Fatal familial insomnia (FFI) is a rare hereditary prion disease characterised by progressive insomnia, autonomic dysfunction and neurodegeneration leading to death. It results from a specific mutation (D178N) in the PRNP gene encoding the prion protein, occurring on a methionine allele at codon 129. This mutation alters the folding of the prion protein, causing it to misfold and aggregate in the brain.
Pathogenesis and Clinical Features
Prion diseases are caused by conformational conversion of the normal cellular prion protein (PrPC) into a pathogenic β-sheet‑rich isoform (PrPSc) that can induce misfolding of additional PrPC molecules. In FFI, the mutated prion protein accumulates predominantly in the thalamus, a region that regulates sleep and autonomic function. Unlike classic spongiform encephalopathies, FFI shows minimal vacuolation but marked neuronal loss and reactive gliosis in the thalamic nuclei and inferior olives. The disease usually manifests in midlife with progressive insomnia accompanied by autonomic symptoms such as tachycardia, hyperhidrosis, hypertension and hyperthermia. Cognitive decline, ataxia and dysarthria develop as the disease progresses. Diagnostic indicators include reduced slow-wave sleep on polysomnography, loss of sleep spindles, thalamic hypometabolism on positron emission tomography and detection of the PRNP mutation. There is no effective therapy; supportive care and symptomatic treatment with sedatives or autonomic stabilisers provide limited relief. The median survival from symptom onset is about 12–18 months.
Notable Cases and Genetic Aspects
FFI was first described in an Italian family in the late 1980s, and more than 100 affected individuals from approximately 30 families worldwide have been documented. Penetrance is high, and all carriers eventually develop the disease. The sporadic counterpart, sporadic fatal insomnia, lacks the familial PRNP mutation but shares clinical and neuropathologic features. FFI exemplifies the pivotal role of thalamic dysfunction in sleep regulation and highlights the impact of single amino acid changes on prion protein conformation and disease phenotype. Genetic counselling and predictive testing are important for at-risk family members. Fatal familial insomnia underscores the diversity of human prion diseases and the devastating consequences of prion protein misfolding on sleep and autonomic regulation. Related Terms: Sporadic fatal insomnia, Creutzfeldt‑Jakob disease, Variably protease‑sensitive prionopathy, Kuru, Prion protein gene