Malignant melanoma of the optic nerve.

A 67-year-old man was diagnosed as having a melanocytoma of the optic disc in the left eye. Observation during a 5-year period showed no change in the lesion. At age 72 years, he had abrupt visual loss to no light perception in the affected left eye. Clinical examination disclosed little enlargement of the papillary tumor but ultrasonographic evidence of optic nerve infiltration. Precontrast magnetic resonance imaging studies disclosed a hyperintense infiltrative lesion in the enlarged left optic nerve. Enhancement features of the lesion excluded a hemorrhagic process. The eye was removed with a 22.5-mm segment of optic nerve. Histopathologic examination showed a large, necrotic, mixed-cell malignant melanoma confined to the optic nerve. No choroidal involvement or viable melanocytoma cells were documented. This case stresses that it may be difficult to differentiate a melanocytoma from a primary malignant melanoma of the optic nerve.