Primary position upbeat nystagmus is a rare clinical finding. We report a patient with clinically isolated syndrome suggestive of multiple sclerosis who presented with primary position upbeat nystagmus. MRI revealed a demyelinating lesion in the lower medulla, which affected the nucleus intercalatus; this type of lesion inhibits the flocculovestibular inhibitory pathway, thereby causing upbeat nystagmus. Nystagmus still persisted after pulsed corticosteroid therapy. This could be due to a loss of central adaptation of the vestibulo-ocular system in our patient, because of more diffuse brainstem damage, shown on vestibular-evoked myogenic potentials as delayed latencies on both sternocleidomastoid muscles and a conduction block for the left extraocular muscles.
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