Downbeat nystagmus caused by a paramedian ponto-medullary lesion.

Downbeat nystagmus (DBN) is rarely caused by lesions in sites other than the cerebellar (para-)floccular lobe. We describe a case of DBN secondary to a hemorrhaged venous cavernoma at the pontomedullary junction. This case provides new insights into the neuro-anatomical substrate of DBN. We propose that DBN arises from lesions in a brainstem-cerebellar feedback loop, which comprises cells of the pontine paramedian tract (PMT).