A 38-year-old man presented with primary position upbeat nystagmus accompanied by peripheral neuropathy. The serum vitamin B12 level was low along with high plasma homocysteine level, indicating vitamin B12 deficiency. Cyanocobalamin supplementation showed partial clinical and electrophysiological improvement. Although brain magnetic resonance imaging did not show any abnormal intensity lesions, the electrophysiological findings suggested that a pontomedullary medial lesion was responsible for the upbeat nystagmus. To our knowledge, this is the first case of upbeat nystagmus with low serum vitamin B12. Physicians need to recognize the possibility of vitamin B12 deficiency as a cause of upbeat nystagmus.
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