Accepted Manuscript The Anterior Subcallosal Approach to Third Ventricular and Suprasellar Lesions: Anatomical Description and Technical Note Brandon D. Liebelt, MD, Kristopher G. Hooten, MD, Gavin W. Britz, MD PII:

S1878-8750(15)01681-2

DOI:

10.1016/j.wneu.2015.12.011

Reference:

WNEU 3474

To appear in:

World Neurosurgery

Received Date: 12 October 2015 Revised Date:

16 December 2015

Accepted Date: 17 December 2015

Please cite this article as: Liebelt BD, Hooten KG, Britz GW, The Anterior Subcallosal Approach to Third Ventricular and Suprasellar Lesions: Anatomical Description and Technical Note, World Neurosurgery (2016), doi: 10.1016/j.wneu.2015.12.011. 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.

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The Anterior Subcallosal Approach to Third Ventricular and Suprasellar Lesions: Anatomical Description and Technical Note

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Brandon D. Liebelt MD1, Kristopher G. Hooten MD2, and Gavin W. Britz MD1

Departments of Neurosurgery, Houston Methodist Neurological Institute, Houston, TX1 and

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University of Florida, Gainesville, FL2

Corresponding author: Gavin W. Britz MD

Scurlock Tower, #944 Houston, TX 77030

Fax: 713-790-5122

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Phone: 713-441-3800

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6560 Fannin St.

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Email: [email protected]

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Abstract: Background: Surgical access to the third ventricle is challenging given the depth of the operative field and

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close proximity of vital neural structures that must be traversed. For anterior third ventricular lesions approach options include anterior transcallosal or transcortical, subfrontal, frontotemporal, or endonasal.

The subcallosal approach, a trans-lamina terminalis approach, is unique in that the surgical

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corridor is just below the corpus callosum, minimizes retraction, and preserves corpus callosum integrity. Case examples are provided and an anatomical study delineating the dimensions of the

Methods:

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surgical corridor is performed.

Two latex injected cadaver heads were utilized to describe the subcallosal corridor. An MRI was obtained and registered with neuronavigation for correlative anatomical illustration. Depth, dimensions, and cross-sectional area were measured for the subcommunicating and

Results:

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supracommunicating corridors.

The surgical depth for anterior transcallosal, subcallosal, and subfrontal approaches was 7.5 cm,

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7.7 cm, and 7.6 cm respectively. The average corridor dimensions for the subcallosal approach was 14.75 x 6.63 mm compared to 8.88 x 5.38 mm for the subcommunicating corridor. Cross sectional area of the subcommunicating corridor was 30.62 mm2 compared to 80.42 mm2 for

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supracommunicating. This was easily enlarged to 156.62 mm2 with gentle retraction. Conclusions:

The anterior subcallosal approach is a safe approach for lesions of the third ventricle that avoids splitting the corpus callosum, resecting unnecessary brain, and minimizes brain retraction. This corridor is superior to the traditional subfrontal approach in terms of working space and compares favorably to the anterior transcallosal approach without disrupting the corpus callosum.

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Key Words: Third Ventricle, Lamina Terminalis, Cavernous Malformation, Surgical Approach

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Running Title: Anterior Interhemispheric Subcallosal Approach

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Introduction Lesions of the third ventricle are particularly difficult to access surgically given their depth from the surface of the skull as well as the intricate neurovascular structures closely

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associated with this region. Pathology of the third ventricle varies by specific location and includes colloid cysts(2), craniopharyngioma(15), chordoid glioma(4,12), choroid plexus

papilloma(11), ependymoma(18), meningioma(16), and rarely vascular lesions(20). Lesions can be stratified according to whether they are primarily intraventricular lesions or arise adjacent to

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the third ventricle and invade into this space(5). Selecting an appropriate approach should be tailored to the specific location the lesion occupies within the third ventricle, broadly classifying lesions as either predominantly anterior or posterior. Further adding complexity to these cases is

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the requirement of traversing and incising certain neural structures in order to gain access to the third ventricular space. This varies by approach and anteriorly can be accomplished through either the lamina terminalis or corpus callosum and fornix.

Pathology of the third ventricle is associated with neurocognitive deficits including impairment in memory, executive function, and manual dexterity(10). Deficits are often present

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prior to surgery and there is not clear data available regarding which surgical approaches are more favorable to avoid causing further neurologic deficits at the time of surgery. Regardless, when choosing a surgical approach it is essential to avoid traversing eloquent areas in order to preserve or restore maximal neurological function for the patient. A subcallosal,

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interhemispheric corridor through the lamina terminalis avoids disruption of both the corpus callosum and fornix. The senior author (GWB) believes that white matter “matters” and modern neurosurgery should minimize the disruption of any white matter tracts including the corpus

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callosum.

Methods

Two latex injected, fixed cadaver heads were used for anatomical dissection and surgical corridor analysis. First, MRI sequences were obtained of each cadaver head using a 1.5 Tesla MRI scanner. The cadaver heads were then fixed in neutral position with a Mayfield three-point fixation device and attached to a table similar to standard operating room conditions. MRI

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images were uploaded to a cranial neuronavigation software (Brainlab, Munich, Germany) and registered accordingly. Microsurgical dissection was aided by the use of an operating microscope (Leica, Wetzlar, Germany) and photographs of the operative corridor were taken for further analysis of the surgical corridors. Craniotomy was performed utilizing an electric drill (Stryker,

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Kalamazoo, MI). Two observers measured each dimension of the operative corridors twice with calipers and mean values were calculated for each dimension in each cadaver. Images were

obtained with a centimeter marker in the field for digital analysis. Cross sectional area of the

presented for further illustration of the approach anatomy.

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Results

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surgical corridors was measured using ImageJ software (Bethesda, MD). Two surgical cases are

Description of Approach

An illustrative overview is provided in Figure 1, depicting the interhemispheric corridor with entry into the third ventricle by way of the lamina terminalis superior to the anterior communicating artery. The patient is positioned in the supine position with head fixed in three-

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point fixation. The neck is extended slightly in order to permit direct inline visualization of the surgical trajectory. The angle of extension will vary depending on if the operating surgeon chooses to sit or stand; however, excessive extension is not necessary to help separate the frontal lobes from the anterior fossa as this is an interhemispheric approach. The use of intraoperative

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neuronavigation is useful in planning the angle of approach in order to permit adequate exposure of the pathology without disrupting the rostrum and genu of the corpus callosum. A bicoronal

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skin incision is made and the scalp flap is retracted forward in a subperiosteal fashion (Figure 2A). Craniotomy is performed between the coronal suture and frontal sinus, avoiding the need for elevating a separate periosteal flap. The bone flap is typically positioned to the right of midline and spanning the superior sagittal sinus in order to allow adequate room for interhemispheric dissection (Figure 2B). The dura is then opened in a C shaped fashion based on the sinus which is left intact. Dissection proceeds along the falx until the corpus callosum and pericallosal arteries are reached. The cistern of the lamina terminalis is then entered and further dissection ensues until clear visualization of the anterior wall of the third ventricle, from optic chiasm to rostrum of corpus callosum, is achieved. The optic chiasm and tracts, anterior

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communicating artery, and bilateral A1 and A2 segments should now be in view. The third ventricle is then entered, by way of the lamina terminalis, above the anterior communicating artery (supracommunicating window). Gentle retraction can be applied to the A2 segments in order to widen the surgical corridor of the third ventricle. After removal of the offending

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pathology, hemostasis is attained and the dura is closed in a watertight fashion. The bone flap is replaced and scalp closed in two-layer fashion with sutures and staples.

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Surgical Corridor Dimensions

First, the depth of the surgical field was compared for the various approach options for

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anterior third ventricular approaches. The distance from the outer table of the skull to the entry point into the third ventricle for the anterior transcallosal, subcallosal, and subfrontal approaches was 7.5cm, 7.7cm, and 7.6cm respectively. The subfrontal and subcallosal entry points through the lamina terminalis and transcallosal entry through the foramen of Monro is illustrated in Figure 3.

The surgical corridor utilized in the subcallosal approach is located superior to the

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anterior communicating artery, in between the A2 segments, and inferior to the rostrum of the corpus callosum (Figure 4A). This is compared to the subcommunicating corridor located inferior to the anterior communicating artery (Acom), between the two A1 segments, and superior to the optic chiasm (Figure 4B). The dimensions of these surgical corridors in the two

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cadavers are summarized in Table 1. Of note, the width of the supracommunicating corridor could be doubled by gently retracting the A2 segments bilaterally. Cross sectional area of the two

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corridors was compared using computer analysis (Table 2). The subcommunicating corridor measured 30.62 mm2 compared to 80.42 mm2 for the supracommunicating corridor. The supracommunicating corridor was easily expanded to 156.62 mm2 with gentle retraction.

Case #1 A 58-year-old woman presented to our hospital with several month history of memory loss and progressive headaches. Imaging at an outside institution revealed a third ventricular mass with obstructive hydrocephalus. She then experienced acute worsening of her headaches

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and presented to the emergency department for further evaluation. CT of the brain revealed a 2.4 x 1.9 cm hyperdense mass in the anterior third ventricle. The mass exhibited T2 hyperintensity with a hypointense rim, heterogeneous T1 signal, and no contrast enhancement consistent with blood products (Figure 5). The patient was taken to the operating room for surgical removal of

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the mass. Skin incision and craniotomy was performed as previously described. The sagittal sinus was retracted laterally to aid in a direct midline view. The third ventricle was opened

through the lamina terminalis and a dark fluid collection consisting of chronic blood products was immediately encountered. The mass was completely evacuated and carefully dissected from

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the lateral walls of the thalamus. A ventricular drain was left in the surgical cavity

postoperatively for transient postoperative drainage. Final pathology was consistent with a

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cavernous malformation of the third ventricle. She did well postoperatively and her memory returned to her preoperative baseline by the time of discharge. She is currently 18 months out from surgery with no complications and stable clinical condition.

Case #2

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A 22-year-old woman presented with a several year history of headaches. She began to experience severe headaches out of proportion to her prior headaches, which were diffuse and throbbing, associated with neck stiffness. Physical exam was within normal limits with no neurologic deficit. MRI at this time revealed a large suprasellar mass concerning for possible

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thrombosed aneurysm, and she was transferred to Houston Methodist Hospital for further workup and care. CT angiogram showed no evidence of aneurysm, but revealed calcification

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within the mass. MRI revealed a 3cm mass in the suprasellar space. The mass exhibited peripheral enhancement, was isointense to white matter on T1, and slightly hyperintense on T2 (Figure 6). The patient was taken to the OR for bicoronal craniotomy. A bone flap was raised exposing the sagittal sinus and the right hemisphere preparing for a subcallosal approach. The dura was opened in a C shaped fashion based on the sagittal sinus and interhemispheric dissection ensued until the tumor was encountered. This was carefully debulked and complete resection obtained; final pathology was consistent with craniopharyngioma. Postoperatively, the patient did have diabetes insipidus (DI) that was controlled and eventually managed with

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intranasal desmopressin upon discharge. She is currently 7 months out from surgery with no further complication but still requiring desmopressin for mild DI.

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Discussion

Dandy initially described surgical approaches to the third ventricle, both for anterior and posteriorly situated lesions; however, Cushing also developed techniques in third ventricular surgery in parallel(6,8). Since that time, surgical options for anterior third ventricular lesions

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have expanded to include two main categories: trans-ventricular or trans-lamina terminalis approaches. Trans-ventricular options encompass the interhemispheric transcallosal and

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transcortical approaches(13). Once entry into the lateral ventricle is achieved, enlargement of the foramen of Monro is often necessary either by incising through an interforniceal incision, between the thalamus and fornix (through the taenia fornicis or taenia choroidea), or a more limited incision sacrificing the anterior septal vein(19). Trans-lamina terminalis approach options include midline interhemispheric or subfrontal(9,22,23), anterolateral supraorbital(1,14), lateral frontotemporal(15), and transnasal trans-sphenoidal trajectories(7). Surgery of the third ventricle

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carries with it significant risk given the proximity of vital neurovascular structures including the corpus callosum, fornix, hypothalamus, anterior cerebral arteries, and optic chiasm and tracts. When selecting a suitable surgical approach, attention should be given to the option that will cause the least disruption to neurovascular and white matter structures while permitting adequate

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access to the lesion.

Detailed knowledge of the anatomy of the third ventricle is imperative to safely remove

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third ventricular pathology and choose an appropriate approach. Briefly, the third ventricle is a narrow corridor bounded laterally by the thalamus and hypothalamus, being separated by the hypothalamic sulcus. The roof of the third ventricle is a four-layered structure spanning the distance between the foramen of Monro back to the suprapineal recess. It is composed of the fornix, two layers of tela choroidea, and the velum interpositum interposed between these layers (housing the internal cerebral veins and medial posterior choroidal arteries). The anterior wall spans from foramen of Monro to optic chiasm and is formed by the lamina terminalis and anterior commissure. The floor extends from optic chiasm to cerebral aqueduct, with the diencephalon and mesencephalon contributing the anterior and posterior halves respectively.

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Finally, the posterior wall spans from suprapineal recess to cerebral aqueduct, encompassing the pineal body and posterior commissure(21). Neuropsychologic testing has been documented in the literature analyzing memory

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impairment, interhemispheric transfer of information, intelligence, fine manual dexterity, and executive function among the various approach options noting no significant differences between different approaches nor differences between surgical and nonsurgical patients (indicating the etiology of symptoms is attributable to the lesions itself)(3,10). However, this is mostly

retrospective analysis and more rigorous testing with both pre and postoperative comparative

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testing is warranted to truly understand any differences. Despite an incomplete picture, minimizing injury to the corpus callosum and fornix is desirable.

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The subcallosal trans-lamina terminalis approach allows preservation of the corpus callosum and fornix and is best suited for lesions in the antero-inferior third ventricle or soft lesions that encompass the whole third ventricle. Modern neurosurgery should aim to preserve all white matter tracts including the corpus callosum. This approach also permits preservation of olfaction since it is an interhemispheric approach, and avoids entry into the frontal sinus with a craniotomy placed directly between the sinus and the coronal suture. A supracommunicating

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approach preserves the A1 perforators to the optic chiasm, but excessive retraction should be avoided due to the close proximity of the anterior nuclei of the hypothalamus. It also gives the surgeon a direct end-on view of the anatomy with a wider entry into the third ventricle than a

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more lateral approach would afford. The subcallosal approach utilizes the supracommunicating corridor situated between the A2 segments, above the Acom, and below the rostrum of the corpus callosum. We found this is a significantly larger working space than a subfrontal

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subcommunicating approach would afford (156.62 mm2 compared to 30.62 mm2). Combining the subcommunicating and supracommunicating windows through sacrifice of the Acom can be dangerous and is unnecessary given the wide cross sectional area allowed by this corridor(17,23). One of the cadaveric specimens had triplication of the A2 segment; despite this anomaly, wide exposure of the anterior third ventricle was still easily achieved with gentle retraction. Limitations of this approach include it is fairly limited to anterior and inferior pathology given the depth of the surgical field as well as relative blind spots situated behind the rostrum of the corpus callosum and behind the optic chiasm in the infundibular recess. If more

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inferior access is desired, combining the supracommunicating corridor with a separate opening in the lamina terminalis below the Acom is also possible. Another option is instead making one larger opening in the lamina terminalis just lateral to one of the anterior cerebral arteries(17).

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Contrast enhanced MRI and neuronavigation are utilized during the subcallosal approach and are helpful for several reasons beyond the usual benefits of defining anatomy and assessing tumor boundaries. MRI can help to identify bridging veins, which can be an obstacle during interhemispheric approaches. A working channel is typically available between veins traversing into the sagittal sinus; however, the approach is far enough anterior that if sacrificing a small

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bridging vein is necessary it should be well tolerated. Neuronavigation is helpful in delineating the boundaries of the craniotomy in order to remain superior to the frontal sinus. As previously

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mentioned a craniotomy spanning between the frontal sinus and coronal suture is usually sufficient for this approach. The adequacy of the bone flap can be assessed using trajectory views in order to view the surgical trajectory and its relationship to the rostrum of the corpus callosum (ideally sliding just underneath this structure). During the interhemispheric dissection, neuronavigation may also be utilized to assist in guiding the surgical corridor just beneath the

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corpus callosum.

Conclusion

The anterior interhemispheric subcallosal approach is a safe and effective approach to

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lesions of the third ventricle that avoids disruption of vital neurovascular elements and white matter tracts including the corpus callosum while permitting a wide surgical corridor for entry

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into the third ventricle.

Disclosure

The authors have no disclosures or conflicts of interest to report.

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Figure Legends: Figure 1: Illustrative depiction of the anterior subcallosal approach. An interhemispheric corridor is utilized with entry into the third ventricle occurring just superior to the anterior

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communicating artery. Figure 2: The skin is incised and the scalp flap is reflected forward in a subperiosteal fashion (A). The bone flap is positioned just above the frontal sinus and spans the sagittal sinus favoring the right side (B).

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Figure 3: Surgical trajectory of approach options for anterior third ventricular lesions: A) Subcallosal approach, B) Anterior transcallosal approach, C) Subfrontal approach.

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Figure 4: A) Visualization of entry into the third ventricle through the supracommunicating corridor. B) Third ventricle visualized through a subcommunicating corridor below the anterior communicating artery and above the optic chiasm.

Acom – anterior communicating artery, * - third ventricle, R – rostrum of corpus callosum Figure 5: Preoperative imaging of T2 axial (A), T1 sagittal post contrast (B), and T1 coronal post

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contrast (C), showing a mass in the anterior third ventricle with heterogeneous signal intensity consistent with blood products. Gross total resection of the cavernous malformation was accomplished through an anterior interhemispheric subcallosal approach (D and E). Figure 6: Preoperative imaging of T1 post contrast sagittal (A), coronal (B), and T2 weighted

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coronal (C), showing a suprasellar mass abutting the anterior third ventricle and inferior to the anterior commissure. Gross total resection of the mass was accomplished through an anterior

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interhemispheric subcallosal approach (D and E).

Specimen 1

Specimen 2

Inter-A2 (rest)

5.0-6.0mm

7.5-8.0mm

Inter-A2 (retraction)

11.5-12mm

13-14.5mm

Acom-rostrum

15mm

14-15mm

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Inter-A1

7.5-8.0mm

Chiasm-Acom

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Subcommunicating corridor

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Supracommunicating corridor

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Specimen 2 10mm

3.5-5.0mm

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Table 1: Comparison of surgical corridor dimensions between a supracommunicating window of the subcallosal approach and a subcommunicating window of the subfrontal approach.

Cross-sectional area

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Supracommunicating

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30.62 mm2

80.42 mm2 156.62 mm2

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Table 2: Comparison of cross sectional area between supracommunicating and subcommunicating windows with and without retraction.

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Third ventricular access is challenging given the depth and surrounding structures



The subcallosal approach preserves the corpus callosum and minimizes retraction



Cadaveric illustration is used to compare corridors above and below the Acom



The subcallosal approach provides a large working area to access third ventricle

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Abbreviations:

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MRI: magnetic resonance imaging Acom: anterior communicating artery

The Anterior Subcallosal Approach to Third Ventricular and Suprasellar Lesions: Anatomical Description and Technical Note.

Surgical access to the third ventricle is challenging, given the depth of the operative field and close proximity of vital neural structures that must...
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