Aseptic meningoencephalitis during nusinersen therapy in a patient with type III spinal muscular atrophy: a case report.

Nusinersen, an intrathecally-administered antisense oligonucleotide for the treatment of spinal muscular atrophy (SMA), may rarely cause mild aseptic meningitis early in treatment. We report a severe late-onset aseptic brain stem meningoencephalitis in a 42-year-old man with type III SMA, occurring 38 months after starting nusinersen. The patient showed clinical and radiological signs of brain stem meningoencephalitis with significant CSF neutrophilic pleocytosis, despite negative infectious disease tests. After high-dose steroids and tapering, symptoms and imaging findings resolved. This case suggests a potential link between nusinersen and aseptic meningoencephalitis in adult SMA, extending previous reports of rare drug-induced aseptic meningitis. Physicians should consider this as a possible adverse event in patients undergoing intrathecal nusinersen therapy.