From OMIM
Fatal familial insomnia is a prion disorder showing autosomal dominant inheritance. It is clinically characterized by insomnia with or without a diurnal dreaming state, hallucinations, delirium, and dysautonomia preceding motor and cognitive deterioration. FFI is specifically associated with the asp178-to-asn mutation of the PRNP gene (D178N; 176640.0010) when the amino acid at position 129 is methionine (M129V; 176640.0005). The D178N mutation and the val129 allele results in Creutzfeldt-Jacob disease (CJD; 123400) (see 176640.0007) (Goldfarb et al., 1992). CJD typically presents with dementia, ataxia, myoclonus, and other abnormal movements; however, there is considerable clinical and pathologic overlap between FFI and CJD, and some individuals with D178N and met129 may present with a phenotype suggestive of CJD. Thus, FFI and CJD may be viewed as extremes of a phenotypic spectrum (summary by Zarranz et al., 2005).  http://www.omim.org/entry/600072

From MedlinePlus Genetics
Prion disease represents a group of conditions that affect the nervous system in humans and animals. In people, these conditions impair brain function, causing changes in memory, personality, and behavior; a decline in intellectual function (dementia); and abnormal movements, particularly difficulty with coordinating movements (ataxia). The signs and symptoms of prion disease typically begin in adulthood and worsen with time, leading to death within a few months to several years.  https://medlineplus.gov/genetics/condition/prion-disease