C9orf72 Gene-Associated Frontotemporal Dementia Mimicking Autoimmune Pathology.

Introduction: The C9orf72 mutation can manifest in diverse clinical ways, including rapid cognitive decline, parkinsonism, or late-life neuropsychiatric symptoms, sometimes mimicking autoimmune encephalitis.

Case Report: A 64-year-old female presented to the autoimmune neurology clinic with rapidly progressive dementia (RPD) associated with episodes of headache, confusion, auditory hallucinations, and abnormal electroencephalogram. She was treated empirically at an outside hospital for possible autoimmune encephalitis with intravenous methylprednisolone, but there was no improvement, and rapid cognitive decline continued.