Abstract Background Fatal familial insomnia (FFI) is a rare autosomal dominant inherited disease and a type of prion diseases. We report a case of fatal familial insomnia (FFI) in a 52-year-old man who was initially misdiagnosed as Alzheimer’s disease. Case presentation The patient presented with persistent insomnia as the initial symptom, accompanied by cognitive impairment, autonomic dysfunction, and disorders of voluntary movement. Cerebrospinal fluid analysis revealed a decrease in Aβ1−40 levels and an increase in total tau protein. Cranial imaging demonstrated bilateral hippocampal atrophy, while long-term video electroencephalography indicated focal abnormalities. The patient’s prion protein gene was D178N/129MM type, confirmed the diagnosis of FFI. Conclusions The key characteristics of FFI include insomnia and rapidly progressive dementia, its differential diagnosis with AD has been extensively discussed in clinical practice. This is the first report of FFI concerning Aβ and tau protein, raises the awareness that the ratio of p-tau/t-tau in cerebrospinal fluid can provide valuable diagnostic clues for FFI.
