Study: A Third of Autistic Children Also Have ADHD

Autism affects about 1 in 50 American children.

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About a third of children who have autism also have symptoms of attention deficit hyperactivity disorder, according to a study released Wednesday.

[HEALTH: What You Need to Know About Autism]

According to researchers at the Baltimore-based Kennedy Krieger Institute, which studies autism, the two disorders may be somehow linked.

"We are increasingly seeing that these two disorders co-occur and a greater understanding of how they relate to each other could ultimately improve outcomes and quality of life for this subset of children," Rebecca Landa, author of the study, said in a statement. The study was published in Autism: The International Journal and Practice.

Experts have long thought the two disorders might be linked, but previous studies have looked at patients who were already exhibiting symptoms of at least one of the disorders, leading to a potential selection bias. Landa's study followed 162 infants and toddlers before they were diagnosed through their eighth birthday. Of the 63 children who were diagnosed with autism, 18 (29 percent) also had "clinically significant" symptoms of ADHD.

A previous study published in 2011 suggested that there may be some "genetic overlap" between genes that cause both ADHD and autism.

[READ: Study Finds Autism Has Strong Genetic Link in Mice]

Though the disorders are seemingly linked, the American Psychiatric Association's recent revisions to the Diagnostic and Statistical Manual of Mental Disorders - a sort of bible for mental health professionals – allows psychiatrists to diagnose the two separately. Previously, the manual suggested that a person diagnosed with autism or Asperger Syndrome could not be diagnosed with ADHD.

Landa found that children diagnosed with both disorders have significantly more trouble in school than students who have only autism, and students who had both scored worse on cognitive and social functioning tests. Children with both disorders were also more likely to display "severe autism mannerisms" such as repetitive behaviors.

Autism is one of the most commonly occurring psychological disorders – a study published in March suggested that about 1 in 50 American children between the ages of 6 and 17 have autism. Researchers are split on whether the uptick (1 in 86 children were diagnosed with autism in a 2007 study) is due to an increase in the actual number of people with autism or due to better diagnostic techniques.

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