Numerous congenital defects can involve the optic nerve, including optic nerve hypoplasia, optic nerve head (ONH) pits (also called optic disc pits), morning glory disc anomaly, Bergmeister papilla, and optic nerve colobomas. These congenital anomalies are discussed further in BCSC Section 5, Neuro-Ophthalmology, and Section 6, Pediatric Ophthalmology and Strabismus.
Typical colobomas of the ONH result from defective closure of the embryonic fissure. They are often observed inferonasally in the ONH; may be associated with colobomatous defects of the retina/choroid, ciliary body, and iris; and may occur at any point along the course of the embryonic fissure (Fig 15-3A).
Histologically, an optic nerve coloboma consists of a large defect in the optic nerve that involves the retina, retinal pigment epithelium, and choroid. An atrophic, gliotic retina lines the defect. The sclera is ectatic and bowed posteriorly. The wall of the defect may contain adipose tissue and even smooth muscle (see Fig 15-3B).
Bacterial or fungal infections of the optic nerve can spread from adjacent anatomical structures, or they may occur as part of a systemic infection, particularly in an immunosuppressed patient. Fungal infections include mucormycosis, cryptococcosis, and coccidioidomycosis. Mucormycosis generally results from contiguous sinus infection. Cryptococcosis results from a direct extension of the infection from the CNS and often produces multiple foci of necrosis with little inflammatory reaction (Fig 15-4). Coccidioidomycosis generally begins as a primary pulmonary infection, and then spreads to the optic nerve and globe. It produces necrotizing granulomas.
Viral infections of the optic nerve are usually associated with a more diffuse CNS process. Acute disseminated encephalomyelitis is an immune-mediated demyelinating disease that often follows bacterial or viral infections. Macrophages remove the damaged myelin. Astrocytic proliferation ultimately produces a glial scar, known as a plaque. The findings of acute disseminated encephalomyelitis are almost identical to those of multiple sclerosis (Fig 15-5).
Noninfectious inflammatory disorders of the optic nerve include giant cell arteritis and sarcoidosis. Giant cell arteritis can produce granulomatous inflammation in the blood vessel wall as well as occlusion of the posterior ciliary vessels, with liquefactive necrosis of the optic nerve. The gold standard for histologic diagnosis of giant cell arteritis is superficial temporal artery biopsy (Fig 15-6). The involvement of the vessel wall in giant cell arteritis can be patchy (ie, skip lesions). Obtaining a biopsy specimen of adequate length (approximately 2 cm) and performing a careful histologic examination of the specimen can increase its diagnostic yield.