Division of Genetics and Metabolism
What is the Prevalence of Autism?

Is there an increasing prevalence of autism spectrum disorder (ASD)?

New information released by the Centers for Disease Control indicates that approximately 1% or 1 in every 110 children will have the diagnosis of an ASD.  This information comes from a retrospective review of medical and school records from 11 states participating in the Autism and Developmental Disabilities Monitoring (ADDM) Network.  This was a surveillance project targeted to children, age 8 years, a time when peak prevalence for the ASD diagnosis occurs.  The project studied approximately 307,000 children in public schools and the ASD prevalence per 1000 children ranged from 4.2 in Florida to 12.1 in Arizona and Missouri.  A similar study by the ADDM occurred in 2002 and for the 10 sites that participated then compared to their 2006 data there was an increase in ASD ranging from 27% to 95%.  The proportion of children with ASD who had cognitive impairment (IQ ≤ 70) ranged from 29 to 51% (average: 41%).  The results of this study are consistent with other surveillance projects in the last several years showing approximate 1% ASD prevalence.  The current study is reported in the December 18, 2009 edition of MMWR (.pdf of report).

Such dramatic increase in autism diagnoses is not easily explainable by genetic mechanisms even though a genetic or heritable component is felt to play a large part in the causation of ASD.  Although genetic inroads toward the causation of autism have made important discoveries, it seems apparent that a large number of genes are involved and that any single gene or group of genes is likely to have only a small effect on the total variation or expression of the autism problem. On the other hand, attempts to identify common environmental factors across various ethnic and geographic populations have been unsuccessful in pinning down any environmental factors that might cause such a large effect.  The anecdotal experience of clinicians who have evaluated children for the past decades is also of importance however in that most clinicians do not recall as frequent occurrences of autism in their developmental clinics. It is also clear that there are institutional and cultural pressures that might make the diagnosis of ASD more tenable or desirable than one of developmental delay or mental retardation.  The conclusion from the ADDM network was that it is not known whether this increasing prevalence trend between 2002 and 2006 is a true increase or whether the increase is due solely to changes in community awareness and identification patterns.

Charlie Williams, MD
edit 1-6-2010

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