PARALYSIS of both medial rectus muscles on attempted conjugate lateral gaze without other evidence of third nerve paralysis is an entity known as internuclear ophthalmoplegia. It is relatively common and almost invariably due to multiple sclerosis. Paralysis of one medial rectus muscle on attempted conjugate lateral gaze is much less common. It is the purpose of this communication to present eight cases of the unilateral syndrome and to describe the anatomic lesions in one. The latter is the first case of unilateral internuclear ophthalmoplegia in which a pathologic study has been carried out. In only a single case of the bilateral syndrome has postmortem examination been made.1
Internuclear ophthalmoplegia is presumed to result from a lesion of the medial longitudinal fasciculus with consequent interruption of the impulses from a center in the medulla on one side to the medial rectus nucleus on the opposite side. The syndrome has been