The optic nerve carries the greater than one million axons that derive from the retinal ganglion cells and project to eight visual nuclei. Actually an extension or tract of the central nervous system, not a true peripheral nerve, the conventional designation of the optic nerve applies to that portion of this tract that extends from the eye to the optic chiasm.
This topic will review functional anatomy of the optic nerve and the differential diagnosis of optic nerve pathologies. Specific disorders are discussed in detail separately. (See "Optic neuritis: Pathophysiology, clinical features, and diagnosis" and "Nonarteritic anterior ischemic optic neuropathy: Clinical features and diagnosis".)
The optic nerve has four anatomic segments: the intraocular, intraorbital, intracanalicular, and intracranial sections [1,2].
- The intraocular portion, or the optic disc, consists of the unmyelinated retinal ganglion cell axons and astrocytes. As the nerve exits the globe, crossing the collagenous lamina cribrosa, it increases in diameter as it becomes myelinated by oligodendrocytes.
- The intraorbital segment of the optic nerve is approximately 25 to 30 mm. Within the orbital apex, the optic nerve is surrounded by dura, arachnoid, and pia mater. The subarachnoid space contains cerebrospinal fluid continuous with that of the brain and spinal cord. The dural covering fuses with the sclera around the optic nerve and more posteriorly with the periosteum of the optic canal. The nerve courses through the annulus of Zinn, which contains the connective tissue origins of the superior, medial, lateral, and inferior rectus muscles.
- The nerve then passes through the optic foramen into the bony optic canal, which lies posteromedially in the lesser wing of the sphenoid bone, and is about 4 to 10 mm in length. The ophthalmic artery and ocular sympathetic nerve fibers also travel through the optic canal.
- Intracranially, the optic nerve courses medially and rises at an angle of 45 degrees toward the chiasm. This segment of the optic nerve is approximately 16 mm in length and lies superior to the cavernous sinus and inferior to the frontal lobe and anterior cerebral and anterior communicating arteries.
The blood supply of the optic disc and nerve is distinct from that of the retina, which is supplied by the central retinal artery. The disc is supplied by anastomosing arterioles supplied by the posterior ciliary arteries, the pial arteriole plexus, and the peripapillary choroid. The intraorbital segment receives blood from perforating branches of the ophthalmic artery, while the intracanalicular and intracranial portions are supplied by pial branches of the ophthalmic, internal carotid, anterior cerebral, and anterior communicating arteries.