Dementia refers to changes in memory and other cognitive skills severe enough to affect a person's ability to perform daily social and occupational activities. Alzheimer's disease (AD), the most common cause of dementia, is a progressive brain disease for which there is no cure, and it affects an estimated 5.2 million Americans over age 65, and approximately 200,000 people younger than 65 (Alzheimer's Association, 2014a). Many of the people affected by younger onset dementia will be those with Down syndrome (NTG, 2012). Individuals with other intellectual disabilities characterized by developmental delays and impairments in social function are also at risk for AD (Strydom et al., 2009). The life expectancy for people with IDD is similar to that of the general population, and the number of adults with IDD aged 60 years and older has been projected to nearly double from 641,860 in 2000 to 1.2 million by 2030, primarily as a result of improved longevity (Heller et al., 2010). The mean age at death ranges from the mid-50s for those with more severe disabilities or Down syndrome to the early 70s for adults with mild/moderate IDD (Heller et al., 2010).
Down syndrome is the best known and most prevalent biologically-based condition associated with IDD (Stancliffe et al., 2012). The prevalence of Down syndrome increases among children born to women who conceived at age 35 or older. The prevalence of Down syndrome rose from 9.0 per 10,000 live births in 1979 to 11.8 in 2003 in 10 representative U.S. regions, largely because of later ages of childbearing (Zigman, 2013). Although estimates vary, a recent study estimated that there were approximately 250,700 people with Down syndrome in the United States as of January 2008 (Presson et al., 2013). Prevalence rates of AD among people with Down syndrome are reported to increase with age. Estimates based on a number of studies range from 10% to 25% in the 40 to 49 age group, 20% to 50% in the 50 to 59 age group, and 30% to 75% among those aged 60 and over (Ball et al., 2008). Perhaps the most dramatic data are found in a longitudinal study of 80 women with Down syndrome aged 40 and older at the start of the study. After 14 years of regular assessments, 89.6% had been clinically diagnosed with dementia, the average age of diagnosis was 55.41 years (SD = 7.14), and there was a median survival of 7 years after diagnosis (McCarron et al., 2014). One study also found that women with Down syndrome have an increased risk of dementia if they start menopause at an early age (Coppus et al., 2010).
Although it is well established that adults with Down syndrome are at high risk for developing AD in middle age, the risk of dementia for people with intellectual disabilities other than Down syndrome has not received as much research attention. Prevalence studies of AD in the intellectual disability population without Down syndrome have found varying results--some with rates greater than the general population and others with rates similar to the general population (Strydom et al., 2010, 2013). For example, a recent study of people with intellectual disabilities other than Down syndrome found an incidence of dementia among people aged 65 and older five times higher than in the general population (Strydom et al., 2013). On the other hand, one study of 117 people with intellectual disability without Down syndrome did not find that the prevalence of dementia or AD was significantly different from the general population (Zigman et al., 2004).
Identifying cognitive decline in people with Down syndrome can be challenging because cognitive difficulties can be related to lifelong deficits or to changes resulting from dementia (Devenny et al., 2000). A number of early symptoms of AD in people with Down syndrome have been identified, including memory loss, disorientation, and changes in personality, behavior, and speech (Ball et al., 2008; Strydom et al., 2010). Behavioral and personality changes can include aggression, irritability, apathy, and decreased social engagement (Strydom et al., 2010). People with Down syndrome and AD also can experience neurological symptoms, such as seizures and myoclonus--the sudden, involuntary jerking of muscle--to a greater extent than the general population with AD (Ball et al., 2006; Urv, Zigman, & Silverman, 2010). Many frontal lobe-related symptoms are more common at an early stage of dementia among adults with Down syndrome. These symptoms include prominent personality and behavior changes such as irritability, aggression, self-abusive behavior, slowness, loss of interest in activities, social withdrawal, apathy, and decreased social engagement (Ball et al., 2006; Deb et al., 2007a; O'Caoimh et al., 2013; Strydom et al., 2010). It is difficult to distinguish AD from new-onset behavioral disorders or other psychiatric illnesses, which are common in older adults with Down syndrome (McCarron et al., 2014; O'Caoimh et al., 2013).
As people with Down syndrome move to the middle stages of dementia, cognitive functioning becomes more impaired, similar to declines in the general population, with complex cognitive functions and visual organization beginning to deteriorate (Alzheimer's Association, 2014a; Strydom et al., 2010). Other changes for people with Down syndrome and dementia can be in the area of functional abilities, changing from mild impairment in personal hygiene in the early stage of dementia to inability to dress or eat as the disease progresses (Strydom et al., 2010). As dementia progresses to the late stages in people with Down syndrome, they are unresponsive to their environment and no longer speak (Strydom et al., 2010).
Much less research has been done on dementia symptoms of people with intellectual disabilities other than Down syndrome. One study found that early symptoms of dementia were marked by a general decline in functioning, whereas memory loss or other changes in cognitive function were not as prominent early on. Behavioral or emotional changes followed declines in functioning, such as changes in mood, lower energy levels, and hallucinations. In the late stages, incontinence and difficulty walking were common (Strydom et al., 2010).
Nearly a quarter of all persons with IDD live with family caregivers who are aged 60 or older (Braddock et al., 2013). By the time people with IDD who are living at home reach early old age, their parents tend to experience adverse health impacts, such as obesity, poorer self-rated health, limitations in activities of daily living (ADLs), and poor health-related quality of life, because of the stress of long-term caregiving (Seltzer et al., 2011). Older family caregivers, particularly parents, often fear that in the event of the parents' death their children will not have adequate services to meet their needs and a place to live (Llewellyn et al., 2004). Family caregivers of adults with an intellectual disability and dementia may have additional challenges in continuing to provide care at home compared to solely caring for individuals with intellectual disabilities or individuals with dementia. Such challenges may include needing certain home modifications or caring for individuals who require assistance with a higher number of ADLs (McCallion et al., 2005).