Progressive visual loss caused by an arachnoidal brain cyst in a patient with an optic nerve coloboma.

We examined a patient with a known coloboma of the optic nerve head who developed progressive visual loss. Documentation of the optic nerve dysfunction included decreased visual acuity, severe visual field loss, and the presence of an afferent pupillary defect. High-resolution computed tomography showed an arachnoidal brain cyst compressing the involved optic nerve.