Dementia may be more common in older adults with intellectual disability (ID) than in the general population. The increased risk for Alzheimer's disease in people with Down syndrome (DS) is well established, but much less is known about dementia in adults with ID who do not have DS. We estimated incidence rates from a longitudinal study of dementia in older adults with ID without DS and compared them to general population rates. 222 participants with ID without DS aged 60 years and older were followed up an average of 2.9 years later to identify those who had declined in functional or cognitive abilities. Those who screened positive had a comprehensive assessment for dementia, diagnosed using ICD 10 and DSM IV criteria. 134 participants who did not have dementia at initial assessment were alive and interviewed at follow up; 21 (15.7%) were diagnosed with dementia. Overall incidence rate for those aged ≥60 was 54.6/1000 person years (95% CI 34.1–82.3). The highest incidence rate (97.8/1000 person years) was in the age group 70–74. Standardised incidence ratio for those aged ≥65 was 4.98 (95% CI 1.62–11.67). Incidence of dementia in older people with intellectual disabilities are up to five times higher than older adults in the general population. Screening may be useful in this population given the high incident rates, particularly as more effective treatments become available. Studies to explore the underlying aetiological factors for dementia associated with intellectual disability could help to identify novel protective and risk factors.
There have been several epidemiological studies of dementia in older adults with Down syndrome (DS) which is known to be associated with an increased risk for Alzheimer's disease (AD), but much less is known about dementia in the intellectual disability population who do not have Down syndrome (non-DS ID) (Strydom et al., 2010b).
Dementia may also be more common in adults with non-DS ID compared to the general population because of specific risk factors. ID is by definition associated with reduced brain reserve (i.e. smaller brain size, fewer neurons or synapse count) (Stern, 2002). The brain reserve hypothesis proposes 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 (Valenzuela and Sachdev, 2006 and Whalley et al., 2004). The brain reserve hypothesis therefore predicts that older adults with ID should have higher rates of dementia than those with normal intelligence, and that dementia rates will be relatively high in younger age groups because those with progressive brain pathology will quickly reach a functional cut-off with early emergence of symptoms (Strydom, Hassiotis, King, & Livingston, 2009). Furthermore, several rare genetic ID syndromes are associated with progressive decline. However, older adults with ID may also be protected against dementia. For instance, they often have better cardiovascular risk profiles such as lower rates of smoking and ischaemic heart disease, which may reduce their risk for dementia, particularly vascular dementia (Haveman et al., 2010).
There have been a small number of dementia prevalence studies in the non-DS ID population, with several studies (Cooper, 1997, Shooshtari et al., 2011 and Strydom et al., 2007) showing an increased prevalence while some found rates similar to those in the general population (Zigman et al., 2004). We have previously shown that dementia prevalence varies by diagnostic criteria used, disability level and age (Strydom et al., 2007 and Strydom et al., 2009). Prevalence is also strongly influenced by mortality rates, and older adults with dementia have much higher mortality rates than those without dementia (Rait et al., 2010). Prevalence rates may therefore underestimate a population's risk for dementia.
The present study reports the annual incidence of dementia in older adults with non-DS ID and compares it with population rates (standardised incidence ratio, SIR).
Incidence of dementia defined with DSMIV or ICD10 criteria is increased in older adults with intellectual disability. Overall incidence rate for those aged ≥60 was 54.6/1000 person years (95% CI 34.1–82.3) and the standardised incidence ratio for those aged ≥65 was 4.98 (95% CI 1.62–11.67); dementia incidence is thus up to 5 times higher than in the general population. Mortality-adjusted rates increased from 24/1000 person-years in those aged 60–64 to 97.8/1000 person-years in those aged 70–74. There were no gender differences in dementia incidence rates. Screening to identify those with cognitive decline may be useful in older people with ID, even those without Down syndrome, particularly as their life expectancy increases and more effective treatments become available, and further research is needed to explore aetiological factors associated with increased risk for dementia in the non-DS ID population.
