By Eleanor McDermid, Senior medwireNews Reporter
Longitudinal analysis of a birth cohort raises the possibility that attention deficit/hyperactivity disorder (ADHD) in children and in adults may be distinct and unrelated disorders.
But Terrie Moffitt (Duke University, Durham, North Carolina, USA) and co-researchers caution that, as their unexpected findings “were not hypothesized in advance, replication in other cohorts is imperative.”
The team’s follow-up of the Dunedin Multidisciplinary Health and Development Study found that 61 (6%) of the 1037 participants had ADHD during childhood, as did 31 (3%) during adulthood, at age 38 years.
But these groups barely overlapped, with only three (5%) of the participants with childhood ADHD retaining the diagnosis in adulthood. Most of the other 58 childhood ADHD patients did not have symptoms that were just below the threshold for an adult diagnosis, suggesting that they had indeed outgrown their childhood diagnosis.
Conversely, participants with ADHD diagnosed during adulthood had no clear signs of the condition during childhood, having parent-rated symptoms at ages 5, 7, 9 and 11 years that did not differ from those of participants without ADHD. They had only “mildly elevated” teacher-rated symptoms, which were “far below the threshold for diagnosis.”
“Interestingly, there is strong scientific consensus that it is extremely difficult to document childhood onset in adults with ADHD”, say the researchers. “The frequent interpretation has been that childhood-onset ADHD was there but patients do not remember it. Our finding suggests the contrary hypothesis that there is an adult-onset form of ADHD.”
Another difference between the groups, and the “most surprising”, was the clear neuropsychological differences.
Participants with childhood ADHD had the expected cognitive deficits during childhood, consistent with ADHD as a neurodevelopmental disorder. Their IQ scores averaged 10 points below the norm and they had impaired executive function and verbal learning–delayed recall.
In adulthood, despite having outgrown their diagnoses, the group retained these impairments. “Consistent with these deficits, few had managed to obtain a university degree, and they were struggling financially”, write the researchers in The American Journal of Psychiatry.
By contrast, adults with ADHD had few measureable cognitive deficits, although they reported subjective cognitive complaints and cash-flow problems.
Finally, male gender predominated among childhood ADHD patients (78.7%) but less so among those with adult-onset ADHD (61.3%), and the childhood ADHD group had an elevated polygenic risk score, whereas the adult-onset group did not.
Moffitt et al therefore propose that adult-onset ADHD “is a bona fide disorder” that is distinct from the neurodevelopmental disorder of childhood-onset ADHD.
If this proves to be the case, they say that the place of adult ADHD in the DSM-5 will need to be reconsidered, and its aetiology must be researched.
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