Downbeat nystagmus associated with dolichoectasia of the vertebrobasilar artery.

We reviewed the case records and radiographic studies of 41 patients with primary position downbeat nystagmus seen during a 5-year period to investigate whether a relationship existed between dolichoectasia of the vertebrobasilar artery and cases of unknown cause. The cause of downbeat nystagmus could not be determined in 12 cases (29%). Two of these cases had dolichoectasia of the vertebrobasilar artery as the only identifiable abnormality. One other case had a dolichoectatic vertebrobasilar artery, but other potential etiologic factors for the development of downbeat nystagmus were present. Compression of the caudal brain stem by an enlarged and tortuous vertebrobasilar arterial system may be the cause of downbeat nystagmus in some cases unassociated with more commonly recognized causes.