Optic neuritis is the leading cause of transient, spontaneous, reversible visual loss in young adults. Characteristically, patients present with central visual loss that peaks within a few days and is often associated with eye pain. Visual loss may be complete. Spontaneous recovery usually begins within 4 weeks, and marked recovery occurs within 1 to 3 months in most patients. Although clinical improvement is the rule, not all patients recover fully, and many are left with residual symptoms. Although there are limited pathological studies in inflammatory ON, the pathological changes are thought to be virtually identical with those seen in MS, with disruption of the blood-nerve (brain) barrier; primary demyelination with axonal sparing; variable degrees of lymphocytic infiltration; an abundance of macrophages around the inflammatory demyelination lesion; various degrees of remyelination; and, later, oligodendrocyte loss, axonal loss, and gliosis.