Almost one-third of children with autism also have attention deficit hyperactivity disorder and symptoms appear to be more severe in kids affected by both conditions, new research suggests.

The findings come from a long-term study of 162 children who were tracked starting when they were still infants or toddlers. By the time the kids reached ages 4 through 8, researchers found that 63 had autism. Of those with the developmental disorder, parent reports indicated that 18 of the children — or about 29 percent — also had clinically significant symptoms of ADHD.

“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,” said Rebecca Landa, director of the Center for Autism and Related Disorders at the Kennedy Krieger Institute and a senior author of the study published online Wednesday in the journal Autism.

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Strikingly, children affected by both disorders were often more impaired than those with autism alone. Such kids were more than twice as likely to have significant cognitive delays, the study found, and they also had more struggles with social abilities and adaptive functioning.

Researchers behind the new study said that their findings are particularly significant because they followed children from a young age, thus reducing opportunities for bias. Other research on co-occurrence has traditionally looked at kids after their parents sought treatment, meaning that such studies may have focused on children with more pronounced symptoms.

“We focused on young school-aged children because the earlier we can identify this subset of children, the earlier we can design specialized interventions,” Landa said.

The findings come just weeks after a new version of the Diagnostic and Statistical Manual of Mental Disorders, or DSM, was released allowing for individuals to be diagnosed with both autism and ADHD simultaneously. The previous edition of the manual instructed doctors not to diagnosis the conditions in a co-occurring fashion.