EPIDEMIOLOGY OF LEARNING [INTELLECTUAL] DISABILITY AND COMORBID CONDITIONS
Vee P Prasher

Basic principles
Epidemiology is essentially the study of a disorder in a given population. Knowledge of the distribution of a disorder in a population can increase understanding of the causes and how best to manage it. Epidemiological research may investigate:

• the causes of a disease
• the natural history of a disease
• the characteristics of a disease, influenced by such factors as age, sex, social class and ethnic background.

Research may be used to test the efficacy and efficiency of healthcare intervention to prevent or treat the disorder. It may also assist in healthcare planning. Figure 1 lists commonly used measures of disease frequency.

Epidemiological strategies
There are a number of general epidemiological strategies which are important in terms of epidemiological research.

Case definition - this is a principal issue in epidemiological research. Although it may appear straightforward, there are a number of issues relating to the questions 'what is a case?' or 'how is any given illness defined?' In psychiatry, this is an important issue, particularly when many disorders are continuous rather than discrete phenomena. For example, in people with intellectual disabilities considerable uncertainty remains regarding the valid detection of autism or dementia. For a meaningful conclusion to be drawn, there must be uniformity in definitions of a given disease. Also, if aetiological factors are to be investigated, what constitutes a given case must be agreed. To this end a number of classification systems have been developed, principally ICD-10 (WHO, 1992) and DSM-IV (APA, 1994).

Sampling methods used - it is usually not possible to examine the entire population; instead, a sample subset of the population is investigated. However, it is important that the sample represents the larger population in an unbiased fashion. For example, investigating the prevalence of an illness in people with severe intellectual disability prevents the conclusions being generalized to people who may have mild or moderate intellectual disability.
There are a number of sampling techniques, including:

• simple random sampling
• systematic sampling
• stratified sampling
• multi-stage sampling
• non-random sampling.
  

Sample size - research proposals often focus on the practicalities of studies and do not place as much importance as they should on the sample size of the study. The sample size will affect whether the original hypotheses can be tested significantly, and the researcher should undertake a power calculation before any study is begun to determine what sample size is needed to answer their hypotheses.

Standardization - to be able to compare and contrast rates of any illness between areas or groups, it is important that underlying compounding factors are controlled for, in particular, age, sex, social class and severity of intellectual disability.

Investigatory studies - there are a number of methods of study in epidemiology, and these may involve observational or interventional studies. They may be restrictive, whereby the pattern of disease in a population is described, or an analytical planned investigation to test a

hypothesis may be undertaken. Further, studies may be retrospective or prospective. In retrospective studies the researcher examines past events of experiences; prospective studies involve collecting new data. There are four main epidemiological research study types, which are listed in Figure 2.

Specific issues in learning [intellectual] disability
Definition of caseness - how applicable accepted standardized diagnostic criteria (ICD-10, DSM-IV) are for people with learning [intellectual] disability has still not been fully researched. Although in general they are applicable, caution may be needed when using such criteria. For example, when investigating schizophrenia, some of the requirements for diagnosis depend on good verbal communication and an ability to verbalize intellectual thoughts and perceptual changes. This requires a reasonable degree of underlying intelligence, which may not be present in all people with learning disability (see also Prasher 2003).

Difficulties in diagnosis - standardized diagnostic assessments and tools are not readily available for the intellectual disability population. Few have been widely accepted or have good validity and reliability. Often the diagnostic assessment is a clinical one and therefore subject to a high degree of interview bias.

Behaviour-mental illness dichotomy - further investigation is required as to whether presenting symptoms are those of a mental illness or part of long-standing behavioural change. Such behaviour may be secondary to an environmental situation, and may be a reaction to a life event rather than an actual psychiatric illness.

Target population - there is a large degree of heterogeneity in intellectual disability, with considerable variation in underlying intellectual functioning, communication skills, adaptive behaviour, associated mental disorders, and in the different causes of disability. Findings from any given study may not be directly representative of the target population.

Sample size - often the number of people with intellectual disability available to participate in studies is limited. Once a number of issues (e.g. ethics, consent/assent, carer agreement) have been addressed the number of individuals recruited may be smaller still, leading to a failure to meet the sample size suggested by the power calculation. Therefore, the conclusions reached may have to be treated with caution.

Direct/indirect studies - owing to limitations of communication and intellectual impairment, subjects often cannot participate directly in studies. The vast majority of studies are thus observational and give limited information.

Specific epidemiological studies
Prevalence and aetiology: prevalence rates vary depending on the study design and the population studied, but according to the World Health Organization the true prevalence of intellectual disability is close to 3%. Roeleveld et al. (1997) undertook a review of prevalence studies and reported 'an enormous gap in our knowledge about learning disability', and that many studies were hampered by imperfections in study design, and estimates of prevalence rates. Individuals with mild disability represent the largest proportion (approximately 2.5% of the whole population); moderate intellectual disability involves approximately 0.4% of the population, and severe and profound levels combined account for approximately 0.1% (see also Cooke 2003).
Epidemiological studies have been undertaken looking at the causes of intellectual disability, including demographic, parental and environmental factors. Down's syndrome, for example, occurs at the same rate in all populations regardless of race, geographical location or season of birth. The principal association appears to be that of an increased rate with increasing maternal age. A range of environmental factors have been studied, including fluoride in drinking water, radiation and thyroid dysfunction in mothers, but generally there is no evidence supporting an environmental agent as a causative factor for Down's syndrome.

Mortality rates: Strauss and Eyman (1996) investigated mortality rates in a large population of people with intellectual disabilities. Up to the age of 35 years, mortality rates for people with Down's syndrome were comparable with those for people with intellectual disability due to other causes. Subsequently, however, the mortality rates for individuals with Down's syndrome doubled every 6.4 years, compared with 9.6 years for people without Down's syndrome. Frid et al. (1999) investigated mortality in Down's syndrome in relation to the presence or absence of congenital malformations, such as heart lesions; mortality rates within 10 years of the birth were 40.6% and 23.5% respectively. The presence of gastrointestinal malformation further increased the risk of mortality. Common causes of death included congenital abnormalities, neonatal complications and respiratory infections. McGuigan et al. (1995) confirmed that the age-specific standardized mortality rates for people with intellectual disabilities are often higher than in the general population. This applies to both men and women.

Prevalence studies of psychiatric disorders: a number of studies have investigated psychiatric disorders among adults with learning [intellectual] disability (e.g. Cooper and Bailey, 2001; Haveman et al., 1989); these have generally been point prevalence studies.

• Cooper and Bailey (2001) assessed 207 adults with intellectual disability, and found a psychiatric disorder rate of 49.2%. Adults with more severe intellectual disability had higher rates of additional psychiatric disorders.
• In contrast, Crews et al. (1994) investigated the prevalence of psychiatric disorders in a residential population (n=1273). Individuals were aged 10-80 years, with a mean age of 40 years. The point prevalence rate of diagnosis based on DSM-III-R criteria was 15.6%. Psychiatric diagnoses were more likely in individuals with mild retardation. Affective disorder was the most common, followed by psychotic disorder.
• Prasher (1995) investigated psychiatric disorders in 201 adults with Down's syndrome aged 16 years and over: the rate of psychiatric disorder was 28.9%. The commonest disorders were dementia of Alzheimer's-type, depression, conduct disorders, and obsessive-compulsive disorders (see also Prasher 2003).

Prevalence studies of physical disorders: a number of studies have looked at the prevalence of physical health problems in people with intellectual disability (e.g. Day and Jancar, 1994; Hand, 1994). As with psychiatric studies, individuals with intellectual disability have been recruited by different means and undergone assessments that at times have not been defined; furthermore, the studies have usually reported point prevalence. Hand (1994) found that epilepsy, cerebral palsy and neurological impairment were the commonest conditions. Forty-two per cent of those studied reported having no major or chronic physical problems. In ageing adults with learning disabilities, common health problems include:

• hearing and visual impairment
• mobility problems
• heart conditions
• diabetes
• fractures and osteoporosis.

Conclusion
Evidence from epidemiological studies suggests that people with intellectual disability are more susceptible to mental and physical illnesses than the general population. The reasons for this include biological risk factors (such as genetic abnormalities and brain damage) and psychological risk factors (such as stigmatization and impaired social integration). However, it is important to recognize the heterogeneity of this population and the inherent difficulties in conducting high-quality epidemiological research with people with learning difficulties.
Although interest is growing in the field of epidemiology in intellectual disability, a number of concerns remain regarding epidemiological methods. Several important issues require further investigation, including:

• the incidence of physical disorders in the people with intellectual disabilities population
• therapeutic intervention studies
• the causes of physical health morbidity.

REFERENCES
American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 4th edition (DSM-IV). Washington, DC: APA, 1994.
Cooke L B. Aetiology of Learning Disability. Psychiatry 2003; 2:8: 4-8.
Crews W D, Bonaventura S, Rowe F. Dual diagnosis: prevalence of psychiatric disorders in a large state residential facility for individuals with mental retardation. American Journal on Mental Retardation 1994; 98: 688-731.
Cooper S-A, Bailey N M. Psychiatric disorders amongst adults with learning disabilities - prevalence and relationship to ability level.
Irish Journal of Psychological Medicine 2001; 18: 45-53.
Day K, Jancar J. Mental and physical health and ageing in mental handicap: a review. Journal of Intellectual Disability Research 1994; 38: 241-56.
Frid C, Drott P, Lundell B, Rasmussen F, Anneren G. Mortality in Down's syndrome in relation to congenital malformations. Journal of Intellectual Disability Research 1999; 43: 234-41.
Hand, J.E. Report of a national survey of older people with lifelong intellectual handicap in New Zealand. Journal of Intellectual Disability Research 1994; 38: 275-87.
Haveman M, Maaskant M A, Sturmans F. Older Dutch residents of institutions, with and without Down's syndrome: comparisons of mortality and morbidity trends and motor/social functioning. Australia and New Zealand Journal of Devopmental Disability 1989; 15: 241-55.
McGuigan S M, Hollins S, Attard M. Age-specific standardized mortality rates in people with learning disability. Journal of Intellectual Disability Research 1995; 39: 527-31.
Prasher V P. Prevalence of psychiatric disorders in adults with Down's syndrome. European Journal of Psychiatry 1995; 9: 77-82.
Prasher V P. Psychiatric Morbidity in Adults with Down's Syndrome. Psychiatry 2003; 2:8: 21-24.
Roeleveld N, Zielhuis G A, Gabreels F. The prevalence of mental retardation: a critical review of recent literature. Developmental Medicine and Child Neurology 1997; 39: 125-32.
Strauss D, Eyman R K. Mortality of people with mental retardation in California with and without Down's syndrome, 1986-1991. American Journal on Mental Retardation 1996; 100: 643-53.
World Health Organization. The ICD-10 Classification of Mental and Behavioural Disorders: Clinical Descriptions and Diagnostic Guidelines. Geneva: WHO, 1992.

FURTHER READING
Bouras N, ed. Mental Health in Mental Retardation: Recent Advances and Practices. Cambridge: Cambridge University Press, 1995.
(Textbook highlighting many of the psychiatric disorders in people with learning disability.)

Haveman M J. Epidemiological issues in mental retardation. Current Opinion in Psychiatry 1996; 6: 305-11.
(Review article of concurrent epidemiological issues affecting the learning-disabled population.)

Freeman C, Tyrer P. Research Methods in Psychiatry: A Beginner's Guide. London: Royal College of Psychiatrists, 1989.
(Textbook describing basic psychiatric epidemiological principles.)

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