This is coextensive with 'severe mental retardation'/'severe mental handicap', 'severe learning disabilities', etc., if these are defined by IQ below 50.
The only criterion of sufficient consistency in the literature to provide comparative data is IQ below 50. Despite problems of measurement, validation, and sampling, which rule out much published work, there are sufficient studies to draw certain conclusions with some confidence (16) for developed countries. They will apply in principle to developing countries, and in practice to some extent, depending upon development and economic status, demographic characteristics, vital statistics, and other social indicators.
1. Point prevalence varies between similar birth cohorts (concurrent age groups) in different communities (e.g. 1.62/1000 children born 1951-55 in Salford, United Kingdom, and 7.34/1000 children born in 1957 in Amsterdam). Greater variation is expected in developing countries, (lJ) especially where there is one dominant cause such as iodine deficiency disease, where congenital hypothyroidism can affect more than 10 per cent of village populations. Down syndrome is often the largest aetiological group, especially in communities with traditions of late marriage, large families, and taboos against contraception and/or abortion, and where early mortality is low. Mortality and survival vary greatly, generally related to the 'development status' of the community.
2. Age-specific prevalence varies over time in the same community. The same factors affecting incidence and mortality which explain differences between communities also change within the same community. In Salford, the prevalence for children aged 5 to 9 was 1.98 in 1000 in 1961, 5.54 in 1000 in 1971, and 3.86 in 1000 in 1980.(16)
3. A similar pattern of temporal variation is common throughout the developed world. Age-specific prevalence was generally low (1.8-4.0 in 1000) for children born in the early 1950s, and high (3.3-5.5 in 1000) for those born in the early 1960s, and has fallen since, at least well into the 1980s. The increase was due largely to decreased early mortality and increased survival associated with better neonatal care, well documented for Down syndrome from life-table studies and no doubt shared by other syndromes.(l, 18) The progressive decrease in prevalence in young children since the late 1960s reflects many processes reducing inceptions.(19) Widespread oral contraception reduced conceptions in older women, greatly reducing the incidence of Down syndrome. This could be reversed. More recently, amniocentesis and abortion programmes have reduced Down syndrome even more. Preventive programmes for several aetiological groups have created a cumulative effect.(l,29 Postnatal screening for inherited metabolic disorders and sporadic congenital hypothyroidism have been very successful. Perinatal factors probably now produce fewer neurological impairments, although proof is difficult to generate. Large-scale immunization has reduced encephalitis, encephalopathy, pertussis, measles, mumps, Haemophilus influenzae type B, and rubella. Early identification and treatment of some syndromes has diminished residual impairments, and early stimulation and training has improved function.
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