• autism;
  • autistic disorder;
  • epidemiology;
  • incidence;
  • PDD

Aim:  Although recent epidemiological studies on the pervasive development disorders (PDD) appear to be reporting higher rates of incidence than previously believed, great variation in the reported figures suggests a need for review of the methodology involved. As such, a survey on the incidence of PDD was conducted and compared with data from a previous survey to examine the effects of screening and diagnostic methodology on incidence.

Methods:  The incidence of pervasive developmental disorders was surveyed in all children (12 589) born between January 1994 and December 1996 in Toyota, Japan.

Results:  Incidence was 1.81% and the ratio of boys to girls was 2.80. Definitive diagnoses were made between 13 months and 7 years 2 months, the average age at diagnosis being 3 years 4 months. Among the cases of PDD, children with normal or borderline intelligence amounted to 66.4%, mild mental retardation (MR) 17.5%, moderate MR 10.3% and severe MR 5.8%.

Conclusion:  An approximately 11-fold increase was noted in prevalence of PDD compared to a previous survey two decades ago, and two main factors were believed to account for this apparent sharp increase. First, inclusion of high-functioning subjects detected during infancy, and second, higher rates of diagnosis resulting from an integrated process of screening.