Isolated unilateral mydriasis with delayed oculomotor nerve palsy secondary to intracranial arachnoid cyst.

A 16-month-old boy presented with isolated unilateral mydriasis. Fourteen months after initial presentation, he developed an acute oculomotor nerve palsy. His initial computed tomography scan, with contrast, was normal. Subsequent magnetic resonance imaging revealed an enlarging arachnoid cyst at the exit of the oculomotor nerve from the midbrain. Isolated unilateral mydriasis may be caused by a slowly enlarging lesion, which is easily missed on neuroimaging.