While the number of children diagnosed with autism each year in the United Kingdom begins to level off following a five-fold increase during the 1990s, newly diagnosed cases in the United States continue to arise. A team of researchers ruled out a link between the MMR vaccine and autism, but said the "actual cause of the dramatic rise in the 1990s remains a mystery."
"The large difference between countries is closely similar to differences in rates reported for children diagnosed and treated for attention deficit hyperactivity disorder in the two countries," the authors added.
According to the Centers for Disease Control and Prevention (CDC), around one in 88 children in the U.S. has been diagnosed with some type of autism spectrum disorder as of 2008. The CDC’s Autism and Developmental Disabilities Monitoring (ADDM) Network reported a 78 percent surge in autism’s prevalence between 2004 and 2008 among 8-year-old children in the U.S.
Researchers used data from the General Practice Research Database (GPRD), which included the records of roughly three million anonymous patients from 300 general practices in the U.K. Starting with 1990, statistics from the GPRD were used to calculate the number of people living with autism — annual prevalence — and the number of newly diagnosed cases, which is the annual incidence tied to 8-year-old children born after 1996.
By dividing the number of children diagnosed with autism by the number of children enrolled in the study, researchers got the annual prevalence rate for 2004-2010. The research team calculated annual incidence by dividing the amount of children newly diagnosed with the condition between 2004 and 2010 by the number of children participating in the study.
Results of the analysis estimated an annual prevalence of 3.8 per 1,000 boys and 0.8 per 1,000 girls, and an estimated annual incidence of 1.2 per 1,000 boys and 0.2 per 1,000 girls. Girls were found to be around 75 percent less likely to be diagnosed with an autism spectrum disorder compared to boys.
This study provides "compelling evidence that a major rise in incidence rates of autism, recorded in general practice, occurred in the decade of the 1990s but reached a plateau shortly after 2000 and has remained steady through 2010," the authors said.
Although the findings from this study fail to pinpoint an actual cause for a rise in the number of children diagnosed with autism, a research team from the University of North Carolina’s School of Medicine recently discovered impairment in a group of enzymes essential to cognitive development that could cause autism. Topoisomerases are enzymes in our DNA that have been heavily associated with brain development. A chemotherapy drug used to inhibit topoisomerases enzymes, topotecan, was likely to interfere with “long” genes, or autism-linked genes.
"Our study shows the magnitude of what can happen if topoisomerases are impaired," explained the study’s lead author, Mark Zylka. "Inhibiting these enzymes has the potential to profoundly affect neurodevelopment — perhaps even more so than having a mutation in any one of the genes that have been linked to autism."