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Finding vertical nystagmus, especially on gaze straight ahead, alerts the neurologist for other signs of cerebellar or brainstem dysfunction (see the supplementary video clip on the Neurology Web site; go to www.neurology.org). Downbeat nystagmus (DBN) can be due to Chiari malformation,1 hereditary spinocerebellar ataxia, long-term lithium therapy or compression of the caudal brainstem by an ectatic vertebral artery.2 DBN is uncommon with vascular or demyelinating lesions.2 In perhaps half of all patients with DBN, no cause is found and the condition progresses slowly, if at all.1 The diagnostic value of DBN owes much to the development of animal models, such as monkeys with bilateral ablation of the cerebellar flocculus and paraflocculus,3 and mice with mutations of the CACNA1 calcium channel gene.4
The neurobiology of upbeat nystagmus (UBN) is less clear; there is no animal model and our understanding of it is based solely on clinical-pathologic studies, which report lesions of the perihypoglossal and medial vestibular nuclei, nucleus intercalatus, and the ventral tegmentum.2 Interestingly, some patients with …
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