Fatal familial insomnia (FFI) is a very rare autosomal dominant inherited prion disease of the brain It is almost always caused by a mutation to the protein PrPc, but can also develop spontaneously in patients with a non-inherited mutation variant called sporadic fatal insomnia (sFI). FFI has no known cure and involves progressively worsening insomnia, which leads to hallucinations, delirium, and confusional states like that of dementia. The average survival span for patients diagnosed with FFI after the onset of symptoms is 18 months.
The mutated protein, called PrPSc, has been found in just 40 families worldwide, affecting about 100 people; if only one parent has the gene, the offspring have a 50% risk of inheriting it and developing the disease. With onset landing around middle age it is essential that any potential patient be tested. Otherwise one would realize they had FFI only after potentially passing it on to their children.