The number of newly diagnosed cases of autism has levelled off in the UK in the past decade after a five-fold surge during the 1990s, researchers have found. 

The findings differ from results issued by the US Centers for Disease Control and Prevention (CDC) last year, which reported a 78% increase in the prevalence of the condition in 8-year-old children between 2004 and 2008 in the US.

Prompted by these results, which found that 1 in 88 8-year-old children in the US had been diagnosed with an autistic spectrum disorder in or before 2008, the authors wanted to find out if there were comparable rates in the UK.

Using data from the General Practice Research Database (GPRD), which contains around three million anonymised active patient records from more than 300 representative general practices in the UK – equivalent to 5% of the UK population – the researchers calculated the annual prevalence and incidence rates for autism among 8-year-olds between 2004 and 2010.

The results showed that the annual prevalence and incidence of autism did not materially change over the study period, for either boys or girls.

They found that the annual prevalence of autistic spectrum disorders was estimated at 3.8 per 1,000 boys and 0.8 per 1,000 girls, while the annual incidence was estimated at 1.2 per 1,000 boys and 0.2 per 1,000 girls. Girls were about 75% less likely to be diagnosed with an autism spectrum disorder as boys.

This means that the UK prevalence of autism – about 4 in 1,000 children – is substantially lower than the equivalent US figure of about 11 in 1,000 children in 2008, which was reported in 2012.

“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 point out.

Their previously published research, based on the same database, showed that the cumulative incidence of autism among children born in UK between 1988 and 1995 increased continuously by a factor of five during that period.

The researchers add that both studies provide “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.”

Similar widespread sharp rises in the number of children diagnosed as autistic were also seen in the 1990s in other parts of Europe and North America, they add, making it unlikely that better understanding of the condition or a broadening of the diagnostic criteria alone could have been responsible for these simultaneous large increases.

But the researchers admit that, given the apparent sudden halt in the rise in rates from early 2000 onwards – at  least in the UK – the “actual cause of the dramatic rise in the 1990s remains a mystery.” 

However, they emphasise that the suggestion that it might be linked to the MMR vaccine has been conclusively ruled out.

The research is published in the online journal BMJ Open. Read the full paper here: http://www.bmjopen.bmj.com/content/3/10/e003219.full