1. Introduction
The world is facing a demographic change without precedents [
1]. Between 2015 and 2050, the percentage of people over the age of 60 will almost double from 12% to 22% according to the WHO [
2]. In Spain, 19.4% of the population in 2019 were above 65 years of age, according to INE [
3]. Longer human lives have led to a global burden of late-life disease [
4]. The lifespan of people with intellectual disabilities (ID) has increased significantly, so 8.9% of the population with ID in 2018 were above 65 years of age, 1% more than in 2014 [
5].
Increases in the life expectancy of people with Intellectual Disability have followed similar trends to those found in the general population. With the exception of people with severe and multiple disabilities or Down syndrome (DS), the life expectancy of this group now closely approximates with that of the general population. Middle and old age, which until 30 years ago were not recognized in this population, are now important parts of the life course of these individuals [
6].
IDs are defined as disorders in neuronal development that lead to deficits in intellectual capabilities and adaptive behavior [
7]. IDs occur in 1–3% of the general population [
8]. The incidence of disorders such as dementia among people with ID is five times greater than among the rest of the population; cognitive problems are generally preceded by behavioral problems [
9]. With the early onset of age-related health problems, dementia is more likely to develop by the age of 40 years in individuals with intellectual disability [
10].
DS is the most common chromosomal disorder, and the main cause of ID worldwide; its incidence is highly dependent on sociocultural variables [
11]. While the life expectancy of the DS population has greatly increased over recent decades, mortality rates are still high and subjects are facing prematurely a phenomenon of atypical and accelerated aging [
12]. The genetic material of people with DS makes them more vulnerable to Alzheimer’s disease [
13]. Chromosome 21, triplicated in DS, contains several genes that are thought to play a critical role in the development of this neuropathology [
14]. All adults with DS demonstrate AD-like brain pathology, including amyloid plaques and neurofibrillary tangles, by age 40, and dementia typically by age 60 [
15].
Consequently, older adults with ID form a small but significant and growing proportion of older people in the community. How these persons grow older and how symptoms and complications of the underlying cause of the Intellectual Disability will influence their life expectancy are of the utmost importance [
6].
Generally, neurological diagnosis comes when the disorder has already caused substantial deterioration, either because the associated behavioral problems can be taken as features of IDs [
13], from the lack of access to appropriate healthcare, or because neuropsychological testing batteries lack specific scales for people with ID [
10,
16,
17].
Initially, neurocognitive changes due to normal aging do not result in the loss of functional abilities [
18]. However, people with ID experience different levels of limitations in basic aspects necessary for independent functioning and living. Satisfactory daily functioning is recognized as a major component of health and well-being by the International Classification of Functioning (ICF) of the World Health Organization [
19,
20,
21].
Concerning associated pathologies, people with ID have a health profile more complex than previously known, being more prone to various chronic health conditions than their peers without ID [
22]. This includes cardiovascular diseases, high cholesterol, obesity, diabetes, epilepsy, and osteoarticular disorders. Some common diseases in people with DS are hypothyroidism, congenital cardiopathy, and gastrointestinal disorders [
11,
22]. Recent studies have identified that people with ID have more than twice the number of mental health conditions, visual impairments, hearing impairments, and physical disabilities [
23]. However, according to Folch-Mas et al., many health problems associated with senior people with ID remain to be identified [
16].
Integrated care has been suggested as a promising solution to the disparities in access and sustained high-quality long-term care emerging in Europe’s ageing population [
24]. Recent studies have shown the importance of factors that improve healthy aging, ensure the social inclusion of the elderly, and prolong the time in which they can live independent lives [
25]. Improving social sustainability in people with ID would mean improving social inclusion [
26].
The literature refers to this gap in our knowledge about aging in people with ID [
27]. More research is needed to assess the effects of aging on mental health [
28], as well as longitudinal studies that also include people without ID, in order to improve healthcare protocols [
29]. Besides that, understanding the possible aging-related mechanisms associated with these clinical manifestations of DS will facilitate therapeutic interventions in mid-to-late adulthood, while at the same time shedding light on basic mechanisms of aging [
21].
The joint study of the influence of cognitive and functional factors, comparing people with and without intellectual disabilities in aging, had not been previously studied.
Therefore, the aim of this study was to analyze cognitive and functional differences between senior people with ID (and DS) and their peers without ID, in order to outline the most appropriate attention protocols.
3. Results
The sample comprised 247 participants, 146 without ID and 101 with ID (29 withtDS and 72 without DS).
Table 1 summarizes the socio-demographic characteristics of the sample. The average age of participants was 70.6 ± 11.5 years (with ID: 60.6 ± 8.3 years; without ID: 77.6 ± 7.6 years).
Table 2 presents the result of cognitive tests. Participants without ID yielded higher MEC scores, both globally and in each separate dominion (
p < 0.01), except STM. All differences were statistically significant after the results were adjusted by the age co-variable, including STM (
p = 0.033). Concerning DS, the only significant differences attested concerned the language variable (
p = 0.001). Regarding the association between cognitive impairment and ID, the study concluded that 88.1% of participants with ID suffered some form of cognitive impairment (84.7% without DS and 96.6% with DS). Logistic regression showed that, regardless of age, participants with ID were 25 times more likely to be afflicted by cognitive impairment than ID-free participants were (
p < 0.05). Concerning verbal fluency, participants without ID yielded higher scores (
p < 0.001) than their disabled peers did. The difference remained significant after the results were adjusted by age. No significant differences were attested between DS and non-DS participants in this variable (
p = 0.062).
Table 3 presents the neurological results. The proportion of participants with mild cognitive impairment (MCI), dementia, and cognitive impairment was higher in the ID-free group of participants (
p < 0.001). No significant differences between the groups were attested concerning memory loss and Parkinson’s.
Table 4 presents the functional results. No significant differences were attested between ID and non-ID participants in terms of ADLs (
p = 0.512), but there were significant differences in terms of IADLs (
p < 0.01), with participants without ID yielding higher overall and Tinetti’s static test scores (both
p < 0.05). After the results were adjusted for age, only the differences in IADLs remained significant. No significant differences were attested between groups in the functional tests (all
p > 0.05). After adjusting the results by age, participants without ID yielded higher values in the Lawton–Brody scale (2.7 vs. 0.9;
p = 0.020), Tinetti’s dynamic test (10.6 vs. 8.9;
p = 0.005), and overall Tinetti’s test score (25.4 vs. 23.0;
p = 0.035).
Figure 1 represents the cognitive and functional differences between groups with and without ID.
In
Figure 1, which represents the cognitive and functional differences between groups with and without ID, it is observed how the group with ID obtained higher scores in the cognitive section, as well as in the Lawton scale. In the rest of the functional tests, hardly any differences were seen between the two groups.
Table 5 presents associated pathologies. The percentage of participants with hypertension, diabetes, cardiopathies, mental health issues, ictus, and technical aid requirements were greater in the group without ID. In contrast, the percentage of participants with obesity and epilepsy were higher in the ID group. No statistically significant differences were attested between groups concerning the other pathologies examined: high cholesterol, tobacco addiction, hearing impairments, vision impairments, and breathing difficulties.
Finally, to analyze the relationship between these measures of cognitive and functional variables, a confirmatory factor analysis (CFA) was performed. After performing a first CFA, we found that the fit indices were not appropriate χ2 (4) = 7.046 p < 0.001; χ2/gl = 1.76; CFI = 0.96; NFI = 0.95; TLI = 0.93; RMSEA = 0.075, 95% CI (0.056–0.093).
Finally, an attempt was made to verify the relationship between cognitive and functional aspects and whether it differed between the group of participants with and without ID.
Figure 2 shows the result of the analysis carried out with structural equations by the maximum likelihood method, which confirmed the suitability of the model composed of the constructs contemplated in the present study. The analysis showed a good correlation (
r = 0.59) between the cognitive variables and the functional variables.
When comparisons of nested models were made (
Table 6), and assuming that the model without restrictions is correct, we obtained a comparison made with the measurement weights, which indicated that the model of equal measurement weights showed the same fit as the model without restrictions did. Therefore, the regression weights in the indicated model were the same. Likewise, the models of measurement intersections, structural covariances, and measurement residuals did not fit the data, and the comparison indicated that the model differed statistically and gave a worse fit. These results indicated that it was not necessary to include the presence of ID as a difference in the established model between the cognitive and functional variables.
4. Discussion
This study examined differences in aging between ID-afflicted and ID-free seniors, taking into consideration cognitive and functional assessments, clinical diagnosis, and pathologies.
In terms of socio-demographic profiles, ID-afflicted seniors are younger, single, institutionalized, and less educated. Age differences are determined by inclusion criteria and premature aging in association with ID [
50]. A study by Vancamptort et al. showed that people with intellectual disability have an increased risk of premature mortality [
51]. Concerning marital status, the historical segregation of ID-afflicted people by sex has limited the scope of personal relationships during youth [
52,
53]. Regarding family relationships, early vulnerability and care requirements often lead to their institutionalization [
54], although those who live with their relatives have a more positive perspective on their health and welfare [
55]. The low educational level presented by ID seniors is confirmed by INE data, according to which 33% of people with ID are illiterate [
3]. Historically, students with intellectual disability were not expected to learn to read, and thus were excluded from reading instruction [
56].
Concerning cognitive performance, both overall and by dominion, ID-afflicted participants yielded lower scores, except in terms of STM, in which the performance was similar. However, other studies that compared groups of ID and non-ID seniors detected memory issues in all periods of life, except the 19–45 age bracket [
57], among people with DS. The ID-related initial cognitive deficits affect executive functions, while in seniors not afflicted by ID, these initially affect episodic memory [
58]. Concerning the relationship of ID and language, some studies have argued that language skills are a significant predictor of MCI, that semantic verbal fluency is the most important predictor of dementia, and that the evaluation of linguistic skills can help to detect dementia in ID-afflicted adults [
59].
In our study, the percentage of participants with MCI and dementia were higher in the non-ID group. However, previous studies have suggested that ID-afflicted people are five times more likely to suffer these conditions [
60,
61]. On the other hand, it is possible that the underrepresentation of these afflictions among ID participants is caused by the tests used in primary healthcare, which are influenced by educative level [
36,
60]; although, in our study, we used Set-Test for this very reason [
37], the literature demands the use of other tools for the diagnosis of ID patients, such as the Barcelona test [
62] or CAMDEX-DS [
13], alongside a clinical diagnosis adapted to ID patients [
60]. Other studies have stated that cognitive instruments used for the general population are not suitable for people with intellectual disability, because of floor effects [
10]. Consensus criteria for both dementia and MCI have been developed for typically developing adults but are of limited applicability for adults with ID, given their pre-existing cognitive impairments [
63].
Concerning the functional variables, our study attested no differences in terms of ADLs, although a smaller proportion of ID participants used technical aids, as their static and dynamic balance was better [
20,
46]. However, another study showed that 40% of adults with ID were completely independent in ADL, but all participants reported activity limitations in at least one IADL. Dynamic balance and MWS, lower-body strength, and manual dexterity showed significant and moderate-to-strong correlations with daily functioning [
21]. Regarding IADLs, ID-afflicted people are more dependent, owing to their cognitive deficit [
20]. After the age of 50, people with ID suffer a faster process of functional deterioration [
64]. The Barthel index and the Lawton–Brody scale are recommended for clinical practice and research with ID patients [
20]. The results of a study by Lobete et al. confirmed that supporting the performance of both ADL and AIDL and promoting physical fitness in community care centers for adults with ID improve their functional Independence [
21].
The literature consistently reports that balance and gait capacities are affected in persons with ID compared to their age-matched peers. These problems start at a young age and remain present during the entire lifespan of persons with ID, with a relatively early occurrence of age-related decline. The relationship between the cognitive and functional variables was analyzed, finding a link between both, with no effect on whether the person presented ID or not. This review demonstrated that balance and gait are potentially trainable in persons with ID [
65].
Regarding associated pathologies, a larger proportion of non-ID participants present hypertension, diabetes, cardiopathies, mental health issues, and ictus. Some studies have argued that these pathologies are more common among ID patients, and that their statistical underrepresentation is due to diagnostic issues, affecting relatives, healthcare professionals, and the patient’s themselves [
16]. Other comparative studies between ID and the general population have shown that many conditions occur more frequently among people with ID, while some conditions occur in the same proportion or at a lower rate. This study concluded that the latter associations may reflect an underdiagnosis [
22].
Obesity [
64] and epilepsy [
66,
67] are more prevalent among people afflicted by ID, both in the sample and in general.
The review of the existing literature highlights the multiple challenges posed by ID and the need to develop a more collaborative approach involving the institutions caring for ID patients, families, and other stakeholders [
68].
Our observational study flagged the different healthcare profiles of people with and without ID. Their comparatively poor cognitive performance is related to the effects of the ID, poor education, cognitive infra-diagnosis, and the use of inadequate measuring tools. Training in adaptive skills, health checks, and education for health programs have led to greater functionality and reduced the incidence of aging-related afflictions in people with ID. These results are clinically relevant insofar as they help prevent cognitive impairment and improve the quality of life of patients, while contributing to a better coordination between primary healthcare and institutions that attend patients afflicted by ID.
The limitations of our study concern the use of cognitive skills-measuring tools that are not adapted to ID but the use of which is widespread in primary healthcare. Another limitation of the study is not having evaluated the lifestyle before and after the diagnosis of ID.
Future research should develop therapeutic activities adapted to patients with ID and carry out comparative studies to assess their effectiveness.