Upbeat nystagmus: clinicopathological and pathophysiological considerations.

We describe an electro-oculographic study of upbeat nystagmus in 4 patients, with neuropathological correlation in one. All patients had lesions in the pontine tegmentum. The electro-oculographic data may be explained by imbalanced vertical vestibular or smooth pursuit eye movement control. The nystagmus stopped or reversed direction during convergence or in supine head positions. We propose that changes in the intensity or direction of upbeat nystagmus that are induced by convergence or changes in head position, are caused by vertical imbalance in the otolithic-ocular reflex, when superimposed on an imbalanced vestibulo-ocular reflex (VOR). Imbalance of the otolithic-ocular reflex and the vertical VOR are caused by damage in the pontomedullary tegmentum.