Attention-deficit/hyperactivity disorder (ADHD) is a disorder characterized by inattentive and/or hyperactive-impulsive behaviors that persist for 6 months or longer. About 6% of children and adolescents and about 3% of adults have ADHD. Because the symptoms are similar in children and adults, it has been assumed that ADHD is the same disorder regardless of age. Thus, most medical professionals have assumed that adults with ADHD have had ADHD since they were young and that many kids with ADHD become adults with ADHD. Despite this widely held belief, there have been few studies directly addressing this issue.

An important paper recently published in the American Journal of Psychiatry by Terrie Moffitt, Avshalom Caspi, and colleagues provides strong evidence that challenges these assumptions. This study is compelling because it uses a longitudinal design involving over 1000 individuals born between April 1972 and March 1973 in Dunedin, New Zealand. This large group of people has been carefully evaluated at regular intervals with the most recent assessment at 38 years of age. This prospective longitudinal design is ideal for determining what happens to children with a diagnosis of ADHD as they get older. In addition, it provides the investigators with the ability to look back at data collected when adults with adult-onset ADHD were young. Importantly, results from this study were unlikely to be influenced by treatment of ADHD because medications were rarely used for ADHD in New Zealand in the 1970’s.

This research team found that 95% of the children with ADHD no longer met criteria for ADHD at age 38, i.e., only 3 of the 61 kids diagnosed with ADHD still had symptoms sufficient for a diagnosis of ADHD when they were 38 years old. Of the 31 adults with a diagnosis of ADHD at age 38, only 3 had ADHD as a youngster.  

There were other significant differences between the children and adults with ADHD. The youngsters with ADHD were mostly male (78%) and had cognitive deficits in addition to symptoms of inattention, hyperactivity, and impulsivity. Even though most were no longer symptomatic as adults, cognitive deficits involving various functions including memory, processing speed, reading comprehension, and rapid visual processing remained. This group’s IQ was about 10 points lower than the comparison group. Those diagnosed with ADHD as adults did not have cognitive deficits, and the numbers of females and males in this adult group were similar. In addition, almost half of the adult-onset group had persistent substance dependence compared to less than a third of adults who had a history of child-onset ADHD. The authors of the article noted that they could not determine the time of onset of either substance abuse disorders or the adult ADHD diagnosis from their data.

These results suggest that individuals with child-onset ADHD and individuals with adult-onset ADHD are two very different groups. Youngsters with ADHD have symptoms of inattention and/or hyperactivity and impulsivity with cognitive deficits. These children maintain some of the cognitive deficits as adults, but most no longer have the large number of ADHD symptoms that they had earlier. Adults with current symptoms of inattention and/or hyperactivity and impulsivity usually did not have many of these symptoms as kids. Also, almost half of these adults have co-existing substance dependence disorders.

It is possible that child-onset ADHD is a developmental disorder and brain regions influencing attention, activity, impulsivity, and cognitive function are influenced by genetic and environmental factors that were present at very early stages of development. Some of the inattentive and impulsive behaviors improve as these children grow up. On the other hand, adults with ADHD symptoms may have different factors influencing brain pathways involving attention, activity, and impulsivity since many of these behaviors don’t appear to be present when these persons were youngsters. Some of these brain mechanisms are likely influenced by substances of abuse. It also seems likely that various genetic and environmental factors influencing the brain circuitry underlying adult-onset symptoms did not start to take effect until the person was older. Exactly what causes the particular group of symptoms described by the diagnosis of ADHD is unknown. It is likely that there are a variety of pathological mechanisms that may lead to adult ADHD symptoms.

Further research will be necessary to better define clinical implications of the data presented in this important study. One take home message might be that it is possible that different underlying mechanisms may cause similar symptoms in children and adults.

This column was written by Eugene Rubin MD, PhD and Charles Zorumski MD.

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