This is usually sporadic, but is occasionally familial. It is due to the same codon 178 mutation as in fatal familial insomnia, but polymorphism at other codons, such as 129, leads to a different phenotypic expression of the genetic abnormality. The cerebral cortex is usually predominantly affected, but in a subtype of the disease there is extensive thalamic atrophy. In these subjects insomnia is a prominent feature, in addition to the dementia and motor abnormalities. There is a progressive loss of both NREM and REM sleep with absence of spindles and K-complexes, and eventually there is no recognizable NREM or REM sleep.
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