One of every five ADHD kids in the study exhibited signs of autism such as slow language development, difficulty interacting with others and problems with emotional control, said study co-author Dr. Joseph Biederman, director of the pediatric psychopharmacology unit at Massachusetts General Hospital.
These kids also showed problems with "executive function," or the ability to plan, organize and conceptualize future action, said Biederman, a professor of psychiatry at Harvard Medical School.
Fewer than 1 percent of kids in the non-ADHD comparison group exhibited any traits linked to autism, according to the study appearing in the September issue of Pediatrics.
"These children are not having the full diagnosis of autism, but they have symptoms of autism," Biederman said. "It may be important to screen children with ADHD for autistic traits because they may need more support, particularly in the educational and interpersonal domains."
Previous studies of children with autism have found that many also have severe ADHD symptoms. This is one of the first studies to turn the tables and see if the reverse is true, said Dr. Alice Mao, an associate professor of psychiatry at Baylor College of Medicine. She was not involved with the study.
"Generally, autistic kids with ADHD are challenging to treat because they don't respond well to ADHD medications," Mao said. "You have to treat the autism symptoms and then treat the ADHD. The conclusion would be that perhaps we should screen these ADHD kids who are not doing well on traditional ADHD treatments to see if they have comorbid autism traits."
The study included 242 kids aged 6 to 18 with ADHD as well as a 227-member "control" group of kids without ADHD. The children were drawn from an existing large-scale sample pool that excluded any kids who had been diagnosed with autism.
The children and their parents filled out a series of questionnaires to grade their behavior and compare it to generally accepted definitions of autistic traits.
The researchers found that 18 percent of kids with ADHD exhibited some behaviors that are common in autism, compared with 0.87 percent of kids from the control group.
The ADHD children with autistic traits had many more social problems than typical ADHD children. They were more likely to fight with and be rejected by other kids, and displayed more school behavior problems, more difficulties using their spare time and more friction with their siblings, the study authors noted.
The children with both ADHD and autistic traits also tended to more frequently suffer additional psychiatric and learning disorders than either kids with only ADHD or children in the control group.
"Those with autism traits have greater severity of symptoms and dysfunction," Mao said. "Certainly it would be useful to screen kids with ADHD who have autism traits to see which kids may need more help socially, as well as to make sure they don't have lower intellectual functioning. You may be able to give other treatments that would be helpful in terms of improving their functioning."
These findings, along with previous research, point to the strong possibility that ADHD and autism share some genetic link, study author Biederman said.
"The genetic markers for ADHD have also been associated with autism," he said. "These autistic traits may be present in other conditions as well. I am quite convinced that these traits may be present in children with mood and anxiety disorders."
For more information on ADHD, visit the U.S. National Library of Medicine.