Steroid-responsive inflammatory leptomeningeal encephalitis temporally associated with lecanemab therapy: A case report

Lecanemab is an anti–amyloid-β monoclonal antibody used for the treatment of early Alzheimer's disease. Herein, we report a 46-year-old woman who developed headache, fever, disorientation, and impaired consciousness 10 months after starting lecanemab. Magnetic resonance imaging revealed a small focal subcortical fluid-attenuated inversion recovery hyperintense lesion and bilateral occipital leptomeningeal enhancement without detectable hemorrhagic findings on T2*-weighted gradient-recalled echo imaging. Cerebrospinal fluid revealed mononuclear pleocytosis and markedly elevated interleukin-6. Infectious studies and neuronal surface antibodies were negative. High-dose corticosteroid therapy led to rapid clinical improvement. These findings support a diagnosis of steroid-responsive inflammatory leptomeningeal encephalitis temporally associated with lecanemab.