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a Centre for Health and Ageing, Edith Cowan University, Perth, Australia
b Centre for Human Genetics, Edith Cowan University, Perth, Australia
c Disability Services Commission of Western Australia, Perth, Australia
d School of Health, University of New England, Armidale, Australia.
A.H. Bittles, Centre for Health and Ageing, Edith Cowan University, 100 Joondalup Drive, Perth WA 6027, Australia E-mail: a.bittles{at}ecu.edu.au.
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Abstract |
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Methods. Since 1953, the Disability Services Commission of Western Australia has maintained a database of persons diagnosed with intellectual disability. The database was used to calculate survival probabilities on a total of 8724 individuals, 7562 of whom were still alive at the time of sampling in December 2000.
Results. Kaplan-Meier survival plots showed a strong negative association between severity of intellectual disability and survival, with median life expectancies of 74.0, 67.6, and 58.6 years for people with mild, moderate, and severe levels of handicap. Significant negative associations also were observed with male gender, Indigenous Australian parentage, and individuals diagnosed with a specific genetic disorder.
Conclusions. The findings indicate a major and expanding increase in the service requirements of this aging, intellectually disabled population during the past two generations.
THROUGHOUT the course of the 20th century, there was a consistent trend toward increased life expectancy in virtually all developed and developing countries. Although much of the research conducted on aging and life expectancy in the general population has been targeted toward assessing health and social needs, relatively little attention has been paid to disadvantaged population subgroups, including those with intellectual disability. On a global basis, 0.3% to 0.5% of children have been reported to be affected by severe intellectual disability (ID), with intelligence quotient (IQ) scores of less than 50 and deficits in adaptive behavior (1). Comparable figures for mild ID, with IQ levels between 50 and 70, show much greater variation and range from 0.2% to 4.0% (2). Estimates of ID in Australia show that between 1.7% and 1.9% of the population are affected (3). However, as in most countries, there may be some degree of underestimation, especially of mild cases and among those living in remote rural areas.
Evidence from the United Kingdom and the United States has indicated significant improvements in the life expectancy of persons with ID (4)(5), and, for milder forms of disability, life expectancies are now almost comparable to those in the general population (6). The principal focus of the present study was to document changes in the age profile of those receiving services for ID within Western Australia during the past 50 years and to determine patterns of life expectancy based on severity and a number of other factors.
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Methods |
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The American Association on Mental Retardation classification is used to define ID (7), with services provided if IQ testing and an adaptive behavior assessment minimally indicate a mild level of ID (IQ <70), with symptoms manifest before 18 years of age. Wechsler intelligence scales are the preferred tool for assessing intellectual ability (mild ID, 55 to 69 points; moderate ID, 40 to 54 points; severe ID, <40 points). Other psychometric tests may be used if physical abilities or communication skills are impaired. Clinical diagnosis is based on a modification of Heber (8).
Information was abstracted from the DSC database for a range of variables, including background demographic characteristics, level of intellectual disability, clinical diagnosis, and the presence of genetic disorders. For deceased individuals, details were abstracted on the date and cause of death. These mortality data were supplemented through linkage with the Mortality Register maintained by the Health Department of Western Australia, which contains information derived from all death certificates issued in the state since 1969.
Statistical Assessment
A total of 9824 cases were identified. In 1100 cases, mainly children younger than 5 years of age, a formal ID assessment had not been conducted, and they were excluded from analysis. Survival estimates were calculated on the remaining 8724 persons, including 242 Indigenous Australians, who met the inclusion criteria, using date of birth as entry and date of death or December 31, 2000 as exit. Differences in survival were compared for gender, indigenous parentage, level of ID, and diagnosed genetic disorder, using Kaplan-Meier survival probabilities and log-rank tests to assess differences in the plotted curves. All data analyses were performed using Stata for PC version 7.0 (Stata Corp., College Station, TX).
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Results |
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The 50% survival probability for the whole ID population was 68.6 years. Comparable 50% survival probabilities in the general Australian population are 75.6 years for men and 81.2 years for women (9). Male participants in the study group had a shorter median lifespan, 66.7 years compared with 71.5 years for the female participants (p < .001). Survival also was significantly reduced for Indigenous Australians compared with non-Indigenous Australians (p < .0001). As shown in Fig. 1, there was a highly significant negative association between survival and severity of ID (p < .0001), with 50% survival probabilities of 74.0, 67.6, and 58.6 years in persons with mild, moderate, and severe levels of ID, respectively. Finally, median survival was reduced in persons with a genetic disorder, 60.1 years, compared to 72.2 years where no genetic diagnosis had been made (p < .0001).
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Discussion |
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The issue of increasing life expectancy has taken on greater significance with the recent call, addressed to the World Health Organization, for research into practices that successfully promote longevity and healthy aging in persons with ID (10). Some studies have reported that morbidity patterns in people with ID do not differ markedly from those of the general aging population (4)(5)(11), which is reflected in the life expectancy figures obtained in this study (Fig. 1). The specifically reduced life expectancy for Indigenous Australians with ID may primarily be indicative of the decreased survival experienced by the indigenous population in general (12).
By the time they reach middle-age, individuals with ID have a greater tendency toward obesity and decreased physical fitness compared with the general population (13)(14). Enhanced predisposition to the development of psychiatric problems (4)(15), osteoporosis (16), thyroid disorders, nonischemic heart disease, various forms of sensory impairment (17), and early onset of dementia (18), has been reported. With this in mind, the results of the present study should prove useful in evaluating the future health, residential, and care needs of this significant sector of the population, especially because substantial increases in life expectancy are occurring across the spectrum of ID severity.
The changing patterns of service delivery introduced during the past decade for people with ID have placed an increasing emphasis on home and/or community care (19). Despite the policy of deinstitutionalization, the demands on health services are likely to increase over the coming years with a consequent need for the development of more comprehensive preventive and promotional health strategies.
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Acknowledgments |
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Received December 19, 2001
Accepted February 5, 2002
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