The Tentorial Bridge to Deep Skull Base Exposure: Anatomic Morphometric Study

Accepted Manuscript The Tentorial Bridge to Deep Skull Base Exposure: Anatomical Morphometric Study Ciro A. Vasquez, M.D, John Thompson, Ph.D., A. Samy Youssef, M.D, Ph.D PII:

S1878-8750(18)30515-1

DOI:

10.1016/j.wneu.2018.03.037

Reference:

WNEU 7649

To appear in:

World Neurosurgery

Received Date: 28 December 2017 Revised Date:

2 March 2018

Accepted Date: 5 March 2018

Please cite this article as: Vasquez CA, Thompson J, Youssef AS, The Tentorial Bridge to Deep Skull Base Exposure: Anatomical Morphometric Study, World Neurosurgery (2018), doi: 10.1016/ j.wneu.2018.03.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

The Tentorial Bridge to Deep Skull Base Exposure: Anatomical Morphometric Study Ciro A. Vasquez, M.D. John Thompson, Ph.D., A. Samy Youssef, M.D, Ph.D. Abbreviated title: Incising the tentorium to expose deep skull base lesions

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Ciro A. Vasquez, M.D. Department of Neurosurgery, University of Colorado School of Medicine, Aurora, Colorado, USA John A. Thompson, Ph.D. Department of Neurosurgery, University of Colorado School of Medicine, Aurora, Colorado, USA

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Corresponding Authors Samy Youssef, MD, PhD [email protected] Department of Neurosurgery, University of Colorado, 12631 E. 17th Ave., C307 Aurora, CO 80045, USA Phone: 303-724-2305 Fax: 303-724-2300

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Samy Youssef, M.D, Ph.D. Department of Neurosurgery, University of Colorado School of Medicine, Aurora, Colorado, USA

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Acknowledgements: Zach Folzenlogen MD for editing of images.Center for Surgical Innovation University of Colorado for aid in surgical dissection and maintenance for cadaver heads.

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Disclosures Authors associated with this submission have no conflicts of interest to disclose.

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ACCEPTED MANUSCRIPT 2 ABSTRACT BACKGROUND: Skull base surgeons have adopted splitting the tentorium in order to expand the exposure, minimize brain retraction and combine the supra and infratentorial compartments for resection of large skull base lesions.

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OBJECTIVE: To describe stepwise techniques for splitting the tentorium to access deeply located skull base lesions and morphometrically assess the gained exposure.

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METHODS: Surgical exposure was performed through a transylvian, subtemporal, posterior transpetrosal and combined posterior supra/infratentorial – trans-sinus approach. We used a custom software program built in Matlab (Mathworks, Natick MA), to trace the surgical exposure region-of-interest (ROI) for area analysis with the ability to accurately assess most irregular areas. Qualitative morphometric assessment was done of the gain in anatomical exposure achieved by splitting the tentorium.

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RESULTS: In the trans-sylvian transtentorial approach, the mean surface area increased 154.17%, from 0.14 cm2 before expansion to 0.32 cm2 after expansion. In the subtemporal transtentorial approach, the mean surface area increased 137.61 %, from 0.66 cm2 before expansion to 1.52 cm2 after expansion. In the posterior transpetrosal transtentorial approach, the mean surface area increased 171.06%, from 1.08 cm2 before expansion to 2.81 cm2 after expansion. In the combined supra/infratentorial transsinus approach, the mean surface area increased 222.03%, from 0.78 cm2 before expansion to 2.38 cm2 after expansion.

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CONCLUSION: With splitting of the tentorium, a substantial area of expansion is obtained thus minimizing the necessity of brain retraction and improving the visualization of deep neurovascular structures in the skull base. KEY WORDS: Tentorium, Trans-sylvian, Subtemporal, Trans-petrosal, Trans-sinus.

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SHORT TITLE: Incising the tentorium to expose deep skull base lesions.

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ACCEPTED MANUSCRIPT 3 INTRODUCTION

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The tentorium is an important dural fold that has a unique location within the intracranial cavity. The unique anatomy

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stems from its dividing the cranial cavity into supra and infratentorial compartments in addition to embracing the

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incisural space containing critical neurovascular structures. The incisural space is further subdivided into anterior middle

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and posterior compartments by the mesencephalon. With the advance of skull base surgery, surgeons adopted splitting

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the tentorium in order to achieve the following goals: 1) expand the surgical exposure, 2) minimize brain retraction, and

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3) combine the supra and infratentorial compartments to focus the surgical approach for resection of large skull base

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lesions. In anterolateral approaches, the tentorium can be incised as an expansive maneuver in order to enlarge the

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anterior incisural space and adequately expose aneurysms or tumors laterally extending below the tentorial edge. In the

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subtemporal approaches, the tentorium is incised in order to expose basilar tip aneurysms, upper petroclival and

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anterior cerebellopontine angle (CPA) lesions. In posterior fossa, the tentorium is incised in the expanded transpetrosal

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approach to the CPA and transtentorial approaches to the pineal region.

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Splitting the tentorium is a technically challenging surgical step associated with risks to nearby critical neurovascular

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structures. Surgical approaches are meticulously tailored to strict surgical needs and any added technical complexity

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should be justified by an objectively assessed surgical benefit.

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We performed cadaveric dissection focused on safe stepwise techniques for splitting the tentorium in different

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compartments of the skull base. The microsurgical anatomy of the tentorium is highlighted with emphasis on the

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neurovascular structures that can be directly impacted by the different incisions. Morphometric assessment was

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performed to measure the gain in anatomical exposure.

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MATERIALS AND METHODS

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Five fixed cadaver heads with no known pathology were dissected on both sides (10 sides). The dissections were done

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using standard microsurgical instruments and a surgical microscope to obtain measurements of each approach. Surgical

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exposure was performed through a frontotemporal trans-sylvian, subtemporal, transpetrosal (presigmoid, expanded

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posterior petrosectomy) and combined posterior supra/infratentorial trans-sinus approach to the pineal region.

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114 Surgical technique:

116 1. The trans-sylvian transtentorial approach

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The patient is positioned supine with 30-degree rotation of the head to the contralateral side and slight extension so

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that the malar eminence is the highest part in the field. With this positioning the tentorial edge and the sphenoid ridge

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are in the same line of vision. Using a high speed Primado 2 Drill (NSK America Corp., IL) a frontotemporal craniotomy is

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performed. The dura is opened in a curvilinear fashion from the temporal fossa across the sylvian fissure into the frontal

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dura. It is then reflected anteriorly so there is a flat line of vision along the surgical trajectory. Under microscopic view

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and using microdissection technique the chiasmatic and carotid cisterns are opened initially followed by the proximal

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sylvian fissure. The sylvian fissure is opened widely distal to proximal and deep to superficial using sharp dissection. The

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internal carotid artery is identified and followed distally. The temporal lobe is then carefully retracted posteriorly and

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laterally to expose the middle fossa floor and the tentorium. The tentorial edge is exposed lateral to the internal carotid

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artery. By continuing with the arachnoid dissection, the 3rd nerve is visualized as it courses under the tentorial edge to

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enter the oculomotor trigone to the cavernous sinus. The anterior petrotentorial ligament is incised from posterior to

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anterior toward the insertion at the anterior clinoid process with extreme caution as the oculomotor nerve is directly

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under it. A diamond arachnoid knife with a protective foot plate (Aesculap, Tuttlingen, Germany) is used to safely split

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the dural sheath around cranial nerves. The tentorium is then retracted with 4-0 stitch to expand the view. (Fig. 1A)

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2. The Subtemporal transtentorial approach

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The patient can be placed in the supine position with the head rotated to the contralateral sided keeping the zygomatic

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arch parallel to the floor and the ipsilateral shoulder elevated. Using a high speed Primado 2 Drill (NSK America Corp., IL)

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a temporal craniotomy is performed with the inferior margin flush with the floor of the middle fossa. The dura is then

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opened in a “U” shape fashion with the base towards the floor of the middle fossa. The temporal lobe is protected with

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cotton patties and carefully elevated approximately 15- 20 mm from the middle fossa floor to expose the free margin of

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the tentorium. As described by Hernesniemi et al. 1, the line of retraction goes first slightly downwards, then across the

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floor of the middle fossa and after that, upwards to the tentorial edge. The trochlear nerve (CN IV) should be exposed

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next, coursing beneath the tentorium prior to entering its dural canal. A key step for trochlear nerve identification and

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preservation is to dissect extra arachnoidal medial to the tentorial edge as the nerve is wrapped in an arachnoid layer

ACCEPTED MANUSCRIPT 5 inferomedial to the tentorial edge. The tentorium is incised parallel to the posterior part of the superior petrosal sinus to

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the free edge. The trochlear nerve is then dissected free of its dural canal up to the posterior clinoidal area in order to

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retract the flap. The incision is then extended toward the superior petrosal sinus. If more exposure is necessary, then the

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incision can be extended across the ligated superior petrosal sinus into the roof of Meckel’s cave. The dural flaps are

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maintained in position with a stitch. The cranial nerves III and IV, and branches of posterior cerebral artery and superior

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cerebellar artery are dissected and protected. (Fig. 1B)

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3. The posterior transpetrosal transtentorial approach

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The patient can be placed in the supine position. The head is rotated to the contralateral side so the sagittal plane is

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parallel to the floor and the vertex facing the floor with the mastoid bone as the highest point. A retroauricular skin

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incision is used but if combined with an anterior petrosectromy, then a horseshoe incision is made from the zygomatic

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arch up to the squamosal suture level and extending 3 finger breadth behind the ear down to 1cm inferior to the

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mastoid tip. The skin and pericranium are elevated until the posterior part of the bony external auditory canal (spine of

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Henle) is visualized. This exposure allows to visualize the asterion, posterior root of the zygoma, mastoid tip, spine of

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Henle and temporal line. We used a transpetrosal retrolabyrinthine approach is performed as described by Sincoff. 2 The

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presigmoid dura and the sigmoid sinus are skeletonized and exposed. The mastoidectomy defect is used as a keyhole for

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the temporal craniotomy. Once the dura of the middle and posterior fossae is exposed, dural cuts are made. The first

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dural incision is made in the middle fossa dura above and parallel to the superior petrosal sinus (SPS). Caution must be

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taken as the underlying vein of Labbe runs posterior and inferior on the temporal lobe and drains at the transverse sinus

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1 cm posterior to the sinodural point. The second incision is made in the posterior fossa dura anterior and parallel to the

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jugular bulb and sigmoid sinus toward the superior petrosal sinus. These two cuts straddle the SPS. The SPS is then

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ligated with hemoclips and sharply divided. The tentorial incision is made from the site of ligation of the SPS and aimed

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anteriorly toward the free edge of the tentorium behind the entrance of the of CN IV into the tentorium. Again,

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extraarachnoidal dissection medial to the edge is a safe technique to visualize and preserve the trochlear nerve.

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Tentorial dural venous structures may be encountered and should be ligated after being ruled out as dominant drainage

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sinuses by a preoperative venogram. (Fig. 1C)

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ACCEPTED MANUSCRIPT 6 172 173

4. Combined supra/infratentrorial trans-sinus approach

174 The patient can be placed in the three-quarter lateral position. The non-dominant transverse sinus to be ligated is placed

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on the dependent side to allow for gravity-aided retraction of the occipital lobe. A “U” shaped skin incision is made to

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expose bilateral parietal and occipital bones for supra and infratentorial exposure. The skin flap and muscles are

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elevated as a single layer and reflected inferiorly. The craniotomy is performed in three pieces as described by Ziyal et al.

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dura is opened in a “L” shape; medial and parallel to the SPS and superior to the transverse sinus. The transverse sinus is

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ligated with hemoclips and divided. The tentorium is incised just lateral and parallel to the straight sinus toward the

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tentorial notch. The occipital lobe and the cerebellum are gently retracted to widely expose the quadrigeminal cistern

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and the pineal region. This approach converts a deep narrow exposure into shallow wide exposure and provides an

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expanded posterolateral view to the pineal region. It facilitates the dissection of large firm pineal region tumors from

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the complex deep venous system. (Fig. 1D)

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The sub-occipital dura is then opened in a linear fashion just inferior and parallel to the transverse sinus. The occipital

Measurements and quantification

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Morphometric assessment was performed in order to measure the gained exposure achieved by splitting the tentorium.

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Measurements were obtained in 2-dimensional planes before and after the splitting of the tentorium under 6x and 10x

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microscopic magnification. Quantitative analyses were conducted for images captured at 6x. Briefly, for each of the four

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surgical approaches (both pre and post bisection of the tentorium) one image was captured with a metric ruler in the

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field of view and a second duplicate image was captured without the ruler. In a custom software program built in Matlab

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(Mathworks, Natick MA), each pair of images (i.e., image with and without the ruler in the field of view) associated with

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one of the four surgical approaches was uploaded for side-by-side visualization. The image with the ruler was used to

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compute the number of pixels per 1 centimeter and the second image was used to trace the surgical exposure region-of-

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interest (ROI) for area analysis (Figure 2 – yellow outlines). For each ROI, the area was computed with the following

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equation
 ∗ (
 ) , where pixelN is the number pixels in the ROI and pixelcm is the number of pixels per

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centimeter. As an internal control for ROI measurements, we assessed the difference between left and right gain in

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exposure for each surgical approach (Figure 3). For this analysis, we computed the percent change in exposure for each

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pre/post ROI. Left and right mean percent change in gain in exposure was compared with a paired t-test for each of the

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4 surgical approaches. To quantify the gain in exposure for each surgical approach, we independent compared left and

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right hemispheres (Figure 4). Within each hemisphere, for each surgical approach, we assessed the difference in mean

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ROI area between pre and post tentorial splitting using a paired t-test. The magnitude of added exposure was

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documented as percentages of the control values. (Table 1). The mean percent change was computed for each

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hemisphere using the following equation: %∆ =

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() 

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ACCEPTED MANUSCRIPT 7 206 For the trans-sylvian transtentorial approach, the area calculated was between the internal carotid artery (ICA) and the

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oculomotor nerve (Fig. 2A). For the subtemporal transtentorial approach, the area calculated before the tentorial cut

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was between the free edge of the tentorium and the lateral edge of the middle temporal gyrus. After the tentorium was

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incised, the area calculated was the triangular view provided after the tentorium leaflets were reflected (Fig. 2B). For the

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posterior transpetrosal- transtentorial approach, the area calculated before the tentorial cut was the posterior fossa

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view after dural opening, is the retrolabyrinthine interval between the semicircular canals, the cerebellopontine angle

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and the tentorium. After the tentorium was incised, the area calculated was the retrolabyrinthine interval in addition to

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the supratentorial area inferior the temporal lobe (Fig. 2C). For the combined posterior supra/infratentorial trans-sinus

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approach, the area calculated before the tentorial cut was between the free edge of the tentorium and the inferior edge

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of the occipital lobe as it is displaced superiorly and laterally. After the tentorium was incised, the area calculated was

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the area inferior to the displaced occipital lobe down to the exposed cerebellomesencephalic fissure including the pineal

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region (Fig. 2D).

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ACCEPTED MANUSCRIPT 8 RESULTS

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We conducted our study on 5 cadaveric heads using 4 separate approaches to assess change in surgical area of exposure

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following splitting of the tentorium. The pre and post tentorial incision areas were interpreted as mean and range before

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and after the expansion of the window. Summary analyses comparing left and right increase in exposure following

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expansion failed to find a significant difference between left and right hemispheres for any surgical approach (Trans-

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sylvian: p = 0.3, Subtemporal: p = 0.8, Transpetrosal: p = 0.5, P. Combined supra-infra tentorial trans-sinus: p = 0.4;

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Figure 3). Statistical quantification of the mean change in exposure between pre and post tentorial splitting indicated

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that on the left hemisphere, the post-incision area of expansion (cm2) increased significantly for the subtemporal,

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transpetrosal and combined trans-sinus approaches (p < 0.05), but did not significantly differ from pre-incision for the

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trans-sylvian approach (p = 0.11). (Figure 4A) For the right hemisphere, for all 4 surgical approaches, the incision

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significantly increased the area of expansion (p < 0.05). (Figure 4B)

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Change in exposure by surgical approach

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In the trans-sylvian transtentorial approach, the mean surface area increased 154.17% (115.18% on the left and 193.16%

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on the right), from 0.14 cm2 (0.14 cm2 on the left and 0.14 cm2 on the right) before expansion to 0.32 cm2 (0.28 cm2 on

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the left and 0.36 cm2 on the right) after expansion.

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In the subtemporal transtentorial approach, the mean surface area increased 137.61 % (133.09% on the left and

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142.13% on the right), from 0.66 cm2 (0.72 cm2 on the left and 0.59 cm2 on the right) before expansion to 1.52 cm2 (1.62

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cm2 on the left and 1.42 cm2 on the right) after expansion.

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In the posterior transpetrosal transtentorial approach, the mean surface area increased 171.06%, (167.02% on the left

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and 175.09% on the right), from 1.08 cm2 (1.08 cm2 on the left and 1.09 cm2 on the right) before expansion to 2.81 cm2

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(2.7 cm2 on the left and 2.92 cm2 on the right) after expansion.

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In the combined supra/infratentorial trans-sinus approach, the mean surface area increased 222.03%, (188.24% on the

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left and 255.83% on the right), from 0.78 cm2 (0.85 cm2 on the left and 0.70 cm2 on the right) before expansion to 2.38

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cm2 (2.22 cm2 on the left and 2.53 cm2 on the right) after expansion. Figure 4

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DISCUSSION

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This study demonstrates that the transcranial exposure to the deep skull base is significantly increased by incising the

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tentorium in four different approaches. The increase in surface area facilitates the exposure and dissection of deep

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neuro-vascular structures during resection of skull base lesions with the ultimate outcome of anatomical and functional

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preservation. We used a novel computerized technique to trace the surgical exposure region-of-interest (ROI) for area

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analysis with the ability to accurately assess most irregular areas.

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In the trans-sylvian transtentorial approach, the 154.17 % increase in surface area allows for better visualization of the

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carotid-oculomotor window, the posterior communicating artery (PCOM) and basilar artery tip. (Fig. 1A) In the

ACCEPTED MANUSCRIPT 9 subtemporal transtentorial approach the 137.61 % increase in surface area grants access to the middle incisural space

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and upper clival region. It provides better visualization of the basilar tip and ambient cistern contents such as the

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posterior cerebral artery (PCA), the superior cerebellar artery (SCA) the third and fourth cranial nerves. (Fig. 1B) In the

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presigmoid transpetrosal transtentorial approach, the 171.06 % increase in surface area allows for better visualization of

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the petroclival area and upper cerebellopontine angle. There is better visualization of the posterior cerebral artery, the

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fourth (IV) cranial nerve and of the contents of the cerebellopontine angle cistern, such as the seventh (VII) and eighth

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(VIII) cranial nerves, the anterior-inferior cerebellar artery (AICA), the petrosal vein and fifth (V) cranial nerve. (Fig. 1C) In

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the combined supra/infratentorial trans-sinus approach to large and firm lesions of the pineal region, the 222.03%

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increase in surface area allows for better visualization of the pineal and supratentorial region. With the slight retraction

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of the occipital lobe and the cerebellum there is better exposure of the splenium of corpus callosum, the pineal gland,

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the posterior third ventricle, and the critical deep venous including the vein of Galen, internal cerebral veins, and basal

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vein of Rosenthal. (Fig. 1D)

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1. The trans-sylvian transtentorial approach

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The pterional trans-sylvian approach is the standard approach for anterior circulation aneurysms and parasellar lesions.

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It can be supplemented by tentorial incision for better exposure of large or laterally projecting posterior communicating

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aneurysms under the tentorial edge. Parasellar tumors extending laterally under the tentorium are better exposed and

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resected by cutting the tentorial edge. Gupta et al.1 published a series of 6 cases in which the trans-sylvian

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transtentorial approach was implemented for resection of trigeminal schwannoma, petroclival meningioma, clival

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chordoma and epidermoid tumor. The pterional trans-sylvian approach was popularized by Yasargil in 1976 to access

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posterior circulation aneurysms through the optico-carotid or carotid oculomotor windows 2. However, visualization is

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limited by the anterior clinoid process, supraclinoid ICA, oculomotor nerve, and posterior clinoid process as limits of the

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carotid-oculomotor window 3. The transylvian approach in combination with anterior and posterior clinoidectomy and

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tentorial incisions to mobilize the oculomotor nerve is advantageous for expanding the exposure of the upper basilar

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artery by 69% 4. In comparison with the subtemporal approach, the trans-sylvian approach provides a combined

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anterolateral exposure of the parasellar region with less retraction of the temporal lobe, and less manipulation of the

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oculomotor nerve.

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Advantages

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This approach offers less retraction of the temporal lobe and less manipulation of the oculomotor nerve. The

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anterolateral view of the interpeduncular cistern and upper basilar artery provides better angle to visualize the critical

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basilar bifurcation perforators.

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Potential complications

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Injury to the oculomotor nerve as it travels under the tentorium can be avoided by maintaining the dissection in an extra

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arachnoidal fashion. In case of altered anatomy by parasellar tumors, the nerve may be laterally displaced and extreme

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caution with adequate visualization of the tips of the scissor blades should be maintained during splitting the tentorium

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in order to avoid cutting through the nerve.

295 2. The Subtemporal transtentorial approach

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In 1961 Drake et al. reported 4 cases of recurrent basilar tip aneurysm bleeding that were treated through a

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subtemporal transtentorial approach into the interpeduncular cistern 5. The temporal lobe was retracted and the

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arachnoid was dissected along the edge of the tentorium. The trochlear nerve was tucked under the tentorium and

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protected with a cotton pattie. Of the 4 cases in only 1 it was necessary to cut the tentorium as the basilar bifurcation

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was low. Drake and Peerless later published their experience with this approach in 1979 6. Subsequent modifications

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have been used to reach lesions of the lateral and posterolateral brainstem. Kawase et al. reported 10 cases of

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petroclival meningiomas that were resected via a subtemporal approach 7. Hernesniemi et al. have reported a technique

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to retract the tentorial edge during a subtemporal approach for clipping of basilar tip aneurysms 8. The tentorial edge is

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reflected downward by 1 cm and tethered with an aneurysm clip to a small incision that is made on the dura of the floor

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of the middle fossa. However, incising the tentorium provides better and wider exposure 9. There have been reports on

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the technique in incising the tentorium as has been described by Tanriover et al. and it is now the most commonly used

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10–12

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dissection and tentorial incision on a subtemporal approach 13. They report that by dissecting the trochlear nerve from

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its dural canal up to the entrance into the cavernous sinus, significant additional tentorial retraction is achievable. In

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their series of 21 patients only one case had a transient trochlear nerve palsy. This is less dangerous than tenting the

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nerve.

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The splitting of the tentorium provides access to the middle incisural space, interpeduncular and ambient cisterns. It also

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provides visualization of the basilar artery, and rostral ventral pons, therefore it has been used in surgery for basilar

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artery trunk aneurysms, superficial temporal-SCA bypass, and brainstem lesions such as cavernous hemangiomas 14.

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Meningiomas that invade the middle fossa can be managed by a subtemporal approach 14,15. Large petroclival

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meningiomas are directly exposed and resected via a subtemporal approach in combination with a pre or retrosigmoid

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approach 16.

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. Subsequently McLaughlin et al. described a modified technique and reported the impact of trochlear nerve

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Advantages

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The subtemporal transtentorial approach provides a shallow wide direct exposure to the middle incisural space and

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upper basilar artery region. Supplemented by an anterior petrosectomy, it can provide access to the upper petroclival

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region. In cases of tentorial meningiomas the arterial blood supply from the tentorial branches can be coagulated at an

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ACCEPTED MANUSCRIPT 11 early stage which facilitates tumor resection from the base of the tumor.16

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The excessive retraction of the temporal lobe can lead to contusions and increase the risk of venous infarction of the

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vein of Labbe, leading to hemiplegia and aphasia.18 Understanding the course of the fourth nerve is paramount to avoid

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its injury when cutting the free edge of the tentorium.16 Dominant tentorial sinuses should be ruled out by a

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preoperative venography study to avoid dominant tentorial venous sinuses drainage compromise from cutting the

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tentorium.

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The posterior transpetrosal approach evolved with continuous advances of surgical microscopes, high speed drills and

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better handling of the sigmoid sinus in order to reach skull base lesions of the cerebello-pontine angle (CPA), the

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ventrolateral brainstem and the clivus, thus providing greater exposure and less brain retraction 17.

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In 1966 Hitselberger and House published their series of 20 patients who underwent a combination of a

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translabyrinthine craniotomy and a suboccipital craniotomy for resection of CPA tumors 18. They report carrying out a

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mastoidectomy and labyrinthectomy and extending the bony removal over the sigmoid sinus. Loss of hearing after

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labyrinthectomy was an unjustifiable collateral damage which lead to the modifications of partial labyrinthectomy,

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retrolabyrinthine and transotic approaches 19. The sigmoid sinus posed a surgical obstacle that limited the transpetrosal

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window for a long time before neurosurgeons were able to mobilize the preserved sigmoid sinus after ligating the

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superior petrosal sinus and splitting the tentorium all the way to the hiatus.21 This approach was described as the

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expanded transpetrosal approach and has been useful in surgery for large petroclival and cerebellopontine angle lesions

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such as meningiomas or schwannomas. Fukushima combined and extended middle fossa anterior petrosal approach

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with a retrolabyrinthine transpetrosal approach for the treatment of petroclival meningioma. This allowed for

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prevention of sigmoid sinus injury and hearing loss.19

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Advantages

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The primary advantage of ligating the superior petrosal sinus and splitting the tentorium is the expanded total working

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area provided by untethering and mobilizing the sigmoid sinus posteriorly and the supratentorial exposure. It provides

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exposure from midbrain and oculomotor nerve (superiorly), vestibulocochlear nerve at midclivus (inferiorly), internal

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acoustic meatus (laterally) and trochlear nerve (medially) and down to the lower cranial nerves IX-XI. There is decreased

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risk of hearing loss and facial palsy since the labyrinth is not sacrificed.

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Potential complications:

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Complications of this approach are related to bone drilling or tentorial splitting. Complications from tentorial splitting

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comprise venous injury such as superior petrosal or sigmoid sinuses, vein of Labbe or abnormal dominant dural venous

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sinus; trochlear nerve injury or temporal lobe retraction injury. Again, a preoperative venogram would identify aberrant

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vein of Labbe drainage into SPS, temporal bridging veins or a sphenopetrosal sinus.24

365 4. Combined supra/infratentrorial – trans-sinus approach

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In 1992 Sekhar and Goel described a combined supratentorial and infratentorial approach to a giant pineal region

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meningioma in order to expand the exposure and allow the dissection of the firm capsule from the deep venous

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system.26 The procedure required sectioning of the less dominant transverse sinus and the tentorium. Two temporary

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clips were placed across the transverse sinus 1 cm lateral to the torcular and then the sinus was sectioned. The

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tentorium was then incised 1 cm lateral from the straight sinus towards the tentorial notch. This technique has been

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implemented for resection of large tumors greater than 4.5 cm extending above and below the tentorium, tumors

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encasing the deep venous system and vascular tumors that cannot be internally debulked prior to capsule dissection.3,26

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It provides good exposure to the venous structures of the Galenic system, and the quadrigeminal cistern can be

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visualized between the pineal gland and cerebellum with no retraction.3

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377 378 Advantages

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This approach allows better dissection of firm vascular tumors from critical neurovascular structures namely the deep

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venous system. As the occipital lobe is dependent with gravity, retraction is minimal and temporary.

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Potential complications

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Major disadvantage is the need to divide the non-dominant transverse sinus. Thorough study of the venous flow should

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be conducted with a conventional venogram prior to surgery.

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Conclusion:

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With splitting of the tentorium, a substantial area of expansion is obtained thus minimizing the necessity of brain

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retraction and improving the visualization of deep neurovascular structures in the skull base.

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REFERENCES

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1.Hernesniemi J, Ishii K, Shen H. Surgical technique to retract the tentorial edge during subtemporal approach: technical

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note. Neurosurgery. 2005 Oct;57(4 Suppl)

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2.Sincoff EH, McMenomey SO, Delashaw JB Jr. Posterior transpetrosal approach: less is more.

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Neurosurgery. 2007 Feb;60(2 Suppl 1): ONS53-8

398 3.Ziyal IM, Sekhar LN, Salas E, Olan WJ. Combined supra/infratentorial-transsinus approach to large pineal region

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tumors. J Neurosurg. 1998 Jun;88(6):1050-7

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4.Gupta SK. Trans-sylvian transtentorial approach for skull base lesions extending from the middle fossa to the upper

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petro-clival region. Br J Neurosurg. 2009 Jun;23(3):287-92

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5.Yasargil MG, Antic J, Laciga R, Jain KK, Hodosh RM, Smith RD. Microsurgical pterional approach to aneurysms of the

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basilar bifurcation. Surg Neurol. 1976 Aug;6(2):83-91

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6.Sabuncuoğlu H, Jittapiromsak P, Cavalcanti DD, Spetzler RF, Preul MC. Accessing the Basilar Artery Apex: Is the

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Temporopolar Transcavernous Route an Anatomically Advantageous Alternative? Skull Base. 2011 Jan; 21(1): 23-30

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7.Youssef AS, Abdel Ayz KM, Kim EY, Keller JT, Zuccarello M, van Loveren HR.

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The carotid-oculomotor window in exposure of upper basilar artery aneurysms: a cadaveric morphometric study.

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Neurosurgery. 2004 May;54(5):1181-7

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8.Drake CG. Bleeding aneurysms of the basilar artery. Direct surgical management in four cases.

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J Neurosurg. 1961 Mar; 18:230-8.

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9.Drake CG. The treatment of aneurysms of the posterior circulation.

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Clin Neurosurg. 1979; 26:96-144

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10.Kawase T, Shiobara R, Toya S. Anterior transpetrosal-transtentorial approach for sphenopetroclival meningiomas:

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surgical method and results in 10 patients. Neurosurgery. 1991 Jun;28(6):869-75; discussion 875-6.

423 11.Hernesniemi J, Ishii K, Shen H. Subtemporal approach to basilar bifurcation aneurysms: advanced technique and

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clinical experience. Acta Neurochir Suppl. 2005;94:31-8.

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12.Tanriover N, Abe H, Rhoton AL Jr, Akar Z. Microsurgical anatomy of the superior petrosal venous complex: new

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classifications and implications for subtemporal transtentorial and retrosigmoid suprameatal approaches. J Neurosurg.

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2007 Jun;106(6):1041-50.

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13.MacDonald JD, Antonelli P, Day AL. The anterior subtemporal, medial transpetrosal approach to the upper basilar

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artery and ponto-mesencephalic junction. Neurosurgery. 1998 Jul;43(1):84-9.

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14.Goel A, Muzumdar D, Desai K. Anterior tentorium-based epidermoid tumours: results of radical surgical treatment in

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96 cases. Br J Neurosurg. 2006 Jun;20(3):139-45.

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15.McLaughlin N, Ma Q, Emerson J, Malkasian DR, Martin NA. The extended subtemporal transtentorial approach: the

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impact of trochlear nerve dissection and tentorial incision. J Clin Neurosci. 2013 Aug;20(8):1139-43.

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16.Yang J, Liu YH, Ma SC, Wei L, Lin RS, Qi JF, Hu YS, Yu CJ. Subtemporal transtentorial petrosalapex approach for giant

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petroclival meningiomas: analyzation and evaluation of the clinical application. J Neurol Surg B Skull Base. 2012

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Feb;73(1):54-63

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17.Samii M, Tatagiba M. Experience with 36 surgical cases of petroclival meningiomas.

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Acta Neurochir (Wien). 1992;118(1-2):27-32.

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18.Zhu W, Mao Y, Zhou LF, Zhang R, Chen L. Combined subtemporal and retrosigmoid keyhole approach for extensive

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petroclival meningiomas surgery: report of experience with 7 cases.

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Minim Invasive Neurosurg. 2008 Apr;51(2):95-9.

450 19.Grossi PM, Nonaka Y, Watanabe K, Fukushima T. The history of the combined supra- and infratentorial approach to

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the petroclival region. Neurosurg Focus. 2012 Aug;33(2): E8.

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20.Hitselberger WE, House WF. A combined approach to the cerebellopontine angle. A suboccipital-petrosal approach.

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Arch Otolaryngol. 1966 Sep;84(3):267-85

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21.Hakuba A, Nishimura S, Tanaka K, Kishi H, Nakamura T. Clivus meningioma: six cases of total removal. Neurol Med

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Chir (Tokyo) 1977 17:63-77

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22.Hitselberger WE, Pulec JL. Trigeminal nerve (posterior root) retrolabyrinthine selective section. Operative procedure

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for intractable pain. Arch Otolaryngol. 1972 Nov;96(5):412-5

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23.Jenkins HA, Fisch U. The transotic approach to resection of difficult acoustic tumors of the cerebellopontine angle.

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Am J Otol. 1980 Oct;2(2):70-6.

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24.Hafez A1, Nader R, Al-Mefty O. Preservation of the superior petrosal sinus during the petrosal approach. J Neurosurg.

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2011 May;114(5):1294-8.

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25.Janjua MB, Caruso JP, Greenfield JP, Souweidane MM, Schwartz TH. The combined transpetrosal approach: Anatomic

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study and literature review. J Clin Neurosci. 2017 Apr 1. pii: S0967-5868(16)31484-9.

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26.Sekhar LN, Goel A. Combined supratentorial and infratentorial approach to large pineal-region meningioma. Surg

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Neurol. 1992 Mar;37(3):197-201

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Figure 1. Collage of the expanded anatomical view after the tentorium has been split through the four different approaches. A, Transylvian approach visualizing the supraclinoid ICA, oculomotor nerve as limits of the carotidoculomotor window. Right side view. B, Subtemporal approach visualizing the middle incisural space, ventral rostral pons and upper basilar artery region. Left side view. C,Transpetrosal approach visualizing the midbrain superiorly, the CN7&8 complex inferiorly, internal acoustic meatus laterally and CN IV medially. Right side view. D, Posterior Transtentorial approach where the venous structures of the Galenic system, and the quadrigeminal cistern can be visualized between the pineal gland and cerebellum. Left side view. TENT (Tentorium). PCOM (posterior communicating artery). CA (internal carotid artery). A.Ch.A (anterior choroidal artery). BA (Basilar artery). PCA (posterior cerebral artery). MB (Midbrain). TL (Temporal lobe). AICA (Anterior inferior cerebellar artery). VG (Vein of Galen). BVR (Basal vein of Rosenthal). ICV (Inferior cerebellar vein). PG (Pineal Gland). SCA (superior cerebellar artery).

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Figure 2. Collage of the measured areas pre. and post. incisions. The yellow line outlines the area interpreted for calculation. A, Transylvian approach the area calculated is between the oculomotor nerve, the internal carotid artery and the temporal pole. B, Subtemporal transtentorial approach the area calculated is between the lateral edge of the middle temporal gyrus and the free edge of the tentorium. After the tentorial incision the area calculated was the triangular view provided after the tentorium leaflets were reflected. C, Posterior transpetrosal-transtentorial approach the area calculated before the tentorial cut was the posterior fossa view after dural opening, this is the retrolabyrinthine interval between the semicircular canals, the cerebellopontine angle and the tentorium. After the tentorium was incised, the area calculated was the retrolabyrinthine interval in addition to the supratentorial area inferior the temporal lobe. D, Combined posterior supra/infratentorial trans-sinus approach, the area calculated before the tentorial cut was between the free edge of the tentorium and the inferior edge of the occipital lobe as it is displaced superiorly and laterally. After the tentorium was incised, the area calculated was the area inferior to the displaced occipital lobe down to the exposed cerebellomesencephalic fissure including the pineal region.

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17 Figure (3). Distributions of percent expansion for left and right hemispheres within each surgical approach. Red dots indicate individual right hemispheric measurements and blue dots indicate left hemispheric measurements, horizontal bars represent the mean percent change.

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Figure (4). Sample distributions for the computed region-of-interest area representing the quantitation of exposure. Grey dots indicate exposure areas pre-tentorial splitting and black dots indicate exposure area measured post-tentorial splitting; red lines represent the comparison between the mean values for paired pre and post sample distributions for each surgical approach A. Right side. B. Left side. * = p < 0.05.

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Table 1. Absolute values of calculated area (cm2) pre and post incision of the tentorium in the four different approaches; Transylvian, Subtemporal, Transpetrosal and Posterior Transtentorial. The area was measured on both right and left sides before and after splitting the tentorium in 5 different specimens.

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525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548

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Area (cm2)

Post cut 1.67 2.74 2.91 3.42 1.89

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Post cut 2.63 3.22 2.65 3.41 2.71

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Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Post. Transtentorial

Post cut 0.99 1.67 0.95 1.81 1.68

Post cut 0.09 0.37 0.16 0.56 0.23

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Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Transpetrosal

Post cut 0.29 0.5 0.36 0.37 0.28

Left Pre cut 0.06 0.27 0.12 0.14 0.09 Left Pre cut 0.58 0.45 0.47 0.9 1.19 Left Pre cut 0.74 0.82 0.93 2.23 0.67 Left Pre cut 0.72 0.63 0.57 1.41 0.94

Post cut 1.57 1.1 1.03 2.27 2.13

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Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Subtemporal

Right Pre cut 0.09 0.16 0.13 0.24 0.07 Right Pre cut 0.38 0.8 0.5 0.72 0.56 Right Pre cut 0.91 0.89 0.99 1.43 1.24 Right Pre cut 0.64 0.68 0.71 0.8 0.68

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Transylvian

Post cut 1.25 3.41 3.05 4.26 1.55 Post cut 2.68 2.02 2.02 2.06 2.33

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Highlights

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First demonstration for quantification of gain in area of expansion using an approach that bifurcates the tentorium With splitting of the tentorium, a substantial area of expansion is obtained thus minimizing the necessity of brain retraction and improving the visualization of deep neurovascular structures in the skull base.

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Abbreviations ROI – Region of interest CPA - cerebellopontine angle

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CN IV – Cranial Nerve 4 SPS - superior petrosal sinus ICA - internal carotid artery PCOM - posterior communicating artery

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PCA - posterior cerebral artery SCA - superior cerebellar artery

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AICA - anterior-inferior cerebellar artery