Present an unusual Opsoclonus-Myoclonus Syndrome (OMS) case secondary to West Nile Virus (WNV) encephalitis.

OMS is a rare disorder that presents with arrhythmic multidirectional conjugate eye movements, diffuse or focal myoclonus, encephalopathy, and ataxia. It is mainly associated with pediatric neoplasms and adult paraneoplastic syndromes. However, it is vital to consider para-infectious causes as well. We present a compelling case of WNV encephalitis and cerebellitis causing OMS.

We identified this patient during routine clinical practice.

A 65-year-old man presented to the hospital with a four-day history of confusion, anomic aphasia, ataxia, and diplopia after sustaining numerous tick bites while camping. His condition continued to worsen despite treatment, later developing opsoclonus and bilateral upper extremity myoclonus. Initial brain and spine magnetic resonance imaging (MRI) revealed no significant findings. Cerebrospinal fluid (CSF) tests were obtained and included infectious, autoimmune, paraneoplastic, and demyelinating etiologies. WNV PCR yielded negative results. However, subsequent testing for WNV antibodies, including IgM and IgG, was positive, leading to the diagnosis of WNV encephalitis. A repeat brain MRI showed patchy, T2 flair hyperintense lesions of bilateral cerebellar hemispheres, suggesting cerebellitis. Treatment with corticosteroids and intravenous immunoglobulin (IVIG) led to the resolution of both opsoclonus and myoclonus, and improved mental status and gait.

OMS is a rare constellation of symptoms with multiple etiologies, including but not limited to paraneoplastic, autoimmune, and in our case, para-infectious causes. Though the pathophysiology is debated, the most supported theory lies within immune mediated dysfunction of Purkinje cells (PCs) in the cerebellum, which is supported by improvement with immunosuppressive therapy. Due to its rarity, identification of OMS may be delayed in the clinical setting. We aim to increase awareness, enabling earlier identification of the syndrome and prompt initiation of immunosuppressive therapy to optimize patient outcomes.