ArticlesPrevalence of disorders of the autism spectrum in a population cohort of children in South Thames: the Special Needs and Autism Project (SNAP)
Introduction
Autism spectrum disorders (ASDs), the common clinical term for the pervasive developmental disorders described in the classification systems,1, 2 are generally regarded as life-long disorders that have a substantial functional and financial effect on the individual and their family.3, 4 Individuals with autism put a heavy demand on educational, social, and medical services, and accurate prevalence estimates are needed for the planning of such services. Until the 1990s, the figure of four to five cases of autism per 10 000 people was widely accepted, although as many as 20 per 10 000 children were reported as showing the triad of impairments in social reciprocity, language impairment, and reduced imagination and restricted activities.5
Studies have shown increased prevalence estimates for all ASDs of between 30 and 90 cases per 10 000.6, 7, 8, 9, 10, 11 In addition to a true increase in prevalence, alternative explanations have been proposed, including changing diagnostic criteria, different methods of ascertainment, varying urban, rural, and country location, and population of study, younger age, and inclusion of individuals with average intelligence quotient (IQ) and those with other neuropsychiatric and medical disorders.12, 13, 14
Our study is a follow-up to a previous one undertaken in the South Thames region of the UK and reported in a series of studies that began with screening a population of 16 235 children aged 18 months, born within a defined geographic area, for autism. Previously we reported on a follow-up at age 7 years when, by a mixture of direct assessment and case-note review, we estimated the prevalence of all ASDs known to services at that time to be 57·9 per 10 000 (95% CI 42·6–77·0).7 We have now extended the population to 56 946 children, screened a high-risk special educational needs group, and directly assessed in depth a sample of 255 children.
Section snippets
Study population
We studied a population cohort of 56 946 children born between July 1, 1990, and Dec 31, 1991, in 12 districts in South Thames, UK. All children currently resident within those 12 districts with a birthdate within the relevant period were included. The study was approved by the South East Multicentre Research Ethics Committee.
Procedures
The special needs register of the child-health services was used in two ways to identify those with a diagnosis of any ASD and to ascertain the sample to be screened.
Results
The SCQ was mailed to 1770 families with 1270 boys and 500 girls (figure). Mean response rate was 70·5% (range across the 12 districts 57·8–87·6%), with 6% of families declining participation. A total of 1066 SCQs were returned completed (mean return rate 60·2%, range 50·0–76·2% across districts), although 2% of families declined further participation, leaving 1035 who returned the SCQ and opted in for further assessments.
A two-way stratified random sample of children from families who returned
Discussion
We have estimated the prevalence of autism and related ASDs in children aged 9–10 years using a screening procedure in a high-risk group in a large population followed by careful diagnosis using face-to-face standardised assessments. Our findings accord with those for high prevalence rates from recent studies.12, 13 Our study did not measure incidence rates, which are more difficult to estimate but which are important in understanding time trends and exploring causality.
Several characteristics
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