scellaneous: Huntington's disease, neoplasms, chronic subdural hematomas, Parkinson's disease, Cruetzfeldt-Jakob disease, AIDS, unknown cause5-20%Life Expectancy and Mortality EstimatesThe following summary is based on Terry's (1988) review of the Wang (1978) and Barclay et al. (1985) studies. The Wang study examined senile dementia (mean age of onset 71.3 years) and presenile dementia (mean age of onset 53.8 years) survival rates during the 1960s. Senile dementia patients survived on the average 6.0 years, close to half of the expected survival rate (11.1 years) of similarly aged non demented people. Presenile demented patients survived slightly longer an average of 6.9 years, against an expected survival of 22.3 years. The Barclay et al. Studies examined survival rates in AD and MID patients in the 1980s. The mean survival rates for AD and multi-infarct dementia were 8.1 and 6.7 years respectively. Interestingly, the survival rate of demented women on the whole is significantly higher than that of men. Terry (1988) suggests that the lower survival rate of demented men is due to a higher incidence of MID in men.Risk FactorsAgeAge is the biggest risk factor for developing dementia. According to a model proposed by Jorm et al. (1987) a doubling of the prevalence rate occurs every 5.1 years. (1987) For the elderly population aged 65 and above the prevalence of dementia is estimated at about 10%. Whereas in the very elderly it can reach up to 40% (Clarfield, 1989).GeneticsGenetic factors are important in some dementing diseases. In HD an autosomal dominant gene on chromosome 4 is directly responsible for the disease. The genetic evidence in AD is less conclusive. On the one hand there are studies (i.e., Breitner et al. , 1988) which have reported a cumulative risk of AD among relatives of patients approaching 50%, thus implying an autosomal dominant mode of transmission (Morris, 1994). But, on the other hand, genetically ...
