A case of progressive multifocal leukoencephalopathy under dimethyl fumarate treatment without severe lymphopenia or immunosenescence

Background: Progressive multifocal leukoencephalopathy (PML) is the main safety concern for dimethyl fumarate (DMF) treatment in persons with multiple sclerosis (pwMS). Risk stratification under DMF is currently based on age above 50 years and prolonged lymphopenia below 500 cells/mu L. Objective: To report a case of PML under DMF without severe lymphopenia or immunosenescence. Methods: Case report. Results: A 39-year-old female pwMS developed DMF-associated oligosymptomatic PML. The patient had not experienced any repeated lymphocyte counts below 800 cells/mu L and was 15 years younger than previously described cases. Conclusion: Despite risk stratification, vigilance for PML is advised in all pwMS under DMF. Severe CD8-lymphopenia is a common feature of all published DMF-associated cases.