Mesencephalic hemorrhage and unilateral pupillary deficit.

An isolated dilated and fixed pupil in the absence of other neurological dysfunction usually suggests ciliary ganglion dysfunction or direct pharmacological blockade of the parasympathetic fibers to the pupil. A unilateral dilated pupil as a manifestation of a mesencephalic hemorrhage has not, to our knowledge, previously been reported. We now describe a 35-year-old woman who had a headache and a dilated and nonreactive right pupil. A cranial computed tomography scan showed a small hemorrhage in the midbrain, and a cerebral angiogram was normal. Mesencephalic hemorrhage should be considered in the differential diagnosis of an isolated dilated pupil.