Lid nystagmus as a sign of intrinsic midbrain disease.

A 6-year-old boy with signs and symptoms of ocular myasthenia gravis had lid nystagmus evoked by horizontal gaze. MR imaging demonstrated an intrinsic midbrain lesion, which was diagnosed by biopsy as a low-grade astrocytoma. In the setting of ocular myasthenia gravis, the finding of lid nystagmus may serve as a useful clinical sign of intrinsic midbrain disease.