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Home , Annulus of Zinn, Optic canal, Superior orbital fissure

Konstantin V Slavin, M.D., Manuel Dujovny, M.D., Gelson Soeira, M.D., and James I. Ausman, M.D., Ph.D.

Optic : Microanatomic Study

The is a bony channel that connects the the optic canal and distance between the folds; the height and the and contains the optic of the (which forms the lateral nerve and the . Detailed knowledge of border of the intracranial opening); distances between the the anatomy of the optic canal and adjacent vital struc- anterior clinoid and the , between the tures, such as the and the hypo- anterior clinoids, and between the inferior margin of the physis, is very important in performing different surgical intracranial opening of the optic canal and the superior procedures on the anterior part of the base and the orbital fissure; and the diameter ofthe intracranial opening orbit. Recently, anatomic data about the optic canal had of the optic canal. After initial dissections and division of included descriptions of bony structures forming its walls the dural fold over the the optic canal was and its size. Improvement in surgical techniques has in- unroofed using the high-speed drill. At this stage the creased interest in the optic canal, but many aspects still second series of measurements was made, which included require investigation. the intracanalicular length of the optic nerve, the length of This study was performed to facilitate understanding the bony roof and floor of the optic canal, the angle of the of the anatomic relationship of the optic canal to the optic canal with the midline in the horizontal plane, and adjacent and soft tissue structures, detail the course the course of the ophthalmic artery relative to the optic of the optic nerve and ophthalmic artery within and near nerve. The diameter of the ophthalmic artery and location the optic canal, and assist in developing a convenient and at the distal and proximal parts of the optic canal were also safe approach to different lesions in this location. noted. For the third stage, unroofing the orbit allowed us to measure the distances between the intraorbital opening of the optic canal and the annulus tendinosus and the bulbus MATERIALS AND METHODS oculi, as well as the width and height of the intraorbital opening ofthe optic canal (foramen opticum). After open- The study was performed on 20 cadaveric speci- ing the entire optic canal, the optic nerve with the ophthal- mens, fixed in glycerin, from patients who had never had mic artery was removed, and the shape and diameter ofthe any intracranial or intraorbital pathology. Optic canal optic nerve in the middle of the optic canal and in its measurements were made before, during, and after ana- orbital exit were measured. The last stage of dissection tomic dissections, which were performed under an opera- included noting the degree of pneumatization of adjacent tive microscope (OPMI I-SH, Carl Zeiss, Inc., New York) bone structures, such as the anterior clinoid process, and using microsurgical instruments and a high-speed drill. the sphenoid and ethmoid sinuses, forming the optic canal. The first series of measurements included the diame- All stages of dissections were photographed and ter and shape of the intracranial portion of the optic nerve; drawn, and appropriate samples were used for laser print- the length of the dural fold covering the proximal part of ing from the slides.

Skull Base Surgery, Volume 4, Number 3, July 1994 Department of Neurosurgery, University of Illinois at Chicago, Chicago, Illinois Reprint requests: Dr. Dujovny, Department ofNeurosurgery, Neuropsychiatric Institute (MIC 799), University of Illinois at Chicago, 912 South Wood Street, 136 Chicago, IL 60612 Copyright C 1994 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved. OPTIC CANAL-SLAVIN ET AL RESU LTS process had a mean height of 9.23 + 1.56 mm. The distance between the anterior clinoid and the tuberculum The results were divided into three groups according sellae was 13.72 ± 1.29 mm, and the distance between the to anatomic portion. We decided that such a division anterior clinoids was 25.67 + 1.52 mm. The distance would better correlate with clinical needs because differ- between the centers of the optic nerves at the level of their ent pathological processes affect the optic canal differ- entrance into the optic canal was 20.2 + 1.49 mm. The ently. was found to lie inferolaterally to The intracranial portion of the optic nerve was a the intracranial opening of the optic canal, the distance horizontal oval shape in all cases (Table 1), with mean between them being 6.41 + 1.34 mm. The dura lining the orbital was to lie over the of greatest diameter of4.25 + 0.98 mm (Table 2). Before the superior fissure found base the optic canal was entered the optic nerve was covered by the anterior clinoid process and continued into the optic canal. falciform dural fold, with a mean length of 3.7 ± 1.9 mm The intracranial opening of the optic canal had an almost (Fig. la-c). The distance between the medial borders of round shape, with a mean diameter of 5.14 + 0.9 mm. The optic canal itself (Table 3) presented as a hollow the dural folds was 10.17 + 1.72 mm. The anterior clinoid cylindrical structure, lined from inside with two layers of Table 1. The Shape of the Optic Nerve (%) dura (Figs. Id and 2). The length of the optic canal was measured as 10.74 ± 1.16 mm, equal to the length of the Horizontal Vertical Oval Round Oval intracanalicular part of the optic nerve. Because of the canal's inclining course the lengths of the bony roof (9.91 Intracranial part 100 Intracanal part 100 + 1.67 mm) and the floor (6.30 + 1.02 mm) were less than Intraorbital part 30 70 the length of the canal (Fig. 2). Inside the optic canal the

Table 2. The Intracranial Opening of the Optic Canal Range Average Standard Deviation Parameter (mm) (mm) (mm) Diameter of the ON intracranially 2-6 4.25 ±0.98 Length of the dural fold 1-6 3.7 +1.9 Distance between the dural folds 7-13 10.17 +1.72 Height of the anterior clinoid 6.5-12 9.23 +1.56 Distance between anterior clinoid and tuberculum sellae 12-17 13.72 +1.29 Distance between anterior clinoids 24-27.5 25.67 ±1.52 Distance between centers of ONs 18-23 20.2 +1.49 Distance between the OC and superior orbital fissure 4-9 6.41 ±1.34 Diameter of the intracranial opening of the OC 4-7 5.14 ±0.9 ON = optic nerve; OC = optic canal

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ON

Figure 1. Photographs of dif- ferent stages of dissection. a: Gen- eral view of intracranial course of the optic nerve. (Figure continued on the next page.) OCh 137 SKULL BASE SURGERYNOLUME 4, NUMBER 3 JULY 1994

S t ~~~DM r _ ~~~~ACLI_ F f~~~~~ON ACUPr PSph

':Figure 1. (Continued). b: Mag- nified view of the intracranial open- ing of the right optic canal. The falci- form dural fold covers the proximal N part of the optic canal. c: Intracranial opening of the right optic canal after excision of the falciform dural fold. d: View of the middle portion of the left optic canal after drilling off its roof. The optic nerve is covered by dura inside the canal. DM = dura mater, PSph = planum sphenoidale, ON = optic nerve, OCh = optic chi- asm, ACLP = anterior clinoid pro- cess, ICA = internal carotid artery, OS = opticstrut, OC = optic canal. 138 OPTIC CANAL-SLAVIN ET AL

Table 3. Middle Part of the Optic Canal periorbit, while the inner layer accompanied the optic Standard nerve to the bulb (Fig. 5). Range Average Deviation At the level of intraorbital opening, or foramen op- Parameter (mm) (mm) (mm) ticum, the optic nerve had the shape of a vertical oval Length of intracanal 9-12 10.74 ±1.16 (70%) or a circle (30%) and its diameter was 2.86 + 0.48 part of ON mm (Table 5). The width ofthe foramen opticum was 5.20 Length of bone roof of 6-12 9.91 +1.67 + 0.68 mm and the height was 5.38 + 0.91 mm. After OC Length of bone floor of 5-8 6.30 ±1.02 leaving the optic canal the optic nerve went through the OC annulus tendinosus (Zinn), which was attached to the Diameter of ON within 2-3 2.62 ±0.38 periorbit and the bone in all cases. The length of the OC intraorbital part of the optic nerve was measured as 22.04 Diameter of ophthalmic 0.8-2 1.24 ±0.12 + 4.7 mm. artery After all of these measurements the degree of pneu- ON = optic nerve; OC = optic canal. matization of bone structures adjacent to the optic canal was evaluated (Table 6). The anterior clinoid process was pneumatized in 65% of cases, including two cases of optic nerve had a round shape in all cases, and its diameter luxury pneumatization. Luxury pneumatization of the in the middle of the canal was 2.62 + 0.38 mm. In the was found in 15% of cases. No cases of horizontal plane the angle of the optic canal in relation to luxury pneumatization of the were found, the midline was 29.56 + 1.520 (220 to 340) (Fig. 3). The but in two cases notable thinning ofthe bone wall separat- ophthalmic artery accompanied the optic nerve during the ing the ethmoid sinus from the optic canal was noted. entire course ofthe optic canal, its external diameter being The comparison of our results with those reported 1.24 ± 0.12 mm. Immediately after entering the optic earlier is given in Table 7. canal the ophthalmic artery lay exactly beneath the optic nerve in 45% and on its inferolateral surface in 55% of cases (Table 4). As the ophthalmic artery progressed to the DISCUSSION orbit it turned laterally in the optic canal. In the distal part of the canal the ophthalmic artery was positioned on the Neurosurgical treatment of the lesions involving the inferomedial (60%), inferior (30%), or inferolateral (10%) optic canal began in 1921 when Dandy operated on a surfaces of the optic nerve (Fig. 4). Both structures were patient with bilateral meningiomas ofthe optic canal using covered by two layers of dura. The outer layer formed the a transcranial approach. He concluded from his own expe-

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A CL Figure 2. Schematic view of the course of the optic nerve and ophthalmic artery in the optic canal. A: Superior view of floor of the anterior cranial fossa. The roof of the orbit and optic canal on the left is drilled off. (Figure continued on the next page.) 139 SKULL BASE SURGERYNOLUME 4, NUMBER 3 JULY 1994

ON-

OA N \N%w\\.. .1 B Figure 2. (Continued). B: Lateral view of the left optic canal. ON = optic nerve, OA = ophthalmic artery, PSph = planum sphenoidal, ICA = internal carotid artery, ACLP = anterior clinoid process, DS = , CL = , ST = .

rience that the transcranial approach for orbit exploration sion of the intracranial lesion, such as in meningioma or in is more convenient and radical compared to the traditional an ophthalmic artery aneurysm. The risk of an injury to lateral approach.",2 Discussion of choice of approach for the optic nerve and adjacent vital structures is quite high, intraorbital tumors still continues, but there is no doubt and this makes detailed knowledge of the microsurgical that with involvement ofthe optic canal or intracanalicular anatomy of this region very important. In spite of its part of the optic nerve the bony roof or wall of the canal importance, the anatomic data concerning the optic canal must be removed,3 at least for nerve decompression.4 The are very limited. In this study we compare our data from larger challenge arises in cases of intracanalicular exten- the series of anatomic dissections with those previously

floor of OC Figure 3. Schematic view of the longitudinal cut of the optic canal. OC = optic canal, ON = optic nerve, 140 OR = orbit, OA = ophthalmic artery. OPTIC CANAL-SLAVIN ET AL

Table 4. Location of Ophthalmic Artery fold, and laterally it is limited by the anterior clinoid Relative to Optic Nerve (%) process. At this level of the optic canal the optic nerve can Inferomedial Inferior Inferolateral be affected by tumor (meningioma, glioma, pituitary ade- In the proximal 45 55 noma, craniopharyngioma, etc), by saccular aneurysms, part of the OC or by a dolichoectatic carotid artery. Excessive pneumat- In the distal part 60 30 10 ization of the sphenoid sinus and the anterior clinoid of the OC process may also cause compression of the optic nerve at OC = optic canal. the entrance to the optic canal.5 The goal of surgical treatment in such cases is preser- vation of visual function by removal of the lesion and decompression of the optic nerve. Our results, as well as reported by others, and make recommendations for clini- previously reported data68 (Table 7), show that the first cal use of this information. 3 mm ofthe canal can be decompressed by resection ofthe We divided the course of the optic canal into three dural fold or by removal of the anterior clinoid. This portions, each one important for solving the specific prob- technique can facilitate the surgical approach to the lesion lems arising during surgery on different lesions of this itself. region. The second portion of the optic canal is the canal The first portion of the optic canal includes the intra- itself. The walls of the canal are formed by two roots ofthe cranial opening of the canal in the . lesser sphenoid ridge. The direction of the canal is from The optic nerve, having a horizontal oval shape, enters the superior, medial, and posterior to inferior, lateral, and optic canal on both sides of the tuberculum sellae. The anterior. The average angle of the optic canal with the upper wall ofthis opening is formed by the falciform dural midline in the horizontal plane is about 300. Now, with

lateral 20%

middle lateral 10% 10%

Figure 4. Five variants of the course of the ophthalmic artery relative to the optic nerve. OA = ophthal- mic artery, SpS = sphenoid sinus, OR = orbit, ON = optic nerve, OS = optic strut, OCh = , ICA = internal carotid artery. From the top left: OA enters canal laterally to ON and leaves it medially to ON (25%), OA enters canal under ON and leaves it medially to ON (35%), OA enters canal laterally to ON and leaves it under ON (20%), OA enters canal under ON and also leaves it under ON (10%), and OA enters canal laterally to ON and also leaves it laterally (10%). 141 SKULL BASE SURGERYNOLUME 4, NUMBER 3 JULY 1994

ernmoic dural covering bone of the optic nerve ~perIorbit

Os

ON

Figure 5. Schematic presentation of the course of the dura mater within and near the optic canal. OS = optic strut, DM = intracranial dura mater, ON = optic nerve.

Table 5. The lntraorbital Opening of the Optic Canal formed without laceration of dura covering of the nerve Standard because, as reported by Walsh, such an incision through Range Average Deviation the dura always interferes with its blood supply and can Parameter (mm) (mm) (mm) impair visual function.9 Nevertheless, delicate sharp dis- Diameter of ON in the 2.5-4 2.86 ±0.48 section of the dural sheath from the dural fold up to the orbit annulus tendinosus of Zinn with detachment of all adhe- Width of intraorbital 4-6 5.20 ±0.68 sions was recommended for better decompression of the opening Height of intraorbital 4-7 5.38 ±0.91 optic nerve.10 opening In our study the ophthalmic artery always accom- Distance between the 15-28 22.04 +4.7 panied the optic nerve in the optic canal and lay intra- intraorbital opening durally in all cases. According to Lang,6"11 in 2% to 4% and the bulb of cases the artery lies separately in a special bony canal ON = optic nerve. parallel to the optic canal. In his series the ophthalmic artery entered the optic canal at the medial border of the optic nerve in 41.4%, exactly in the middle and below the nerve in and lateral to the in use of computed tomography and magnetic resonance optic 32.7%, optic nerve imaging in preoperative evaluation of patients, it is pos- 25.9% ofcases.6 In our study the artery lay in the proximal sible to determine the angle of the optic canal before the part of the optic canal on the inferior and inferomedial surgery. This parameter is important during unroofing of surface of the optic nerve in 45% and 55% of cases, the optic canal for decompression or for excision of tumor respectively (Table 5). Also, in Lang's material at the inside the canal. orbital aperture of the optic canal the ophthalmic artery The dural lining of the cranium continues into the was situated medial to the nerve in 15.5% and lateral to it in optic canal, where the optic nerve and ophthalmic artery 84.5% of cases.6 We found the ophthalmic artery at the are covered by two layers of dura. The process of decom- distal part ofthe optic canal on the inferomedial surface of pression of the optic canal in its bony part must be per- the nerve in 60%, on the inferior surface in 30%, and on the inferolateral surface in 10% of cases. These observa- tions show the great variability of the course of the oph- thalmic artery within the optic canal. Table 6. Pneumatization of Bone Structures Around the Optic Canal (%) The third portion of the optic canal is located at the intraorbital opening of the canal, which is known as the Regular Extensive No. optic foramen.12 The shapes and the sizes of the optic Anterior clinoid 55 10 35 canal found in our study corresponded with radiological Sphenoidal sinus 85 15 measurements to et a113: The Ethmoidal sinus 100 according Hammerschlag 142 optic canal is a 4.5 x 6-mm horizontal oval at the intra- OPTIC CANAL-SLAVIN ET AL

Table 7. Literature Data: Anatomy of the Optic Canal Lang6 Renn and Manisalco Lang and Parameter Walsh12 Rhoton8 and HabaP Housepian14 Kageyamall Slavin et al Diameter of intracranial 7.18 7.18 5.14 opening (mm) (5.0-9.5) (4-7) Diameter of ON intracranially 4.5 3 x 5 4 x 6 4.25 (mm) (2-5 x 3.5-6) (2-6) Length of dural fold (mm) 3.0 3.0 3.11; 2.58 3.7 (0.5-8.0) (0.5-8.5) (1-6.5) Length of ON within OC 9.22 10-12 10.24 (mm) (5.5-11.5) (9-12) Diameter of ON within OC 5 2.62 (mm) (2-3) Length of bone roof of OC - 9.80 9.90 (mm) (7.3-12.0) (6-12) Length of bone floor of OC 4.80 6.30 (mm) (3.0-9.3) (5-8) Angle of OC with midline (0) 39.1 29.56 (22-34) Diameter of arteria - 2.0 1.5 1.24 ophthalmica (mm) (0.5-3.0) (1.1-1.9) (0.8-2.0) Diameter of orbital opening - 4.7 5.2 x 5.38 (mm) (1.0-6.0) (4-6 x 4-7) Diameter of ON intraorbitally 3-4 4 x 6 3.5 2.86 (mm) (2.5-4) ON = optic nerve, OC = optic canal.

cranial end, a 5 x 5-mm circle at the middle, and a 5 x 6-mm vertical oval at the orbital end. REFERENCES The optic nerve enters the orbit with the ophthalmic artery lying under it. The annulus tendinosus of Zinn, 1. Dandy WE: Prechiasmal intracranial tumors of the optic nerves. Am J Ophthalmol 5:169-188, 1922 which is a fibrous band serving as the origin of five 2. Dandy WE: Results following the transcranial operative attack on , attaches the optic nerve to the orbital orbital tumors. Arch Ophthalmol 25:191-216, 1941 apex. At the level ofthe annulus tendinosus the meningeal 3. Newman SA, Jane JA: Meningiomas of the optic nerve, orbit, and anterior visual pathways. In Al-Mefty 0: Meningiomas. New sheaths of the optic nerve are fused dorsomedially and York: Raven Press, 1991 ventrally, and the subarachnoidal space continues just on 4. Long D: Neurosurgical involvement in tumors of the orbit. Clin the lateral surface of the optic nerve. The Neurosurg 32:514-523, 1985 5. Unsold R, Seeger W, De Groot J: A narrow passage-anatomic does not provide a barrier for tumor growth through the considerations. In Unsold R, Seeger W: Compressive Optic optic nerve and cannot serve as a border ofnerve resection. 14 Nerve Lesions at the Optic Canal. Berlin: Springer-Verlag, 1989 The numbers found in our study show that in spite of 6. Lang J: Clinical Anatomy of the Head. , Orbit, Craniocervical Regions. Berlin: Springer-Verlag, 1983 great variability there are some approximate anatomic 7. Manisalco JE, Habal MB: Microanatomy of the optic canal. J standards that can be used in neurosurgical practice. We Neurosurg 48:402-406, 1978 also think that division of the optic canal into three por- 8. Renn WH, Rhoton AL Jr: Microsurgical anatomy of the sellar region. J Neurosurg 43:288-298, 1975 tions can facilitate the understanding of pathological in- 9. Walsh FB: Meningiomas primary within the orbit and optic canal. volvement of the canal and its contents. Knowledge of the In Glasser JS, Smith JL: Neuro-ophthalmology. Vol. 8. St. size of both openings of the optic canal, the length of its Louis: CV Mosby Co, 1975 10. Eggert H-R: Pterional approach for microsurgical decompression walls, and its angle and shape may be helpful in perform- of the optic nerve. In Unsold R, Seeger W: Compressive Optic ing surgical exploration ofthe canal for its decompression, Nerve Lesions at the Optic Canal. Berlin: Springer-Verlag, 1989 removal of tumor, or approaches to the or 11. Lang J, Kageyama I: Clinical anatomy of the blood spaces and blood vessels surrounding the siphon of the internal carotid sella. Knowledge of the course of the ophthalmic artery artery. Acta Anat 139:320-325, 1990 will make all optic canal explorations safer. The use of 12. Walsh FB: Clinical Neuro-ophthalmology. 2nd ed. Baltimore: Wil- preoperative angiography for this purpose is highly rec- liams & Wilkins, 1957 13. Hammerschlag SB, Hesselink JR, Weber AL: Computed Tomog- ommended. raphy of the Eye and Orbit. Norwalk, CT: Appleton-Century- In the near future, the three-dimensional reconstruc- Crofts, 1983 tion of high-resolution imaging and computer-assisted vi- 14. Housepian EM: Microsurgical anatomy of the orbital apex and principles of transcranial orbital exploration. Clin Neurosurg sualization of the skull base will improve the anatomic 25:556-573, 1978 understanding of the optic canal and adjacent structures and make the surgery more safe and effective. 143 SKULL BASE SURGERYNOLUME 4, NUMBER 3 JULY 1994

REVIEWER'S COMMENTS

This is an excellent study of the microanatomy of the optic canal. The measurements are made sequentially as the canal would be opened by a neurosurgeon. As surgeons open the optic canal more frequently, relying on studies such as this, other questions (possibly harder to study in cadavers) will arise: For example, does unroofing the optic canal usually provide sufficient decompression of the optic nerve? Should the annulus of Zinn be left intact or opened? Does opening the canal more completely bring more risk to the vascular supply of the nerve? In patients with traumatic optic neuropathy the optic canal is often intact (no fracture). For these patients, it would be important to know where in the canal the optic nerve is most tightly confined and what length of optic canal must be opened. This microanatomic study is a good example of how these questions can be answered.

Michael Joseph

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