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Support for persons with mental retardation (MR, Note 1) (1) draws on a range of social, health, and educational services. Direct costs are considerable. In the Netherlands, for example, MR, is the greatest single source of health care costs, at 8.1% of the total (2). This population is also described as the most costly diagnostic group in the UK (3). Research on the prevalence of MR and increasing understanding of prevalence patterns permit more rational planning of services and targeting of resources. According to the World Health Organization (WHO), the true overall prevalence of MR in industrialized countries is close to 3% (4,5). In the US, controversy exists as to whether the rate is 1% or 3%, while Scandinavian countries consider their true prevalence rate to be 1% (6). Community prevalence studies of MR span over 150 years of research and have been reviewed elsewhere (6–13). In the most recent review, rates of severe mental retardation (SMR) among those aged 5 to 19 years varied around an average rate of 3.8/1000 and were reported to be relatively stable across international studies conducted between 1960 and 1986 (6). Ascertainment of service prevalence closely approximated true prevalence for SMR. In contrast, there was huge variation in the prevalence rates for mild mental retardation (MMR), which ranged between 3.9 and 79.3/1000 for the 43 studies meeting inclusion criteria. In general, lower rates for MMR are thought to underestimate true prevalence; however, lower rates may also reflect such factors as the influence of improved environments, increased mean IQs in the populations studied, or altered awareness about the presence of persons with MR in communities where they are more integrated into the mainstream culture. Roeleveld and others comment that a striking association is found between MMR and social class, race, or parental occupation (6). Moreover, these authors note that specific etiologic factors, some of which are associated with poor medical care, may prevail in lower social classes and may explain some of the variation in MMR prevalence rates. Based on these observations, it might be predicted that the prevalence of MMR would be lower in a country such as Canada, where universal health care is available regardless of income and education, and where social policies are sensitive to the rights of minority groups and are directed at minimizing the impact of personal disabilities. Unlike Europe, the US and Canada do not have a tradition of maintaining registers for persons with MR from which prevalence data can be derived. Rather, target populations are typically ascertained by identifying those who receive services (defined as “service prevalence”). Estimates from such studies are therefore influenced by the extent to which persons with MR have access to and use services and the extent to which service personnel recognize, acknowledge, and report that their clients have MR. Three previous studies have examined the prevalence of MR in Canadian populations. McDonald reports a rate of 3.8/1000 for SMR, derived from a population aged 8 to 10 years and living in Quebec during 1966 to 1969 (14). McQueen and others, who also focused on children with SMR, found a similar rate of 3.7/1000 in a population aged 7 to 10 years living in the Maritime Provinces during 1980 (15). Baird and Sadovnick studied all levels of MR in British Columbia and quote a minimum prevalence of 7.7/1000 among a population aged 15 to 29 years, for whom ascertainment was best (16) . We provide new Canadian prevalence data for MR, derived from a large population-based study undertaken to explore mental health disturbances in young persons with MR. Our data were obtained for teenagers and young adults living in the Niagara Region of Ontario, where normalization and integration have been the prevailing philosophies of care over the past 25 years. The findings supplement existing literature on the prevalence of mild and severe MR and illuminate some of the issues outlined above. MethodsThe Study Setting The Niagara region is located in the southeastern corner of the province of Ontario, 140 km southeast of Toronto, Canada’s largest city. Its eastern border is separated from the US by the Niagara River. The region has a population of around 400 000 (17), with a mix of rural and urban lifestyles and socioeconomic circumstances reflecting the diversity found in other parts of the province (total population = 11.5 million [18]). At the time of the study, the region had a range of educational options and various generic and specialized services aimed at optimizing independence and integration of persons with disabilities into the larger community. Target Population Individuals were drawn from the population aged 14 to 20 years living in the Niagara region on 1 June 1994 (those born between 1 June 1973 and 31 May 1980). Here, we use the term “mental retardation” to refer to individuals we subsequently confirmed to be functioning at or below an IQ level of 75. We reserve the term “developmental problems” to refer to those identified by their community as likely to be eligible for the study, but for whom IQ had yet to be confirmed. Study Design We conducted the study in 3 stages: First, we identified young persons with developmental problems; second, we collected background information on this population (participants and nonparticipants); and third, we tested participants and screened nonparticipants to confirm MR. Identification of Young Persons With Developmental Problems We contacted all community agencies and institutions serving the population aged 14 to 20 years, including, but not restricted to, those mandated to serve persons with MR. These included educational (statutory and private), mental health, correctional, social, and provincial residential services and Native centres. School is mandatory in Canada up to age 16 years, and thus virtually all children aged 16 years or less with developmental problems can be identified through the school system. Extensive consultation took place, during which we described the study and provided information about participant eligibility. We consulted with personnel in each of the school boards, schools and specialized educational settings, and with the classroom teachers in individual schools. In our contact with statutory social services for the defined age range, we included generic services such as family and childrens’ services, centres for youth care, training and employment centres, and the 6 regional associations for community living (ACL). Each of the latter is responsible for providing services to persons with MR within defined geographical areas. We also consulted extensively with senior agency staff through to frontline workers. At this stage, criteria for possible inclusion in the study were broad so as to be overinclusive. They included any individual aged 14 to 20 years with the following characteristics:
Provided that criteria 1 or 2, or both, were met, there were no exclusionary criteria: individuals with sensory impairments (blindness or deafness), motor or muscular impairments, neurological impairment, or recognizable or unidentified genetic disorders were included. To maintain confidentiality, the investigators did not have direct access to the names of individuals identified by staff (although their initials, sex, and date of birth were provided for cross-referencing). School boards and agencies sent letters to all identified persons and their families describing the study and inviting them to participate. A further letter was sent out to families who had not responded, and this was followed by a telephone call from the student’s teacher or from agency staff. Research staff worked closely with teachers and agency staff to ensure that those families who had not responded were contacted and could decide whether they wished to participate. Once research staff made contact, they explained the study to the individual or family member, asked them whether they would like to participate, and obtained verbal consent from the individuals or their legal guardian (usually a parent). Arrangements were made to meet with the individuals to conduct a psychological assessment, usually in their home. Prior to the assessment, informed written consent was obtained from both participant (wherever possible, and using a simplified form) and parent.
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