The prevalence of mental retardation: a critical review of recent literature

Abstract

Mental retardation (AIR) is a serious and lifelong disability that places heavy demands on society and the health system. Since the first publication on this topic',theprevalenceofMR has been thoroughly studied for different purposes. Most prevalence studies are designed for the planning of services and establish an 'ascertained' prevalence rate, which is the number of cases officially recorded by the authorities'.The 'true'prevalence rate is the total number of mentally retarded people in a population, whether or not they require services,and is defined by the prevalence of M R at birth and the mortality rate.For mild mental retardation (MWR, I& 50-70) ' the true prevalence rate is more difficult to estimate than for severe mental retardation (SMR, IQ <50).Very often ascertained prevalence rates are mistaken for true prevalence rates. Moreover, the estimates of both rates are influenced by the design of the study, the assessment criteria used, and the method applied for the identification of case^^-^.As a result, differences in prevalence rates might partly reflect the true variation over populations and partly reveal discrepancies between studies and in the interpretation of the prevalence measure used. For instance, different frequencies are yielded by uses of the organic, psychological and social WHO criteria, which are reflected in impairment, disability and handicaps. The prevalence rates observed range from two to 85 per 1000. According to the WHO, the true prevalence rate of total MR in industrialised countries comes close to 3%".O, but in the United States controversy exists over whether the rate is 1% versus 3%5.7.'0.1', whereas the Scandinavian countries claim that the 1 % figure is their true pre~alence~.'~.'~. 'The aim ofthis annotation is to establish valid estimates of the true prevalence rates for SMR and AIMR in children of school age and to elucidate the variation in prevalence rates. Therefore the methodology of prevalence studies performed since 1960 was critically evaluated and a distinction was made between ascertained and true prevalence estimates. Methods of selectionA computerised literature search was conducted on MED-LINE regarding publications from 1981 to 1995, using the keywords'MR' and 'occurrence'. Only a few original articles and reviews were found; most papers were traced through references listed in these reviewss.'.'" l i and by browsing through relevant journals.Thirteen papers were unobtainable and 31 studies were excluded in accordance with the following criteria: 1) the study was restricted to institutionalised cases; 2) the population size was not given; 3) the age group studied exceeded the age range 5-19 years (school age) and no age structure was specified; 4) the I& levels studied were not specified; 5) hIR could not be distinguished from other disabilities. If rates for a specific population were described more than once, only the most recent publication was selected.As a result,43 original articles were included in this review. These are summarised in Table 1,l8 wi...

Implications of gene copy-number variation in health and diseases

“…Studies of large cohorts may help in detecting and confirming the roles of rare de novo CNVs in MR. MR occurs in 2-3% of newborns in the general population, however, its cause has remained elusive. 19 X-linked MR showed six overlapping duplications at the Xp11.22 in six unrelated males. Further, it was noted that this duplication covered a 320-kb region involving four genes (SMC1A, RIBC1, HSD17B10 and HUWE1), three candidates of which may convey the phenotype of MR. 19 Apart from duplication, many other forms of genetic variations such as point mutation in SMC1A and HUWE1 genes and a silent mutation in HSD17B10 gene conveyed the phenotypes of MR along with other distinguished characteristic.…”

“…19 X-linked MR showed six overlapping duplications at the Xp11.22 in six unrelated males. Further, it was noted that this duplication covered a 320-kb region involving four genes (SMC1A, RIBC1, HSD17B10 and HUWE1), three candidates of which may convey the phenotype of MR. 19 Apart from duplication, many other forms of genetic variations such as point mutation in SMC1A and HUWE1 genes and a silent mutation in HSD17B10 gene conveyed the phenotypes of MR along with other distinguished characteristic. 20 The conclusion drawn from the above findings showed that it is a dosagesensitive gene, which confers the MR phenotype in the patients with duplicated genes.…”

A novel microdeletion in 1(p34.2p34.3), involving the SLC2A1 (GLUT1) gene, and severe delayed development

“…Delayed development, with or without additional malformations, occurs in 2 to 3% of the general population. 1 The aetiology of delayed development often remains unresolved. Depending on the clinical selection criteria and techniques used, delayed development is caused by chromosome abnormalities detectable by microscopic analysis in 5 to 10% of cases.…”

Moderate learning difficulties: searching for clarity and understanding

“…Roeleveld, Zeilheis, and Gabreels (1997) reported widely varying ranges from 0.5 to 8% for those identified as having mild ID. According to Simonoff et al (2006), the reasons for this variation are not well understood as age, the cognitive measures used and socio-economic levels are all seen as relevant factors.…”