The optic nerves constitute the axonal projections of the retinal ganglion cells to the lateral geniculate bodies and other sites. Their proximal portion, the optic nerve head, can be visually inspected and changes in the optic disc are therefore of particular importance. These changes may reflect the presence of raised intracranial pressure as already described; optic neuritis (“papillitis”); infarction of the optic nerve head; congenital defects of the optic nerves (optic pits and colobomas); hypoplasia and atrophy of the optic nerves; and glaucoma. Illustrations of these and other abnormalities of the disc and ocular fundus can be found in the atlas by E.M. Chester and in the text by Biousse and Newman. In general, optic neuropathies are distinguished from other causes of visual loss by a predominance of loss of color vision and by the presence of an afferent pupillary defect.
Table 12-3 lists the main causes of optic neuropathy, which are discussed in the following portions of this chapter.
Optic Neuritis (See Chap. 35)
This inflammatory process causes unilateral acute impairment of vision that may appear in one or both eyes, either simultaneously or successively. It develops in a number of clinical settings, but has a special relationship to multiple sclerosis. The most common situation is one in which an adolescent or young adult woman has a rapid diminution of vision in one eye as though a veil had covered the eye, progressing within hours or days. Pain on movement of the globe is a common, but not universal reported symptom (Table 12-2). The swinging flashlight test usually confirms a relative afferent pupillary defect on the affected side, unless the fellow eye has previously been affected.
The optic disc and retina appear normal in the majority of cases, in which the condition is of the more common retrobulbar variety. However, if the inflammation is near the nerve head, there is swelling of the disc (papillitis) (Fig. 12-14). The disc margins are then seen to be elevated, blurred, and, rarely, surrounded by hemorrhages. Inflammatory sheathing of the retinal veins, as described by Rucker, is known to occur but has been uncommon in our patients. In extreme cases, edema may suffuse from the disc to cause a rippling in the adjacent retina.
After several weeks to months, there is spontaneous recovery; vision returns to normal in more than two-thirds of cases. Recovery of vision occurs spontaneously, or may be hastened by the intravenous administration of high doses of corticosteroids (see “Treatment of Optic Neuritis” in Chap. 35). Diminution of brightness, dyschromatopsia, or a scotoma may remain; rarely, the patient is left with severe permanent visual deficits. Often, patients report a transient increase in blurring with exertion or with exposure to heat (Uhthoff phenomenon).
Despite the return of visual acuity in the majority of patients with optic neuritis, a degree of optic atrophy almost always results. The disc then appears pale, particularly in its temporal sector (temporal pallor), and the pallor extends beyond the margins of the disc into the peripapillary retinal nerve fibers. The pattern-shift visual evoked potential remains delayed; as a result, this test is a highly sensitive indicator of previous, even asymptomatic, episodes of optic neuritis.
As time progresses, more than half of adults with idiopathic optic neuritis will develop other symptoms and signs of multiple sclerosis within 5 years, and even more do so when they are observed for longer periods. Conversely, in approximately 15 percent of patients with multiple sclerosis, the history discloses that retrobulbar neuritis was the first symptom. A proportion of patients with acute optic neuritis are found at the time of an acute attack to have characteristic subclinical radiographic features of multiple sclerosis on MRI of the cerebrum and spine. The treatment of optic neuritis is taken up with multiple sclerosis in Chap. 35.