Selective Impairment of Downward Gaze Holding Observed in One-and-a-Half Syndrome

Abstract

A 43-year-old man with pontine cavernous hemangioma in the left dorsal pons showed left facial nerve palsy and right hemiparesis and paretic pontine exotropia of the right eye. His conjugate horizontal eye movements were restricted except for abduction of the right eye. His convergence, however, was preserved. His eye movements were investigated using DC-EOG and magnetic search coil methods. Abduction of the right eye was hypermetric and followed by dissociated nystagmus, indicating left internuclear ophthalmoplegia. Downward visually guided saccades were hypermetric and followed by exponential drifts with an average time constant of 0.9 s. Leaky integration was not observed after upward saccades. Upward smooth pursuit was well preserved, while downward smooth pursuit was impaired. Impairment of neural integration for downward eye movements may accompany one-and-a-half syndrome. We speculate that downward and upward velocity-to-position integration signals may flow separately in the medial longitudinal fasciculus and its vicinity.

KEYWORDS:

• One-and-a-half syndrome
• neural integration
• vertical eye movements
• medial longitudinal fasciculus (MLF)