Study Suggests Rise in Autism Cases May Not Be Real
CHICAGO — A rise in autism cases is not evidence of a feared epidemic but reflects that schools are diagnosing autism more frequently, a study said on Monday.
Children classified by U.S. school special education programs as mentally retarded or learning disabled have declined in tandem with the rise in autism cases between 1994 and 2003, the author of the study said, suggesting a switch of diagnoses.
Government health authorities have been trying to allay widely publicized concerns that vaccines containing the mercury-containing preservative therimerosal, which is no longer used, were behind an autism epidemic.
There may be as yet unknown environmental triggers behind autism, study author Paul Shattuck of the University of Wisconsin at Madison said, but his research suggested the past decade’s rise in autism cases was more of a labeling issue.
Autism was fully recognized in 1994 by all U.S. states as a behavioral classification for schoolchildren, who receive individualized attention whatever their diagnosis, he wrote in the journal Pediatrics.
Subsequent increases in the number of autism cases have varied widely by state but the average prevalence among 6- to 11-year-olds enrolled in special education programs increased from 0.6 per 1,000 pupils in 1994 to 3.1 per 1,000 in 2003.
During the same period, diagnoses of mental retardation fell by 2.8 per 1,000 students and diagnoses of learning disabilities dropped by 8.3 per 1,000 students.
Autism is a spectrum of disorders caused by abnormal brain development that can lead to diminished social skills, as well as unusual ways of learning and reactions to sensations. As many as 6 in 1,000 children are ultimately diagnosed with it to some degree, according to the Autism Society of America.
Shattuck’s analysis was challenged in an accompanying commentary by autism researcher Craig Newschaffer of Johns Hopkins Bloomberg School of Public Health in Baltimore.
"We do not know whether individual children have switched classifications, and of course we can never know whether a given child in a particular birth cohort would have been classified differently had they been born either earlier or later. At best, analyses of this type are merely trying to determine if trends in one classification have the potential to offset those in another," he wrote.
There was a clear need for definitive studies into the roles played by genetic susceptibility and environmental triggers in autism, Newschaffer wrote.