Fatal familial insomnia presenting with agrypnia excitata and very low atonia index level

Rationale: Fatal familial insomnia (FFI) is a human prion disease that is characterized by sleep-wake cycle deterioration, loss of slow-wave sleep, and motor overactivation over the daily 24-hour period. Patient concerns: Here, we report the case of a 57-year-old man who had an irregular sleep-wake cycle and exhibited frequent movements and vocalizations during sleep. Diagnoses: Video-polysomnography showed disrupted sleep structure, rapid alternation between sleep stages, and an absence of sleep spindles and slow-wave sleep. Moreover, body movements persisted throughout the entire sleep period, including rapid eye movement (REM) sleep. The atonia index was very low (<0.025) during REM sleep. Genetic testing revealed a prion protein gene mutation at codon 178, and the patient was diagnosed with FFI. Interventions: We tried to treat with amantadine, doxycycline, and immunotherapies, but the disease progressed. Outcomes: Sleep disturbance is the most frequent and essential symptom of FFI. Lessons: FFI is difficult to diagnose due to the low sensitivity of diagnostic tools. Diagnoses can be further supported by better knowledge of typical polysomnographic findings.