An unusual variant of the dorsal midbrain syndrome in MS: clinical characteristics and pathophysiologic mechanisms.

Patients with MS exhibit a broad diversity of ocular motor syndromes. We describe a patient with relapsing-remitting MS who developed an unusual variation of the dorsal midbrain syndrome, characterized by monocular convergent-retraction nystagmus in the right eye, accompanied by divergent-retraction nystagmus in the fellow eye upon attempted upward gaze. Examination also revealed a skew deviation with a left hyperdeviation and severe adduction limitation in the left eye during attempted right gaze. We propose that a left INO accounted for the inability of the left eye to adduct (and result in convergent-retraction) during attempted upward saccades. We consider the pathophysiologic mechanisms responsible for our observations and review important details of the dorsal midbrain ocular motor circuitry.