Optic neuritis is also a main component of neuromyelitis optica (Devic disease; see Chap. 35); the vision loss is often more severe and the prognosis for recovery is poorer than for optic neuritis in multiple sclerosis.
Postinfectious demyelinating disease is a possible cause in some cases that do not later show signs of multiple sclerosis. Less is known about children with retrobulbar neuropathy, in whom the disorder is more often bilateral and frequently relates to a preceding viral infection (“neuroretinitis,” see the following text). Their prognosis is better than that of adults. Some cases of optic neuritis have been convincingly attributed to paranasal sinus disease, but this association only occurs rarely, as discussed further on. Neuroretinitis is a rare postor parainfectious process seen mostly in children and young adults, sometimes in association with exposure to the Bartonella henselae bacteria, the cause of cat scratch fever. Papillitis is accompanied by macular edema and exudates situated radially in the Henle layer, producing a “macular star” appearance.
In persons older than 50 years of age, a common cause of a persistent monocular loss of vision is ischemic infarction of the optic nerve head (Fig. 12-15). With anterior ischemic optic neuropathy, the optic disc is involved and optic disc edema is visualized in the acute phase. In contrast, with posterior ischemic optic neuropathy, there are no changes of the optic disc until disc pallor develops in the chronic stages. Ischemic optic neuropathy can be either arteritic (discussed in the following text) or nonarteritic. With nonarteritic anterior ischemic optic neuropathy (NAION), the onset is typically abrupt and painless, but on occasion the visual loss is progressive for several days. Usually, there are no premonitory symptoms or episodes of transient visual loss. The field defect is often altitudinal. If the area of central fixation is involved, then acuity can be severely compromised. Swelling of the optic disc is typical, and can be sectoral, with certain portions of the disc circumference visibly spared. Associated small, flame-shaped hemorrhages are also typical. The retina and retinal vessels are not affected, as they are in cases of embolic occlusion of the central retinal artery.
Despite these distinctive features, ischemic optic neuropathy can sometimes be difficult to differentiate from optic neuritis, as pointed out by Rizzo and Lessell. This proves particularly problematic when visual loss evolves over days and mild pain accompanies the ischemic condition. However, the age of the patient and nature of the field defect (central in optic neuritis in contrast to sometimes altitudinal in ischemic neuropathy) further serve to clarify the situation. Furthermore, arteritic and nonarteritic forms of ischemic optic neuropathy are distinguished, the former being the result of temporal (giant cell) arteritis.
The prognosis for visual recovery from nonarteritic anterior ischemic optic neuropathy is generally poorer than for other forms of optic neuropathy. As the disc edema subsides, optic atrophy becomes evident. The second eye may be similarly affected at a later date, particularly in those patients with hypertension and diabetes mellitus.
As to the pathogenesis of nonarteritic anterior ischemic optic neuropathy, it has been attributed by Hayreh to insufficiency of the posterior ciliary artery circulation and more specifically to occlusion of the branches of the peripapillary choroidal arterial system. A “crowded” optic disc, with small cup-to-disc ratio, is a risk factor. Most cases occur on a background of hypertensive vascular disease and diabetes, Nocturnal hypotension may be a contributing factor, and patients are often counseled to avoid medications that could cause the blood pressure to decline overnight. Nonarteritic anterior ischemic optic neuropathy may also complicate intraocular surgery.