The one-and-a-half syndrome--a unilateral disorder of the pontine tegmentum: a study of 20 cases and review of the literature.

The one-and-a-half syndrome is a clinical disorder of extraocular movements characterized by a conjugate horizontal gaze palsy in one direction plus an internuclear ophthalmoplegia in the other. The syndrome is usually due to a single unilateral lesion of the paramedian pontine reticular formation or the abducens nucleus on one side (causing the conjugate gaze palsy), with interruption of internuclear fibers of the ipsilateral medial longitudinal fasciculus after it has crossed the midline from its site of origin in the contralateral abducens nucleus (causing failure of adduction of the ipsilateral eye). Twenty cases are reported; 14 had multiple sclerosis.