Refining Operative Strategies for Optic Nerve Decompression: A Morphometric Analysis of Transcranial and Endoscopic Endonasal Techniques Using Clinical Parameters

Refining Operative Strategies for Optic Nerve Decompression: A Morphometric Analysis of... Abstract BACKGROUND Various approaches can be considered for decompression of the intracanalicular optic nerve. Although clinical experience has been reported, no quantitative study has yet compared the extent of decompression achieved by an endoscopic endonasal versus transcranial approach. OBJECTIVE Toward this aim, our morphometric analysis compared both approaches by quantifying the circumferential degree of optic canal decompression that is possible before any meningeal violation, which would result in cerebrospinal fluid (CSF) leak. METHODS From 10 cadaver heads, 20 optic canals were sequentially decompressed using an endoscopic endonasal approach and pterional craniotomy with extradural clinoidectomy. Dissections ended before violation of the sphenoid sinus during the transcranial approach, and before intracranial transgression from the endonasal corridor. Based on our study criteria, decompressions were not maximal for either approach, but were maximal before violating the other compartment. Decompression achieved from each approach was quantified using CT scans for each stage. RESULTS Greater circumferential bony optic canal decompression was obtained from transcranial (245.2°) than endonasal (114.8°) routes (P < .001). By endonasal perspective, the anatomical point where the optic nerve traverses intracranially was approximated by the medial border of the anterior ascending cavernous internal carotid artery. CONCLUSION Our morphometric analysis comparing optic canal decompression for endonasal and transcranial corridors provides important guidance for this location. Ample visualization and wide exposure can be achieved via a transcranial approach with limited risk of CSF leak. A landmark, where the intracanalicular segment ends and optic nerve traverses intracranially, can mark the extent of decompression safely obtained before risking CSF leak. Cadaver, Decompression, Surgical, Endoscopy/methods, Nasal cavity/surgery, Optic nerve/surgery, Optic neuropathy, Procedures, Neurosurgical ABBREVIATIONS ABBREVIATIONS CSF cerebrospinal fluid ICA internal carotid artery A wide array of pathology has been cited in the literature that may compromise the intracanalicular optic nerve, including traumatic and nontraumatic optic neurop-athy,1-14 metabolic derangement15-18, and com-pression due to neoplasms19-25 or other les-ions.26-28 Given the vulnerability of this vital structure, an operative approach to this location should ideally provide ample visualization, ade-quate exposure, and safety to nearby neurova-scular structures. Historically, the preferred route to the optic canal was a craniotomy with subfrontal exposure via a pterional, supraorbital, or orbitozygomatic approach.6,10,13-14,21-24,26,29,30 However, modern technological advancements have greatly enhanced the neurosurgeon's ability to access pathology via minimally invasive surgical approaches. Most notably, the widespread use of the rigid endoscope has enhanced visibility in narrow corridors and expanded surgical indications for their application.3,5,7-9,11,16,17,25,28,31-35 Recent anatomic contributions have shown that an endoscopic endonasal approach can provide up to 270° of circumferential decompression of the intracanalicular optic nerve.31 However, significant intradural extension of this approach becomes necessary and the procedure carries significant risk of cerebrospinal fluid (CSF) leak. To this point, the degree of optic nerve exposure obtainable endonasally before arachnoid violation has yet to be elucidated. Additionally, the extent of optic canal decompression obtained via craniotomy has not been quantified. We sought to quantitatively compare the extent of bony canal removal obtained via a transcranial with endonasal approach before the violation of the other compartment. That is, the maximal transcranial decompression achievable before entering the sphenoid sinus vs the extent of endonasal decompression attainable before traversing intracranially. This information would accurately detail the available exposure of the optic nerve via each approach while minimizing the risk of CSF leak. METHODS From 10 cadaveric heads, 20 optic canals were sequentially decompressed using an endoscopic endonasal approach and pterional craniotomy with extradural clinoidectomy. Dissections ended before violation of the sphenoid sinus during the transcranial approach and before intracranial transgression from the endonasal corridor. Based on our study criteria, decompressions were not maximal for either approach, but were maximal before violating the other compartment. Extent of decompression was quantified by CT scans for each stage in thin-cut (0.625 mm) slices. Endoscopic Endonasal Approach In 6 of 10 specimens, an endoscopic endonasal decompression of the optic canal was performed using a Stryker Nav II System running iNtellect ENT software (Version 1.0-14; Stryker, Kalamazoo, Michigan). A rigid endoscope (4 mm diameter, 14 cm length) was used with both 0° and 30° lenses (Stryker). Using a binostril approach, the sphenoid sinus was accessed via posterior ethmoidectomy, partial superior turbinectomy, wide sphenoidotomy, and posterior nasal septectomy. A medial maxillary antrostomy was then utilized to expand the space available for instrument manipulation, which decreases the risk of maxillary scarring from procedural manipulation. Decompression proceeded with a combination of a high-speed Medtronic drill (Medtronic, Dublin, Ireland) using 3-mm diamond burrs, Kerrison punches, microdissectors, and microcurettes. Medial decompression was halted before becoming intracranial. The position of this point relative to the anterior ascending cavernous internal carotid artery (ICA) was noted in each specimen, thus replicating the procedure performed in clinical settings. Decompression superiorly ended before any violation of the meningeal plane along the anterior skull base. The lateral border of canal decompression was limited by the annulus of Zinn. Finally, the optic nerve sheath was opened to evaluate the position of the ophthalmic artery within the canal (Figure 1). FIGURE 1. View largeDownload slide Image represents a completed endoscopic endonasal decompression of a left optic nerve with the opened optic nerve sheath reflected inferiorly. Asterisk (*) shows the point where the optic nerve (ON) exits the canal and courses intracranially. Note the approximation of this point with the medial border of the cavernous ICA (dashed line). Printed with permission from Mayfield Clinic. ICA, internal carotid artery. FIGURE 1. View largeDownload slide Image represents a completed endoscopic endonasal decompression of a left optic nerve with the opened optic nerve sheath reflected inferiorly. Asterisk (*) shows the point where the optic nerve (ON) exits the canal and courses intracranially. Note the approximation of this point with the medial border of the cavernous ICA (dashed line). Printed with permission from Mayfield Clinic. ICA, internal carotid artery. Transcranial Approach After repeating a thin-cut head CT for calculations, heads were positioned supine, fixed in a Mayfield® Modified Skull Clamp (Integra, Plainsboro, New Jersey), rotated 5° to 10° to the contralateral side, and extended 10° to 15°. A curved incision, made immediately behind the hairline, extended from the zygoma to the midline. The temporalis muscle was then dissected subperiosteally and retracted in a single myocutaneous flap until the pterion was exposed. A standard pterional craniotomy (Figure 2A) and extradural anterior clinoidectomy were performed following our previously published technique,36,37 using a Budde® Halo Retractor System (Integra) for exposure. The lesser wing of the sphenoid was removed to the lateral superior orbital fissure using a 4-mm drill. Decompression proceeded with an operating microscope (Leica Microsystems Inc, Buffalo Grove, Illinois) via a combination of a high-speed 3-mm drill and microdissectors. Bony decompression included the complete unroofing of the superolateral optic canal and optic strut, stopping prior to violation of the sphenoid sinus (Figure 2B). A C-shaped incision was then made in the dura and optic nerve decompression completed by splitting the falciform ligament and optic nerve sheath (Figure 3). Thin-cut CT scans were then repeated to confirm circumferential decompression. In 4 specimens, the transcranial approach preceded the endonasal procedure with imaging between stages to ensure accuracy of measurements. FIGURE 2. View largeDownload slide Overview of the transcranial approach. A, Standard pterional craniotomy with extradural dissection of the anterior skull base. This degree of temporal exposure is not typically required for this procedure. B, Extent of bony removal required to unroof the optic canal and remove the anterior clinoid and optic strut. II = cranial nerve II (optic nerve); DDR = distal dural ring; ICA = internal carotid artery; A1 = first segment of anterior cerebral artery; M1 = first segment of middle cerebral artery. Printed with permission from Mayfield Clinic. FIGURE 2. View largeDownload slide Overview of the transcranial approach. A, Standard pterional craniotomy with extradural dissection of the anterior skull base. This degree of temporal exposure is not typically required for this procedure. B, Extent of bony removal required to unroof the optic canal and remove the anterior clinoid and optic strut. II = cranial nerve II (optic nerve); DDR = distal dural ring; ICA = internal carotid artery; A1 = first segment of anterior cerebral artery; M1 = first segment of middle cerebral artery. Printed with permission from Mayfield Clinic. FIGURE 3. View largeDownload slide Cadaveric demonstration of a completed transcranial optic nerve decompression following anterior clinoidectomy and removal of the optic strut. This degree of intracranial neurovascular dissection is not required for standard optic canal decompression. Printed with permission from Mayfield Clinic. FIGURE 3. View largeDownload slide Cadaveric demonstration of a completed transcranial optic nerve decompression following anterior clinoidectomy and removal of the optic strut. This degree of intracranial neurovascular dissection is not required for standard optic canal decompression. Printed with permission from Mayfield Clinic. Statistical Analysis The 3 CT scans of each specimen were uploaded and analyzed using OsiriX MD software (Pixmeo SARL, Geneva, Switzerland). Optic canal boundaries and surface area were calculated according to the conical frustrum model originally described by Hart et al38 as follows:   \begin{equation*} \text{surface area} = \pi \text{(r + R) s} \end{equation*} Specifically, “r” represents the radius of the canal at the conus elasticus, “R” the radius at the sellar face, and “s” the canal length.38 Angles of bony decompression obtained via an endonasal and transcranial approach were then measured using coronal images of the 20 optic canals (Figure 4). All data were then uploaded into Microsoft® Excel (Microsoft, Redmond, Washington), and a Student's t-test was performed to determine statistical difference between approaches, with P < .05 considered as statistically significant. FIGURE 4. View largeDownload slide Representative measurement of the degree of bony optic canal decompression obtained in 2 specimens using OsiriX software following an endonasal A and transcranial B approach. Printed with permission from Mayfield Clinic. FIGURE 4. View largeDownload slide Representative measurement of the degree of bony optic canal decompression obtained in 2 specimens using OsiriX software following an endonasal A and transcranial B approach. Printed with permission from Mayfield Clinic. RESULTS Anatomical Observations An endoscopic endonasal approach provided quick access to the inferomedial optic canal protruding into the sphenoid sinus. Thickness of the lamina papyracea varied greatly among specimens; some required the use of a high-speed drill to access the proximal canal, whereas others had bone dehiscent or thin enough to permit the use of dissectors and micropunches to decompress the intracanalicular optic nerve. Decompression extended from the orbital apex, where the nerve passes through the annulus of Zinn, to the point where the optic nerve sheath transitions into cranial dura mater and the nerve becomes intracranial medially. The position of this transition point relative to the ICA in all specimens was consistently approximated by the medial border of the anterior ascending cavernous segment of the ICA (Figure 1). Likewise, the ophthalmic artery origin and course were evaluated. The artery branched off the cavernous segment of the ICA in 1 specimen and was reliably seen coursing in the inferior or inferomedial optic canal. The pterional approach combined with an extradural clinoidectomy provided an extensive view of the superolateral optic canal. After removal of the lesser wing of the sphenoid down to the lateral superior orbital fissure, drilling proceeded along the superior canal. This step establishes the proper orientation of the optic nerve and location of the optic strut, taking care not to violate the sphenoid sinus medially. Drilling then proceeded from the proximal canal at the annulus of Zinn to the falciform ligament. Removal of the anterior clinoid and optic strut then followed, which allowed for an expansive view of the nerve. Dissection of the falciform ligament and nerve sheath completed the procedure and a wide optic decompression could be clearly visualized in all specimens. Optic Canal Decompression The 20 intact optic canals had a mean canal length of 1.09 cm (range, 0.83-1.23 cm), mean diameter of 0.44 cm (0.31-0.60 cm) at the conus elasticus, and mean diameter of 0.60 cm (0.47-0.80 cm) at the sellar face. Using the conical frustrum formula, the canal surface area averaged 1.80 cm2 (range, 1.21-2.50 cm2; Table 1). TABLE 1. Average Measurements Taken From 20 Optic Canals Measurements  Mean  Range  Optic canal length (s, cm)  1.09  0.83-1.23  Proximal canal diameter (2r, cm)  0.44  0.31-0.6  Distal canal diameter (2R, cm)  0.6  0.47-0.8  Canal surface area (cm2)  1.8  1.21-2.5  Measurements  Mean  Range  Optic canal length (s, cm)  1.09  0.83-1.23  Proximal canal diameter (2r, cm)  0.44  0.31-0.6  Distal canal diameter (2R, cm)  0.6  0.47-0.8  Canal surface area (cm2)  1.8  1.21-2.5  Surface area = π(R+r)s View Large Morphometric analysis revealed that an endoscopic endonasal approach achieved a mean circumferential optic canal decompression of 114.8 ± 18.8° (median 116.7°, range 82.5°-149.0°) before intracranial transgression; this correlated to 0.57 cm2 (31.9%) of canal surface area. Conversely, a transcranial corridor yielded a mean circumferential decompression of 245.2 ± 18.8° (median 243.3°, range 211.0°-277.5°), or 1.23 cm2 (68.1%) of canal surface area before violating the sphenoid sinus (Figure 5). These differences in circumferential decompression reached statistical significance (P < .001) for all calculations (Table 2). FIGURE 5. View largeDownload slide Depiction of the mean circumferential bony canal removal obtained by endonasal and transcranial approaches as viewed directly through the optic canal from a posterior-to-anterior A and anterior-to-posterior B vantage point. Shading shows bony decompression from an endoscopic approach (green) and transcranial bony removal (orange). Printed with permission from Mayfield Clinic. FIGURE 5. View largeDownload slide Depiction of the mean circumferential bony canal removal obtained by endonasal and transcranial approaches as viewed directly through the optic canal from a posterior-to-anterior A and anterior-to-posterior B vantage point. Shading shows bony decompression from an endoscopic approach (green) and transcranial bony removal (orange). Printed with permission from Mayfield Clinic. TABLE 2. Quantitative Analysis Shows the Mean Optic Canal Decompression Gained via Each Approach With Associated P Values Analysis  Endonasal  Transcranial    Mean circumference of decompression (degrees)  114.8  245.2  P < .001  Mean surface area (cm2)  0.57  1.23  P < .001  Mean surface area (%)  31.9  68.1  P < .001  Analysis  Endonasal  Transcranial    Mean circumference of decompression (degrees)  114.8  245.2  P < .001  Mean surface area (cm2)  0.57  1.23  P < .001  Mean surface area (%)  31.9  68.1  P < .001  View Large DISCUSSION As endoscopic endonasal approaches gain popularity, surgeons must weigh this trend in the context of which approach—transcranial vs endoscopic endonasal—will be most effective for a particular clinical scenario. To our knowledge, this study provides clinicians with the first quantitative analysis of the circumferential degree of optic canal decompression that can be obtained by these 2 approaches. A small pterional craniotomy with anterior clinoidectomy and removal of the optic strut was simple to perform and provided a wide exposure of the optic nerve with minimal temporal exposure. Conversely, optic decompression via an endoscopic endonasal route was anatomically limited when avoiding intracranial extension and the creation of a CSF fistula, but was a less invasive approach. Endonasal Decompression A wide range of pathologies may compromise the optic nerve and require decompression of its intracanalicular segment. Some authors have previously described the potential to decompress up to 270° of the canal from an endoscopic endonasal approach.31 However, this strategy requires extensive intracranial extension and delicate manipulation around the optic nerve, which may be more challenging from this approach for less experienced surgeons. Additionally, it could significantly increase the risk of CSF leak from the anterior skull base, and thus would require a nasoseptal flap or other precautions to prevent this complication. Another limitation of the endonasal approach is the inability to transect the falciform ligament to free the optic nerve without violating the arachnoid plane and risking a CSF leak. Given the extent of technical difficulty associated with this degree of endonasal decompression, we quantified the extent of bony canal removal that is possible before entering the intracranial compartment, since this strategy may be more practical in many clinical situations. Not surprisingly, the extent of this decompression highly depends on a patient's unique anatomy because the canal surface area exposed in the sphenoid sinus varies widely. In a study of 191 normal canals from their clinical database, Hart et al38 found an average of 101.3° of medial optic canal wall exposure in the sphenoid sinus. Our endonasal decompression averaging 114.8° slightly surpassed the circumference directly exposed within the sinus, as would be expected. However, we halted bone removal prior to intracranial extension, which significantly limited the extent of this approach. Nonetheless, the overall range of our decompression was consistent with previous studies.38 Of particular note is the anatomical landmark where the intracanalicular segment ends and the optic nerve traverses intracranially. This point was consistently observed in line with the medial border of the anterior ascending cavernous segment of the ICA from a rostral-caudal viewpoint (Figure 1). This relationship, which has not been previously described, can serve as a useful intraoperative marker for the extent of decompression that one may obtain before creating violation of arachnoid. Further exposure likely provides no further decompressive benefit, but enhances the risk of a spinal fluid leak. Transcranial Decompression By taking care to first establish the orientation of the intracanalicular optic nerve near the orbital apex with a high-speed drill under constant irrigation, the neurosurgeon can safely unroof the nerve longitudinally, identify and remove the optic strut, and avoid any potential injury to the underlying ICA. Importantly, a transcranial approach to the superolateral canal avoids injury to the ophthalmic artery, which we observed in the inferior or inferolateral optic canal in all specimens consistent with prior reports.31,39-42 Decompression of the medial border of the optic canal from above is limited by the sphenoid sinus. However, a substantial decompression of the superolateral optic nerve averaging 245.2° may still be obtained without entering the sinus. This was significantly greater than the exposure gained via the endonasal route and obviates the temptation to drill medially and risk a CSF leak, since this degree of decompression should be more than adequate in most clinical situations. Furthermore, sectioning of the falciform ligament is easily performed via the transcranial route without creating communication between the 2 compartments. This allows for safe procedural mobility of the optic nerve when necessary for decompression, as traction on the nerve may cause irreparable injury.36 Optic nerve mobilization may be required to safely remove tumor from the nerve or other specific scenarios, but was not used for our standard decompression. The retraction of the frontal lobe required with this approach was very minimal despite the rigidity of formalin-fixed brain tissue. Techniques to improve brain relaxation intraoperatively, including arachnoid dissection, cisternal opening, CSF diversion, or hyperosmolar therapy, may further relegate the utility of a retractor to a simple protective device during drilling. The chief limitation of the transcranial approach lies in the invasive nature of a craniotomy. Although the procedure can be completed extradurally, it carries the risk of injury to the frontal lobes during retraction or exposure. Additionally, the inferomedial optic canal is difficult to access from this angle. Choice of Surgical Approach Given the various pathologies that can compromise the intracanicular optic nerve, one must tailor the approach to an individual patient's anatomy, pathology, and clinical scenario. Clearly, if the goal of the procedure is to remove tumor encroaching upon the medial or inferomedial canal and resection will adequately decompress the nerve, an endonasal approach would be preferred. An endonasal approach may also be ideal for tumors encroaching upon the superolateral canal that have failed transcranial surgery and/or radiation. Of note, when it is necessary to enter the optic nerve sheath from an endoscopic endonasal approach, it should be opened superiorly so as to avoid injury to the ophthalmic artery coursing inferiorly. However, a wide decompression of the optic nerve is sometimes required either because of significant edema or the need for extensive procedural manipulation of the optic nerve. In these cases, we have shown that a substantial optic decompression may be obtained comfortably from a transcranial corridor with limited risk of CSF leak to the patient. One could posit that the extent of decompression need only exceed 180° to adequately unroof this structure and allow for nerve expansion in most scenarios. However, such a maneuver cannot be completed endonasal without arachnoid violation. Conversely, a craniotomy easily yields this degree of exposure before entering the sphenoid sinus and the optic sheath may be split without increasing the risk of CSF leak.30 Thus, a transcranial route provides the most extensive decompression of the optic nerve with limited risk of a CSF fistula. Furthermore, prior authors have argued that the additional removal of the anterior clinoid process and optic strut obviates the need for transecting the annulus of Zinn for adequate nerve decompression, which maintains the integrity of this structure.30 An extradural dissection and removal of the anterior clinoid is preferred by the authors, as it may optimize orientation, decrease trauma to intracranial structures, and require less operative time than an intradural clinoidectomy.19,30,37 Other minimally invasive cranial approaches to this location have been reported, including the supraorbital approach with or without endoscopic assistance.43-48 These techniques could conceivably yield a similar decompression to our modified pterional craniotomy with anterior clinoidectomy. However, our approach provides a wider exposure, is more generalizable and comfortable to most neurosurgeons, and provides the highest degree of safety to nearby neurovascular structures. The bony canal removal afforded by these minimally invasive transcranial approaches has yet to be quantified for comparison, but in theory should be similar to our study. Finally, in the rare patient where circumferential decompression of the nerve is indicated, such as extensive tumor invasion, a combination of the endonasal and transcranial approaches would be preferred. This combination yielded 360° of decompression in all of our specimens. In this case, precautions would be undertaken up front to prevent a CSF leak, as this would inevitably create a wide communication between the intracranial compartment and sphenoid sinus. Overall, the choice of approach should be determined by the individual patient's anatomy and pathology, and the preference of the surgical team in each clinical scenario. The transcranial and endoscopic endonasal approaches should be considered as complementary options for accessing this location, not mutually exclusive. The goal of this study was to assess the ability of each approach to decompress the optic canal with relatively low risk of a CSF leak. A leak should not be feared, however, and is certainly preferred over risk of injury to the optic nerve or major arteries. Furthermore, a CSF leak can be locally repaired in most situations from either approach when encountered. This anatomical knowledge may serve as additional information to be incorporated by a surgeon in preoperative planning. Limitations As with any cadaveric study, our model does not perfectly simulate the clinical environment in regard to tissue compliance and assessment of procedural morbidity. Variation of sinonasal anatomy can be significant, including the degree of pneumatization of the sphenoid sinus, anterior clinoid process, and optic strut. Of our 20 examined optic canals, only 1 specimen had notable pneumatization of the anterior clinoid process; this variation could lead to violation of the sphenoid sinus during clinoidectomy if not recognized preoperatively and a high risk of CSF leak. Finally, by stopping our dissection before entering the other compartment, our results underestimate the maximal decompression that could be obtained via both corridors. Overall, we believe that our anatomical measurements were consistent with previous large case series38 with respect to regional anatomy. We stress that careful study of preoperative imaging is essential for determining the most appropriate corridor for decompression. CONCLUSION Our quantitative analysis shows that a transcranial approach to the optic canal, compared with an endoscopic endonasal corridor, provided a substantially greater circumferential degree of decompression and exposure of the optic nerve before creation of a CSF leak. Although a significant exposure may be obtained via craniotomy, selection of approach should be tailored to each individual patient's regional anatomy and particular clinical scenario. Decompressions were not maximal from either approach, but rather maximal before violation of the other compartment. Thus, this represents an arbitrary—albeit clinically relevant—delineation of the limits of endonasal and transcranial approaches to the optic canal. Further quantitative analysis of minimally invasive cranial approaches to this region would be clinically useful. Disclosures Materials and costs related to this research project were entirely funded by a Mayfield Education Research Foundation (MERF) grant awarded by the MERF board and UC Department of Neurosurgery Executive Committee. No industry sponsorship was utilized. The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. 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Transsphenoidal optic nerve decompression: an endoscopic anatomic study. J Craniofac Surg . 2008; 19( 6): 1670- 1674. Google Scholar CrossRef Search ADS PubMed  33. Hassler W, Eggert HR. Extradural and intradural microsurgical approaches to lesions of the optic canal and the superior orbital fissure. Acta Neurochir (Wien) . 1985; 74( 3–4): 87- 93. Google Scholar CrossRef Search ADS PubMed  34. Abuzayed B, Tanriover N, Gazioglu N, Eraslan BS, Akar Z. Endoscopic endonasal approach to the orbital apex and medial orbital wall: anatomic study and clinical applications. J Craniofac Surg . 2009; 20( 5): 1594- 1600. Google Scholar CrossRef Search ADS PubMed  35. Yilmazlar S, Saraydaroglu O, Korfali E. Anatomical aspects in the transsphenoidal transethmoidal approach to the optic canal an anatomic cadaveric study. J Craniomaxillofac Surg . 2012; 40( 7): e198- e205. Google Scholar CrossRef Search ADS PubMed  36. Andaluz N, Beretta F, Bernucci C, Keller JT, Zuccarello M. Evidence for the improved exposure of the ophthalmic segment of the internal carotid artery after anterior clinoidectomy: morphometric analysis. Acta Neurochir (Wien) . 2006; 148( 9): 971- 975. Google Scholar CrossRef Search ADS PubMed  37. Froelich SC, Aziz KM, Levine NB, Theodosopoulos PV, van Loveran HR, Keller JT. Refinement of the extradural anterior clinoidectomy: surgical anatomy of the orbitotemporal periosteal fold. Neurosurgery . 2007; 61( 5 suppl 2): 179- 185. Google Scholar PubMed  38. Hart CK, Theodosopoulos PV, Zimmer LA. Anatomy of the optic canal: a computed tomography study of endoscopic nerve decompression. Ann Otol Rhinol Laryngol . 2009; 118( 12): 839- 844. Google Scholar CrossRef Search ADS PubMed  39. Chou PI, Sadun AA, Lee H. Vasculature and morphometry of the optic canal and intracanalicular optic nerve. J Neuroopthalmol . 1995; 15( 3): 186- 190. 40. Habal MB, Maniscalco JE, Rhoton AL Jr. Microsurgical anatomy of the optic canal: correlates to optic nerve exposure. J Surg Res . 1977; 22( 5): 527- 533. Google Scholar CrossRef Search ADS PubMed  41. Kerr RG, Tobler WD, Leach JL et al.   Anatomic variation of the optic strut: classification schema, radiologic evaluation, and surgical relevance. J Neurol Surg B Skull Base . 2012; 73( 6): 424- 429. Google Scholar CrossRef Search ADS PubMed  42. Akdemir G, Tekdemir I, Altin L. Transethmoidal approach to the optic canal: surgical and radiological microanatomy. Surg Neurol . 2004; 62( 3): 268- 274 Google Scholar CrossRef Search ADS PubMed  43. Chen YH, Lin SZ, Chiang YH, Ju DT, Liu MY, Chen GJ. Supraorbital keyhole surgery for optic nerve decompression and dura repair. J Neurotrauma . 2004; 21( 7): 976- 981. Google Scholar CrossRef Search ADS PubMed  44. Lin Y, Zhang W, Luo Q, Jiang J, Qiu Y. Extracranial microanatomic study of supraorbital keyhole approach. J Craniofac Surg . 2009; 20( 1): 215- 218. Google Scholar CrossRef Search ADS PubMed  45. Hassler W, Schick U. The supraorbital approach—a minimally invasive approach to the superior orbit. Acta Neurochir (Wien) . 2009; 151( 6): 605- 611. Google Scholar CrossRef Search ADS PubMed  46. Beer-Furlan A, Evins AI, Rigante L et al.   Endoscopic extradural anterior clinoidectomy and optic nerve decompression through a pterional port. J Clin Neurosci . 2014; 21( 5): 836- 840. Google Scholar CrossRef Search ADS PubMed  47. Beer-Furlan A, Evins AI, Rigante L, Anichini G, Stieg PE, Bernardo A. The pterional port in dual-port endoscopy: a 2D and 3D cadaveric study. J Neurol Surg B Skull Base . 2015; 76( 1): 80- 86. Google Scholar CrossRef Search ADS PubMed  48. Rigante L, Evins AI, Berra LV, Beer-Furlan A, Stieg PE, Bernardo A. Optic nerve decompression through a supraorbital approach. J Neurol Surg B Skull Base . 2015; 76( 3): 239- 247. Google Scholar CrossRef Search ADS PubMed  Acknowledgments Special thanks to Tonya Hines for her assistance with image editing and preparation, and Mary Kemper for her editorial contribution. Copyright © 2017 by the Congress of Neurological Surgeons http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Operative Neurosurgery Oxford University Press

Refining Operative Strategies for Optic Nerve Decompression: A Morphometric Analysis of Transcranial and Endoscopic Endonasal Techniques Using Clinical Parameters

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Copyright © 2017 by the Congress of Neurological Surgeons
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Abstract

Abstract BACKGROUND Various approaches can be considered for decompression of the intracanalicular optic nerve. Although clinical experience has been reported, no quantitative study has yet compared the extent of decompression achieved by an endoscopic endonasal versus transcranial approach. OBJECTIVE Toward this aim, our morphometric analysis compared both approaches by quantifying the circumferential degree of optic canal decompression that is possible before any meningeal violation, which would result in cerebrospinal fluid (CSF) leak. METHODS From 10 cadaver heads, 20 optic canals were sequentially decompressed using an endoscopic endonasal approach and pterional craniotomy with extradural clinoidectomy. Dissections ended before violation of the sphenoid sinus during the transcranial approach, and before intracranial transgression from the endonasal corridor. Based on our study criteria, decompressions were not maximal for either approach, but were maximal before violating the other compartment. Decompression achieved from each approach was quantified using CT scans for each stage. RESULTS Greater circumferential bony optic canal decompression was obtained from transcranial (245.2°) than endonasal (114.8°) routes (P < .001). By endonasal perspective, the anatomical point where the optic nerve traverses intracranially was approximated by the medial border of the anterior ascending cavernous internal carotid artery. CONCLUSION Our morphometric analysis comparing optic canal decompression for endonasal and transcranial corridors provides important guidance for this location. Ample visualization and wide exposure can be achieved via a transcranial approach with limited risk of CSF leak. A landmark, where the intracanalicular segment ends and optic nerve traverses intracranially, can mark the extent of decompression safely obtained before risking CSF leak. Cadaver, Decompression, Surgical, Endoscopy/methods, Nasal cavity/surgery, Optic nerve/surgery, Optic neuropathy, Procedures, Neurosurgical ABBREVIATIONS ABBREVIATIONS CSF cerebrospinal fluid ICA internal carotid artery A wide array of pathology has been cited in the literature that may compromise the intracanalicular optic nerve, including traumatic and nontraumatic optic neurop-athy,1-14 metabolic derangement15-18, and com-pression due to neoplasms19-25 or other les-ions.26-28 Given the vulnerability of this vital structure, an operative approach to this location should ideally provide ample visualization, ade-quate exposure, and safety to nearby neurova-scular structures. Historically, the preferred route to the optic canal was a craniotomy with subfrontal exposure via a pterional, supraorbital, or orbitozygomatic approach.6,10,13-14,21-24,26,29,30 However, modern technological advancements have greatly enhanced the neurosurgeon's ability to access pathology via minimally invasive surgical approaches. Most notably, the widespread use of the rigid endoscope has enhanced visibility in narrow corridors and expanded surgical indications for their application.3,5,7-9,11,16,17,25,28,31-35 Recent anatomic contributions have shown that an endoscopic endonasal approach can provide up to 270° of circumferential decompression of the intracanalicular optic nerve.31 However, significant intradural extension of this approach becomes necessary and the procedure carries significant risk of cerebrospinal fluid (CSF) leak. To this point, the degree of optic nerve exposure obtainable endonasally before arachnoid violation has yet to be elucidated. Additionally, the extent of optic canal decompression obtained via craniotomy has not been quantified. We sought to quantitatively compare the extent of bony canal removal obtained via a transcranial with endonasal approach before the violation of the other compartment. That is, the maximal transcranial decompression achievable before entering the sphenoid sinus vs the extent of endonasal decompression attainable before traversing intracranially. This information would accurately detail the available exposure of the optic nerve via each approach while minimizing the risk of CSF leak. METHODS From 10 cadaveric heads, 20 optic canals were sequentially decompressed using an endoscopic endonasal approach and pterional craniotomy with extradural clinoidectomy. Dissections ended before violation of the sphenoid sinus during the transcranial approach and before intracranial transgression from the endonasal corridor. Based on our study criteria, decompressions were not maximal for either approach, but were maximal before violating the other compartment. Extent of decompression was quantified by CT scans for each stage in thin-cut (0.625 mm) slices. Endoscopic Endonasal Approach In 6 of 10 specimens, an endoscopic endonasal decompression of the optic canal was performed using a Stryker Nav II System running iNtellect ENT software (Version 1.0-14; Stryker, Kalamazoo, Michigan). A rigid endoscope (4 mm diameter, 14 cm length) was used with both 0° and 30° lenses (Stryker). Using a binostril approach, the sphenoid sinus was accessed via posterior ethmoidectomy, partial superior turbinectomy, wide sphenoidotomy, and posterior nasal septectomy. A medial maxillary antrostomy was then utilized to expand the space available for instrument manipulation, which decreases the risk of maxillary scarring from procedural manipulation. Decompression proceeded with a combination of a high-speed Medtronic drill (Medtronic, Dublin, Ireland) using 3-mm diamond burrs, Kerrison punches, microdissectors, and microcurettes. Medial decompression was halted before becoming intracranial. The position of this point relative to the anterior ascending cavernous internal carotid artery (ICA) was noted in each specimen, thus replicating the procedure performed in clinical settings. Decompression superiorly ended before any violation of the meningeal plane along the anterior skull base. The lateral border of canal decompression was limited by the annulus of Zinn. Finally, the optic nerve sheath was opened to evaluate the position of the ophthalmic artery within the canal (Figure 1). FIGURE 1. View largeDownload slide Image represents a completed endoscopic endonasal decompression of a left optic nerve with the opened optic nerve sheath reflected inferiorly. Asterisk (*) shows the point where the optic nerve (ON) exits the canal and courses intracranially. Note the approximation of this point with the medial border of the cavernous ICA (dashed line). Printed with permission from Mayfield Clinic. ICA, internal carotid artery. FIGURE 1. View largeDownload slide Image represents a completed endoscopic endonasal decompression of a left optic nerve with the opened optic nerve sheath reflected inferiorly. Asterisk (*) shows the point where the optic nerve (ON) exits the canal and courses intracranially. Note the approximation of this point with the medial border of the cavernous ICA (dashed line). Printed with permission from Mayfield Clinic. ICA, internal carotid artery. Transcranial Approach After repeating a thin-cut head CT for calculations, heads were positioned supine, fixed in a Mayfield® Modified Skull Clamp (Integra, Plainsboro, New Jersey), rotated 5° to 10° to the contralateral side, and extended 10° to 15°. A curved incision, made immediately behind the hairline, extended from the zygoma to the midline. The temporalis muscle was then dissected subperiosteally and retracted in a single myocutaneous flap until the pterion was exposed. A standard pterional craniotomy (Figure 2A) and extradural anterior clinoidectomy were performed following our previously published technique,36,37 using a Budde® Halo Retractor System (Integra) for exposure. The lesser wing of the sphenoid was removed to the lateral superior orbital fissure using a 4-mm drill. Decompression proceeded with an operating microscope (Leica Microsystems Inc, Buffalo Grove, Illinois) via a combination of a high-speed 3-mm drill and microdissectors. Bony decompression included the complete unroofing of the superolateral optic canal and optic strut, stopping prior to violation of the sphenoid sinus (Figure 2B). A C-shaped incision was then made in the dura and optic nerve decompression completed by splitting the falciform ligament and optic nerve sheath (Figure 3). Thin-cut CT scans were then repeated to confirm circumferential decompression. In 4 specimens, the transcranial approach preceded the endonasal procedure with imaging between stages to ensure accuracy of measurements. FIGURE 2. View largeDownload slide Overview of the transcranial approach. A, Standard pterional craniotomy with extradural dissection of the anterior skull base. This degree of temporal exposure is not typically required for this procedure. B, Extent of bony removal required to unroof the optic canal and remove the anterior clinoid and optic strut. II = cranial nerve II (optic nerve); DDR = distal dural ring; ICA = internal carotid artery; A1 = first segment of anterior cerebral artery; M1 = first segment of middle cerebral artery. Printed with permission from Mayfield Clinic. FIGURE 2. View largeDownload slide Overview of the transcranial approach. A, Standard pterional craniotomy with extradural dissection of the anterior skull base. This degree of temporal exposure is not typically required for this procedure. B, Extent of bony removal required to unroof the optic canal and remove the anterior clinoid and optic strut. II = cranial nerve II (optic nerve); DDR = distal dural ring; ICA = internal carotid artery; A1 = first segment of anterior cerebral artery; M1 = first segment of middle cerebral artery. Printed with permission from Mayfield Clinic. FIGURE 3. View largeDownload slide Cadaveric demonstration of a completed transcranial optic nerve decompression following anterior clinoidectomy and removal of the optic strut. This degree of intracranial neurovascular dissection is not required for standard optic canal decompression. Printed with permission from Mayfield Clinic. FIGURE 3. View largeDownload slide Cadaveric demonstration of a completed transcranial optic nerve decompression following anterior clinoidectomy and removal of the optic strut. This degree of intracranial neurovascular dissection is not required for standard optic canal decompression. Printed with permission from Mayfield Clinic. Statistical Analysis The 3 CT scans of each specimen were uploaded and analyzed using OsiriX MD software (Pixmeo SARL, Geneva, Switzerland). Optic canal boundaries and surface area were calculated according to the conical frustrum model originally described by Hart et al38 as follows:   \begin{equation*} \text{surface area} = \pi \text{(r + R) s} \end{equation*} Specifically, “r” represents the radius of the canal at the conus elasticus, “R” the radius at the sellar face, and “s” the canal length.38 Angles of bony decompression obtained via an endonasal and transcranial approach were then measured using coronal images of the 20 optic canals (Figure 4). All data were then uploaded into Microsoft® Excel (Microsoft, Redmond, Washington), and a Student's t-test was performed to determine statistical difference between approaches, with P < .05 considered as statistically significant. FIGURE 4. View largeDownload slide Representative measurement of the degree of bony optic canal decompression obtained in 2 specimens using OsiriX software following an endonasal A and transcranial B approach. Printed with permission from Mayfield Clinic. FIGURE 4. View largeDownload slide Representative measurement of the degree of bony optic canal decompression obtained in 2 specimens using OsiriX software following an endonasal A and transcranial B approach. Printed with permission from Mayfield Clinic. RESULTS Anatomical Observations An endoscopic endonasal approach provided quick access to the inferomedial optic canal protruding into the sphenoid sinus. Thickness of the lamina papyracea varied greatly among specimens; some required the use of a high-speed drill to access the proximal canal, whereas others had bone dehiscent or thin enough to permit the use of dissectors and micropunches to decompress the intracanalicular optic nerve. Decompression extended from the orbital apex, where the nerve passes through the annulus of Zinn, to the point where the optic nerve sheath transitions into cranial dura mater and the nerve becomes intracranial medially. The position of this transition point relative to the ICA in all specimens was consistently approximated by the medial border of the anterior ascending cavernous segment of the ICA (Figure 1). Likewise, the ophthalmic artery origin and course were evaluated. The artery branched off the cavernous segment of the ICA in 1 specimen and was reliably seen coursing in the inferior or inferomedial optic canal. The pterional approach combined with an extradural clinoidectomy provided an extensive view of the superolateral optic canal. After removal of the lesser wing of the sphenoid down to the lateral superior orbital fissure, drilling proceeded along the superior canal. This step establishes the proper orientation of the optic nerve and location of the optic strut, taking care not to violate the sphenoid sinus medially. Drilling then proceeded from the proximal canal at the annulus of Zinn to the falciform ligament. Removal of the anterior clinoid and optic strut then followed, which allowed for an expansive view of the nerve. Dissection of the falciform ligament and nerve sheath completed the procedure and a wide optic decompression could be clearly visualized in all specimens. Optic Canal Decompression The 20 intact optic canals had a mean canal length of 1.09 cm (range, 0.83-1.23 cm), mean diameter of 0.44 cm (0.31-0.60 cm) at the conus elasticus, and mean diameter of 0.60 cm (0.47-0.80 cm) at the sellar face. Using the conical frustrum formula, the canal surface area averaged 1.80 cm2 (range, 1.21-2.50 cm2; Table 1). TABLE 1. Average Measurements Taken From 20 Optic Canals Measurements  Mean  Range  Optic canal length (s, cm)  1.09  0.83-1.23  Proximal canal diameter (2r, cm)  0.44  0.31-0.6  Distal canal diameter (2R, cm)  0.6  0.47-0.8  Canal surface area (cm2)  1.8  1.21-2.5  Measurements  Mean  Range  Optic canal length (s, cm)  1.09  0.83-1.23  Proximal canal diameter (2r, cm)  0.44  0.31-0.6  Distal canal diameter (2R, cm)  0.6  0.47-0.8  Canal surface area (cm2)  1.8  1.21-2.5  Surface area = π(R+r)s View Large Morphometric analysis revealed that an endoscopic endonasal approach achieved a mean circumferential optic canal decompression of 114.8 ± 18.8° (median 116.7°, range 82.5°-149.0°) before intracranial transgression; this correlated to 0.57 cm2 (31.9%) of canal surface area. Conversely, a transcranial corridor yielded a mean circumferential decompression of 245.2 ± 18.8° (median 243.3°, range 211.0°-277.5°), or 1.23 cm2 (68.1%) of canal surface area before violating the sphenoid sinus (Figure 5). These differences in circumferential decompression reached statistical significance (P < .001) for all calculations (Table 2). FIGURE 5. View largeDownload slide Depiction of the mean circumferential bony canal removal obtained by endonasal and transcranial approaches as viewed directly through the optic canal from a posterior-to-anterior A and anterior-to-posterior B vantage point. Shading shows bony decompression from an endoscopic approach (green) and transcranial bony removal (orange). Printed with permission from Mayfield Clinic. FIGURE 5. View largeDownload slide Depiction of the mean circumferential bony canal removal obtained by endonasal and transcranial approaches as viewed directly through the optic canal from a posterior-to-anterior A and anterior-to-posterior B vantage point. Shading shows bony decompression from an endoscopic approach (green) and transcranial bony removal (orange). Printed with permission from Mayfield Clinic. TABLE 2. Quantitative Analysis Shows the Mean Optic Canal Decompression Gained via Each Approach With Associated P Values Analysis  Endonasal  Transcranial    Mean circumference of decompression (degrees)  114.8  245.2  P < .001  Mean surface area (cm2)  0.57  1.23  P < .001  Mean surface area (%)  31.9  68.1  P < .001  Analysis  Endonasal  Transcranial    Mean circumference of decompression (degrees)  114.8  245.2  P < .001  Mean surface area (cm2)  0.57  1.23  P < .001  Mean surface area (%)  31.9  68.1  P < .001  View Large DISCUSSION As endoscopic endonasal approaches gain popularity, surgeons must weigh this trend in the context of which approach—transcranial vs endoscopic endonasal—will be most effective for a particular clinical scenario. To our knowledge, this study provides clinicians with the first quantitative analysis of the circumferential degree of optic canal decompression that can be obtained by these 2 approaches. A small pterional craniotomy with anterior clinoidectomy and removal of the optic strut was simple to perform and provided a wide exposure of the optic nerve with minimal temporal exposure. Conversely, optic decompression via an endoscopic endonasal route was anatomically limited when avoiding intracranial extension and the creation of a CSF fistula, but was a less invasive approach. Endonasal Decompression A wide range of pathologies may compromise the optic nerve and require decompression of its intracanalicular segment. Some authors have previously described the potential to decompress up to 270° of the canal from an endoscopic endonasal approach.31 However, this strategy requires extensive intracranial extension and delicate manipulation around the optic nerve, which may be more challenging from this approach for less experienced surgeons. Additionally, it could significantly increase the risk of CSF leak from the anterior skull base, and thus would require a nasoseptal flap or other precautions to prevent this complication. Another limitation of the endonasal approach is the inability to transect the falciform ligament to free the optic nerve without violating the arachnoid plane and risking a CSF leak. Given the extent of technical difficulty associated with this degree of endonasal decompression, we quantified the extent of bony canal removal that is possible before entering the intracranial compartment, since this strategy may be more practical in many clinical situations. Not surprisingly, the extent of this decompression highly depends on a patient's unique anatomy because the canal surface area exposed in the sphenoid sinus varies widely. In a study of 191 normal canals from their clinical database, Hart et al38 found an average of 101.3° of medial optic canal wall exposure in the sphenoid sinus. Our endonasal decompression averaging 114.8° slightly surpassed the circumference directly exposed within the sinus, as would be expected. However, we halted bone removal prior to intracranial extension, which significantly limited the extent of this approach. Nonetheless, the overall range of our decompression was consistent with previous studies.38 Of particular note is the anatomical landmark where the intracanalicular segment ends and the optic nerve traverses intracranially. This point was consistently observed in line with the medial border of the anterior ascending cavernous segment of the ICA from a rostral-caudal viewpoint (Figure 1). This relationship, which has not been previously described, can serve as a useful intraoperative marker for the extent of decompression that one may obtain before creating violation of arachnoid. Further exposure likely provides no further decompressive benefit, but enhances the risk of a spinal fluid leak. Transcranial Decompression By taking care to first establish the orientation of the intracanalicular optic nerve near the orbital apex with a high-speed drill under constant irrigation, the neurosurgeon can safely unroof the nerve longitudinally, identify and remove the optic strut, and avoid any potential injury to the underlying ICA. Importantly, a transcranial approach to the superolateral canal avoids injury to the ophthalmic artery, which we observed in the inferior or inferolateral optic canal in all specimens consistent with prior reports.31,39-42 Decompression of the medial border of the optic canal from above is limited by the sphenoid sinus. However, a substantial decompression of the superolateral optic nerve averaging 245.2° may still be obtained without entering the sinus. This was significantly greater than the exposure gained via the endonasal route and obviates the temptation to drill medially and risk a CSF leak, since this degree of decompression should be more than adequate in most clinical situations. Furthermore, sectioning of the falciform ligament is easily performed via the transcranial route without creating communication between the 2 compartments. This allows for safe procedural mobility of the optic nerve when necessary for decompression, as traction on the nerve may cause irreparable injury.36 Optic nerve mobilization may be required to safely remove tumor from the nerve or other specific scenarios, but was not used for our standard decompression. The retraction of the frontal lobe required with this approach was very minimal despite the rigidity of formalin-fixed brain tissue. Techniques to improve brain relaxation intraoperatively, including arachnoid dissection, cisternal opening, CSF diversion, or hyperosmolar therapy, may further relegate the utility of a retractor to a simple protective device during drilling. The chief limitation of the transcranial approach lies in the invasive nature of a craniotomy. Although the procedure can be completed extradurally, it carries the risk of injury to the frontal lobes during retraction or exposure. Additionally, the inferomedial optic canal is difficult to access from this angle. Choice of Surgical Approach Given the various pathologies that can compromise the intracanicular optic nerve, one must tailor the approach to an individual patient's anatomy, pathology, and clinical scenario. Clearly, if the goal of the procedure is to remove tumor encroaching upon the medial or inferomedial canal and resection will adequately decompress the nerve, an endonasal approach would be preferred. An endonasal approach may also be ideal for tumors encroaching upon the superolateral canal that have failed transcranial surgery and/or radiation. Of note, when it is necessary to enter the optic nerve sheath from an endoscopic endonasal approach, it should be opened superiorly so as to avoid injury to the ophthalmic artery coursing inferiorly. However, a wide decompression of the optic nerve is sometimes required either because of significant edema or the need for extensive procedural manipulation of the optic nerve. In these cases, we have shown that a substantial optic decompression may be obtained comfortably from a transcranial corridor with limited risk of CSF leak to the patient. One could posit that the extent of decompression need only exceed 180° to adequately unroof this structure and allow for nerve expansion in most scenarios. However, such a maneuver cannot be completed endonasal without arachnoid violation. Conversely, a craniotomy easily yields this degree of exposure before entering the sphenoid sinus and the optic sheath may be split without increasing the risk of CSF leak.30 Thus, a transcranial route provides the most extensive decompression of the optic nerve with limited risk of a CSF fistula. Furthermore, prior authors have argued that the additional removal of the anterior clinoid process and optic strut obviates the need for transecting the annulus of Zinn for adequate nerve decompression, which maintains the integrity of this structure.30 An extradural dissection and removal of the anterior clinoid is preferred by the authors, as it may optimize orientation, decrease trauma to intracranial structures, and require less operative time than an intradural clinoidectomy.19,30,37 Other minimally invasive cranial approaches to this location have been reported, including the supraorbital approach with or without endoscopic assistance.43-48 These techniques could conceivably yield a similar decompression to our modified pterional craniotomy with anterior clinoidectomy. However, our approach provides a wider exposure, is more generalizable and comfortable to most neurosurgeons, and provides the highest degree of safety to nearby neurovascular structures. The bony canal removal afforded by these minimally invasive transcranial approaches has yet to be quantified for comparison, but in theory should be similar to our study. Finally, in the rare patient where circumferential decompression of the nerve is indicated, such as extensive tumor invasion, a combination of the endonasal and transcranial approaches would be preferred. This combination yielded 360° of decompression in all of our specimens. In this case, precautions would be undertaken up front to prevent a CSF leak, as this would inevitably create a wide communication between the intracranial compartment and sphenoid sinus. Overall, the choice of approach should be determined by the individual patient's anatomy and pathology, and the preference of the surgical team in each clinical scenario. The transcranial and endoscopic endonasal approaches should be considered as complementary options for accessing this location, not mutually exclusive. The goal of this study was to assess the ability of each approach to decompress the optic canal with relatively low risk of a CSF leak. A leak should not be feared, however, and is certainly preferred over risk of injury to the optic nerve or major arteries. Furthermore, a CSF leak can be locally repaired in most situations from either approach when encountered. This anatomical knowledge may serve as additional information to be incorporated by a surgeon in preoperative planning. Limitations As with any cadaveric study, our model does not perfectly simulate the clinical environment in regard to tissue compliance and assessment of procedural morbidity. Variation of sinonasal anatomy can be significant, including the degree of pneumatization of the sphenoid sinus, anterior clinoid process, and optic strut. Of our 20 examined optic canals, only 1 specimen had notable pneumatization of the anterior clinoid process; this variation could lead to violation of the sphenoid sinus during clinoidectomy if not recognized preoperatively and a high risk of CSF leak. Finally, by stopping our dissection before entering the other compartment, our results underestimate the maximal decompression that could be obtained via both corridors. Overall, we believe that our anatomical measurements were consistent with previous large case series38 with respect to regional anatomy. We stress that careful study of preoperative imaging is essential for determining the most appropriate corridor for decompression. CONCLUSION Our quantitative analysis shows that a transcranial approach to the optic canal, compared with an endoscopic endonasal corridor, provided a substantially greater circumferential degree of decompression and exposure of the optic nerve before creation of a CSF leak. Although a significant exposure may be obtained via craniotomy, selection of approach should be tailored to each individual patient's regional anatomy and particular clinical scenario. Decompressions were not maximal from either approach, but rather maximal before violation of the other compartment. Thus, this represents an arbitrary—albeit clinically relevant—delineation of the limits of endonasal and transcranial approaches to the optic canal. Further quantitative analysis of minimally invasive cranial approaches to this region would be clinically useful. Disclosures Materials and costs related to this research project were entirely funded by a Mayfield Education Research Foundation (MERF) grant awarded by the MERF board and UC Department of Neurosurgery Executive Committee. No industry sponsorship was utilized. The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. 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Operative NeurosurgeryOxford University Press

Published: Mar 1, 2018

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