Impairment and disability

The 1992 General Household Survey in the UK recorded 36.5% of people surveyed as describing themselves suffering from a serious disease or disability. Clearly, not all of these disorders will confer a level of impairment to produce ‘disability’ as the term is normally used in the general population. However, this figure supports pressure groups and professionals who argue for people with more obvious disabilities being seen as part of continuum within the population (‘everyone is not so good at something’).

When IQ is used to define those with learning disabilities (LD), the assumption of normal variation in the population does not entirely determine the prevalence found, i.e. if one defines LD as an IQ of less than 70, the prevalence is 2.2% whereas the actual prevalence is 3.7% (Figure 17.3). For severe LD (IQ<50), the prevalence is approximately 0.4%. The skew to the left in Figure 17.3 is due to a combination of ‘statistical’ low IQ with those individuals with genetic/chromosomal abnormalities. Those with an IQ between 50 and 70 rarely have an obvious cause for their impairment, whereas individuals with an IQ below 50 frequently do have a more obvious aetiology such as a genetic abnormality.

Figure 17.3 • Frequency distribution in intelligence quotients.

There is a high correlation between different impairments (see Figure 17.5). A third of children with cerebral palsy are also found to have severe LD (but conversely this means that two-thirds are above this IQ level). A study of residential hospital admissions showed that 38% of those with severe LD also had cerebral palsy (with the majority having spastic paralysis of all four limbs). This latter figure must be viewed with caution, as the likelihood of hospital admission was increased by greater disability, even when the emphasis was not on care in the community. When children with LD were admitted to residential hospital, a third of the hospital population were found also to have epilepsy. Although the same reservations about a hospital population as above must apply, similar figures have been found in Swedish community studies (Table 17.3).

Figure 17.5 • Incidence of Down’s syndrome with increasing maternal age.

Table 17.3 Levels of coexistence of different impairments

Specific syndromes

The frequencies of a number of the more common or well-known conditions are shown in Figure 17.4.

Figure 17.4 • Frequencies of specific syndromes (per 1000 births).

For Down’s syndrome there is a clear correlation between affected births and increasing maternal age (Figure 17.5). It must be noted, however, that the majority of children with Down’s syndrome are not born to women over the age of 35, despite the risk being so much greater. The figures for cytogenetic analysis of amniotic fluid at 15–16 weeks’ gestation show an even higher incidence of trisomy 21 than shown in Figure 17.5, suggesting a significant fetal loss rate.

Psychiatric and psychological aspects of disability

The Isle of Wight study of psychiatric disorder in nine to 12-year-olds, carried out in the 1970s, found that the prevalence of psychiatric disorder was:

Comparing studies examining the frequency of mental disorder in a learning-disabled population requires careful consideration of the definitions of both mental disorder and LD and also the population studied. Hospital studies have found prevalences of psychiatric disorder of between 32% and 59% in the learning-disabled population. In a study comparing hospital and community populations, significant psychiatric disorder was found in 31% of the former but only 13% of the latter. If minor personality quirks and behavioural ‘problems’ were added in, the figures were 52% and 41% respectively.

Corbett studied learning-disabled adults in Camberwell, south London, and found a prevalence of schizophrenia of 3.5%. This is very similar to the figures for hospital populations of adults with learning disabilities. He also found a further 3% with a previous history of schizophrenia. In the same study, he diagnosed 25% of the learning-disabled group as suffering from ‘personality disorder’ of varying types (see Chapter 15 for a discussion of the issues relating to such diagnoses). In a study of adults over the age of 50 with LD, 11.4% were found to be suffering from a psychiatric disorder, and 11.4% from dementia (combined figure 21%.)

In the Isle of Wight study, five out of the 38 severely learning-disabled children identified were also diagnosed as having a ‘neurotic disorder’ (13%). The equivalent figure in Camberwell was 4%. In more recent community studies in Scotland and Australia, around 40% of children with severe learning disability were found to have significant psychiatric disorder.

For bipolar mood disorders, the hospital point prevalence in a learning-disabled population was found to be 1.2%. A community study found that 1.5% of people older than 16 in contact with a LD service had a bipolar disorder, and a further 2% had depression.

Severe self-injurious behaviour is almost always associated with profound LD and occurs in 1.7% of the LD population. Milder stereotyped behaviours occur in 10–15% of children and adults.

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