JC polyomavirus granule cell neuronopathy in a patient treated with rituximab.

IMPORTANCE Progressive multifocal leukoencephalopathy results from lytic infection of the glia by the JC polyomavirus (JCV); JCV granule cell neuronopathy is caused by infection with a mutated form of JCV, leading to a shift in viral tropism from the glia to cerebellar granule cells. This shift results in a clinical syndrome dominated by progressive cerebellar dysfunction that might elude standard diagnostic workup strategies for ataxia. OBSERVATIONS We present the case report of a patient receiving long-term rituximab therapy who developed progressive cerebellar ataxia and marked isolated cerebellar degeneration. This syndrome resulted from JCV granule cell neuronopathy associated with a novel JCV mutation. CONCLUSIONS AND RELEVANCE New onset or worsening of isolated cerebellar ataxia in patients being treated with rituximab or natalizumab warrants early assessment for JCV infection.