West Nile Virus (WNV) encephalomyelitis is a mosquito-borne illness, now endemic to the Northeast United States, that is known to commonly have neuromuscular manifestations. Typically, this presents as a poliomyelitis-like flaccid monoparesis or quadriparesis. Acute brachial diplegia is a rare presentation of WNV infection.  

A 50-year-old man from Connecticut presented with eight days of fever, night sweats, and myalgia. A day later, he developed sudden onset of painless, flaccid paralysis of the upper extremities with intact sensation.

WNV infections are primarily asymptomatic; only a minority progress to febrile illness or develop neuroinvasive disease. Neuroinvasive disease typically presents with meningitis or encephalitis, with about 50% of patients developing flaccid paralysis described as quadriparesis or monoparesis. This patient was highly atypical as evidenced by his hyperacute brachial diplegia. The diffuse enhancement of the cauda equina in this case, suggested inflammatory involvement of the ventral nerve roots in addition to loss of cervical motor neurons/anterior horn cells. The dual pathology may explain why he had such severe weakness in the upper extremities with sparing of the lower extremities, both on clinical exam and the EMG. Further studies are needed to reconcile the discrepancy between clinical, electrodiagnostic and radiographic findings.