Mental illness is common among adults with learning disabilities. Cooper’s epidemiological study⁴ comparing the prevalence rates of psychiatric disorder among older (≥65 years) and younger (20–65 years) adults with learning disabilities found significantly higher rates among the elderly cohort (68.7% compared with 47.9%). This was mainly accounted for by the higher point prevalence rates for dementia, generalized anxiety disorder and depression in the older cohort groups. While the ageing process can bring about a similar increased vulnerability to mental illness as seen in the general population, additional risk factors apply for people with learning disabilities. These include: the detrimental effect of pre-existing brain damage; co-morbid physical illnesses like epilepsy; specific psychiatric or behaviour disorders associated with genetic risk factors; low self-esteem which may be related to frequent placement breakdowns, past exploitation, neglect or abuse; and the complications of learned maladaptive behaviours such as self-injurious head injury.

Mental illness may go unrecognized due to a number of barriers in the pathways to health care. Service barriers can include the lack of specialist services or the lack of expertise in mainstream services, complex appointment systems that largely presume reading ability, and inaccessible health-care delivery. Problems with the doctor–user interface include communication difficulties, ‘power imbalance’ exacerbating the suggestibility of the person with learning disability, capacity issues and the complexities of atypical clinical presentations. Diagnostic overshadowing (assuming that any changes in behaviour or presentation are due to the learning disability, rather than a co-morbid illness) remains one of the major barriers preventing people with learning disabilities from accessing adequate health care⁵. User–carer barriers include pre-existing cognitive impairments making it difficult to obtain an accurate timeline of symptoms, the effects of past negative experiences of health-care delivery and the failure of carers to recognize symptoms. Older people with learning disabilities from ethnic minority groups may have particular problems accessing appropriate services⁶.

The rates of dementia in older people with learning disabilities are four times higher than in the general population⁷. However, these high rates cannot be attributed just to those with Down syndrome and Alzheimer’s disease. Strydom et al.’s epidemiological survey⁸ of dementia in older adults (aged ≥60 years) with learning disabilities, but without Down syndrome, has confirmed this. Alzheimer’s disease was found to be the most common sub-type, but the prevalence rates (8.6%) were almost three times higher than expected. While vascular dementia is the second commonest sub-type in the general population, Lewy body and frontotemporal dementias were found to be commoner in the older learning disabled population. Vascular dementia had a prevalence rate of 2.7%. Explanations for these increased prevalence rates include the effects of poorly controlled epilepsy or other physical illnesses, the detrimental effects of brain damage during birth and early life, and genetic risk factors⁸.

Diagnosing dementia in people with learning disabilities can be difficult due to the wide range of pre-existing baseline cognitive, functional and behavioural impairments. Individuals are more likely to present with the behavioural and psychological symptoms of dementia, or with atypical symptoms such as unexplained seizures. Greater emphasis should be placed on personality and behavioural changes, in association with functional change, as diagnostic indicators⁹. Diagnostic difficulties can also result from altered social or communication skills making it difficult to identify subjective symptoms (psychosocial masking/intellectual distortion), and from diagnostic overshadowing¹⁰.

Good collateral information from the person’s carers is essential. The exacerbation of pre-existing deficits (‘baseline exaggeration’) may only be detected in day-care environments, where the person has more demands placed upon them.