To report a case of opsoclonus-myoclonus syndrome after Powassan virus meningitis.

Opsoclonus-myoclonus syndrome is rare in adults and has been associated with viral infection, including West Nile virus (WNV).  Powassan virus (PWV) is a tick-borne flavivirus causing a spectrum of disease similar to other flaviviruses such as WNV, which is mosquito-borne. Most human cases in the United States occur in the northeastern and Great Lakes regions from Spring through Fall when ticks are most active. 

A 44-year-old man from Minnesota recovering from a meningitic syndrome characterized by fever, headache, and lymphocytic pleocytosis (860 cells/μL) presented with oscillopsia and gait instability. He reported a history of recent tick exposure. Exam demonstrated opsoclonus most prominent after gaze refixation, myoclonus, prominent tremor and dysmetria of the upper extremities, truncal ataxia, and a wide-based ataxic gait. Magnetic resonance imaging brain was normal. Lyme serology and serum testing for autoimmune/paraneoplastic autoantibodies were unremarkable. Serologic testing detected IgM antibodies against WNV in serum and cerebrospinal fluid. Given history of tick exposure, PWV serology was also performed and demonstrated the presence of IgM antibodies against the virus in both serum and cerebrospinal fluid. Confirmatory plaque reduction neutralization test (PRNT), however, detected neutralizing antibodies against PWV only. He received 5 days of 1g methylprednisolone and 2g/kg intravenous immune globulin, and his symptoms slowly resolved over three months.

Opsoclonus-myoclonus is uncommon in adults and may be a rare complication of arthropod-borne viral infections, including PWV. Clinicians should consider testing for PWV in patients from endemic regions presenting with opsoclonus-myoclonus and potential tick exposure.