(See also Overview of Prion Diseases.)
Fatal familial insomnia (FFI) results from an autosomal dominant mutation in the PrP gene. Average age at onset is 40 years (ranging from the late 20s to the early 70s). Life expectancy is 7 to 73 months. Early symptoms of FFI include increasing difficulty falling asleep and maintaining sleep, as well as cognitive decline, ataxia, and psychiatric symptoms. Sympathetic hyperactivity (eg, hypertension, tachycardia, hyperthermia, sweating) may occur later.
Sporadic fatal insomnia (sFI) lacks a PrP gene mutation. Average age at onset is slightly older and life expectancy is slightly longer than in FFI. Early symptoms include cognitive decline and ataxia. Sleep abnormalities are not commonly reported but can usually be observed during a sleep study.
Fatal insomnia should be considered as a rare possibility when patients have rapidly progressive cognitive impairment accompanied by behavioral or mood changes, ataxia, and sleep disturbances. Suspicion of FFI or sFI should prompt a sleep study by polysomnography. Genetic testing can confirm the diagnosis of the familial form. MRI and measurement of 14-3-3 protein and tau in cerebrospinal fluid (CSF) are not useful, but polysomnography and positron emission tomography (PET), which shows thalamic hypometabolism, can confirm the diagnosis.
There is only supportive treatment for fatal insomnia.