Definition of Fatal familial insomnia

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Fatal familial insomnia: Abbreviated FFI. An hereditary prion disease characterized by disrupted sleep (insomnia), motor abnormalities (myoclonus, ataxia, dysarthria, dysphagia, and pyramidal signs), and hyperactivation of the autonomic nervous system. Due to a missense mutation at codon 178 of the prion protein gene on chromosome 20. A sporadic form of fatal insomnia is also known.