People with learning disabilities are at a greater risk of developing dementia than other people. People with Down's syndrome have a genetic predisposition for dementia, related to the APP gene on chromosome 21 but not all people with learning disabilities have Down's syndrome. The cognitive-reserve theory holds that everyone starts out with different levels of brain power. Dementia reduces this but only when the brain's capacity falls below a certain level do the symptoms of dementia appear. This would explain why people with higher IQs have a smaller risk of dementia; the same processes of decline are going on but they have enough spare capacity to cope without symptoms developing. According to this theory people with learning disabilities should develop dementia symptoms earlier as they have less capacity to begin with. 281 adults over 60, and with learning disabilities, took part in a study organised by researchers at University College Medical School. The study found that the rate of dementia was 2.77 times as high in this group as among the general population but that there was little difference in the rates of dementia between those with mild, moderate and severe learning disabilities. Younger people with learning disabilities were also at a higher risk of developing Alzheimer's disease than people of the same age without learning disabilities.
Strydom, A. ... [et al] - The relationship of dementia prevalence in older adults with intellectual disability (ID) to age and severity of ID Psychological Medicine January 2009, 39(1), 13-21