Background Diagnosing dementia in people with intellectual disabilities can be challenging due to pre-existing cognitive impairment. In this population, functional loss could be an early indicator of dementia, but the relationship between functional loss and dementia is not well understood. This means there is a risk of delayed diagnosis and therefore a delay in care planning.

Aim To identify and compare the prevalence and age distribution of dementia in people with intellectual disabilities, differentiating between those with Down’s syndrome and those with an intellectual disability not attributable to Down’s syndrome, and to identify and compare functional loss in people with intellectual disabilities who have developed dementia, differentiating between those with Down’s syndrome and those with an intellectual disability not attributable to Down’s syndrome.

Method This was a secondary analysis of data from waves 1 and 3 of the Intellectual Disability Supplement to the Irish Longitudinal Study on Ageing (IDS-TILDA), a nationally representative study in Ireland of older adults with intellectual disabilities as they age. Functional loss was determined by participants’ level of difficulty with activities of daily living (ADLs) and instrumental activities of daily living (IADLs).

Results The prevalence of dementia was higher among participants with Down’s syndrome than among those with an intellectual disability not attributable to Down’s syndrome, and the age distribution of dementia differed markedly between the two groups. Both groups experienced similar patterns of difficulty with ADLs and IADLs. IADLs posed more difficulty than ADLs, and among ADLs self-care activities posed the most difficulty.

Conclusion It is important to continue to investigate functional loss in older adults with intellectual disability who have developed dementia, as this should lead to earlier assessment and care planning.