A 71-year-old man with hypertension and alcohol use disorder presented to the clinic with a 3-month history of vertigo, diplopia, and ataxia. A neurologic examination showed ataxia, dysarthria, action tremor, and myoclonus of the proximal legs. Rapid, nonrhythmic, saccadic, and multidirectional ocular movements were present, a condition referred to as opsoclonus (see video). Results of initial laboratory tests, including thiamine tests, were normal. An analysis of the cerebrospinal fluid was negative for infection and showed 8 nucleated cells per microliter (reference value, ≤5), a protein level of 88 mg per deciliter (reference value, ≤35), and 5 oligoclonal bands (reference value, <4). The patient’s serum was positive for Ma2 IgG antibodies. Findings on magnetic resonance imaging of the head were unremarkable, and no evidence of cancer was found on fluorodeoxyglucose positron-emission tomography. A diagnosis of opsoclonus–myoclonus syndrome due to anti-Ma2 brain-stem encephalitis was made. Opsoclonus–myoclonus syndrome consists of opsoclonus, myoclonic jerks, and ataxia and may be associated with autoimmune conditions affecting the brain stem in adults. Treatment with glucocorticoids, plasma exchange, and rituximab was started, with initial abatement of the opsoclonus and myoclonus. The ataxia progressed, and despite having received additional therapy with cyclophosphamide, the patient was using a wheelchair 1 year after presentation.
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