Natalizumab-related PML 2 weeks after negative anti-JCV antibody assay

Progressive multifocal leukoencephalopathy (PML) is a serious complication of natalizumab pharmacotherapy, caused by the JC virus (JCV), against which 60% of adults have detectable serum antibodies.1 The most common symptoms of PML are cognitive impairment, motor dysfunction, visual abnormalities, and speech deficit.2 Although the mortality rate of 23%3 in natalizumab-associated cases is lower compared to PML from other causes, most survivors have a poor functional outcome.4 As of March 3, 2015, Biogen had reported 541 cases of natalizumab-related PML. Of 278 cases with available data, only 2 were negative for anti-JCV antibody; these 2 patients had tests dating from 8 and 9 months prior to diagnosis.3 Acknowledgment: The authors thank Dr. David Clifford, Washington University at St. Louis, Missouri, for therapeutic management advice; the patient and her family; and Hillary Lipe and Deb Cramer, University of Washington, Seattle, for clinical support.