Alzheimers disease has been linked to which cause of intellectual disabilities?

People with intellectual disability are at a higher risk of developing dementia than the general Australian population (Evans & Troller 2018). Down syndrome in particular carries a higher risk of dementia, specifically for Alzheimer’s disease (Evans & Troller 2018; see Box 13.2 for more information). Less research has been undertaken on dementia in Australians with other intellectual and physical disabilities, but the findings show the onset of dementia in people with intellectual disabilities (other than Down syndrome) is on average 10 years younger than the general population (Evans & Troller 2018).

Box 13.2: Down syndrome and Alzheimer’s disease

Down syndrome, or trisomy 21, is a congenital condition where a person is born with extra genetic material – an additional copy of chromosome 21 (Dementia Australia 2021). Usually people are born with 46 chromosomes (23 pairs) in most of their cells, but people with Down syndrome are born with 47 chromosomes. The additional genetic material causes developmental and health issues in individuals, presenting as Down syndrome.

As advances in health care and knowledge have increased the average life span of individuals with Down syndrome, a relationship between Down syndrome and Alzheimer’s disease has become increasingly apparent. Autopsy studies have shown that by the age of 40, almost all individuals with Down syndrome have the physiological hallmarking of Alzheimer’s disease (this includes significant levels of beta-amyloid plaques and tau tangles) (Alzheimer’s Association 2021). However, it is important to note that not all individuals who present with the physiological markers of Alzheimer’s disease will display symptoms.

There are limited national statistics on people with both dementia and Down syndrome. However, some information is available for this population group within permanent residential aged care and mortality data:

• In 2019–20, 62% of people with Down syndrome living in permanent residential aged care also had dementia recorded as a condition contributing to their care needs (Table S13.23). In contrast, dementia was recorded among 54% of residents who did not have Down syndrome.
• In 2020, for those under the age of 65 years who had dementia listed as an associated cause of death, Down syndrome was the leading underlying cause of death (Table S13.24), accounting for 28% of these deaths.

Note, there are limitations with these data. The ACFI (see Box 10.3 in Residential aged care for information on this tool) only collects up to 3 behavioural conditions and 3 medical conditions that impact a persons’ care needs. Therefore, people with multiple health conditions may not have all of their conditions listed. Furthermore, dementia may not be recorded as an associated cause of death among people with Down syndrome who also had dementia. This is because as associated cause of death is only recorded if it directly contributes to the underlying cause of death. This may mean early stages of dementia in people with Down syndrome are not noted. 

Further explorative studies are needed to accurately quantify the relationship between the two conditions. Developments in data linkage studies that capture both dementia diagnosis and a person’s detailed health history may contribute towards furthering our understanding of the relationship between Down syndrome and dementia.

While the strongest risk factor for dementia in people with intellectual disabilities is having Down syndrome, other factors found to increase the risk of dementia include:

• poor physical and mental health, in particular depression and epilepsy
• poor cardiovascular health, which can be common in this group
• a high rate of sensory impairments, including vision and hearing loss, and undiagnosed impairments (Evans & Troller 2018; Prasher et al. 2016).

There are also many factors known to increase the risk of dementia in the general population that are common in people with intellectual disability, such as poor diet and exercise, poorer social, employment and education engagement and head injury (Evans & Troller 2018).

While this section focuses on the relationship between intellectual disability and dementia, it should be noted that dementia may impact anyone who is managing a pre-existing health condition. The additional needs of those living with disability, be it intellectual, physical or a pre-existing medical condition, and dementia are not well understood. The Royal Commission into Aged Care Quality and Safety (Royal Commission) discussed findings regarding the unsuitability of residential aged care for younger people with disability. The Royal Commission called for ‘personalised care from well-trained staff’, and noted that ‘there is not one correct model of care for those with dementia’, highlighting the need for further insight to tailor support services for these individuals (Royal Commission 2021).  

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