To the Editor:
In 2001, we reported that the first-time (incident) diagnosis of autism in the United Kingdom increased nearly 4-fold among boys aged 2-5 years born between 1988 and 1993.1 Subsequently, we reported that the incidence of autism continued to rise for boys age 2-4 years old through the 1996 birth cohorts, and may have reached a plateau in 1997.2 To estimate the incidence of autism beyond the 1997 birth cohort, we updated the findings for additional birth cohorts of boys from 1998 through 2001.
This study, as in our previous 2 studies,1,2 was derived from information in the population-based General Practice Research Database. These data have been shown to be of high quality and completeness.3 The diagnosis of autism has been confirmed from original clinical records.4 The specificity of these autism diagnoses have been found to be highly concordant with the DSM-IV definition of autism spectrum disorders.5
As in our previous studies,1,2 we identified boys who had a first-time diagnosis of autism. We required the identified boys (cases) to have at least 18 months of recorded information in the computerized practice record before the first recorded diagnosis of autism, to be born in 1988 to 2001, and to be age 2-4 years at the time of the first recorded diagnosis. We calculated the 3-year cumulative incidence (risk) of autism by dividing the number of cases of autism diagnosed in each birth cohort by the number of boys present in the birth cohort for the 3-year period. Each annual birth cohort (1988-2001) was analyzed separately.
We identified 328 males with a first-time diagnosis of autism. As previously reported,1,2 the 3-year risk of autism rose consistently from about 4 per 10,000 in the 1988 birth cohort to about 25 per 10,000 in the 1997 birth cohort. The cumulative incidence remained fairly stable for birth cohorts 1998 through 2001 (Fig.).
Hertz-Picciotto and Delwiche6 recently published results on the incidence of autism in young children, based on the California Department of Developmental Services databases for birth cohorts from years 1990-2003.6,7 The authors concluded that there continued to be an increase in autism incidence over time that was not accounted for by changing age at diagnosis and by shifting diagnostic criteria. By contrast, in our study the incidence of diagnosed autism in the United Kingdom appeared to have reached a plateau for the last 5 birth cohorts (1997-2001) in young boys.
We have suggested that a main cause of the apparent increase in the incidence of autism in the United Kingdom in the early 1990s was a change in criteria for diagnosing autism in children previously diagnosed as having only delayed development.8 The current study provides support for that proposition. The criteria for the diagnosis of autism seem to have become stable in the United Kingdom in young boys born in the late 1990s and early 2000s.
Katrina W. Hagberg
Boston Collaborative Drug Surveillance Program
Boston University School of Medicine
1. Kaye JA, Melero-Montes M, Jick H. Mumps, measles, and rubella vaccine and the incidence of autism recorded by general practitioners: a time trend analysis. BMJ
2. Jick H, Kaye JA, Black C. Changes in risk of autism in the UK for birth cohorts 1990-1998. Epidemiology
3. Jick SS, Kaye JA, Vasilakis-Scaramozza C, et al. Validity of the general practice research database. Pharmacotherapy
4. Black C, Kaye JA, Jick H. Relation of childhood gastrointestinal disorders to autism: nested case-control study using data from the UK General Practice Research Database. BMJ
5. Fombonne E, Heavey L, Smeeth L, et al. Validation of the diagnosis of autism in general practitioner records. BMC Public Health
. 2004; 4:5–13.
6. Hertz-Picciotto I, Delwiche L. The rise of autism and the role of age at diagnosis. Epidemiology
7. Hertz-Picciotto I, Delwiche L. Estimating the incidence of autism [letter]. Epidemiology
8. Jick H, Kaye JA. Epidemiology and possible causes of autism. Pharmacotherapy