Upbeat nystagmus influenced by posture: report of two cases.

Two cases of vertical gaze nystagmus, which were influenced by posture are reported. In both cases, upbeat nystagmus (UBN) on forward gaze in upright and supine positions changed to downbeat nystagmus (DBN) in prone position. Convergence had no influence on the vertical nystagmus. A CT scan revealed an atrophy of the anterior vermis of the cerebellum in the first case. The UBN of the second case was suspected to be associated with lesions in the brachium conjunctivum. The authors concluded that both UBN and DBN were not completely separate, but they could coexist in cases with certain lesions in the posterior cranial fossa.