PATHOMECHANISM OF MAMMALIAN DOWNBEAT NYSTAGMUS DUE TO CEREBELLAR LESION - A SIMPLE HYPOTHESIS

Most of the various hypotheses on the pathomechanism of the slight ocu lar upward drift in normal mammals and on the prominent downbeat nysta gmus following cerebellar lesions assume an inherent vertical asymmetr y in the central vestibuloocular pathways. In this paper we propose th at this vertical asymmetry is simply based on the anatomical orientati on of the six semicircular canals in the head which is right-left symm etrical but lacks symmetry in the cranio-caudal direction. Presuming t hat each semicircular canal elicits eye movements in a direction rough ly in its anatomical plane, vectorial addition of the tonic resting ac tivity of all six canals leads to a cancellation of horizontal and tor sional eye movement components but leaves an important vertical (slow phase) upward component. This peripheral vestibular bias is centrally cancelled by floccular and parafloccular inhibitory pathways which are related to the smooth pursuit system, but becomes disinhibited in the presence of posterior cerebellar lesions. (C) 1998 Elsevier Science I reland Ltd. All rights reserved.