Herpes simplex virus encephalitis temporally associated with dimethyl fumarate-induced lymphopenia in a multiple sclerosis patient

Background Dimethyl fumarate (DMF) is approved as first line therapy for relapsing-remitting multiple sclerosis (RRMS). In some (3%) patients, DMF induces a marked lymphopenia. Herpes simplex encephalitis (HSE) may occur in lymphopenic subjects under treatment with immune-suppressive drugs. Case presentation We report a case of a 39-year-old female patient with RRMS that developed HSE temporally associated with a marked and sudden drop in lymphocyte count, from 1200/µl to 600/µl, in the peripheral blood. HSE DNA was demonstrated in the cerebrospinal fluid. HSE had the features that characterize HSE occurring in immunosuppressed subjects, i.e. less prominent CSF pleocytosis, bilateral and mainly cortical involvement and less extensive tissue necrosis. Antiviral therapy determined a progressive, although incomplete, improvement. Three months later the patient presented only a mild short-term memory deficit and sporadic episodes of inappropriate emotionality. Lymphocyte count returned to normal values (1120/µl) after DMF discontinuation. Conclusion Our case of HSE in a lymphopenic DMF-treated RRMS patient, points out the necessity of further studies on DMF-related lymphopenia, especially whether it implies an impaired immunity against viruses.