Unilateral internuclear ophthalmoplegia. The lack of inhibitory involvement in medial rectus muscle activity.

Inability to adduct the left eye beyond the midline was found in a patient after brain stem infarction. Pursuit movements, refixation saccades, and the vestibulo-ocular reflex were restricted to the temporal hemifield. Only near convergence led to an adduction of 15 degrees in the left eye. Nasally directed saccades were slow, and temporally directed saccades showed glissadic back-drift. Electromyography revealed a lack of burst activity in the left medial rectus; however, inhibition of the left (homolateral) and the right (contralateral) medial rectus muscles during off-saccades and in off-positions of gaze was normal. These observations suggest that inhibition for medial rectus motor neurons is not mediated by the medial longitudinal fasciculus. The findings are compatible with the neurophysiological data, which show that reductions of activity in the medial rectus motoneurons are largely caused by disfacilitation via internuclear neurons rather than by inhibition.