The extent of multi-morbidity experienced by adults with intellectual disabilities, and its direct comparison with the general population, is a novel and important finding, with implications for services, including the age at which they are likely to be needed. Consequently, any policy initiatives or guidelines on multi-morbidity need to be relevant at a much earlier age in people with intellectual disabilities. This difference has not been previously reported. Morbidity burden and multimorbidity is higher in the population with intellectual disabilities than in the general population, due to higher rates of some physical conditions, for example epilepsy, gastro-intestinal disorders, and sensory impairments, and higher rates of a range of mental health conditions. Whereas multi-morbidity is common in the general population only in older age groups, particularly those aged 50–54 and older, it is common in all age groups in adults with intellectual disabilities. The pattern of disease also differs from the general population with some conditions being less common, such as cardiovascular disease. Additionally, prevalence of multi-morbidity did not follow the typical gradient seen in the general population across areas of increasing neighbourhood deprivation, importantly highlighting that services are equally needed in all areas.
Two previous studies from the Netherlands and Ireland have reported high rates of multimorbidity in older people with intellectual disabilities [6, 7], and we also found this, and extended this finding down the age range to all adults with intellectual disabilities.
Strengths and limitations
Scottish GP practices have held a register of people with intellectual disabilities since a change in their contract introducing pay-for-performance, which precedes the data extraction this study used. Intellectual disability is a lifetime diagnosis, and once coded at birth or in childhood this remains on the medical record indefinitely. The sample with intellectual disabilities appears to be representative, and benefits from its very large size. As expected, compared with people without intellectual disabilities, there were more men, they were younger, and they were more likely to live in deprived areas. Rates of morbidity were compared with the general population registered at the same general practices, and standardised by age, gender, and neighbourhood deprivation. It is possible that some people with intellectual disabilities were not coded as such, for example people with Down syndrome, however the prevalence of the population identified is similar to that reported for adults with intellectual disabilities in a recent meta-analysis of prevalence studies (0.5 %) , and the odds ratio for dementia for the intellectual disabilities group compared with the general population suggests people with Down syndrome, who have dementia at a much earlier age than the general public, were included.
There may be under-reporting of health conditions in the population with intellectual disabilities. This may be so for conditions that are not overtly obvious to paid carers, or where carers attribute the effects of conditions to other reasons. The similarity in extent of blood pressure recording in the population with intellectual disabilities compared with the general population is reassuring in this regard. If there was under-reporting, then the difference between the two groups would be even more marked than that we report, and the key message of our paper still stands i.e. that multi-morbidity is markedly more common in adults with intellectual disabilities than in the general population, and occurs at a much younger age.
Problem behaviours, which occur in 22.5 % of adults with intellectual disabilities  were not included in the study, due to the lack of suitable Read codes for these disorders, hampering their recording/consistent recording. Comparable problem behaviours are rare in the general population, hence the extent of the difference in multimorbidity would have been greater if these could have been included. We also did not include autism and attention deficit hyperactivity disorder, both of which are known to be more common in people with intellectual disabilities than in the general population. Conditions are coded during routine health care, including primary care encounters and based on letters from secondary care, and there could be some variation between practices.
We do not have information on type of accommodation/support the people with intellectual disabilities had.
Interpretation of findings
Some causes of intellectual disabilities also cause physical and/or mental ill-health, for example Down syndrome is associated with thyroid disorder and sensory impairments; however, Down syndrome accounts for only about 15 % of the population with intellectual disabilities. Adults with intellectual disabilities are also more likely to lead sedentary lives and not exercise , have more mobility problems , obesity , and are less likely to eat healthily  than the general public, and about a quarter take antipsychotic drugs , which may contribute to some of these conditions. They are also more likely to be prescribed multiple drugs, which can adversely affect health through side-effects and drug interactions . They do not always have the knowledge or understanding to make healthy choices, and are reliant on others for support and communication. These issues are often compounded by difficulties accessing the health services they need.
Eleven of the conditions were recorded statistically less commonly in adults with intellectual disabilities than in the general population. The lower rates of smoking and alcohol use among the population with intellectual disability may well account for several of these conditions being diagnosed at a lower frequency, particularly cardiovascular disease and chronic obstructive pulmonary disease. The majority of adults with intellectual disabilities do not drink alcohol at all, although some do misuse it, and at a slightly higher rate than the general population in this study.
Despite the higher prevalence of comorbidity experienced by the adults with intellectual disabilities, the extent of their morbidities may be under-recorded. Mental and physical health conditions may be unrecognised, under-investigated and untreated [23–26], with ill-heath presenting late, at more severe stages of disease progression which may be less responsive to treatment. Chronic disease monitoring is also less well addressed [27, 28]. Several factors are implicated, such as limited verbal communication skills, impaired mobility, and problem behaviours. People with intellectual disabilities are reliant on carers recognising they may have a problem and seeking help, and dependent upon carers communicating effectively within the team, and indeed across care teams (e.g. day care team and home care team). Sometimes, health conditions are misattributed by paid carers or health professionals as being part of the adult’s intellectual disabilities (diagnostic overshadowing), and not addressed for this reason. These problems are compounded across the entire lifecourse, rather than just being due to communication problems in late life.
The apparent drop-off in the rate of multimorbidity in men aged 75 and older is likely to be a reflection of the very small numbers in these age groups. Most people with intellectual disabilities do not live to such old ages [29, 30], so these individuals are the “healthy survivors”. Older people with intellectual disabilities typically have milder intellectual disabilities than those who die earlier, and people with milder intellectual disabilities are likely to have fewer health problems than people with more severe intellectual disabilities. Of the total of 4518 men with intellectual disabilities in the study, there were only 60 (1.32 %) aged 75–79, 32 (0.71 %) aged 80–84, and 16 (0.35 %) aged 85 or older. This compares with 24,831 (3.57 %) aged 75–79 out of the total of 694,911 men without intellectual disabilities, 15,921 (2.29 %) aged 80–84, and 11,017 (1.59 %) aged 85 or older.
The lack of association between neighbourhood deprivation and multimorbidity in this population is likely to be due to area based measures of deprivation not accurately reflecting the relative degree of affluence or poverty experienced by people with intellectual disabilities, in the face of the extensive difficulties they have to cope with in life. Many adults with intellectual disabilities are not integrated within their communities. They do not necessarily have shared values and lifestyles with their local community. Rented accommodation in which adults with intellectual disabilities are placed with individual tenancies, or shared tenancies with other adults with intellectual disabilities, tend to be in less affluent areas. One can speculate that their paid carers are more likely to live in the local area, but the adult may still have regular contact with family, whom they grew up with and who may have different levels of affluence and lifestyles compared to the area their adult child with intellectual disabilities now lives in. The interaction of these factors is likely to be complex. Additionally, some of the more congregate care style of housing is more likely to be in affluent areas where there are larger houses; but large group living can result in less individual time from paid carers who are shared by several adults, and less time for community integration. Very few adults with intellectual disabilities have paid employment, so are likely to be of low socio-economic status, and dependant on state benefits, regardless of the area they live in.
Generalisability of findings
The broader dataset is representative of the Scottish population in terms of age, sex, and deprivation . Intellectual disabilities was found in 0.56 % of the sample. This is slightly higher than the 0.5 % recorded in GP registers for pay-for-performance, reflecting that we used a somewhat broader set of Read Codes (http://www.isdscotland.org/Health-Topics/General-Practice/Quality-And-Outcomes-Framework/ Accessed 23.12.14.). As expected, there were more men than women with intellectual disabilities (as more boys than girls are born with intellectual disabilities), a smaller proportion at older age groups than in the general population (due to premature death [29, 30]), and more lived in areas of neighbourhood deprivation. This suggests that the sample with intellectual disabilities is representative of the Scottish population, and hence that these findings are generalisable.