The incidence of mental ill-health in adults with intellectual disabilities.
MD thesis, University of Glasgow.
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Adults with intellectual disabilities account for a minor proportion of the population, with reported prevalence rates in developed countries in the range of 3-6 per 1000 adults (Beange & Taplin, 1996; McGrother et al, 2002; McConkey et al, 2006) but have very high health needs and thus make up a proportionally larger section of the population with illness. Although it has been demonstrated that adults with intellectual disabilities have a higher prevalence of mental ill-health when compared to that reported for the general population (Bailey, 2008; Hassiotis et al, 2008; Cooper et al, 2007; Cooper & Bailey, 2001; Lund, 1985a; Corbett,1979), and some studies have shown a degree of persistence of behavioural problems and affective symptoms over time (Thompson & Reid, 2002; Collishaw et al, 2003), there is insufficient evidence to answer the question of whether this high prevalence is due to a high level of enduring mental ill-health or a high incidence of mental health, or indeed a combination of the two. To date, three studies (Holland et al, 2000; Zigman et al, 2004; van Schrojenstein Lantman-de Valk et al, 1997) have measured the incidence of dementia and only one study (van Schrojenstein Lantman-de Valk et al, 1997) has attempted to measure the incidence of affective disorder. No study has measured the overall incidence of mental ill-health in this population.
Similarly, although the population based prevalence studies by Deb et al (2001a), Cooper et al (2007) and Bailey (2008) have identified some associations with mental ill-health (female gender, severe intellectual disabilities, past psychiatric history, not living with a family carer, smoking, life events, urinary incontinence and not having immobility), it is unknown whether some of these factors are cause or effect. No study to date has identified any adult risk factors for the onset of all types of mental-ill health in adults with all levels and causes of intellectual disabilities. As a consequence, our current knowledge of the epidemiology of mental ill-health in this population is limited and almost non-existent with regard to incidence and predictive factors of mental ill-health. The aim of this study was to measure the 2 year incidence rate of all types of mental ill-health in a population based sample of adults with intellectual disabilities with sufficient numbers to allow investigation of possible predictive factors for the onset of mental ill-health and examine the 2 year chronicity of mental-ill health in this population.
A large scale population based study of the prevalence of mental ill-health in adults with intellectual disabilities living in Glasgow, undertaken during 2002-2003 (Cooper et al, 2007), provided the opportunity to carry out a prospective longitudinal cohort design study, with the prevalence study providing the sample and baseline data. The sample size was 651 with a cohort retention rate of 70%. All participants were assessed using a two stage process (screening then detailed psychiatric assessment of potential cases) at baseline and at the 2 year follow-up interview. A modified version of the PAS-ADD checklist (Moss et al, 1998), with a reduced threshold for caseness to increase sensitivity, plus a problem behaviour checklist, pervasive developmental disorders checklist, and past 2 years mental health needs questionnaire was used to screen for mental ill-health occurring at any point during the two year follow up. All participants with identified episodes of mental ill-health were referred to the Glasgow University Centre for Excellence in Developmental Disabilities for detailed psychiatric assessment using PPS-LD (Cooper, 1997), checklists for problem behaviour, ADHD and pervasive developmental disorders and the Test for Severe Impairment (Albert & Cohen, 1992) (for possible dementia) and consensus diagnosis according to clinician, DC-LD, ICD-10-DCR and DSM-IV–TR criteria. Incidence and recovery rates were calculated. Standardised incident ratios were calculated by comparing the findings with reported rates for the general population. Stepwise binary logistic regression was used to examine factors independently related to the incidence and chronicity of mental ill-health.
The two year incidence of episodes of mental ill-health in adults with intellectual disabilities according to clinical diagnosis was 16.3%. This incidence rate is similar to the incidence rate of mental disorders in the general population but the type and proportion of individual disorders that accounted for this rate was different. Approximately 20% of this incidence rate was accounted for by problem behaviour, the incidence of psychosis, bipolar affective disorder and early onset dementia was very much higher than that reported for the general population with standardised incident ratios of 9.93 (95% CI 2.05-29.02), 100.20 (95% CI 12.14-361.96) and 66.67 (95% CI 18.16-170.69) respectively. The incidence of substance misuse and anxiety disorders was lower than that reported for the general population with standardised incident ratios of 0.04 (95% CI 0.00-0.24) and 0.17 (95% CI 0.06-0.37) respectively, although the lowered rate of anxiety disorders might be due to the methodological limitations of this study. Factors found to be predictive of episodes of mental ill-health (excluding problem behaviour, dementia, and delirium) were, in order of decreasing strength of association: not living with a family carer, not having immobility, mental ill-health in the past, more severe intellectual disabilities, abuse/adversity in adulthood, and urinary incontinence.
A high level of chronic mental ill-health was found with a 2 year recovery rate of only 32.5%. Factors identified as associated with the endurance of mental ill-health (excluding problem behaviours) in adults with intellectual disabilities were, in decreasing order of strength of association: problem behaviour, not having Down’s syndrome, not having immobility and smoking.
The overall incidence of mental ill-health in adults with intellectual disabilities is similar to that reported for the general population but the type and proportion of disorders accounting for this is different. There is high level of enduring mental ill-health in adults with intellectual disabilities. It appears that the high point prevalence of mental ill-health in adults with intellectual disabilities compared to the general population is accounted for more by a higher level of endurance of mental ill-health than by a higher incidence. The identification of risk factors for the onset of mental ill-health means that hypothesis based studies, leading on to the development of interventions and then randomised controlled trials are now possible.
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