medical Definition of fatal familial insomnia
: a rare fatal prion disease that is inherited as an autosomal dominant trait, that is marked by progressive neurodegenerative changes especially in the thalamus, and that tends to follow a clinical course exhibiting disturbances of the sleep cycle, intractable insomnia, motor disturbances (as ataxia and myoclonus), disturbances in autonomic nervous system functioning, dementia, coma, and death
