Primary position upbeat nystagmus. Clinicopathologic study of four patients.

We report an electro-oculographic study (EOG) of upbeat nystagmus and neuroradiological correlations in 4 patients and neuropathological findings in 1 patient. All 4 patients revealed responsible lesions in the lower pontine tegmentum. The EOG data suggest that our patients had a deficit in vertical smooth eye movement balance. The nystagmus stopped or reversed direction during convergence or changes of head position. These EOG findings might be caused by vertical imbalance in the otolithic ocular reflex, superimposed on an imbalanced vestibulo-ocular reflex (VOR), secondary to a damage to the pontomedullary tegmentum.