Abstract

This report presents the 4th documented case worldwide of herpes simplex encephalitis in multiple sclerosis (MS) patients on natalizumab and the first case in the UK. Natalizumab is indicated for relapsing remitting multiple sclerosis in patients with high disease activity despite treatment with β-interferon and patients with rapidly evolving severe, multiple sclerosis. It works as a monoclonal antibody targeted against α-4 integrin with the proposed mechanism of attenuating the migration of immune cells into the central nervous system. Reactivation of the JC virus causing progressive multifocal leucoencephalopathy and its association with natalizumab is well documented. This case adds weight to the suggestion that natalizumab also increases the reactivation risk of herpes simplex infection. A 34-year-old woman was admitted with a generalised tonic-clonic seizure, fever and confusion following her 40th infusion of natalizumab. Her relapsing remitting MS was previously treated with β-interferon but was discontinued due to intolerable side effects. MRI and EEG demonstrated increased temporal lobe activity and PCR confirmed herpes simplex virus in the cerebrospinal fluid. The patient made an excellent recovery after 21 days of intravenous acyclovir therapy followed by 14 days of oral treatment. It is important to recognise that patients treated with Natalizumab are not only at risk of PML and early initiation of IV acyclovir should always be considered upon such presentation.