[A case report of leptomeningeal amyloidosis presenting with meningoencephalitis-resembling manifestations in a 54-year-old woman].

We report a 54-year-old woman previously healthy who developed acute fever, headache, and impaired consciousness. We administered corticosteroid therapy as autoimmune mediated meningoencephalitis, which improved her symptoms and reduced cerebrospinal IL-6 level, but her cognitive impairment persisted. Contrast-enhanced MRI showed diffuse meningeal enhancement lesion, which led us to the possibility of leptomeningeal amyloidosis (LA). We performed transthyretin gene analysis identified heterozygosity for the c.265T>C, p.Y89H (Y69H) variant, which is known as one of the causative mutations of familial LA. Given the responsiveness to steroid treatment, it is possible that the inflammatory pathology related to amyloid deposition in leptomeninges with encephalitis-like symptoms contributed to the prolonged duration of symptoms lasting from hours to days.