Prevalence and time trends of disabilities in school-age children.

Background. Although the evolution of the prevalence of cerebral palsy is now well documented, much less is known about the evolution of the prevalence of other disabilities such as mental retardation, sensorial defects, autism and psychosis. The aim of this paper is to determine those trends. Methods. A population-based survey was carried out in 1992-1993 in three French 'departements'. All disabled children born between 1976 and 1985 and receiving a special education and/or financial assistance were systematically registered. Results. The comparison of three cohorts of children born in 1976-1978, 1979-1981 and 1982-1984 using the test for trend in proportion showed a significant decrease (P = 0.03) in the prevalence of severe mental retardation, after exclusion of Down syndrome. This decrease was significant for severe mental retardation associated with psychosis. The time trend prevalence for cerebral palsy increased (P 0.03) but was irregular. The time trend prevalence of other disabilities (other motor defects, severe sensorial disabilities, autism and psychosis) did not change significantly. A detailed analysis of severe mental retardation and cerebral palsy was performed by geographical area, age at first registration and type of disability. Conclusion. The increase in prevalence of cerebral palsy is possibly due to earlier registration of disabled children. The decrease in prevalence of severe mental retardation does not seem to be due to recruitment bias, but there is a possibility of classification bias.