- Email: [email protected]
Scala tympani cochleostomy for cochlear implantation
Operative Techniques in Otolaryngology (2010) 21, 218-222
Scala tympani cochleostomy for cochlear implantation Gregory J. Basura, MD, PhD, Oliver F. Adunka, MD, Craig A. Buchman, MD From the Department of Otolaryngology/Head and Neck Surgery, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina. KEYWORDS Cochlear implantation; Surgical technique; Human temporal bone; Hearing preservation
The use of cochlear implantation to treat sensorineural hearing loss continues to evolve as an increasing number of both adult and pediatric patients undergo this life-changing surgery. As our understanding of electrical stimulation of the auditory system unfolds, the importance of proper cochleostomy placement and intracochlear electrode positioning continues to evolve. Currently, atraumatic intracochlear electrode array implantation into scala tympani appears to optimize performance while providing the opportunity for hearing preservation for the purposes of bimodal stimulation. With these objectives in mind, this article describes the authors’ surgical approach to gain access to scala tympani with the intent of minimizing trauma to the underlying structures. This approach is based on the results of multiple clinical and anatomical studies as well as on data from various temporal bone experiments. Based herein, the authors perform either direct round window insertions or create a round window-related cochleostomy either with or without a bony partition. Blind drilling procedures on the convexity of the promontory are avoided because this can result in either scala vestibuli access or substantial intracochlear damage. © 2010 Elsevier Inc. All rights reserved.
Historically, cochlear implantation (CI) has been a treatment option for both children and adults suffering from bilateral profound sensorineural hearing loss. More recent advances have demonstrated that CIs can help many patients with substantial hearing remnants.1 Further, some individuals have retained measurable hearing following CI surgery and have been able to combine the acoustic and electric signals to gain a speech perception benefit in noise, a traditional weakness of conventional electrical stimulation alone.2,3 To offer this performance advantage to a broader group of patients, efforts have been made to systematically preserve hearing following CI and to include patients with even greater amounts of residual hearing in candidacy. Surgical protocols have now been developed in an effort to consistently save residual hearing.4-7 Most efforts have centered on surgical techniques that avoid intracochlear trauma through modifications in cochleostomy technique, electrode design, and insertion mechanics. With regard to electrodes, Address reprint requests and correspondence: Craig A. Buchman, MD, Department of Otolaryngology/Head and Neck Surgery, The University of North Carolina at Chapel Hill, 170 Manning Drive, Physicians Office Building, Chapel Hill, NC 27599. E-mail address: [email protected]. 1043-1810/$ -see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.otot.2010.08.001
softer and shorter arrays have been developed to limit insertion depths and to either atraumatically deflect off critical intracochlear structures or to avoid such contact altogether.6 These arrays are also smaller in cross section than conventional arrays, thereby limiting adverse mechanical–physiological consequences on the traveling wave. Future developments are underway to minimize immune reactivity in an effort to avoid secondary or delayed hearing losses. The process of hearing preservation CI has led to a critical reappraisal of the basic cochleostomy concepts. Initially, the cochleostomy was described as an alternative access route, in lieu of the round window (RW), to facilitate the insertion of relatively large and rigid multielectrode arrays. This cochleostomy was designed to avoid the hook region of the basal turn and to provide a relatively straight trajectory into scala tympani (ST). This approach allowed for the generation of greater insertion forces while reducing buckling vectors. Modern CI electrode arrays, and especially those used for hearing-preservation procedures, are now quite flexible and thin. Therefore, the potential for implanting these electrodes through the RW has reemerged.7,8 Both temporal bone experiments as well as clinical studies have clearly demonstrated the feasibility of RW insertions and possibly
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indicate a superiority of this approach for avoiding intracochlear trauma.9,10 Further supporting this approach has been the fact that several investigators have demonstrated a relatively high incidence of electrode misplacements into scala vestibuli (SV) and basal cochlear damage when using the old promontory-based cochleostomy approach.6,11,12 As a better understanding of hearing-preservation CI surgery has evolved, two independent groups of researchers have identified performance advantages among patients who have electrode arrays that are placed solely within the ST, irrespective of residual hearing maintenance.13,14 Specifically, patients who have devices that have been placed exclusively within ST demonstrate clear improvements in speech discrimination abilities when compared with similar individuals with either isolated SV insertions or cross scalar (ST¡ SV) insertions. The findings of these studies have been fundamental for the field. One can now surmise that the ideal location for CI electrodes for the purpose of tonotopic electrical stimulation (in addition to hearing preservation) is the ST. Thus, precise and exclusive placement of electrode arrays into ST should be sought for all patients undergoing CI surgery, unless otherwise not possible or practical. This article will summarize the author’s current surgical decision making and the techniques used when performing ST cochleostomy for the purposes of standard CI placement as well as for hearing-preserving CI.
Technique Overview When describing the surgical technique for cochleostomy creation, one should keep the basic objectives in mind: open ST (and not SV), minimize collateral trauma to physiologically relevant intracochlear structures, and provide a relatively straight insertion trajectory along the longitudinal axis of the basal turn in an effort to allow for buckle-free electrode insertion. Currently, a variety of differing cochleostomy techniques exist that can be adapted to the clinical situation depending on the following: the electrode array to be used, the cochlear morphology, and the desire for hearing preservation. Generally speaking, in the setting of normal cochlear anatomy, insertions that are closer to the RW have the potential benefit for less traumatic scalar openings but are limited by insertion trajectories that can predispose to buckling. Conversely, more anteriorly placed openings have a greater propensity for linear insertion trajectories, and thus the application of greater insertion forces, but are clearly associated with more extensive drilling and, thus, trauma.15 These generalities might also vary somewhat among individual patients as well as within populations. For instance, children with normal anatomy often have cochlear basal turns that appear to be rotated somewhat more superiorly and medially relative to the facial recess view. Thus, this fortuitously creates the opportunity for inline insertion trajectories, even when the cochleostomy is located at or very near the RW membrane (RWM).
Figure 1 Axial cross section schematic demonstrating insertion trajectories (red arrows) with electrode insertions through the RWM in different anatomic settings. (a) The large image shows a typical insertion trajectory with a mostly horizontal, inferiorly facing RWM. Even with proper removal of the overhang (hook), the resulting electrode insertion will most likely aim at the critical cochlear structures (ie, modiolus, osseous spiral lamina, basilar membrane). (b) By contrast, when the viewed RWM has a vertical orientation and is posteriorly facing (inlay image), an almost straight trajectory along the axis of ST can be expected. The small red arrows depict the differences in the positioning of the bony overhang and the hook region. (Color version of figure is available online.)
Clearly, these anatomical vagrancies create the need for an experienced surgeon who can adapt. This requires significant experience and attention to somewhat subtle details. Our current practice uses an adaptive cochleostomy approach whereby electrode arrays that are long, rigid, and inserted with a stylet or rigid insertion tool are placed through a separate cochleostomy (Figures 1 and 2). Such a cochleostomy is always RW-related and might actually communicate with the RWM if higher insertion forces are not anticipated. For very long electrode arrays that are designed for complete cochlear coverage, a cochleostomy that has circumferential bony walls is critical to apply the necessary insertion forces for deep insertion. For cases in which a shorter array is to be used in a hearing-preservation capacity, openings that include the RWM are the rule (Figures 1 and 2).
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Operative Techniques in Otolaryngology, Vol 21, No 4, December 2010 carried out when the pathology only involves a short segment or a separate SV opening is created. Occasionally, two arrays can be placed within the cochlea for the purposes of complete coverage in the setting of extensive ossification. When considering developmental cochlear malformations, cochleostomy locations might need to be individualized for the purposes of cerebrospinal fluid leakage control or anatomical variations in scalar direction and caliber.
Round window niche approach (ie, facial recess) and round window niche overhang
Figure 2 Schematic illustration of the typical view of the promontory and adjacent structures through a facial recess. The inlay image demonstrates a common anatomic scenario in which most of the RWM is hidden by a bony overhang (RW overhang). When that has been removed, the membrane can be viewed and its position can be assessed. The main image illustrates the three cochleostomy sites: directly though the RWM by an incision in its lateral aspect (red line), a cochleostomy expanding the RWM (RW marginal cochleostomy, illustrated in blue, the actual RWM can be folded superrior and posterior to enhance the exposure), and a separate cochleostomy inferior and slighly anterior to the RWM (green line). S, stapes; FN, facial nerve (mastoid segment shown); SM, stapedius muscle; P, promontory. (Color version of figure is available online.)
With regard to RWM insertions, this very much depends on the orientation of the membrane relative to both the facial recess view and the longitudinal axis of the first segment of ST (Figures 1 and 2). Issues with the RWM approach include depth of insertion that may not be satisfactory, a trajectory of insertion at the RW that may increase resistance because of friction at the margins of the window, and electrode tip trajectories that course toward the modiolus, osseous spiral lamina, basilar membrane, or even the vestibule in some instances. In general, when the visible RWM is facing posteriorly (ie, vertical orientation) and lies in a plane roughly parallel to the facial nerve and perpendicular to the ST long axis, a favorable insertion angle can be expected with minimal collateral trauma. Conversely, an inferior-facing (ie, horizontal orientation) RWM can offer a poor view as well as a less favorable insertion angle and will likely be more traumatic (Figure 1). When cochlear anatomy has been altered by ST ossification or obstruction, either a drill-through procedure is
In the vast majority of cases, CI is performed via a transmastoid approach. The facial recess (a.k.a. chordafacial angle or posterior tympanotomy) typically provides access to the region of the RW and the adjacent promontory. A properly drilled facial recess more often than not allows visualization of the RW niche (RWN) with its overhang. To achieve such an exposure may require judicious skeletonization of the posterior external auditory canal wall, facial nerve, and even removal of the pyramidal process in rare instances. We prefer to expose the facial nerve canal in all cases to maximize exposure and working room. The superior extent of facial recess exposure is the short process of the incus. The degree of inferior exposure is dictated somewhat by the chorda tympani–facial nerve anatomy and the inferior extension of the RWN overhang.15-18 Occasionally, the free edge of the RWN overhang will be quite inferior, completely obscuring the view of any portion of the RWM. Although inferior drilling might resolve this, careful observation is often needed to differentiate the RWN from a closely related hypotympanic air cell.19 One clue to this anatomic region is the fact that the RWM always lies within 1.5 mm of the inferior border of the stapes footplate.15 In most cases, the RWN overhang needs to be at least partially removed before the actual window can be properly viewed.16,17 When drilling the overhang, it is important to remember that the posterior portion of the overhang and the RWM are in close proximity to the basilar membrane at its most basal portion. Moreover, extensive excavation of the posterior limb of the niche overhang may result in a singular neurectomy.18,20 Thus, drilling of the bony overhang should mainly involve the anterior limb of the RWN. When dissecting the RWN, one frequently encounters a mucosal membrane mimicking the actual RWM, which is found underneath.21 Careful soft tissue dissection allows adequate exposure of the RW proper without creating a premature perilymph leak.
Assessment of the round window After proper (maximized) exposure of the RWM, its detailed anatomical position should be carefully assessed. When the RWM has a posteriorly angled vertical orientation, the surgeon typically obtains an almost perpendicular (en face) view (Figure 1). In this situation, an almost straight line of sight trajectory down ST can be assumed. In cases in which hearing preservation is to be attempted, this
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Scala Tympani Cochleostomy
Figure 3 Axial cross section of the basal end of the cochlea, including the region of the RW in an inferiorly facing example. In this situation, the angle of the RWM and the bony hook region (*) block a straight insertion trajectory along the axis of scala tympani. Note the close anatomic relationship of the posterior portion of the RWM and the basilar membrane. Thus, drilling procedures on the posterior aspect of the RWN should be avoided. CF, crista fenestra (semilunaris); OSL, osseous spiral lamina; BM, basilar membrane. (Color version of figure is available online.)
represents a favorable orientation for short flexible electrode array insertion. When the RWM is angled inferiorly in a horizontal orientation, a tangential line of sight is common through the facial recess, and longitudinal access to the ST lumen will be difficult through the membrane (Figures 1 and 3). In such cases, electrode insertions perpendicular to the RWM will result in trajectories toward the osseous spiral lamina, basilar membrane, and bony modiolus in the basal turn hook region. Tangential insertions through the RWM that are directed anteroinferiorly down the lumen might meet resistance at the bony edges of the RWM. In these cases, an RW-related cochleostomy might improve this angle and provide a straight trajectory into ST while avoiding intracochlear damage. Naturally, most anatomical situations encountered are variations between the two extremes, and the specific choice of cochlear opening remains a judgment call for the surgeon. It is worth emphasizing that because the RWM provides the only reliable landmark for ST, this structure must be identified and used for cochleostomy creation (Figure 2). Random drilling procedures on the convexity of the promontory should be strictly avoided.
Round window insertions When implanting through the RWM, one should recall the underlying anatomy of the lateral wall of the basal turn: the spiral ligament is formed by the lateral attachment of the basilar membrane. The ligament represents a relatively broad-based attachment, and only the inferior portions of the lateral wall are lined by cochlear endosteum alone. Also, the spiral ligament can be found directly anterior to the RWM annulus. Thus, the anterior portion of the window
221 should be avoided whenever possible. The inferior circumference of the membrane is relatively safe, and the electrode should be implanted through this portion. The crista semilunaris (crista fenestra) is a delicate halfmoon-shaped bony crest just medial to the inferior and anterior portions of the window. The crista fenestra can help identify the anatomical extension of the RWM, but it is to be distinguished from the bony portion of the lateroinferior wall of ST forming the hook region. To create an opening in the RWM for the purposes of electrode insertion, the inferior and adjacent anterior–inferior portion of the membrane should be opened using a sharp knife and small angled hook. The incision is usually an arc along the window margin (Figure 2). Then, the opening should be enlarged enough to host the electrode array. Most modern electrodes have a basal diameter of less than 1 mm. Thus, an opening of that size is typically sufficient. In rare instances, the RWM itself is smaller than this, in which case an RW-related inferior cochleostomy should be used to create a slightly larger opening.
Round window-related cochleostomy Despite clear visualization of the RWM through the facial recess, the longitudinal luminal direction or axis of ST remains unknown before opening surgically. Temporal bone histologic evidence indicates that the bone inferior and slightly anterior to the RWM annulus provides a relatively safe corridor for access to the ST. However, finding this corridor can be tricky when the scalar direction is unknown. Our approach for the inferiorly angled and horizontally oriented RWM is to commence drilling at the membrane margin directly inferior to the window. Using the side of the drill, the surgeon sways anteriorly along the margin in search of the cochlear endosteum that is in contact with the window annulus. When the promontory anterior to the window is large, significant drilling may be required. The bony hook should be removed until the cochlear endosteum has been exposed at a size large enough to allow for electrode insertion along the trajectory of the basal turn ST (Figure 2). Occasionally, the white spiral ligament is evident following endosteal decompression. Bony excavation directly anterior to the window should be avoided to reduce the risk of damage to the spiral ligament and the basilar membrane.19
Cochlear opening and electrode insertion Before opening the perilymph space, careful hemostasis should be obtained to avoid contamination. Also, once the cochlea has been opened, any suctioning should be strictly avoided.4 Some clinicians advocate the use of both topical as well as systemic steroids to reduce the postinsertion inflammatory reaction, although their use remains empiric.22 It is important to emphasize the need for a large enough opening when performing the cochleostomy. This will allow for compensatory perilymph leak and displacement during electrode insertion. Lastly, following electrode insertion, many advocate the use of a small piece of tempo-
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ralis muscle or fascia placed next to the cochleostomy to seal the opening.
Conclusions Surgical access to the cochlea for the purposes of cochlear implantation has evolved to reflect recent advances in the field.23 The notion that placing the electrode into any intracochlear compartment will result in the same quality of electric stimulation has recently been disproved. Strong evidence points toward ST as the ideal location for a CI array, and intracochlear damage seems to negatively affect performance with the implant. The anatomy of the RW and cochlear promontory represents a region of great anatomical variation for the CI surgeon.16 Correct identification of this variation is critical for carrying out an atraumatic cochlear opening for the purposes of cochlear implant electrode array placement in to ST. An adaptive approach to cochleostomy creation has evolved in an effort to account for these anatomical vagrancies as well as the variety of electrode arrays that are needed in modern CI practice. The RWM remains the only consistent landmark for the ST lumen and must be identified in all cases before cochlear opening. This can require significant RWN excavation. Variations in RWM anatomy further confound the picture. In our current practice, electrode arrays that are long, rigid, and inserted with a stylet or rigid insertion tool are placed through a separate cochleostomy. Such a cochleostomy is always RW-related and might actually communicate with the RWM if higher insertion forces are not anticipated. For very long electrode arrays that are designed for complete cochlear coverage, a cochleostomy that has circumferential bony walls is critical to apply the necessary insertion forces for deep insertion. For cases in which a shorter array is to be used in a hearingpreservation capacity, openings that include the RWM are the rule. Blind drilling procedures, as proposed in the early days of multichannel CI, should be strictly avoided.
References 1. Cullen RD, Higgins C, Buss E, et al: Cochlear implantation in patients with substantial residual hearing. Laryngoscope 114:2218-2223, 2004 2. von Ilberg C, Kiefer J, Tillein J, et al: Electric-acoustic stimulation of the auditory system. New technology for severe hearing loss. ORL J Otorhinolaryngol Relat Spec 61:334-340, 1999
3. Gantz BJ, Turner CW: Combining acoustic and electrical hearing. Laryngoscope 113:1726-1730, 2003 4. Kiefer J, Gstoettner W, Baumgartner W, et al: Conservation of lowfrequency hearing in cochlear implantation. Acta Otolaryngol 124: 272-280, 2004 5. Adunka O, Unkelbach MH, Mack MG, et al: Predicting basal cochlear length for electric-acoustic stimulation. Arch Otolaryngol Head Neck Surg 131:488-492, 2005 6. Adunka O, Kiefer J, Unkelbach MH, et al: Development and evaluation of an improved cochlear implant electrode design for electric acoustic stimulation. Laryngoscope 114:1237-1241, 2004 7. Adunka O, Unkelbach MH, Mack M, et al: Cochlear implantation via the round window membrane minimizes trauma to cochlear structures: a histologically controlled insertion study. Acta Otolaryngol 124:807812, 2004 8. Roland PS, Wright CG, Isaacson B: Cochlear implant electrode insertion: the round window revisited. Laryngoscope 117:1397-1402, 2007 9. Skarzynski H, Lorens A, Piotrowska A, et al: Preservation of low frequency hearing in partial deafness cochlear implantation (PDCI) using the round window surgical approach. Acta Otolaryngol 127:4148, 2007 10. Skarzynski H, Lorens A, Piotrowska A: A new method of partial deafness treatment. Med Sci Monit 9:CS20-CS24, 2003 11. Aschendorff A, Kubalek R, Turowski B, et al: Quality control after cochlear implant surgery by means of rotational tomography. Otol Neurotol 26:34-37, 2005 12. Aschendorff A, Kubalek R, Hochmuth A, et al: Imaging procedures in cochlear implant patients— evaluation of different radiological techniques. Acta Otolaryngol Suppl May(552):46-49, 2004 13. Aschendorff A, Kromeier J, Klenzner T, et al: Quality control after insertion of the nucleus contour and contour advance electrode in adults. Ear Hear 28:75S-79S, 2007 14. Skinner MW, Holden TA, Whiting BR, et al: In vivo estimates of the position of advanced bionics electrode arrays in the human cochlea. Ann Otol Rhinol Laryngol Suppl 197:2-24, 2007 15. Briggs RJ, Tykocinski M, Stidham K, et al: Cochleostomy site: implications for electrode placement and hearing preservation. Acta Otolaryngol 125:870-876, 2005 16. Franz BK, Clark GM, Bloom DM: Surgical anatomy of the round window with special reference to cochlear implantation. J Laryngol Otol 101:97-102, 1987 17. Proctor B, Bollobas B, Niparko JK: Anatomy of the round window niche. Ann Otol Rhinol Laryngol 95:444-446, 1986 18. Su WY, Marion MS, Hinojosa R, et al: Anatomical measurements of the cochlear aqueduct, round window membrane, round window niche, and facial recess. Laryngoscope 92:483-486, 1982 19. Adunka OF, Radeloff A, Gstoettner WK, et al: Scala tympani cochleostomy. II. Topography and histology. Laryngoscope 117:21952200, 2007 20. Sato H, Takahashi H, Sando I: Bony dehiscence between singular canal and round window niche. Laryngoscope 103:78-81, 1993 21. Alzamil KS, Linthicum FH Jr: Extraneous round window membranes and plugs: possible effect on intratympanic therapy. Ann Otol Rhinol Laryngol 109:30-32, 2000 22. Kiefer J, Ye Q, Tillein K, et al: Protecting the cochlea during stapes surgery: is there a role for corticosteroids? Adv Otorhinolaryngol 65:300-307, 2007 23. Adunka OF, Buchman CA: Scala tympani cochleostomy. I. Results of a survey. Laryngoscope 117:2187-2194, 2007
Scala tympani cochleostomy for cochlear implantation Gregory J. Basura, MD, PhD, Oliver F. Adunka, MD, Craig A. Buchman, MD From the Department of Otolaryngology/Head and Neck Surgery, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina. KEYWORDS Cochlear implantation; Surgical technique; Human temporal bone; Hearing preservation
The use of cochlear implantation to treat sensorineural hearing loss continues to evolve as an increasing number of both adult and pediatric patients undergo this life-changing surgery. As our understanding of electrical stimulation of the auditory system unfolds, the importance of proper cochleostomy placement and intracochlear electrode positioning continues to evolve. Currently, atraumatic intracochlear electrode array implantation into scala tympani appears to optimize performance while providing the opportunity for hearing preservation for the purposes of bimodal stimulation. With these objectives in mind, this article describes the authors’ surgical approach to gain access to scala tympani with the intent of minimizing trauma to the underlying structures. This approach is based on the results of multiple clinical and anatomical studies as well as on data from various temporal bone experiments. Based herein, the authors perform either direct round window insertions or create a round window-related cochleostomy either with or without a bony partition. Blind drilling procedures on the convexity of the promontory are avoided because this can result in either scala vestibuli access or substantial intracochlear damage. © 2010 Elsevier Inc. All rights reserved.
Historically, cochlear implantation (CI) has been a treatment option for both children and adults suffering from bilateral profound sensorineural hearing loss. More recent advances have demonstrated that CIs can help many patients with substantial hearing remnants.1 Further, some individuals have retained measurable hearing following CI surgery and have been able to combine the acoustic and electric signals to gain a speech perception benefit in noise, a traditional weakness of conventional electrical stimulation alone.2,3 To offer this performance advantage to a broader group of patients, efforts have been made to systematically preserve hearing following CI and to include patients with even greater amounts of residual hearing in candidacy. Surgical protocols have now been developed in an effort to consistently save residual hearing.4-7 Most efforts have centered on surgical techniques that avoid intracochlear trauma through modifications in cochleostomy technique, electrode design, and insertion mechanics. With regard to electrodes, Address reprint requests and correspondence: Craig A. Buchman, MD, Department of Otolaryngology/Head and Neck Surgery, The University of North Carolina at Chapel Hill, 170 Manning Drive, Physicians Office Building, Chapel Hill, NC 27599. E-mail address: [email protected]. 1043-1810/$ -see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.otot.2010.08.001
softer and shorter arrays have been developed to limit insertion depths and to either atraumatically deflect off critical intracochlear structures or to avoid such contact altogether.6 These arrays are also smaller in cross section than conventional arrays, thereby limiting adverse mechanical–physiological consequences on the traveling wave. Future developments are underway to minimize immune reactivity in an effort to avoid secondary or delayed hearing losses. The process of hearing preservation CI has led to a critical reappraisal of the basic cochleostomy concepts. Initially, the cochleostomy was described as an alternative access route, in lieu of the round window (RW), to facilitate the insertion of relatively large and rigid multielectrode arrays. This cochleostomy was designed to avoid the hook region of the basal turn and to provide a relatively straight trajectory into scala tympani (ST). This approach allowed for the generation of greater insertion forces while reducing buckling vectors. Modern CI electrode arrays, and especially those used for hearing-preservation procedures, are now quite flexible and thin. Therefore, the potential for implanting these electrodes through the RW has reemerged.7,8 Both temporal bone experiments as well as clinical studies have clearly demonstrated the feasibility of RW insertions and possibly
Basura et al
Scala Tympani Cochleostomy
219
indicate a superiority of this approach for avoiding intracochlear trauma.9,10 Further supporting this approach has been the fact that several investigators have demonstrated a relatively high incidence of electrode misplacements into scala vestibuli (SV) and basal cochlear damage when using the old promontory-based cochleostomy approach.6,11,12 As a better understanding of hearing-preservation CI surgery has evolved, two independent groups of researchers have identified performance advantages among patients who have electrode arrays that are placed solely within the ST, irrespective of residual hearing maintenance.13,14 Specifically, patients who have devices that have been placed exclusively within ST demonstrate clear improvements in speech discrimination abilities when compared with similar individuals with either isolated SV insertions or cross scalar (ST¡ SV) insertions. The findings of these studies have been fundamental for the field. One can now surmise that the ideal location for CI electrodes for the purpose of tonotopic electrical stimulation (in addition to hearing preservation) is the ST. Thus, precise and exclusive placement of electrode arrays into ST should be sought for all patients undergoing CI surgery, unless otherwise not possible or practical. This article will summarize the author’s current surgical decision making and the techniques used when performing ST cochleostomy for the purposes of standard CI placement as well as for hearing-preserving CI.
Technique Overview When describing the surgical technique for cochleostomy creation, one should keep the basic objectives in mind: open ST (and not SV), minimize collateral trauma to physiologically relevant intracochlear structures, and provide a relatively straight insertion trajectory along the longitudinal axis of the basal turn in an effort to allow for buckle-free electrode insertion. Currently, a variety of differing cochleostomy techniques exist that can be adapted to the clinical situation depending on the following: the electrode array to be used, the cochlear morphology, and the desire for hearing preservation. Generally speaking, in the setting of normal cochlear anatomy, insertions that are closer to the RW have the potential benefit for less traumatic scalar openings but are limited by insertion trajectories that can predispose to buckling. Conversely, more anteriorly placed openings have a greater propensity for linear insertion trajectories, and thus the application of greater insertion forces, but are clearly associated with more extensive drilling and, thus, trauma.15 These generalities might also vary somewhat among individual patients as well as within populations. For instance, children with normal anatomy often have cochlear basal turns that appear to be rotated somewhat more superiorly and medially relative to the facial recess view. Thus, this fortuitously creates the opportunity for inline insertion trajectories, even when the cochleostomy is located at or very near the RW membrane (RWM).
Figure 1 Axial cross section schematic demonstrating insertion trajectories (red arrows) with electrode insertions through the RWM in different anatomic settings. (a) The large image shows a typical insertion trajectory with a mostly horizontal, inferiorly facing RWM. Even with proper removal of the overhang (hook), the resulting electrode insertion will most likely aim at the critical cochlear structures (ie, modiolus, osseous spiral lamina, basilar membrane). (b) By contrast, when the viewed RWM has a vertical orientation and is posteriorly facing (inlay image), an almost straight trajectory along the axis of ST can be expected. The small red arrows depict the differences in the positioning of the bony overhang and the hook region. (Color version of figure is available online.)
Clearly, these anatomical vagrancies create the need for an experienced surgeon who can adapt. This requires significant experience and attention to somewhat subtle details. Our current practice uses an adaptive cochleostomy approach whereby electrode arrays that are long, rigid, and inserted with a stylet or rigid insertion tool are placed through a separate cochleostomy (Figures 1 and 2). Such a cochleostomy is always RW-related and might actually communicate with the RWM if higher insertion forces are not anticipated. For very long electrode arrays that are designed for complete cochlear coverage, a cochleostomy that has circumferential bony walls is critical to apply the necessary insertion forces for deep insertion. For cases in which a shorter array is to be used in a hearing-preservation capacity, openings that include the RWM are the rule (Figures 1 and 2).
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Operative Techniques in Otolaryngology, Vol 21, No 4, December 2010 carried out when the pathology only involves a short segment or a separate SV opening is created. Occasionally, two arrays can be placed within the cochlea for the purposes of complete coverage in the setting of extensive ossification. When considering developmental cochlear malformations, cochleostomy locations might need to be individualized for the purposes of cerebrospinal fluid leakage control or anatomical variations in scalar direction and caliber.
Round window niche approach (ie, facial recess) and round window niche overhang
Figure 2 Schematic illustration of the typical view of the promontory and adjacent structures through a facial recess. The inlay image demonstrates a common anatomic scenario in which most of the RWM is hidden by a bony overhang (RW overhang). When that has been removed, the membrane can be viewed and its position can be assessed. The main image illustrates the three cochleostomy sites: directly though the RWM by an incision in its lateral aspect (red line), a cochleostomy expanding the RWM (RW marginal cochleostomy, illustrated in blue, the actual RWM can be folded superrior and posterior to enhance the exposure), and a separate cochleostomy inferior and slighly anterior to the RWM (green line). S, stapes; FN, facial nerve (mastoid segment shown); SM, stapedius muscle; P, promontory. (Color version of figure is available online.)
With regard to RWM insertions, this very much depends on the orientation of the membrane relative to both the facial recess view and the longitudinal axis of the first segment of ST (Figures 1 and 2). Issues with the RWM approach include depth of insertion that may not be satisfactory, a trajectory of insertion at the RW that may increase resistance because of friction at the margins of the window, and electrode tip trajectories that course toward the modiolus, osseous spiral lamina, basilar membrane, or even the vestibule in some instances. In general, when the visible RWM is facing posteriorly (ie, vertical orientation) and lies in a plane roughly parallel to the facial nerve and perpendicular to the ST long axis, a favorable insertion angle can be expected with minimal collateral trauma. Conversely, an inferior-facing (ie, horizontal orientation) RWM can offer a poor view as well as a less favorable insertion angle and will likely be more traumatic (Figure 1). When cochlear anatomy has been altered by ST ossification or obstruction, either a drill-through procedure is
In the vast majority of cases, CI is performed via a transmastoid approach. The facial recess (a.k.a. chordafacial angle or posterior tympanotomy) typically provides access to the region of the RW and the adjacent promontory. A properly drilled facial recess more often than not allows visualization of the RW niche (RWN) with its overhang. To achieve such an exposure may require judicious skeletonization of the posterior external auditory canal wall, facial nerve, and even removal of the pyramidal process in rare instances. We prefer to expose the facial nerve canal in all cases to maximize exposure and working room. The superior extent of facial recess exposure is the short process of the incus. The degree of inferior exposure is dictated somewhat by the chorda tympani–facial nerve anatomy and the inferior extension of the RWN overhang.15-18 Occasionally, the free edge of the RWN overhang will be quite inferior, completely obscuring the view of any portion of the RWM. Although inferior drilling might resolve this, careful observation is often needed to differentiate the RWN from a closely related hypotympanic air cell.19 One clue to this anatomic region is the fact that the RWM always lies within 1.5 mm of the inferior border of the stapes footplate.15 In most cases, the RWN overhang needs to be at least partially removed before the actual window can be properly viewed.16,17 When drilling the overhang, it is important to remember that the posterior portion of the overhang and the RWM are in close proximity to the basilar membrane at its most basal portion. Moreover, extensive excavation of the posterior limb of the niche overhang may result in a singular neurectomy.18,20 Thus, drilling of the bony overhang should mainly involve the anterior limb of the RWN. When dissecting the RWN, one frequently encounters a mucosal membrane mimicking the actual RWM, which is found underneath.21 Careful soft tissue dissection allows adequate exposure of the RW proper without creating a premature perilymph leak.
Assessment of the round window After proper (maximized) exposure of the RWM, its detailed anatomical position should be carefully assessed. When the RWM has a posteriorly angled vertical orientation, the surgeon typically obtains an almost perpendicular (en face) view (Figure 1). In this situation, an almost straight line of sight trajectory down ST can be assumed. In cases in which hearing preservation is to be attempted, this
Basura et al
Scala Tympani Cochleostomy
Figure 3 Axial cross section of the basal end of the cochlea, including the region of the RW in an inferiorly facing example. In this situation, the angle of the RWM and the bony hook region (*) block a straight insertion trajectory along the axis of scala tympani. Note the close anatomic relationship of the posterior portion of the RWM and the basilar membrane. Thus, drilling procedures on the posterior aspect of the RWN should be avoided. CF, crista fenestra (semilunaris); OSL, osseous spiral lamina; BM, basilar membrane. (Color version of figure is available online.)
represents a favorable orientation for short flexible electrode array insertion. When the RWM is angled inferiorly in a horizontal orientation, a tangential line of sight is common through the facial recess, and longitudinal access to the ST lumen will be difficult through the membrane (Figures 1 and 3). In such cases, electrode insertions perpendicular to the RWM will result in trajectories toward the osseous spiral lamina, basilar membrane, and bony modiolus in the basal turn hook region. Tangential insertions through the RWM that are directed anteroinferiorly down the lumen might meet resistance at the bony edges of the RWM. In these cases, an RW-related cochleostomy might improve this angle and provide a straight trajectory into ST while avoiding intracochlear damage. Naturally, most anatomical situations encountered are variations between the two extremes, and the specific choice of cochlear opening remains a judgment call for the surgeon. It is worth emphasizing that because the RWM provides the only reliable landmark for ST, this structure must be identified and used for cochleostomy creation (Figure 2). Random drilling procedures on the convexity of the promontory should be strictly avoided.
Round window insertions When implanting through the RWM, one should recall the underlying anatomy of the lateral wall of the basal turn: the spiral ligament is formed by the lateral attachment of the basilar membrane. The ligament represents a relatively broad-based attachment, and only the inferior portions of the lateral wall are lined by cochlear endosteum alone. Also, the spiral ligament can be found directly anterior to the RWM annulus. Thus, the anterior portion of the window
221 should be avoided whenever possible. The inferior circumference of the membrane is relatively safe, and the electrode should be implanted through this portion. The crista semilunaris (crista fenestra) is a delicate halfmoon-shaped bony crest just medial to the inferior and anterior portions of the window. The crista fenestra can help identify the anatomical extension of the RWM, but it is to be distinguished from the bony portion of the lateroinferior wall of ST forming the hook region. To create an opening in the RWM for the purposes of electrode insertion, the inferior and adjacent anterior–inferior portion of the membrane should be opened using a sharp knife and small angled hook. The incision is usually an arc along the window margin (Figure 2). Then, the opening should be enlarged enough to host the electrode array. Most modern electrodes have a basal diameter of less than 1 mm. Thus, an opening of that size is typically sufficient. In rare instances, the RWM itself is smaller than this, in which case an RW-related inferior cochleostomy should be used to create a slightly larger opening.
Round window-related cochleostomy Despite clear visualization of the RWM through the facial recess, the longitudinal luminal direction or axis of ST remains unknown before opening surgically. Temporal bone histologic evidence indicates that the bone inferior and slightly anterior to the RWM annulus provides a relatively safe corridor for access to the ST. However, finding this corridor can be tricky when the scalar direction is unknown. Our approach for the inferiorly angled and horizontally oriented RWM is to commence drilling at the membrane margin directly inferior to the window. Using the side of the drill, the surgeon sways anteriorly along the margin in search of the cochlear endosteum that is in contact with the window annulus. When the promontory anterior to the window is large, significant drilling may be required. The bony hook should be removed until the cochlear endosteum has been exposed at a size large enough to allow for electrode insertion along the trajectory of the basal turn ST (Figure 2). Occasionally, the white spiral ligament is evident following endosteal decompression. Bony excavation directly anterior to the window should be avoided to reduce the risk of damage to the spiral ligament and the basilar membrane.19
Cochlear opening and electrode insertion Before opening the perilymph space, careful hemostasis should be obtained to avoid contamination. Also, once the cochlea has been opened, any suctioning should be strictly avoided.4 Some clinicians advocate the use of both topical as well as systemic steroids to reduce the postinsertion inflammatory reaction, although their use remains empiric.22 It is important to emphasize the need for a large enough opening when performing the cochleostomy. This will allow for compensatory perilymph leak and displacement during electrode insertion. Lastly, following electrode insertion, many advocate the use of a small piece of tempo-
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ralis muscle or fascia placed next to the cochleostomy to seal the opening.
Conclusions Surgical access to the cochlea for the purposes of cochlear implantation has evolved to reflect recent advances in the field.23 The notion that placing the electrode into any intracochlear compartment will result in the same quality of electric stimulation has recently been disproved. Strong evidence points toward ST as the ideal location for a CI array, and intracochlear damage seems to negatively affect performance with the implant. The anatomy of the RW and cochlear promontory represents a region of great anatomical variation for the CI surgeon.16 Correct identification of this variation is critical for carrying out an atraumatic cochlear opening for the purposes of cochlear implant electrode array placement in to ST. An adaptive approach to cochleostomy creation has evolved in an effort to account for these anatomical vagrancies as well as the variety of electrode arrays that are needed in modern CI practice. The RWM remains the only consistent landmark for the ST lumen and must be identified in all cases before cochlear opening. This can require significant RWN excavation. Variations in RWM anatomy further confound the picture. In our current practice, electrode arrays that are long, rigid, and inserted with a stylet or rigid insertion tool are placed through a separate cochleostomy. Such a cochleostomy is always RW-related and might actually communicate with the RWM if higher insertion forces are not anticipated. For very long electrode arrays that are designed for complete cochlear coverage, a cochleostomy that has circumferential bony walls is critical to apply the necessary insertion forces for deep insertion. For cases in which a shorter array is to be used in a hearingpreservation capacity, openings that include the RWM are the rule. Blind drilling procedures, as proposed in the early days of multichannel CI, should be strictly avoided.
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