Flaccid Brachial Monoplegia As Initial Presentation in a Patient With Progressive Multifocal Leukoencephalopathy.

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease caused by activation of John Cunningham virus (JCV) replication in the setting of impaired cellular immunity. A positive polymerase chain reaction (PCR) assay for JCV DNA in the cerebrospinal fluid (CSF) in conjunction with clinical findings and neuroimaging are diagnostic of PML. A false negative JCV PCR in the CSF may occur, necessitating PML confirmation by brain biopsy. We describe the unique clinical profile of a patient with no prior history of immunocompromise, referred to us for electrodiagnostic evaluation, who initially presented with rapidly progressive weakness of the right upper extremity. The unusual pattern of motor weakness suggested a conduction block or disconnection at the subcortical level. The patient was later diagnosed with atypical small cell lymphocytic lymphoma although not treated with monoclonal antibodies or other forms of chemotherapy. The CSF was negative for JCV, and PML was subsequently confirmed by brain biopsy. This case illustrates an uncommon presentation of PML and highlights the need for a high index of suspicion to diagnose PML.