Paresis of contralateral smooth pursuit and normal vestibular smooth eye movements after unilateral brainstem lesions.

Pursuit and vestibular smooth eye movements were measured in patients with lesions of the caudal brainstem tegmentum identified by magnetic resonance imaging (MRI) and computed tomography (CT), with neuropathological correlation in 1 patient. Contralateral smooth pursuit gain was significantly lower than ipsilateral gain in each patient. Ipsilateral smooth pursuit gain was also subnormal in patients with unilateral pontine damage that caused slowing of ipsilateral saccades. Horizontal vestibulo-ocular reflex gain and phase were normal. These quantitative correlations indicate that lesions of the pontine tegmentum that paralyze ipsilateral saccades can spare the vestibulo-ocular reflex, and that smooth pursuit movement and the vestibulo-ocular reflex can be impaired independently by pontine or medullary lesions. In contrast to lesions at other sites, unilateral lesions of the pontine or medullary tegmentum impair contralateral smooth pursuit more than ipsilateral pursuit movements. These findings provide evidence that a double decussating pathway mediates smooth pursuit; the first decussation is from the pons to the cerebellum, and the second decussation is from the vestibular nucleus to the contralateral abducens nucleus.