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Epidemiology of learning disability and comorbid conditions
Aetiology and epidemiology
Epidemiology of learning disability and comorbid conditions
What’s new? • White et al. (2005) looked at the prevalence of learning disability and comorbid mental illness in an Australian community sample of 42,664 individuals. Fourteen percent had an anxiety disorder; depressive disorder was also common, with 8% fulfilling the criteria for this diagnosis
Vee P Prasher Hassan M Kapadia
• Zigman et al. (2004) found rates of dementia in adults with learning disability without Down syndrome to be equivalent or lower than would be expected compared with the general population
Abstract This contribution describes the basic principles of epidemiology and epidemiological strategies, covering specific issues in learning disability related to the definition of caseness, difficulties in diagnosis, the behaviour–mental illness dichotomy, the target population, sample size, and direct and indirect studies. Examples from specific epidemiological studies relating to aetiology, mortality rates and prevalence studies of psychiatric and physical disorders are used as illustration. Evidence from epidemiological studies suggests that people with learning 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 learning disability, a number of concerns remain regarding epidemiological methods. Several important issues require further investigation, including the incidence of physical disorders in the learning-disabled population; therapeutic intervention studies; and the causes of physical health morbidity.
• Malfa et al. (2004) carried out a clinical review of 166 people with learning disability and found the prevalence of epilepsy in patients without disability was 1%; in learning disability it was 25.5%. In patients with both cerebral palsy and learning disability, this rate was 40%
• 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. Table 1 lists commonly used measures of disease frequency.
Epidemiological strategies
Keywords epidemiology; learning disability; mental illness; physical
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 learning 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-101 and DSM-IV.2 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 learning disability prevents the conclusions being generalized to people who may have mild or moderate learning disability. There are a number of sampling techniques, some of which are described below.
disorder; psychiatric disorder
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:
Vee P Prasher MMedSci MRCPsych MD PhD FIASSID is a Consultant Psychiatrist and Honorary Senior Research Fellow in Neurodevelopmental Psychiatry based in Birmingham, UK. He qualified from Birmingham University and has completed three postgraduate degrees. His main research interests include ageing and physical health issues of adults with Down syndrome. He has published numerous research articles and edited a number of textbooks. He was recently made a fellow of IASSID. Hassan M Kapadia BMedSc MBChB is a Senior House Officer in Neurodevelopmental Psychiatry, working for Dr Prasher in Birmingham, UK. He qualified from Birmingham University and has been on the All Birmingham Psychiatry Rotation since completing House Jobs.
PSYCHIATRY 5:9
302
© 2006 Published by Elsevier Ltd.
Aetiology and epidemiology
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 specific 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 Table 2.
Commonly used measures of disease frequency Measure
Definition
Point prevalence rate
Refers to the proportion of people in a defined population who are affected by the disorder at a given point in time Proportion of people who are affected by a disorder at any time within a stated period Measure of new episode of illness: the proportion of formerly well subjects who developed an illness in a defined period of time (usually 1 year) The ratio of incidence of an outcome in those that are exposed to a certain risk factor compared to the incidence in an exposed group The ratio of the odds of disease in exposed individuals relative to the unexposed Meaningful way of expressing the benefit of any intervention: relates to how many individuals need to be treated for one individual to benefit
Period prevalence rate
Incidence rate
Relative risk (RR)
Odd ratio (OR)
Number needed to treat (NNT)
Specific issues in learning disability Definition of caseness – how applicable accepted standardized diagnostic criteria (i.e. ICD-10, DSM-IV) are for people with learning 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. Difficulties in diagnosis – standardized diagnostic assessments and tools are not readily available for the learning disability population. Few have been widely accepted or have good
Epidemiological research methods Cross-sectional study Used to measure the prevalence of an illness or event. Observational and descriptive. A single measurement at one moment in time. Results usually limited by the study’s inability to identify cause or relationship
Table 1
• Simple random sampling (the basic sampling technique in which a group of subjects (a sample) is selected for study from a larger group (a population). Each individual is chosen entirely by chance and each member of the population has an equal chance of being included in the sample). • Systematic sampling (units are selected from the population at a regular interval (e.g. once an hour, every other lot). • Stratified sampling (a stratified sample is obtained by taking samples from each stratum or sub-group of the population being studied). • Multi-stage sampling (sampling is undertaken In two or more stages). • Non-random sampling (data collection in which some individuals in the population have a greater chance of being selected). Sample size – research proposals often focus on the practicalities of studies and do not place as much importance as they should on size of the sample. The sample size of a study 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 learning disability.
PSYCHIATRY 5:9
Case-control study Usually involves two groups: one group of people with the disease and another unaffected (control group). The relationship of aetiological factors for the disease can then be examined by comparing measures between the groups. Matching of underlying variables is an important issue. Selection of subjects needs to be unbiased Cohort study A defined group of individuals is studied, usually over a defined period of time, to ascertain the frequency with which selected characteristics change or develop. Can be both prospective and retrospective. Often used to ascertain the effect of exposure to particular hazards. Can be time-consuming and expensive, and can rapidly become out of date Controlled clinical trial Intervention studies that are usually prospective and experimental. Aims to determine the effects of an intervention or therapeutic measure. Two groups must be matched prior to any intervention: one group is given the treatment and the other usually the placebo. Groups are followed up over time and compared on a number of given measures Table 2
303
© 2006 Published by Elsevier Ltd.
Aetiology and epidemiology
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 longstanding 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 learning 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 learning 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 provide limited information.
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.6 McGuigan et al. confirmed that the age-specific standardized mortality rates for people with learning disabilities are often higher than in the general population (this applies to both men and women).7 Prevalence studies of psychiatric disorders: a number of studies have investigated psychiatric disorders among adults with learning disability8,9; these have generally been point prevalence studies. • Cooper and Bailey assessed 207 adults with learning disability, and found a psychiatric disorder rate of 49.2%. Adults with more severe learning disability had higher rates of additional psychiatric disorders.8 • In contrast, Crews et al. 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.10 • More recently, White et al. looked at the prevalence of learning disability and comorbid mental illness in an Australian community sample of 42,664 individuals living at home or in cared accommodation. The prevalence of learning disability in the sampled population was 1.25%. Fourteen percent of these people had an anxiety disorder. In concordance with Crews’ study, depressive disorder was also common, with 8% fulfilling the criteria for this diagnosis. Psychotic disorder had been diagnosed in 1.3% of the learning disability population.11 • Prasher investigated psychiatric disorders in 201 adults with DS aged 16 years and over: the rate of psychiatric disorder was 28.9%. The commonest disorders were dementia of Alzheimer’stype, depression, conduct disorders and obsessive–compulsive disorders.12 • Although dementia of Alzheimer’s-type is common in DS, Zigman et al. found rates of dementia in adults with learning disability without DS to be equivalent to or lower than would be expected compared to the general population.13 A longitudinal study focusing on the incidence and prevalence of dementia in 126 adults with learning disability without DS over the age of 65 was conducted. Participants were tested up to three times each at 18-month intervals over 4.5 years. This study was limited by a relatively small sample size and possibly by a ‘healthy survivor effect’. • La Malfa et al. carried out a clinical review of 166 people with learning disability living in an institution, using the scale of Pervasive Developmental Disorder in Mentally Retarded Persons (PDD-MRS) to assess the prevalence of PDD. This raised the prevalence of pervasive developmental disorder in this population from 7.8% to 39.2%. However, a limitation of the study was that the sample was derived entirely from an institutionalized population.14
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 learning disability is close to 3%. Roeleveld et al. 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.3 Individuals with mild disability represent the largest proportion (approximately 2.5% of the whole population); moderate learning disability involves approximately 0.4% of the population, and severe and profound levels combined account for approximately 0.1%. Epidemiological studies have been undertaken looking at the causes of learning disability, including demographic, parental and environmental factors. Down syndrome (DS), 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 DS.4 Mortality rates: Strauss and Eyman investigated mortality rates in a large population of people with learning disabilities. Up to the age of 35 years, mortality rates for people with DS were comparable with those for people with learning disability due to other causes. Subsequently, however, the mortality rates for individuals with DS doubled every 6.4 years, compared with 9.6 years for people without DS.5 Frid et al. investigated mortality in DS in relation to the presence or absence of congenital malformations,
PSYCHIATRY 5:9
Prevalence studies of physical disorders: a number of studies have looked at the prevalence of physical health problems 304
© 2006 Published by Elsevier Ltd.
Aetiology and epidemiology
in people with learning disability.15,16 As with psychiatric studies, individuals with learning 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 found that epilepsy, cerebral palsy and neurological impairment were the commonest conditions. Fortytwo percent of those studied reported having no major or chronic physical problems.17 In ageing adults with learning disabilities, common health problems include: • hearing and visual impairment • mobility problems • heart conditions • diabetes • fractures and osteoporosis. McDermott et al. investigated the prevalence of epilepsy in adults with learning disability using a retrospective cohort study to compare 741 adults with learning disability and related diagnoses with 1806 adults without a disability. Epilepsy prevalence in patients without disability was 1%, where as in learning disability it was 25.5%. However, in patients with both cerebral palsy and learning disability, this rate was 40%.17
Evidence from epidemiological studies suggests that people with learning 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 learning disability, a number of concerns remain regarding epidemiological methods. Several important issues require further investigation, including: • the incidence of physical disorders in the learning-disabled population • therapeutic intervention studies • the causes of physical health morbidity. ◆
4 Stolwijk A M, Jongbloet P H, Zielhuis G A, Gabreels F J. Seasonal variation in the prevalence of Down syndrome at birth: a review. J Epidemiol Community Health 1997; 51: 350–3. 5 Strauss D, Eyman R K. Mortality of people with mental retardation in California with and without Down’s syndrome, 1986–1991. Am J Ment Retard 1996; 100: 643–53. 6 Frid C, Drott P, Lundell B, Rasmussen F, Anneren G. Mortality in Down’s syndrome in relation to congenital malformations. J Intellect Disabil Res 1999; 43: 234–41. 7 McGuigan S M, Hollins S, Attard M. Age-specific standardized mortality rates in people with learning disability. J Intellect Disabil Res 1995; 39: 527–31. 8 Cooper S-A, Bailey N M. Psychiatric disorders amongst adults with learning disabilities – prevalence and relationship to ability level. Ir J Psychol Med 2001; 18: 45–53. 9 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. Aust NZ J Dev Disabil 1989; 15: 241–55. 10 Crews W D, Bonaventura S, Rowe F. Dual diagnosis: prevalence of psychiatric disorders in a large state residential facility for individuals with mental retardation. Am J Ment Retard 1994; 98: 688–731. 11 White P, Chant D, Edwards N, Townsend C, Waghorn G. Prevalence of intellectual disability and comorbid mental illness in an Australian community sample. Aust NZ J Psychiatry 2005; 39: 395–400. 12 Prasher V P. Prevalence of psychiatric disorders in adults with Down’s syndrome. Eur J Psychiatry 1995; 9: 77–82. 13 Zigman W B, Schupf N, Devenny D A et al. Incidence and prevalence of dementia in elderly adults with mental retardation without Down’s syndrome. Am J Ment Retard 2004; 109: 126–41. 14 La Malfa G, Lassi S, Bertelli M, Salvini R, Placidi G F. Autism and intellectual disability: a study of prevalence on a sample of the Italian population. J Intellect Disabil Res 2004; 48: 262–7. 15 Day K, Jancar J. Mental and physical health and ageing in mental handicap: a review. J Intellect Disabil Res 1994; 38: 241–56. 16 Hand J E. Report of a national survey of older people with lifelong intellectual handicap in New Zealand. J Intellect Disabil Res 1994; 38: 275–87. 17 McDermott S, Moran R, Platt T, Wood H, Isaac T, Dasari S. Prevalence of epilepsy in adults with mental retardation and related disabilities in primary care. Am J Ment Retard 2005; 110: 48–56.
References 1 World Health Organization. The ICD-10 classification of mental and behavioural disorders: clinical descriptions and diagnostic guidelines. Geneva: WHO, 1992. 2 American Psychiatric Association. Diagnostic and statistical manual of mental disorders. 4th edn. (DSM-IV). Washington, DC: APA, 1994. 3 Roeleveld N, Zielhuis G A, Gabreels F. The prevalence of mental retardation: a critical review of recent literature. Dev Med Child Neurol 1997; 39: 125–32.
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.) Freeman C, Tyrer P. Research methods in psychiatry: a beginner’s guide. London: Royal College of Psychiatrists, 1989. (Textbook describing basic psychiatric epidemiological principles.) Haveman M J. Epidemiological issues in mental retardation. Curr Opin Psychiatry 1996; 6: 305–11. (Review article of concurrent epidemiological issues affecting the learning-disabled population.)
Conclusion
PSYCHIATRY 5:9
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© 2006 Published by Elsevier Ltd.
Epidemiology of learning disability and comorbid conditions
What’s new? • White et al. (2005) looked at the prevalence of learning disability and comorbid mental illness in an Australian community sample of 42,664 individuals. Fourteen percent had an anxiety disorder; depressive disorder was also common, with 8% fulfilling the criteria for this diagnosis
Vee P Prasher Hassan M Kapadia
• Zigman et al. (2004) found rates of dementia in adults with learning disability without Down syndrome to be equivalent or lower than would be expected compared with the general population
Abstract This contribution describes the basic principles of epidemiology and epidemiological strategies, covering specific issues in learning disability related to the definition of caseness, difficulties in diagnosis, the behaviour–mental illness dichotomy, the target population, sample size, and direct and indirect studies. Examples from specific epidemiological studies relating to aetiology, mortality rates and prevalence studies of psychiatric and physical disorders are used as illustration. Evidence from epidemiological studies suggests that people with learning 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 learning disability, a number of concerns remain regarding epidemiological methods. Several important issues require further investigation, including the incidence of physical disorders in the learning-disabled population; therapeutic intervention studies; and the causes of physical health morbidity.
• Malfa et al. (2004) carried out a clinical review of 166 people with learning disability and found the prevalence of epilepsy in patients without disability was 1%; in learning disability it was 25.5%. In patients with both cerebral palsy and learning disability, this rate was 40%
• 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. Table 1 lists commonly used measures of disease frequency.
Epidemiological strategies
Keywords epidemiology; learning disability; mental illness; physical
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 learning 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-101 and DSM-IV.2 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 learning disability prevents the conclusions being generalized to people who may have mild or moderate learning disability. There are a number of sampling techniques, some of which are described below.
disorder; psychiatric disorder
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:
Vee P Prasher MMedSci MRCPsych MD PhD FIASSID is a Consultant Psychiatrist and Honorary Senior Research Fellow in Neurodevelopmental Psychiatry based in Birmingham, UK. He qualified from Birmingham University and has completed three postgraduate degrees. His main research interests include ageing and physical health issues of adults with Down syndrome. He has published numerous research articles and edited a number of textbooks. He was recently made a fellow of IASSID. Hassan M Kapadia BMedSc MBChB is a Senior House Officer in Neurodevelopmental Psychiatry, working for Dr Prasher in Birmingham, UK. He qualified from Birmingham University and has been on the All Birmingham Psychiatry Rotation since completing House Jobs.
PSYCHIATRY 5:9
302
© 2006 Published by Elsevier Ltd.
Aetiology and epidemiology
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 specific 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 Table 2.
Commonly used measures of disease frequency Measure
Definition
Point prevalence rate
Refers to the proportion of people in a defined population who are affected by the disorder at a given point in time Proportion of people who are affected by a disorder at any time within a stated period Measure of new episode of illness: the proportion of formerly well subjects who developed an illness in a defined period of time (usually 1 year) The ratio of incidence of an outcome in those that are exposed to a certain risk factor compared to the incidence in an exposed group The ratio of the odds of disease in exposed individuals relative to the unexposed Meaningful way of expressing the benefit of any intervention: relates to how many individuals need to be treated for one individual to benefit
Period prevalence rate
Incidence rate
Relative risk (RR)
Odd ratio (OR)
Number needed to treat (NNT)
Specific issues in learning disability Definition of caseness – how applicable accepted standardized diagnostic criteria (i.e. ICD-10, DSM-IV) are for people with learning 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. Difficulties in diagnosis – standardized diagnostic assessments and tools are not readily available for the learning disability population. Few have been widely accepted or have good
Epidemiological research methods Cross-sectional study Used to measure the prevalence of an illness or event. Observational and descriptive. A single measurement at one moment in time. Results usually limited by the study’s inability to identify cause or relationship
Table 1
• Simple random sampling (the basic sampling technique in which a group of subjects (a sample) is selected for study from a larger group (a population). Each individual is chosen entirely by chance and each member of the population has an equal chance of being included in the sample). • Systematic sampling (units are selected from the population at a regular interval (e.g. once an hour, every other lot). • Stratified sampling (a stratified sample is obtained by taking samples from each stratum or sub-group of the population being studied). • Multi-stage sampling (sampling is undertaken In two or more stages). • Non-random sampling (data collection in which some individuals in the population have a greater chance of being selected). Sample size – research proposals often focus on the practicalities of studies and do not place as much importance as they should on size of the sample. The sample size of a study 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 learning disability.
PSYCHIATRY 5:9
Case-control study Usually involves two groups: one group of people with the disease and another unaffected (control group). The relationship of aetiological factors for the disease can then be examined by comparing measures between the groups. Matching of underlying variables is an important issue. Selection of subjects needs to be unbiased Cohort study A defined group of individuals is studied, usually over a defined period of time, to ascertain the frequency with which selected characteristics change or develop. Can be both prospective and retrospective. Often used to ascertain the effect of exposure to particular hazards. Can be time-consuming and expensive, and can rapidly become out of date Controlled clinical trial Intervention studies that are usually prospective and experimental. Aims to determine the effects of an intervention or therapeutic measure. Two groups must be matched prior to any intervention: one group is given the treatment and the other usually the placebo. Groups are followed up over time and compared on a number of given measures Table 2
303
© 2006 Published by Elsevier Ltd.
Aetiology and epidemiology
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 longstanding 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 learning 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 learning 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 provide limited information.
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.6 McGuigan et al. confirmed that the age-specific standardized mortality rates for people with learning disabilities are often higher than in the general population (this applies to both men and women).7 Prevalence studies of psychiatric disorders: a number of studies have investigated psychiatric disorders among adults with learning disability8,9; these have generally been point prevalence studies. • Cooper and Bailey assessed 207 adults with learning disability, and found a psychiatric disorder rate of 49.2%. Adults with more severe learning disability had higher rates of additional psychiatric disorders.8 • In contrast, Crews et al. 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.10 • More recently, White et al. looked at the prevalence of learning disability and comorbid mental illness in an Australian community sample of 42,664 individuals living at home or in cared accommodation. The prevalence of learning disability in the sampled population was 1.25%. Fourteen percent of these people had an anxiety disorder. In concordance with Crews’ study, depressive disorder was also common, with 8% fulfilling the criteria for this diagnosis. Psychotic disorder had been diagnosed in 1.3% of the learning disability population.11 • Prasher investigated psychiatric disorders in 201 adults with DS aged 16 years and over: the rate of psychiatric disorder was 28.9%. The commonest disorders were dementia of Alzheimer’stype, depression, conduct disorders and obsessive–compulsive disorders.12 • Although dementia of Alzheimer’s-type is common in DS, Zigman et al. found rates of dementia in adults with learning disability without DS to be equivalent to or lower than would be expected compared to the general population.13 A longitudinal study focusing on the incidence and prevalence of dementia in 126 adults with learning disability without DS over the age of 65 was conducted. Participants were tested up to three times each at 18-month intervals over 4.5 years. This study was limited by a relatively small sample size and possibly by a ‘healthy survivor effect’. • La Malfa et al. carried out a clinical review of 166 people with learning disability living in an institution, using the scale of Pervasive Developmental Disorder in Mentally Retarded Persons (PDD-MRS) to assess the prevalence of PDD. This raised the prevalence of pervasive developmental disorder in this population from 7.8% to 39.2%. However, a limitation of the study was that the sample was derived entirely from an institutionalized population.14
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 learning disability is close to 3%. Roeleveld et al. 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.3 Individuals with mild disability represent the largest proportion (approximately 2.5% of the whole population); moderate learning disability involves approximately 0.4% of the population, and severe and profound levels combined account for approximately 0.1%. Epidemiological studies have been undertaken looking at the causes of learning disability, including demographic, parental and environmental factors. Down syndrome (DS), 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 DS.4 Mortality rates: Strauss and Eyman investigated mortality rates in a large population of people with learning disabilities. Up to the age of 35 years, mortality rates for people with DS were comparable with those for people with learning disability due to other causes. Subsequently, however, the mortality rates for individuals with DS doubled every 6.4 years, compared with 9.6 years for people without DS.5 Frid et al. investigated mortality in DS in relation to the presence or absence of congenital malformations,
PSYCHIATRY 5:9
Prevalence studies of physical disorders: a number of studies have looked at the prevalence of physical health problems 304
© 2006 Published by Elsevier Ltd.
Aetiology and epidemiology
in people with learning disability.15,16 As with psychiatric studies, individuals with learning 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 found that epilepsy, cerebral palsy and neurological impairment were the commonest conditions. Fortytwo percent of those studied reported having no major or chronic physical problems.17 In ageing adults with learning disabilities, common health problems include: • hearing and visual impairment • mobility problems • heart conditions • diabetes • fractures and osteoporosis. McDermott et al. investigated the prevalence of epilepsy in adults with learning disability using a retrospective cohort study to compare 741 adults with learning disability and related diagnoses with 1806 adults without a disability. Epilepsy prevalence in patients without disability was 1%, where as in learning disability it was 25.5%. However, in patients with both cerebral palsy and learning disability, this rate was 40%.17
Evidence from epidemiological studies suggests that people with learning 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 learning disability, a number of concerns remain regarding epidemiological methods. Several important issues require further investigation, including: • the incidence of physical disorders in the learning-disabled population • therapeutic intervention studies • the causes of physical health morbidity. ◆
4 Stolwijk A M, Jongbloet P H, Zielhuis G A, Gabreels F J. Seasonal variation in the prevalence of Down syndrome at birth: a review. J Epidemiol Community Health 1997; 51: 350–3. 5 Strauss D, Eyman R K. Mortality of people with mental retardation in California with and without Down’s syndrome, 1986–1991. Am J Ment Retard 1996; 100: 643–53. 6 Frid C, Drott P, Lundell B, Rasmussen F, Anneren G. Mortality in Down’s syndrome in relation to congenital malformations. J Intellect Disabil Res 1999; 43: 234–41. 7 McGuigan S M, Hollins S, Attard M. Age-specific standardized mortality rates in people with learning disability. J Intellect Disabil Res 1995; 39: 527–31. 8 Cooper S-A, Bailey N M. Psychiatric disorders amongst adults with learning disabilities – prevalence and relationship to ability level. Ir J Psychol Med 2001; 18: 45–53. 9 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. Aust NZ J Dev Disabil 1989; 15: 241–55. 10 Crews W D, Bonaventura S, Rowe F. Dual diagnosis: prevalence of psychiatric disorders in a large state residential facility for individuals with mental retardation. Am J Ment Retard 1994; 98: 688–731. 11 White P, Chant D, Edwards N, Townsend C, Waghorn G. Prevalence of intellectual disability and comorbid mental illness in an Australian community sample. Aust NZ J Psychiatry 2005; 39: 395–400. 12 Prasher V P. Prevalence of psychiatric disorders in adults with Down’s syndrome. Eur J Psychiatry 1995; 9: 77–82. 13 Zigman W B, Schupf N, Devenny D A et al. Incidence and prevalence of dementia in elderly adults with mental retardation without Down’s syndrome. Am J Ment Retard 2004; 109: 126–41. 14 La Malfa G, Lassi S, Bertelli M, Salvini R, Placidi G F. Autism and intellectual disability: a study of prevalence on a sample of the Italian population. J Intellect Disabil Res 2004; 48: 262–7. 15 Day K, Jancar J. Mental and physical health and ageing in mental handicap: a review. J Intellect Disabil Res 1994; 38: 241–56. 16 Hand J E. Report of a national survey of older people with lifelong intellectual handicap in New Zealand. J Intellect Disabil Res 1994; 38: 275–87. 17 McDermott S, Moran R, Platt T, Wood H, Isaac T, Dasari S. Prevalence of epilepsy in adults with mental retardation and related disabilities in primary care. Am J Ment Retard 2005; 110: 48–56.
References 1 World Health Organization. The ICD-10 classification of mental and behavioural disorders: clinical descriptions and diagnostic guidelines. Geneva: WHO, 1992. 2 American Psychiatric Association. Diagnostic and statistical manual of mental disorders. 4th edn. (DSM-IV). Washington, DC: APA, 1994. 3 Roeleveld N, Zielhuis G A, Gabreels F. The prevalence of mental retardation: a critical review of recent literature. Dev Med Child Neurol 1997; 39: 125–32.
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.) Freeman C, Tyrer P. Research methods in psychiatry: a beginner’s guide. London: Royal College of Psychiatrists, 1989. (Textbook describing basic psychiatric epidemiological principles.) Haveman M J. Epidemiological issues in mental retardation. Curr Opin Psychiatry 1996; 6: 305–11. (Review article of concurrent epidemiological issues affecting the learning-disabled population.)
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