Objective:

To report a case of adult-onset opsoclonus-myoclonus-ataxia syndrome (OMAS) following West Nile virus infection, highlighting the diagnostic dilemma and recovery with immunotherapy.

Background:

Design/Methods:

We retrospectively reviewed the clinical presentation, diagnostic workup, and hospital course of a 69-year-old woman presenting with acute encephalopathy, abnormal ocular movements, and generalized myoclonus. Diagnostic evaluation included neuroimaging, cerebrospinal fluid analysis, and serologic testing. Management strategies and clinical response to immunotherapy were documented.

Results:

Neurologic examination demonstrated multidirectional opsoclonus, diffuse myoclonus, severe truncal and gait ataxia, and encephalopathy. Brain MRI revealed no acute pathology, and cerebrospinal fluid showed mild lymphocytic pleocytosis. Given the absence of structural findings, functional neurologic disorder was initially considered. Subsequent testing confirmed serum and CSF positivity for WNV IgM, establishing WNV neuroinvasive disease as the underlying etiology. The patient required ICU-level care for agitation and delirium. Treatment with high-dose intravenous methylprednisolone followed by intravenous immunoglobulin led to gradual improvement in opsoclonus, myoclonus, and mental status. At transfer to inpatient rehabilitation, she had persistent cognitive-linguistic deficits, urinary retention, and residual ataxia, but demonstrated progressive functional recovery.

Conclusions:

WNV-associated OMAS represents a rare but important post-infectious autoimmune encephalopathy in adults. This case illustrates the diagnostic dilemma posed by initial consideration of functional disorder and highlights the role of immunotherapy in promoting neurological recovery. Clinicians should maintain a broad differential for adult-onset OMAS, including infectious etiologies, to ensure timely recognition and management.