The apparent increase in autism disorders reflects an actual increase in prevalence, rather than a reclassification of other developmental disorders as autism, reported Craig Newschaffer, Ph.D., of Johns Hopkins University, Baltimore, and his colleagues.
Some researchers have suggested that children who would once have been classified in other categories—such as mental retardation or speech disorders—are now being diagnosed as autistic and that this “diagnostic shifting” accounts for the increase in autism. This is not the case, the investigators maintained, because although autism diagnoses have risen, there has been no corresponding decrease in other diagnostic categories (Pediatrics 2005;115:e277-82).
Dr. Newschaffer and his associates examined data from the U.S. Department of Education's office of special education programs for 1992-2001. These records reflect state counts of children who received free public education services. The children were classified into 13 primary disability categories defined under the Individuals with Disabilities Education Act.
The researchers calculated the prevalence of autism, traumatic brain injury, mental retardation, speech/language impairment, and other health impairments in children aged 6-17 years during each of these years. They then superimposed those data onto birth cohorts extending as far back as 1975.
There were clear, significant increases in the prevalence of autism among younger birth cohorts, especially among those born between 1987 and 1992. In those years, the prevalence of autism rose by about 50% every 2 years; the prevalence was 5.3/10,000 in 1984, 7.8/10,000 in 1986, 11.8/10,000 in 1988, and 18.3/10,000 in 1990.
There were no changes, however, in the prevalence of mental retardation, speech/language impairment, or traumatic brain injury, which suggests that the increase in autism is real and not the result of either reclassification of diagnoses or across-the-board increases in special education classification.
The yearly increases seemed to begin leveling off after 1992. It's impossible to know if that observation represents a true decrease in prevalence, however. Since 1997, federal law has allowed state and local education agencies to classify as “developmentally delayed” children as old as 9 years, Dr. Newschaffer and his associates noted.
“It is possible that increasing proportions of children in younger cohorts who would have been classified previously as having autism as they transitioned out of preschool special education retain developmental delay classifications,” the investigators said. This may mean that children are now simply being diagnosed with autism at later ages.
They also pointed out that these administrative data can't explain why autism is increasing. Additionally, Thomas Burns, Psy.D., said in an interview, the numbers paint the spectrum of autism diagnoses with the broadest brush possible.
The Department of Education uses only one autism classification, which includes all students receiving services who have been diagnosed with any one of the autism spectrum disorders. Thus, the study's prevalence numbers include an enormous array of children whose disabilities range from severe to mild, Dr. Burns said.
“The study makes it a little hard to compare apples to apples,” said Dr. Burns, director of neuropsychology at Children's Healthcare of Atlanta. “In this category of autism, you will certainly have kids who are severely mentally handicapped as well as kids with IQs of 130 who are delayed socially.”
Upcoming studies by the Centers for Disease Control and Prevention may further illuminate the issue since they will use uniform diagnostic criteria. “Some of these other disorders are really objective and easy to identify. You either have traumatic brain injury or you don't. You either have a low IQ and mental retardation or you don't. In autism and Asperger's, you can be dealing with vague symptoms and diagnostic criteria that vary from physician to physician and from study to study,” he said.