Surgical Anatomy of the Optic Nerves and Chiasm

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.

Surgical Anatomy

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.

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