Rituximab-induced mild encephalopathy with a reversible splenial lesion syndrome (MERS): An adverse effect to add to the list.

Mild encephalopathy with a reversible splenial lesion syndrome (MERS) is a rare clinico-radiological entity. Rituximab (RTX)-induced MERS has never been described before. Herein, we report the case of a 33-year-old patient diagnosed since 2017, with an IgG4-related disease (IgG4-RD). This diagnosis was retained in the face of a prolonged fever, sicca syndrome, hepatic damage and renal pseudotumour associated to a high level of IGg4 at 2.8 g/L with suggestive renal histology. The patient was treated with corticosteroid therapy with persistence of renal impairment and nephrotic syndrome indicating RTX therapy. The patient received his first dose of RTX and presented neurological and respiratory impairments a few hours afterwards. An infectious investigation comprising a SARS CoV-2 PCR and viral PCRs (VZV, herpes and CMV) on cerebrospinal fluid (CSF) were negative. The HBV, HCV, HIV, Parvo B19, CMV, EBV, herpes, mycoplasma and syphilis serologies as well as Legionella antigenuria were also negative. The patient had received intravenous immunoglobulins (IVIG) and methylprednisone, associated with sodium valproate with favourable outcome. The diagnosis of MERS induced by RTX was retained in our patient according to clinical and radiological features. We herein report the first case of MERS following RTX in a patient treated for IgG4-RD.