The optic nerve and chiasm can be involved with pathology affecting the orbit, orbital apex, and skull base. When surgically addressing these structures, it is critical to maintain not only their neural integrity but also their vascular supply, as this will allow for superior visual outcomes. Even relatively common procedures such as endoscopic endonasal transsellar resections of pituitary adenomas require a robust surgical knowledge of the optic apparatus.
Surgical decompression of the optic nerve and chiasm may be indicated for either decompression or tumor resection. Perhaps the most common indication for isolated decompression of the optic nerve is traumatic optic neuropathy; however, this may also be performed for nontraumatic optic neuropathy related to compressive pathologies such as Graves ophthalmopathy, fibrous dysplasia, or mucocele. Anterior skull base tumors involving the optic canals or chiasm are particularly characteristic of tuberculum sellae and planum sphenoidale meningiomas, with the incidence of optic canal invasion reported to be approximately 27% to 77% and as high as 97% in one series. These tumors necessitate surgical access to the optic apparatus for curative resection, improvement of visual deficits, or exploration to delineate tumor extent. Access can be achieved by either open or endonasal endoscopic approaches and is influenced by the compartment of the optic canal that needs to be addressed (e.g., medial or lateral), tumor size and location, and the goals of surgery. Lateral approaches include transorbital or craniotomy approaches. The endoscopic endonasal approach (EEA) ideally addresses medial lesions; allows for direct access to the orbital apex, optic nerve, and suprasellar cistern; and provides enhanced visualization of the subchiasmatic space. EEA approaches also potentially confer the advantages of less morbidity, less brain or orbital retraction, and superior cosmesis, as there are typically no external incisions.
The optic nerve should be considered an extension of the brain, as it contains meninges including a cerebrospinal fluid–containing subarachnoid space. There are four segments of the optic nerve distal to the optic chiasm: intracranial, intracanalicular, intraorbital, and intraocular. The optic chiasm and nerve are covered in this chapter proximally to distally.
Anatomy of the Optic Chiasm
The optic chiasm lies within the suprasellar cistern. The bony chiasmatic groove or sulcus, a bony depression bordered anteriorly by the limbus sphenoidale and posteriorly by the tuberculum sellae, is a consistent anatomic landmark for the level of the optic chiasm. The optic chiasm usually lies above the diaphragm and pituitary gland in 70% of cases, but in the remaining 30%, the optic chiasm can overlie the tuberculum sella in a “prefixed” configuration or the dorsum sellae in a “postfixed” configuration. Superior to the optic chiasm are the anterior cerebral and anterior communicating arteries ( Fig. 34.1 ). Immediately posterior to the optic chiasm is the pituitary infundibulum. Laterally, the optic chiasm is abutted by the supraclinoid internal carotid arteries (ICAs).
As the optic chiasm traverses the circle of Willis, it receives blood supply from it via the anterior cerebral and communicating arteries, posterior cerebral and communicating arteries, and the basilar artery. The optic chiasm also receives significant blood supply from the superior hypophyseal artery (SHA). The SHA is typically composed of two arteries (one proximal and the other distal) that arise from each ICA. The proximal artery typically has three main branches: infundibular (supplies the pituitary stalk and optic chiasm), optic (supplies the ventral and anterior optic chiasm as well as the proximal optic nerves), and descending (supplies the sellar diaphragm, stalk, and adenohypophysis) ( Fig. 34.2 ). Unilateral injury to or sacrifice of the SHA is unlikely to cause endocrine or chiasmal deficits owing to redundant blood supply but may pose significant risk to the proximal optic nerves as they have minimal collateral blood supply.
Anatomy of the Intracranial Segment of the Optic Nerve
The intracranial segment is the portion of the optic nerve between the optic chiasm and intracanalicular segment, and is approximately 12 to 16 mm in length and 4.5 mm in caliber. This segment is perfused by the ophthalmic, anterior cerebral, anterior communicating, and SHAs. Lateral to the optic nerve in this segment is the supraclinoid ICA ( Fig. 34.3 ). The ophthalmic artery originates from the supraclinoid ICA and generally courses inferolaterally to the nerve within its meninges as they both enter the optic canal. However, in approximately 15% of cases, the ophthalmic artery lies inferomedial to the nerve, potentially posing risk during EEA approaches that require the optic canals to be drilled. The ophthalmic artery gives off many small emissary vessels that supply the surrounding meninges and underlying optic nerve. Above the optic nerve in the intracranial segment are the gyri recti of the frontal lobes. Just proximal to where the intracranial optic nerve enters into its osseous canal, a fibrous band termed the falciform ligament ( Fig. 34.4 ) runs anteromedially from the anterior clinoid to the limbus sphenoidale, forming the roof of this preforaminal portion of the intracranial optic nerve. The falciform ligament covers the nerve superiorly for approximately 3 mm in length.
Anatomy of the Intracanalicular Segment of the Optic Nerve
The intracanalicular segment of the optic nerve bridges the intracranial and intraorbital segments and spans approximately 9 mm in length ( Fig. 34.5 ). This segment is supplied by the plial arterial network from the ophthalmic artery. The intracanalicular portion lies within an oblong cylinder of bone formed by the confluence of the optic strut and anterior clinoid process. As the nerve courses through the canal and transitions into its intraorbital segment, this bony exit is known as the optic foramen. The diameter of the intracanalicular portion is greater mediolaterally than superoinferiorly, and the anteroposterior distance is greater laterally than medially. However, as the nerve courses toward the optic foramen to exit into the orbit, the dimensions of the optic foramen are wider superoinferiorly than mediolaterally. The roof of the optic canal is the anterior root of the lesser sphenoid wing, which is continuous laterally with the anterior clinoid process and medially with the limbus sphenoidale ( Fig. 34.6 ). The optic strut is the bony floor of the optic canal that connects the anterior clinoid process to the lateral sphenoid sinus and separates the optic canal from the superior orbital fissure inferiorly. The medial wall is formed by the body of the sphenoid and is often thin (78%) or even dehiscent in up to 28% of cases. Distally, the intracanalicular optic canal is narrower and the bone composing it is thicker. The thickness of the medial bony wall at this point, where the optic canal courses near the orbit, is 0.57 mm on average, in contrast to the medial wall proximally toward the chiasm, where it is markedly thinner, measuring approximately 0.21 mm. The distal aspect of the canal as it terminates at the optic foramen contains particularly thick bone medially and is termed the optic tubercle. The optic tubercle is typically too hardy to be fractured off with instruments and instead requires a high-speed drill. The optic tubercle can be visualized to varying degrees and may lie within the sphenoid sinus or at the sphenoethmoidal junction, depending on the pneumatization pattern of the sphenoid and posterior ethmoid sinuses.