neurologic disorders
Internuclear Ophthalmoplegia
Key Points
Internuclear ophthalmoplegia results from a lesion in the medial longitudinal fasciculus, which coordinates abduction of one eye with adduction of the other. Common causes are multiple sclerosis in young people (often bilateral) and stroke in older people (typically unilateral). Distinguish internuclear ophthalmoplegia (which impairs adduction of the ipsilateral eye but not convergence) from 3rd cranial nerve palsy (which impairs adduction and convergence of the ipsilateral eye). One-and-a-half syndrome is a rare disorder in which a lesion affects the horizontal gaze center and the medial longitudinal fasciculus on the same side; the eye on the affected side cannot move horizontally to either side, but the other eye can abduct; convergence is unaffected. 