Timing Is Everything in Diagnosis at Overlap of ADHD and ASD

A percentage of children ultimately diagnosed with autism spectrum disorder (ASD) were previously diagnosed with attention-deficit/hyperactivity disorder (ADHD), an error that may delay the correct diagnosis and appropriate treatment, according to a study by Amir Miodovnik, M.D., M.P.H., and colleagues.

Symptoms like inattention, hyperactivity, and impulsivity can appear in children with either disorder. One Canadian study found that ASD was diagnosed 1.29 years after a comorbid ADHD diagnosis but did not address the question of age, said Miodovnik, a developmental pediatrician at Boston Children’s Hospital.

The American Academy of Pediatrics recommends that screening for ASD occur between 18 and 24 months, but diagnosis often comes much later than that, they wrote in the October Pediatrics. If ADHD is diagnosed before ASD, clinicians might be inclined to stick with the former because its symptoms appear dominant. ASD would then go untreated for several years until the correct diagnosis was made.

The researchers looked at a sample of 1,496 children with a current diagnosis of ADHD, drawn from the 2011-2012 National Survey of Children’s Health. Of those, 705 also had been diagnosed with ASD, including 313 who were diagnosed with ADHD before ASD. On average, there was about a three-year delay in the age of diagnosis for this latter group, compared with children whose ADHD was diagnosed at the same time as or after their ASD, or those with ASD only.

The ADHD-before-ASD group exhibited fewer speech problems than the other two groups, a factor that may have contributed to delays in ASD diagnosis.

Also, the children with ADHD before ASD were 16.7 times more likely to be diagnosed with ASD after their sixth birthday than the ASD-only cohort and 29.5 times more likely than the ADHD same/after group.

Part of the problem may be that pediatricians usually see their patients for short clinic visits, during which the typical social and behavioral deficits that characterize autism may not appear. Any problematic behavior may be ascribed to a more familiar condition.

“[G]eneral practitioners may be inclined to attribute maladaptive behaviors to ADHD, the most common neurobehavioral disorder of childhood,” wrote Miodovnik. “Our study supports the hypothesis that receiving a diagnosis of ADHD before ASD may delay the diagnosis of ASD, and that this delay persists across age and severity of the ASD.”

The longer the delay before an autism diagnosis, the worse patient outcomes are likely to be, they concluded.

More work is needed to define how and when the symptoms of ADHD and ASD overlap and how that knowledge can be best applied to screening and diagnosis.

“For now, clinicians should consider ASD when evaluating young children presenting with ADHD symptoms,” they said. ■