The relationship of dementia prevalence in older

It has often been assumed that dementia occurs more commonly in the intellectual disability (ID) population than in the general population (Torr, 2005). Although it is now accepted that those with Down syndrome(DS) have a genetic predisposition for dementia related to the APP gene on chromosome 21, dementia may also be more common in the ID population who do not have DS (Cooper, 1997). Furthermore, it has been proposed that dementia in the ID population should occur at a younger age than is usual. Tredgold, a London physician during the first half of the previous century, asserted that ‘as would be expected,in most cases of primary amentia, [the] senile form of dementia sets in at an earlier age than the normal. It often begins to show itself in the fourth decade […], and the majority of aments who live much after this usually show definite and progressive mental deterioration ’ (Tredgold, 1952). Thompson (1951) believed the earlier age of decline to be related to arrested brain development. More recently, the cognitive reserve hypothesis has been proposed to explain how adults with similar brain insults may present with differing clinical pictures. It proposes that intelligence, education and occupational level can influence the occurrence and course of many central nervous system disorders Whalley et al. 2004). Stern (2002) proposed two components to cognitive reserve. The first comprises passive components such as brain size and synapse count or ‘hardware ’ of the brain, which differs between individuals. Proxies for it include measurements such as brain volume and pre-morbid intelligence (Staff et al. 2004). Active components or ‘software ’ of the brain are developed through educational, leisure and occupational activities that develop the use of different neuronal pathways (Stern,2003). The hypothesis assumes that there is a critical threshold of reserve capacity that needs to be breached by pathological processes before clinical or functional symptoms will develop. Those with more reserve have been found to be less likely to develop dementia or cognitive decline (Whalley et al. 2000 ; Verghese et al.2003 ; Valenzuela & Sachdev, 2006). Although these studies are consistent with the theory of cognitive reserve, none specifically studied participants in the ID (mental retardation) range of ability.
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