterior to this compartment. In cases in which the nerves are pushed inferiorly from superior lesions, the nerves should first be exposed in the CPA and then followed into the canal before debulking of the tumor is
attempted. For small lesions located mainly in the CPA with minimal extension into the canal, it is safer to open the dura at the posterior aspect of the IAC on the side of the arcuate eminence. The loop of the anterior inferior cerebellar artery is easily controlled in the CPA and in cases in which the loop enters the IAC. The ability for transtubular synthetic tumor resection was confirmed with gross total resections (Grade I) achieved in 5 cases with greater than 98% of the tumor recovered, including the tumors located anterior, posterior, and superior to the CN VIIeVIII complex. The remaining tumor, located inferior to the CN VIIeVIII complex, was only near totally resected (Grade II) with 96% of the tumor recovered because of its location at the fundus of the IAC, where excessive manipulation of the facial nerve would have been required to achieve a gross total resection. Because of
Table 4. Exposure of Target Surgical Structures Anatomical Structure
Microscopic Exposure, degree
Endoscopic Exposure, degree
Anterior inferior cerebellar artery loop
220
360
CN VII
220
360
CN VIII
120
220
Geniculate ganglion
180
180
Inferior vestibular nerves
120
220
Internal auditory (labyrinthine) artery
220
360
Superior vestibular nerve
220
360
CN, cranial nerve.
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
the limited size of the surgical corridor, the transtubular MFA should be limited to tumors less than 1.5 cm in diameter and with minimal extension into the CPA. Use of the 3D endoscope provided valuable stereoscopic perception (6), although because of its small size no specific advantage was noted when the endoscope was used within the IAC. The 3D endoscope, however, proved useful in exploring the CPA, checking for any potential anatomical variance, and inspecting for residual tumor. The 3D endoscope facilitated resection of the portions of synthetic tumor located within the CPA. We believe that the endoscope also may be safely used on its own as an alternative to the microscope; however the microscope—with use of bayonetted and tube shaft instruments—provided sufficient exposure through the narrow working channel and of the CPA. The minimally invasive 15-cm drill attachment, with a 10-degree curvature and the ability to telescope 3 mm, allowed for unobstructed visualization of the surgical field while unroofing the IAC. Despite the detailed multiangled anatomical exposure provided by the endoscope, surgeons should be extremely familiar with the associated anatomy, as well as the use of instruments through a tubular retractor, before attempting a transtubular approach. Although the retractor provides a safe corridor for the approach, it should not provide the surgeon with a false sense of safety— awareness of the position and anatomical surroundings of each instrument is still paramount to prevent tissue damage. Each surgeon who participated in this study spent several hours
REFERENCES 1. Abdel Aziz KM, Sanan A, van Loveren HR, Tew JM Jr, Keller JT, Pensak ML: Petroclival meningiomas: predictive parameters for transpetrosal approaches. Neurosurgery 47:139-152, 2000. 2. Angeli S: Middle fossa approach: indications, technique and results. Otolaryngol Clinic N Am 45:417-438, 2012. 3. Ansari SF, Terry C, Cohen-Gadol AA: Surgery for vestibular schwannomas: a systematic review of complications by approach. Neurosurg Focus 33: E14, 2012. 4. Baidya NB, Berhouma M, Ammirati M: Endoscope-assisted retrosigmoid resection of a medium size vestibular schwannoma tumor model: a cadaveric study. Clin Neurol Neurosurg 119:35-38, 2014. 5. Barlas O, Karadereler S: Stereotactically guided microsurgical removal of colloid cysts. Acta Neurochir 146:1199-1204, 2004. 6. Bernardo A, Boeris D, Evins AI, Anichini G, Stieg PE: A combined dual-port endoscopeassisted pre- and retrosigmoid approach to the cerebellopontine angle: an extensive anatomosurgical study. Neurosurg Rev 37:597-608, 2014. 7. Bernardo A, Evins AI, Visca A, Stieg PE: The intracranial facial nerve as seen through different surgical windows: an extensive anatomosurgical study. Neurosurgery 72 (2 Suppl Operative): ons194-207, 2013.
practicing using different instruments to handle objects, such as small paper cubes and tissue paper, through the retractor in order to gain familiarity with transtubular technique before attempting it on cadavers. Despite the learning curve associated with transtubular technique, the principles are mainly the same as those of microsurgery (14, 30). Development of instruments specifically designed to accompany this system could contribute to the safety of this technique. Future designs of the VBAS may benefit from incorporation of additional shapes and multiple beveled tip options. Although a neuronavigation pointer was used without difficulty here, a retractor with integrated neuronavigation markers would greatly ease insertion and enable constant monitoring of position and trajectory relative to anatomical landmarks. CONCLUSION The microscopic and/or endoscopic percutaneous transtubular MFA is a valid minimally invasive and potentially safer option for the management of small intracanalicular lesions. The tubular retractor provides reduced and conically distributed pressure onto the temporal lobe and may protect surrounding tissues from accidental instrumental and/or thermal injury. Improved cosmetic outcomes may be achieved from the minimally invasive incision and bone opening. The combination of 3D-endoscopic visualization and a clear plastic retractor facilitates safe direct monitoring of the surgical field.
8. Dorman J: Tumor resection utilizing a minimally invasive spinal retractor with a novel cranial adaptor. Minim Invasive Neurosurg 51:358-360, 2008. 9. Evins AI, Santillan A, Burrell J, Tebo C, Stieg PE, Bernardo A: External landmarks for identifying the drainage site of the vein of Labbé. Paper presented at: Congress of Neurological Surgeons 2012 Annual Meeting; October 6e10, 2012; Baltimore, MD. Available at: http://w3.cns.org/dp/ 2012CNS/1213.pdf. Accessed June 3, 2014. 10. Evins AI, Shoakazemi A, Burrell JC, Kapoor R, Stieg PE, Bernardo A: 3D endoscopic transtubular anterior petrosectomy for petroclival meningiomas: assessment of resection in varying tumor volumes utilizing a synthetic tumor model [NASBS abstract A036]. J Neurol Surg B Skull Base 75 (Suppl 1):A036, 2014. 11. Ginzkey C, Scheich M, Harnisch W, Bonn V, Ehrmann-Müller D, Shehata-Dieler W, Mlynski R, Hagen R: Outcome on hearing and facial nerve function in microsurgical treatment of small vestibular schwannoma via the middle cranial fossa approach. Eur Arch Otorhinolaryngol 270: 1209-1216, 2013. 12. Gragnaniello C, Nader R, van Doormaal T, Kamel M, Voormolen EH, Lasio G, Aboud E, Regli L, Tulleken CA, Al-Mefty O: Skull base tumor model. J Neurosurg 113:1006-1111, 2010. 13. Greenfield JP, Cobb WS, Tsouris AJ, Schwartz TH: Stereotactic minimally invasive tubular retractor system for deep brain lesions. Neurosurgery 63 (4 Suppl 2):334-340, 2008.
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14. Harris AE, Hadjipanayis CG, Lunsford LD, Lunsford AK, Kassam AB: Microsurgical removal of intraventricular lesions using endoscopic visualization and stereotactic guidance. Neurosurgery 62 (Suppl 2):622-629, 2008. 15. House JW, Cullen RD: Complications of neurotologic surgery. In: Friedman RA, ed. Lateral skull base surgery. New York: Thieme; 2012:165-172. 16. House WF, Shelton C: Middle Fossa Approach for acoustic tumor removal. Otolaryngol Clin North Am 25:347-359, 1992. 17. House WF: Surgical exposure of the internal auditory canal and its contents through the middle cranial fossa. Laryngoscope 71:1363-1385, 1961. 18. Kartush JM, Kemink JL, Graham MD: The arcuate eminence. Topographic orientation in middle cranial fossa surgery. Ann Otol Rhinol Laryngol 94:25-28, 1985. 19. Kawase T: Endoscopic middle fossa approach for central skull base. World Neurosurg 80:505-506, 2013. 20. Kelly PJ, Goerss SJ, Kall BA: The stereotaxic retractor in computer-assisted stereotaxic microsurgery. Technical note. J Neurosurg 69:301-306, 1988. 21. Komatsu F, Komatsu M, Di Ieva A, Tschabitscher M: Endoscopic extradural subtemporal approach to lateral and central skull base: a cadaveric study. World Neurosurg 80: 591-597, 2013.
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22. Komatsu M, Komatsu F, Di Ieva A, Inoue T, Tschabitscher M: Endoscopic reconstruction of the middle cranial fossa through a subtemporal keyhole using a pedicled deep temporal fascial flap: a cadaveric study. Neurosurgery 70 (1 Suppl Operative):157-162, 2012. 23. Perneczky A, Reisch R: Subtemporal approach. In: Perneczky A, Reisch R, eds. Concept and Surgical Technique. Keyhole Approaches in Neurosurgery, Volume I. Vienna: Springer; 2008:97-158. 24. Pichierri A, D’Avella E, Ruggeri A, Tschabitscher M, Delfini R: Endoscopic assistance in the epidural subtemporal approach and Kawase approach: anatomic study. Neurosurgery 67 (3 Suppl Operative):ons29-37, 2010.
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26. Raza SM, Recinos PF, Avendano J, Adams H, Jallo GI, Quinones-Hinojosa A: Minimally invasive trans-portal resection of deep intracranial lesions. Minim Invasive Neurosurg 54:5-11, 2011. 27. Recinos PF, Raza SM, Jallo GI, Recinos VR: Use of a minimally invasive tubular retraction system for deep-seated tumors in pediatric patients. J Neurosurg Pediatr 7:516-521, 2011. 28. Rosenørn J, Diemer N: The risk of cerebral damage during graded brain retractor pressure in the rat. J Neurosurg 63:608-611, 1985.
31. Whitfield PC, Hardy DG: Cerebello-pontine angle tumors. In: Moore AJ, Newell DW, eds. Neurosurgery: Principles and Practice. London: Springer; 2005:247-262.
Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Received 3 September 2014; accepted 25 February 2015
29. Ross DA: A simple stereotactic retractor for use with the Leksell stereotactic system. Neurosurgery 32:475-476, 1993.
Citation: World Neurosurg. (2015) 84, 1:132-146. http://dx.doi.org/10.1016/j.wneu.2015.02.042 Journal homepage: www.WORLDNEUROSURGERY.org
25. Post KD, Eisenberg MB, Catalano PJ: Hearing preservation in vestibular schwannoma surgery: what factors influence outcome? J Neurosurg 83: 191-196, 1995.
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30. Shoakazemi A, Evins AI, Burrell JC, Stieg PE, Bernardo A: A 3D-endoscopic transtubular transcallosal approach to the third ventricle. J Neurosurg 122:564-573, 2015.
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A Percutaneous Transtubular Middle Fossa Approach for Intracanalicular Tumors Antonio Bernardo1, Alexander I. Evins1, Apostolos J. Tsiouris2, Philip E. Stieg1
OBJECTIVE: In cases of small intracanalicular tumors (£1.5 cm), the middle fossa approach (MFA) provides the ability for adequate tumor removal with preservation of existing auditory function. Application of a minimally invasive tubular retractor in this approach may help mitigate the risk of postoperative seizures, aphasia, and venous complications by minimizing intraoperative retraction of the temporal lobe. We propose a minimally invasive microscopic and/or endoscopic percutaneous transtubular MFA for the management of intracanalicular tumors.
-
METHODS: Subtemporal keyhole craniectomies were performed on 5 preserved cadaveric heads (10 sides), with 6 sides previously injected with a synthetic tumor model. A ViewSite Brain Access System tubular retractor (Vycor Medical, Inc., Boca Raton, Florida, USA) was used to provide minimal temporal retraction and protection of the surrounding anatomy. An extradural dissection of the internal auditory canal was performed under microscopic and endoscopic visualization with a minimally invasive surgical drill and tube shaft instruments, the intracanalicular tumors were removed, and degree of resection was assessed.
-
RESULTS: All 10 approaches were completed successfully through the tubular retractor with minimal retraction of the temporal lobe. Excellent visualization of the structures within the internal auditory canal was achieved with both the microscope and 3-dimensional endoscope. On the 6 synthetic intracanalicular tumors resected, 5 gross total (Grade I) and 1 near total (Grade II) resections were achieved.
-
Key words Endoscopic - Internal auditory canal - Middle fossa - Minimally invasive - Transtubular - Tubular retractor -
Abbreviations and Acronyms CN: Cranial nerve CPA: Cerebellopontine angle CT: Computed tomography GSPN: Greater superior petrosal nerve IAC: Internal auditory canal ICA: Internal carotid artery MFA: Middle fossa approach
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CONCLUSION: A percutaneous transtubular MFA is a feasible minimally invasive option for resection of small intracanalicular tumors with potential preservation of auditory function, reduced temporal retraction, and enhanced protection of surrounding structures.
-
INTRODUCTION
T
he middle fossa approach (MFA), first proposed in the earlier 20th century for the treatment of otosclerosis and later for the resection of acoustic neuromas by House and Shelton, is indicated for the management of small tumors (1.5 cm) of the internal auditory canal (IAC) with an extension of less than 1 cm into the cerebellopontine angle (CPA) in patients with no preoperative auditory deficits (2, 16, 17, 31). The MFA provides exposure of the entire length of the IAC and the facial (cranial nerve [CN] VII) and vestibular (CN VIII) nerves from the inner ear to the pons (2, 16, 17). Compared with other approaches used for the resection of intracanalicular vestibular tumors, the MFA provides the greatest preservation of auditory and facial nerve function (3, 25). However, the complex microsurgical anatomy of the petrous bone and the neurovascular structures it encases, and the difficulty of identifying landmarks on the petrous ridge, make this a technically challenging approach (Figure 1). The main drawback of the classic MFA is the risk of retraction injury to the temporal lobe (19, 31). Extensive temporal retraction can result in postoperative seizures, aphasia, or other focal neurologic deficits, and/ or venous complications from the compromise of the inferior anastomotic cerebral vein (vein of Labbé) (2, 3, 7, 9, 15). Stereotactic cylindrical retractors, first introduced by Kelly et al. (20) in 1988 for the excision of deep intraparenchymal lesions and MMA: Middle meningeal artery 3D: Three-dimensional VBAS: ViewSite Brain Access System From the 1Department of Neurological Surgery, Weill Cornell Medical College, New York Presbyterian Hospital, New York, New York, USA; and 2Department of Neuroradiology, Weill Cornell Medical College, New York Presbyterian Hospital, New York, New York, USA To whom correspondence should be addressed: Antonio Bernardo, M.D. [E-mail: [email protected]] Citation: World Neurosurg. (2015) 84, 1:132-146. http://dx.doi.org/10.1016/j.wneu.2015.02.042 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2015 Elsevier Inc. All rights reserved.
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ORIGINAL ARTICLE ANTONIO BERNARDO ET AL.
MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 1. Three-dimensional model of the neurovascular structures around and within the temporal bone, including the trigeminal nerve with the Gasserian ganglion and its ophthalmic, maxillary, and mandibular divisions; geniculate ganglion; greater superficial petrosal nerve; CN VII-VIII complex; chorda tympani; superior, lateral, and posterior semicircular canals; and cochlea. The intrapetrous internal carotid artery, inferior and superior petrosal sinuses, and the sigmoid sinus are shown in red and blue. The Eustachian tube, endolymphatic sac, and endolymphatic duct also are visible among other anatomical structures.
proposed for use in surgical approaches to deep-seated intraparenchymal and intraventricular lesions (30), may help mitigate the aforementioned postoperative complications. A transtubular approach to a skull base pathology that would allow for a smaller bone opening, minimal extradural cortical retraction, enhanced protection of surrounding tissues, and an improved cosmetic outcome has yet to be described. We propose and evaluate the feasibility of a minimally invasive microscopic and/ or endoscopic percutaneous transtubular MFA for the management of intracanalicular tumors. METHODS Microscopic (5 sides) and 3-dimensional (3D) endoscopic (5 sides) percutaneous MFAs were performed through a tubular retractor system on 5 preserved cadaveric heads (10 sides), all previously injected with colored latex—red for arteries, blue for veins (Figure 2). Six sides also were injected previously with a synthetic intracanalicular tumor model. Dissections were completed with a neurosurgical microscope (Zeiss OPMI Neuro/NC 4 System, Carl Zeiss Meditec AG; Jena, Germany) and 3D endoscope (VSIII; Visionsense Ltd.; New York, New York, USA) with 0 , 30 , and 90 angled optics. An Anspach eMax 2 Plus (Synthes Inc.; Palm Beach Gardens, Florida, USA) electric neurosurgical drill was used with a minimally invasive attachment. Synthetic Tumor Model Three specimens (6 sides) were injected preoperatively with synthetic tumor models (ST-540 Injection Resin, Strata-Tech; Des Moines, Iowa, USA) bilaterally using a Teflon integrated curved IV catheter through a retrosigmoid route to simulate small
WORLD NEUROSURGERY 84 [1]: 132-146, JULY 2015
Figure 2. The transtubular middle fossa approach. Depiction of the percutaneous keyhole opening, surgical trajectory, and key anatomical landmarks.
intracanalicular tumors (1.5 cm in diameter) with minimal extension into the posterior fossa (4, 10, 12). The tumors were placed anterior (2 sides), posterior (2 sides), superior (1 side), and inferior (1 side) to the CN VIIeVIII complex. The synthetic tumor resin was mixed with a radiopaque solution (Omnipaque [iohexol] solution, GE Healthcare Inc.; Little Chalfont, United Kingdom) to appear hyperdense on computed tomography (CT) (Figure 3). Neuronavigation For image-guided neuronavigation, 6 skin markers (fiducials) that appear on CT scans were affixed to the cranium. One-millimeter spiral CT axial slices were obtained (Biograph TruePoint PETCT, Siemens AG; Munich, Germany) of each specimen and transferred to the neuronavigation workstation (Kolibri ImageGuided Surgery Platform, Brainlab AG; Feldkirchen, Germany) for spatial registration. Entry and Trajectory Planning Because of the small size of the bone opening and the rigid nature of the tubular retractor, determination of an optimal entry point is essential for ensuring an accurate trajectory to the petrous bone. To define an optimal entry zone, the lateral petroclival angle—the angle between the petrous bone and a sagittal line passing through the petroclival suture at the level of petrous ridge—and the distance from the petroclival suture to the porus acusticus
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On the basis of the calculated optimal entry zone, a 2.5- to 3-cm diameter burr hole was fashioned centered 4 cm posterior to the lateral orbital wall and 2 cm anterior to the external auditory meatus with its inferior margin placed as basal as possible and flush with the superior rim of the zygomatic arch (Figure 7). The dura was detached gently from the margins of the burr hole and raised from the floor of the middle fossa. Tubular Retractor System The tubular retractor system (17L [17 mm width 11 mm height 70 mm length] ViewSite Brain Access System [VBAS], Vycor Medical Inc.; Boca Raton, Florida, USA) consisted of an introducer inside of a working channel port. The working channel used herein was modified to provide a beveled tip that facilitated dural retraction when positioned against the angled petrous ridge (Figure 8). The tubular retractor was mounted onto an extension arm (Vycor Medical Inc.) attached to a self-retaining snake retractor (Mizuho America, Inc., Union City, California, USA) and inserted gently into the extradural space through the burr hole under direct microscopic visualization.
Figure 3. Synthetic tumor model. Computed tomography scan of a cadaveric specimen with a small (1. 5 cm in diameter) hyperdense contrast-enhancing synthetic intracanalicular tumor (yellow arrow) with extension into the cerebellopontine angle.
internus, the petroclival-acoustic distance, were first determined with the 5 obtained CT scans and an additional 20 normal adult head CT scans (50 sides) (Figure 4) (1). A trajectory, perpendicular to the petrous ridge, was then plotted and measured from the porus acusticus internus to an external point on the skull flush with the floor of the middle fossa. Mean lateral petroclival angles and corresponding distances were calculated to determine an optimal entry zone (Figure 5). The calculated optimal entry zone was correlated to external landmarks and then verified for target accuracy using the trajectory planning feature of the neuronavigation software on all ten cadaveric sides (Figure 6). Positioning, Incision, and Burr Hole Placement Three-point fixation was achieved using a Mayfield head holder and each head was positioned supine with 90 rotation to the contralateral side to provide an unobstructed view of the middle fossa floor and 20 lateroflection to facilitate gravitational retraction of the temporal lobe. A 4-cm vertical linear skin incision was placed beginning at the inferior rim of the zygomatic arch 4 cm anterior to the external auditory meatus and continued superiorly along the hairline. Care was taken to preserve the frontal branch of the facial nerve, and the fibers of the temporalis muscle were dissected and retracted bilaterally to expose the squamous temporal bone.
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Identification and Dissection of the IAC The microscope was angled so that visualization through the retractor could be achieved, the blunt end of the introducer was advanced into the opening, and the dura was gently peeled off of the floor of the middle fossa. The retractor could yaw slightly along the axial plane allowing for identification of the middle meningeal artery (MMA) and foramen spinosum. The greater superior petrosal nerve (GSPN) was identified at its exit from the facial hiatus and carefully dissected from the outer layer of temporal dura and middle fossa periosteum. The tubular retractor was yawed slightly to follow the GSPN posteriorly until the arcuate eminence was identified. The introducer was removed, the working channel was positioned up against the petrous ridge, and the arcuate eminence was fully exposed (Figure 9). Improper positioning of the retractor could result in obstruction of the surgical field by temporal dura. The tubular retractor was mounted onto a self-retaining snake retractor, via an extension arm, and locked in place. For the procedures being conducted endoscopically, the 3D endoscope was introduced into the tubular retractor at this point. Before proceeding with dissection of the IAC, correct surgical positioning was verified by confirming the location of anatomic structures in relation to the tubular retractor. The transtubular surgical field was divided into 4 quadrants: anterosuperior, anteroinferior, posterosuperior, and posteroinferior. The GSPN and facial hiatus were visualized and confirmed to be in the anterosuperior quadrant and the arcuate eminence in the posterosuperior quadrant (Figure 10). Because the arcuate eminence has a constant relationship to the location of the IAC, an imaginary line bisecting the angle between the arcuate eminence and the GSPN was located and corresponded to the location of the IAC. The location of the cochlea was identified at the vertex of the angle between the GSPN and the IAC, where the GSPN corresponds anatomically to the course of the horizontal portion of the intrapetrous internal carotid artery (ICA) (Figure 11). A minimally invasive surgical drill was used through the retractor to remove the thin layer of bone along this
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 4. Entry point geometry. (A) A midsagittal line was drawn between the nasion and inion, and the distance, A, to a parallel sagittal line passing through the petroclival suture was measured. The lateral petroclival angle, a, was measured between the petrous bone and the sagittal line passing through the petroclival suture at the level of petrous ridge. The petroclival-acoustic distance, B, also was measured at the level of petrous ridge from the petroclival suture to the porus acusticus internus. A trajectory perpendicular to the petrous ridge, C, was then plotted and measured from the porus acusticus internus to an external point on the skull. (B) A representative computed tomography scan showing collection of the midsagittal-petroclival suture distance, lateral petroclival angle, petroclival-acoustic distance, and trajectory length. (Note: To calculate the true petroclival angle—the angle between the posterior surface of the clivus and the posterior surface of the petrous bone at the level of the petrous ridge—90 degrees should be added to the lateral petroclival angle).
bisecting line to expose the entire length of the IAC from the fundus to the porus acusticus (Figure 12). Drilling of the IAC was initiated medially at the intersection with the petrous ridge, where the bone is thickest, and continued laterally toward the fundus where Bill’s bar was exposed. To further confirm surgical accuracy, the superior semicircular canal was blue-lined in 4 cadaveric sides and a line 60 anterior to the blue-line was used to indicate the course of the IAC. With the high-speed drill, bone was removed along this imaginary line down to the porus acusticus. Bone removal extended from the premeatal triangle to the postmeatal triangle, where it was limited posteriorly to avoid damage to the superior semicircular canal. Care was taken to avoid damage to the cochlea, located in the vertex of the angle between the ICA and the most lateral aspect of the IAC (Figures 1 and 11). The IAC was unroofed by approximately 270 and exposed in its entire length from the
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lateral aspect of the fundus to the porus acusticus. The dura of the IAC was exposed and incised at the posterior aspect of the IAC toward the petrous ridge. After the dura was opened, the facial nerve, Bill’s bar, the superior vestibular nerve, the loop of the anterior inferior cerebellar artery, and the lateral surface of the pons were observed (Figure 13). Care was taken to preserve the internal auditory (labyrinthine) artery, which was identified between the facial and the cochlear nerves. In specimens containing a synthetic tumor model, resection was completed. After closure of the IAC, the retractor was removed, and the field was irrigated. Assessment of Tumor Resection, Exposure, and Maneuverability The degree of tumor resection was assessed quantitatively for all specimens with synthetic intracanalicular tumors (Table 1). Additionally, the degree of exposure of important surgical
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Figure 5. Optimal entry zone. (A) Three-dimensional graphical representation of the mean optimal entry zone, not to scale. The black dots indicate the calculated optimal entry trajectories from each of the 50 sides evaluated, and the green circle indicates the location of the optimal entry zone. All individual trajectories fell within the margins of the burr hole centered on the optimal entry zone. (B) Three-dimensional view of the optimal entry zone with the tubular retractor placed through the subtemporal burr hole.
landmarks was accomplished to evaluate surgical maneuverability on and around key structures (Table 2) (7, 30). Accordingly, a value of exposure less than 90 indicates that the structure can be exposed from a single angle, but circumferential control is absent and surgical maneuverability is not possible. A degree of exposure between 90 and 180 indicates that the structure can be exposed from different angles, but full circumferential control and surgical maneuverability are still somewhat difficult, particularly if the structure is completely encircled within the lesion. A degree of exposure greater than 180 indicates that the structure is fully exposed from different angles, control along its entire circumference is complete, and surgical maneuvers using a combination of microinstruments, suction, and/or bayoneted and tube shaft instruments through the tubular retractor are possible. RESULTS Surgical Opening and Trajectory The mean lateral petroclival angle was 41 5 degrees and mean petroclival-acoustic distance was 21 2 mm (Table 3). The corresponding mean optimal entry point was located 4 cm posterior to the lateral orbital wall, 2 cm anterior to the external auditory meatus, and flush with the superior rim of the zygomatic arch (Figure 5). This calculated optimal entry zone and trajectory was verified successfully for target accuracy on all 10 cadaveric sides via use of the trajectory planning feature of the neuronavigation software (Figure 6). Tubular Retractor All 10 approaches, microscopic and endoscopic, were successfully completed through the tubular retractor and minimally invasive burr hole with minimal retraction of the temporal lobe. The 17L VBAS provided sufficient space to allow for simultaneous
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placement of the endoscope and any combination of 2 microsuction aspirators, microinstruments, bayoneted instruments, and/or tube shaft instruments. Use of bayonetted and/or tube shaft instruments in the retractor did not obstruct the microscopic visual field. The conical shape of the retractor greatly facilitated its insertion and advancement, and its transparent walls provided excellent visualization of the peripheral anatomy—especially the key bony landmarks. Micro suction, microinstruments, and bayonetted and tube shaft instruments all were used through the retractor without difficulty. A neuronavigation pointer was also easily used through the retractor, and irrigation was applied through the retractor as needed without difficulty. Transtubular MFA The accurate placement of key visible anatomical structures in the correct transtubular visual quadrants helped ensure safe drilling of the petrous bone (Figure 10). The arcuate eminence and GSPN provided superficial landmarks to identify the deep intrapetrous locations of the superior semicircular canals and the intrapetrous ICA, respectively. The cochlea was identified in the anterosuperior quadrant, the tegmen tympani in the posterosuperior quadrant, and the intrapetrous ICA—running underneath the GSPN—in the anterosuperior quadrant. The medial portion of the IAC coursed within the anteroinferior quadrant. In 2 cases, dehiscent geniculate ganglia were observed and required additional care while drilling. After correct confirmation of surgical orientation, the cancellous bone covering the IAC was removed in the proximity of the petrous ridge using a cutting burr (Figure 12). As the bone became more compact and the dura of the IAC was approached, a diamond burr was used with copious irrigation to mitigate the risk of thermal injury to any neurovascular structures pushed up against the dura by mass effect from a tumor. In some cases,
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 6. Neuronavigation trajectory planning screenshot. The calculated optimal entry zone was verified for target accuracy using the trajectory planning feature of the neuronavigation software before each procedure.
a thin shell of bone was left over the dura and subsequently removed using a blunt dissector or fine Kerrison punch. A curved microsuction aspirator provided an unobstructed transtubular field and allowed for enhanced visualization of the surgical field. Drilling proceeded laterally using the diamond burr and bone was removed until the Bill’s bar was exposed at the fundus of the canal. At this step, extreme care was taken as the facial nerve becomes increasingly superficial as it nears the fundus and joins the geniculate ganglion—which in some cases may be dehiscent. Once the dura was properly identified, a cutting burr was again used to remove bone in the pre- and postmeatal triangles. The tubular retractor appeared to adequately limit the amount of exposure needed. The dura of the IAC was exposed and incised linearly at the posterior aspect of the IAC toward the petrous ridge. In most cases, sacrifice of the superior petrosal sinus was not necessary unless there is tumor extending medially and rostrally into the CPA. Clinically, proper intraoperative management of venous bleeding from venous plexuses and sinuses with
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hemostatic agents and cerebrospinal fluid leaks caused by opening of the tympanic cavity is essential (22). After the dura was opened, the intracanalicular segment of CN VII and the superior vestibular nerve were well observed. The inferior vestibular nerve was found inferior to the superior vestibular nerve and the cochlear nerve was observed underneath CN VII. The 90-degree medial rotation of the nerves at their entrance into the porus acusticus and the resulting anatomy, with the superior vestibular nerve posterior, the inferior vestibular nerve inferoposterior, and the cochlear nerve inferior relative to CN VII, was clearly visualized (7). Within the CPA, CN VII and the superior vestibular nerve were observed superior to the cochlear nerve, and the inferior vestibular nerve was observed anterior to CN VII and posterior to the superior vestibular nerve (Figure 13). At the fundus, Bill’s bar was observed clearly dividing the superior aspect of the IAC. Anterior to Bill’s bar, in the anterosuperior quadrant of the transtubular visual field, CN VII and the nervus intermedius were observed. The superior vestibular nerve was observed posteriorly.
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3D endoscope (Table 4). The magnification and angled view provided by the 3D endoscope allowed for enhanced anatomical exposure. DISCUSSION
Figure 7. Skin incision, burr hole placement, and tubular retractor insertion. A 4-cm vertical linear skin incision was placed beginning at the inferior rim of the zygomatic arch 4 cm anterior to the external auditory meatus and continued superiorly along the hairline. Based on the calculated optimal entry zone, a 2.5- to 3-cm burr hole was fashioned centered 4 cm posterior to the lateral orbital wall and 2 cm anterior to the external auditory meatus with its inferior margin placed as basal as possible and flush with the superior rim of the zygomatic arch. The tubular retractor was inserted gently into the extradural space through the burr hole and advanced into the field under direct microscopic visualization.
Tumor Resection Of the 6 synthetic intracanalicular tumors (1.5 cm in diameter) resected, gross total (Grade I) resections were achieved in 5 cases with greater than 98% of the tumor recovered as determined by weight and included the tumors placed anterior, posterior, and superior to the CN VIIeVIII complex. A near-total (Grade II) resection, with 96% of the tumor recovered, was achieved in the remaining case in which the tumor was placed inferior to the CN VIIeVIII complex. All minute portions of tumor that extended into the posterior fossa were resected without difficulty. Surgical Exposure Excellent visualization of the structures within the IAC was achieved through the tubular retractor with both the microscope and
Figure 8. The ViewSite Brain Access System (VBAS). The VBAS consists of an introducer inside of a working channel port. The working channel used here was modified to provide a beveled tip that facilitates dural retraction when positioned against the angled petrous ridge.
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The middle fossa, translabyrinthine, transcochlear, and retrosigmoid approaches are commonly used routes to access lesions of the IAC and CPA, including vestibular and facial schwannomas, intracanalicular meningiomas, and vascular pathologies. Determinants in selecting the most suitable surgical route to an intracanalicular lesion include pathologic characteristics and extension, size, status of auditory function, and the surgeon’s preferences. In specific cases, the MFA provides the possibility of preserving auditory and facial nerve function without the extensive cerebellar retraction required in the retrosigmoid approach. The classic microscopic MFA is generally considered for the management of small tumors (1.5 cm) of the IAC with an extension of less than 1 cm into the CPA in patients with serviceable hearing. Intraoperatively, the MFA provides exposure of the entire length of the IAC and its contents from the inner ear to the pons allowing the removal of both medially and laterally located tumors (2, 3, 16, 17). Compared with the other common approaches used for the resection of vestibular tumors, the MFA provides the greatest preservation of auditory and facial nerve function—despite the fact that CN VII often is located between the surgeon and the tumor and thus subject to surgical manipulation (3, 11, 25). However, the complex microsurgical anatomy of the petrous bone, and the neurovascular structures it encases, and the difficulty of identifying landmarks on the petrous ridge make this a technically challenging approach (Figures 1 and 11). The main drawback of the classic MFA is the risk of retraction injury to the temporal lobe as traditionally deep midline dissection of the skull base has necessitated a certain degree of temporal retraction to gain sufficient surgical exposure (3, 11, 19, 31). Extensive temporal retraction can result in postoperative seizures, aphasia, or other focal neurologic deficits, and/or venous complications (2, 3, 7, 9, 15, 31). Compromise of the inferior anastomotic vein (vein of Labbé) is a potentially serious complication that could lead to massive infarction and death in cases with a small or obstructed superior anastomotic vein (vein of Trolard) wherein the vein of Labbé may be the only site of venous drainage for the entire hemisphere (7, 9, 15). To mitigate this risk, keyhole and endoscopic subtemporal approaches to this region have been proposed (19, 21, 23, 24). However, fully endoscopic transcranial approaches carry the risk of iatrogenic injury to neurovascular structures from instruments behind the tip of the endoscope and not visible to the surgeon (21). Therefore, endoscope-assisted approaches through conventional craniotomy openings may be used but sacrifice any potential for minimally invasiveness. To overcome this, we propose the first transtubular approach to a skull base pathology and evaluate the feasibility of performing a minimally invasive percutaneous 3D endoscopic or microscopic MFA for the resection of intracanalicular lesions that provides minimal bone resection, minimal temporal retraction, protection of surrounding tissues from iatrogenic or thermal injury, and the potential for hearing preservation and an improved cosmetic outcome.
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 9. Intraoperative placement of the tubular retractor. (A) Two-dimensional and (B) 3-dimensional intraoperative images of the working channel positioned up against the right petrous ridge with exposure of the arcuate eminence in the posterosuperior quadrant (see Figure 10) after removal of the introducer.
Stereotactic cylindrical retractors, first introduced by Kelly et al. in 1988 for the excision of intraparenchymal lesions, have been since proposed for use in transcortical and transventricular surgical approaches (5, 8, 20, 29, 30). The use of frameless stereotactic techniques combined with minimally invasive tubular retractor systems have shown evidence of faster recovery times and lower morbidity rates (14). Tubular retraction in cranial surgery has been preliminarily shown to reduce pressure on cortical surfaces and, compared to brain spatulas, exerts reduced and conically distributed pressure as low as less than 10 mm Hg onto impacted surface areas, reducing the risk of neuronal damage (27). This is significant as an increase in cortical damage from retractors held in place for more than 15 minutes with 20 mm Hg of pressure has been observed in a rat model (28). Furthermore, systemic intraoperative factors, such as hypotension and blood loss, can increase vulnerability for retractor mediated cortical ischemia (27). Additionally, by limiting the range of instrument movement and protecting the surrounding tissues from instruments within the
working channel, the tubular retractor may reduce inadvertent iatrogenic instrumental injury and thermal injury from the endoscope light or electrocautery (30). Compared with freehand manipulation of the retractor, the use of a self-retaining snake retractor and extension arm may also decrease the risk of injury from torque effect (14). Postoperative magnetic resonance imaging data, following use of the tubular retractor, has shown minimal T2/fluid-attenuated inversion recovery changes along the surgical corridor indicating minimal tissue damage and ischemia (13, 14, 26). A 17L VBAS was found to be suitable for this approach. The tubular retractor was modified to provide a beveled tip that would contour to the slope of the petrous ridge and mitigate the ability for temporal dura to obscure the surgical field (Figure 8). The transparent construction allowed for visual monitoring of the retracted surfaces throughout the procedure and provided good visualization of the surrounding anatomy. There was sufficient space within the retractor to allow for simultaneous placement of the endoscope and any combination of 2 microsuction
Figure 10. Intraoperative transtubular surgical anatomy and segmentation. The key surgical landmarks, including the middle meningeal artery, greater superior petrosal nerve (GSPN), facial hiatus (FH), arcuate eminence (AE), mandibular nerve (V3), petrous ridge, and tegman tympani, as seen through the tubular retractor (top) and through the petrous bone (bottom). The transtubular surgical field was divided into 4 quadrants: anterosuperior, anteroinferior, posterosuperior, and posteroinferior. The GSPN and facial hiatus were visualized and confirmed to be in the anterosuperior quadrant and the arcuate eminence in the posterosuperior quadrant. The tip of the retractor could achieve 20 degrees of yaw and the opening of the retractor could achieve 10 degrees of negative pitch (downward in relation to the visual field, superior/rostral toward to the top of the head in relation to the anatomy) at full insertion.
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Figure 11. Three-dimensional temporal bone model. (ALC) A left temporal bone seen in increasing transparency to reveal the neurovascular structures within including the trigeminal nerve with the Gasserian ganglion and its ophthalmic, maxillary, and mandibular divisions; geniculate ganglion; greater superficial petrosal nerve; cranial nerve VII-VIII complex; chorda tympani; superior, lateral, and posterior semicircular canals, and cochlea. The intrapetrous internal carotid artery, inferior and superior petrosal sinuses, and the sigmoid sinus are shown in red and blue. The Eustachian tube, endolymphatic sac, and endolymphatic duct also are visible among other anatomical structures.
Figure 12. Drilling of the internal auditory canal (IAC). (A, C) Two-dimensional and (B, D) three-dimensional images showing the surgical drill and micro-suction aspirator in the transtubular surgical field (A, B) and the ensuing removal of bone overlying the IAC (C, D). Drilling was initiated medially at the intersection with the petrous ridge, where the roof of the IAC is thickest, and continued laterally toward the fundus.
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Figure 13. Intradural exposure. (A, C) Two-dimensional and (B, D) three-dimensional images of the facial nerve, Bill’s bar, the superior vestibular nerve (cut to expose the inferior vestibular nerve), the loop of the anterior inferior cerebellar artery, the labyrinthine artery, and the lateral surface of the pons following dural opening. (E) Endoscopic view of the same anatomy with residual tumor seen in the cerebellopontine angle.
aspirators, microinstruments, bayoneted instruments, and/or tube shaft instruments without visually obstructing the surgical field. Instruments could be interchanged easily and quickly—with the surrounding tissues protected from accidental injury. If the use of a larger instrument or device is necessary, a retractor with a
larger distal port size may be used, but would necessitate a larger bone opening. Tube shaft, bayonetted, and long endoscopic instruments were most efficient geometrically and ergonomically, and provided more space for movement within the retractor.
Table 1. Synthetic Tumor Resection Scale Grade
Description
Percentage Removed*
I
Gross total removal of all visible macroscopic tumor model.
98100%
II
Near-total removal with only a small amount of inaccessible or unobservable tumor left in situ.
9197%
III
Subtotal removal with significant portions of inaccessible tumor left in situ.
5090%
IV
Partial removal with portions of visible tumor left in situ.
< 50%
V
Biopsy-like access with removal of only a small portion of tumor.
< 20%
*By weight.
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Table 2. Characteristics of the Different Degrees of Exposure (9) Degree of Exposure
Angles Exposed
Circumferential Control
Single
Absent
90180
Multiple
Difficult
> 180
All
Present
< 90
The rigid nature of the tubular retractor coupled with the use of a small minimally invasive bone opening required accurate determination of an optimal entry point that would provide a direct trajectory to the petrous bone and adequate control of the entire surgical field (Figures 2 and 4). The calculation of a mean lateral petroclival angle, petroclival-acoustic distance, and trajectory perpendicular to the petrous ridge at the porus acusticus from a small series of 25 CT scans (50 sides) enabled determination of an optimal entry zone—located 4 cm posterior to the lateral orbital wall, 2 cm anterior to the external auditory meatus, and flush with the superior rim of the zygomatic arch. Placement of the opening flush with the floor of the middle fossa was essential for achieving minimal temporal lobe retraction. The 2.5- to 3-cm diameter burr hole facilitated easy insertion of the retractor and allowed for the tip of the retractor to achieve 20 degrees of yaw and the opening of the retractor to achieve 10 degrees of negative pitch (downward in relation to the visual field, superior/rostral toward to the top of the head in relation to the anatomy) at full insertion (Figure 10). Insertion of the tubular retractor into the surgical field was performed from a lateral to medial direction with the tip aimed a few degrees anterior to the planned trajectory toward the MMA and V3. The MMA, a tethering point that hinders further elevation of the temporal lobe dura from the floor of middle fossa, was in most cases not incised as complete elevation of the temporal lobe dura was not necessary. Careful dissection of the layers of the temporal dura around foramen ovale and V3 coupled with minimal elevation of the temporal lobe can help to reduce epidural venous bleeding in clinical cases. The periosteal dura encasing GSPN was separated from the temporal dura to allow for the nerve to remain on the floor of the middle fossa while elevation of the temporal dura was continued posteriorly. The GSPN, identified medial to the MMA in its anterior course toward V3, was followed to its exit from the facial hiatus by yawing the tip of the retractor 10 degrees posteriorly. Slight elevation of the GSPN and periosteal fibers enabled visualization of the facial hiatus. The tip of the retractor was yawed a few additional degrees posteriorly to enable identification of the most lateral segment of the arcuate eminence. Bisecting the 120-degree angle between the GSPN, overlying the course of the intrapetrous ICA, and the arcuate eminence is an effective technique for locating the course of the IAC. A perpendicular surgical trajectory to the superior surface of the petrous bone between these 2 structures is thus essential for correctly identifying this angle and its bisecting line. In the approximately 15% of patients who lack an identifiable arcuate eminence (18), an imaginary line at a 60-degree angle medial to the GSPN may be used to locate the course of the IAC. Because of the complex microsurgical anatomy of this region, the transtubular surgical field was divided into 4 quadrants to ease
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surgical orientation, including anterosuperior, anteroinferior, posterosuperior, and posteroinferior. Before proceeding with dissection of the IAC, correct surgical positioning was verified by confirming the location of anatomic structures in relation to these quadrants (Figure 10). The anterosuperior quadrant contains the facial hiatus, proximal portion of the GSPN, initial horizontal portion of the intrapetrous ICA, and the cochlea. Of these, the facial hiatus and GSPN should be visualized in this quadrant. The posterosuperior quadrant contains the lateral segment of the arcuate eminence, posterior semicircular canal, a portion of the tegmen tympani, and the posterior portion of the fundus. Of these, the lateral segment of the arcuate eminence should be visualized in this quadrant. The anteroinferior quadrant contains the majority of the anterior aspect of the medial portion of the IAC and a part of the cancellous bone of the petrous apex. There are no visible landmarks in this quadrant. The posteroinferior quadrant contains the posterior aspect of the medial portion of the IAC and the bone of the postmeatal triangle. There are no visible landmarks in this quadrant, although in 2 specimens it was possible to visualize the superficial portion of the arcuate eminence here. Because of the 45-degree angle between the course of the IAC and the superior surface of the petrous bone, the tubular retractor was pitched 10 degrees superiorly to align the surgical trajectory as perpendicular as possible to the petrous bone and with the roof of the IAC while applying only minimal retraction of the temporal lobe (Figure 9). This maneuver allows for full exposure of the fundus of the IAC. Final placement of the tip of the retractor—with the facial hiatus in the anterosuperior visual quadrant and the lateral segment of the arcuate eminence in the posterosuperior visual quadrant—proved in all cases to be an accurate location for uncomplicated exposure of the IAC. Multiangled surgical exposure within the IAC was present and enabled sufficient intraoperative maneuverability (Table 4). After the dura of the IAC was opened, the facial and superior vestibular nerves were immediately exposed from the CPA up to their divergence at Bill’s bar and into the fundus of the canal. CN VII may be moved slightly anteriorly or, after the superior vestibular nerve is cut, slightly posteriorly to allow for improved access to the cochlear nerve. The 270-degree exposure of the IAC with removal of bone in both the pre- and postmeatal triangles provides the ability for full surgical control of the contents of the canal, especially in cases in which tumor displaces the anatomy anteriorly or posteriorly within the canal. The use of a synthetic tumor model provided a means for more closely simulating clinical conditions and provided an additional method of quantitatively evaluating the surgical applicability of this approach through the assessment of the
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
Table 3. Petroclival Angles and Petroclival-Auditory Distances Midsagittal*-Petroclival Suture Distance, mm
Lateral Petroclival Angle*
Petroclival-Acoustic Distance, mm
Trajectory Length, mm
1
11
39
22
42
2
10
31
23
44
3
11
43
21
42
4
9
41
21
37
5
10
44
21
37
6
9
34
22
39
7
10
43
27
40
8
10
28
25
38
9
12
43
23
43
10
12
45
24
40
11
10
36
23
43
12
10
39
22
44
13
12
40
20
39
14
11
39
21
41
15
11
40
22
41
16
10
46
22
41
17
12
33
22
42
18
10
36
23
43
19
12
48
21
37
20
11
47
21
36
21
10
48
19
38
22
10
44
19
39
23
9
37
20
39
24
9
35
20
39
25
11
41
21
42
26
11
42
23
40
27
10
44
18
34
28
10
41
20
36
29
8
42
20
43
30
9
41
21
42
31
10
41
19
33
32
10
45
20
35
33
11
38
21
36
34
11
45
24
40
35
11
44
20
47
36
11
32
21
46
37
12
42
18
44
38
12
45
23
42
39
10
41
21
40
No.
Continues
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Table 3. Continued No.
Midsagittal*-Petroclival Suture Distance, mm
Lateral Petroclival Angle*
Petroclival-Acoustic Distance, mm
Trajectory Length, mm
40
10
37
21
38
41
11
42
19
39
42
11
45
20
40
43
11
37
21
39
44
10
36
21
38
45
8
33
20
38
46
8
32
20
40
47
12
46
22
47
48
11
42
23
47
49
11
49
24
37
12
48
24
39
Mean
10 1
41 5
21 2
40 3
Range
8e12
28e49
18e27
33e47
50 y
*The true petroclival angle can calculated by adding 90 degrees to the lateral petroclival angle. yThe midsagittal line was placed between the nasion and the inion.
degree of tumor resection (Figure 3 and Table 1). To more closely recreate lesions encountered clinically, tumors were placed anterior (2 sides), posterior (2 sides), superior (1 side), and inferior (1 side) to the CN VIIeVIII complex. In cases in which tumor was pushing the nerve complex toward the superior aspect of the dura of the IAC, the dural opening was fashioned near to the petrous ridge on the side of the postmeatal triangle where there is increased distance between the dura and the nerves. When attempting to expose tumors that follow CN VII in the space anterior to Bill’s bar, care should be taken to avoid damaging the cochlea, which is located immediately anterior to this compartment. In cases in which the nerves are pushed inferiorly from superior lesions, the nerves should first be exposed in the CPA and then followed into the canal before debulking of the tumor is
attempted. For small lesions located mainly in the CPA with minimal extension into the canal, it is safer to open the dura at the posterior aspect of the IAC on the side of the arcuate eminence. The loop of the anterior inferior cerebellar artery is easily controlled in the CPA and in cases in which the loop enters the IAC. The ability for transtubular synthetic tumor resection was confirmed with gross total resections (Grade I) achieved in 5 cases with greater than 98% of the tumor recovered, including the tumors located anterior, posterior, and superior to the CN VIIeVIII complex. The remaining tumor, located inferior to the CN VIIeVIII complex, was only near totally resected (Grade II) with 96% of the tumor recovered because of its location at the fundus of the IAC, where excessive manipulation of the facial nerve would have been required to achieve a gross total resection. Because of
Table 4. Exposure of Target Surgical Structures Anatomical Structure
Microscopic Exposure, degree
Endoscopic Exposure, degree
Anterior inferior cerebellar artery loop
220
360
CN VII
220
360
CN VIII
120
220
Geniculate ganglion
180
180
Inferior vestibular nerves
120
220
Internal auditory (labyrinthine) artery
220
360
Superior vestibular nerve
220
360
CN, cranial nerve.
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
the limited size of the surgical corridor, the transtubular MFA should be limited to tumors less than 1.5 cm in diameter and with minimal extension into the CPA. Use of the 3D endoscope provided valuable stereoscopic perception (6), although because of its small size no specific advantage was noted when the endoscope was used within the IAC. The 3D endoscope, however, proved useful in exploring the CPA, checking for any potential anatomical variance, and inspecting for residual tumor. The 3D endoscope facilitated resection of the portions of synthetic tumor located within the CPA. We believe that the endoscope also may be safely used on its own as an alternative to the microscope; however the microscope—with use of bayonetted and tube shaft instruments—provided sufficient exposure through the narrow working channel and of the CPA. The minimally invasive 15-cm drill attachment, with a 10-degree curvature and the ability to telescope 3 mm, allowed for unobstructed visualization of the surgical field while unroofing the IAC. Despite the detailed multiangled anatomical exposure provided by the endoscope, surgeons should be extremely familiar with the associated anatomy, as well as the use of instruments through a tubular retractor, before attempting a transtubular approach. Although the retractor provides a safe corridor for the approach, it should not provide the surgeon with a false sense of safety— awareness of the position and anatomical surroundings of each instrument is still paramount to prevent tissue damage. Each surgeon who participated in this study spent several hours
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MIDDLE FOSSA APPROACH FOR INTRACANALICULAR TUMORS
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Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Received 3 September 2014; accepted 25 February 2015
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Citation: World Neurosurg. (2015) 84, 1:132-146. http://dx.doi.org/10.1016/j.wneu.2015.02.042 Journal homepage: www.WORLDNEUROSURGERY.org
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