Nystagmus

An 86-year-old woman presented with acute, painless, “crossing out” of the left eye. She denied any other systemic or neurological symptoms. Past medical history was significant for coronary artery disease, hypertension, congestive heart failure, and hypothyroidism. The remainder of her medical, surgical, social, and family history was unremarkable. Best-corrected visual acuity was 20/25 in both eyes (OU). The pupils measured 4 mm in the dark and 2 mm in the light OU, with no relative afferent pupillary defect (RAPD). Extraocular motility exam demonstrated full ocular motility in the right eye (OD) with a bilateral, conjugate upbeat nystagmus in primary position and a dissociated monocular horizontal abducting nystagmus OD with an adduction deficit in the left eyes (OS) on attempted gaze to the right. There was a 40-prism-diopters exotropia (XT) OS in primary gaze. Slit lamp examination, intraocular pressure measurements, and dilated funduscopic exam were normal OU. Magnetic resonance imaging (MRI) of the brain with and without contrast is shown in Fig. 19.1.