Midbrain paresis of horizontal gaze.

Unilateral paramedian involvement of the midbrain tegmentum causes monocular paralysis of adduction in the ipsilateral eye, paresis of contralateral saccades in the opposite eye, and conjugate paresis of ipsilateral smooth pursuit. The adduction paralysis can be nuclear, or internuclear from a lesion in the medial longitudinal fasciculus. This distinctive midbrain syndrome of horizontal gaze paresis is exemplified by means of quantitative infrared oculographic, radiological, and neuropathological correlation in two patients with predominantly paramedian midbrain tumors involving the mesencephalic reticular formation and the oculomotor nucleus. Binocular paralysis of elevation provided evidence that one human oculomotor nucleus contains axons to both superior rectus muscles, as does the simian oculomotor nucleus. The midbrain tectum was spared. These pathophysiological correlations indicate that the mesencephalic reticular formation contains pathways that control contralateral saccades and ipsilateral smooth pursuit.