The dark side of the (half) moon.

A 71-year-old man was referred to neuro-ophthalmology for evaluation of reading problems associated with pituitary adenoma. Perimetry showed a right temporal crescent syndrome ("half moon" syndrome) suggesting left occipital disease. Cranial magnetic resonance imaging, electroencephalogram, and cerebrospinal fluid examination (14-3-3 protein) were consistent with a diagnosis of Creutzfeldt-Jakob disease. His neurologic condition rapidly deteriorated with memory problems, myoclonus, and apathy followed by death, 4 months later. This case shows how visual problems may be the first manifestation of this rare prion disease (Heidenhain variant of Creutzfeldt-Jakob disease).