The localizing value of downbeat nystagmus.

Downbeat nystagmus (DBN) is a primary position nystagmus with the fast phase in a downward direction. It is a rare but distinctive disorder of ocular motility and usually localizes the lesion at the posterior fossa. Four patients with DBN were seen in the department. One had a medullary glioma, and another congenital basilar invagination. The other two were initially diagnosed as demyelinating disease. One was subsequently found to have Arnold Chiari Malformation on magnetic resonance imaging (MRI). Review of the literature showed that cerebellar ectopia (Arnold Chiari Malformation) is the commonest cause of DBN. However 1/3 of reported cases have no obvious cause. DBN is of such high localizing value that we recommend MRI of cervicomedullary junction for all patients with DBN to exclude cerebellar ectopia or medullary lesion.