Sovereign Health/Shutterstock
Source: Sovereign Health/Shutterstock

Attention deficit hyperactivity disorder, or ADHD, is typically thought of as a childhood illness. When you use a search engine to look up signs and symptoms of the disorder (for instance, shortened attention span, impulsivity, excessive speech and restlessness) or treatments (such as behavioral therapy and medications), many of the websites focus exclusively on children. ADHD in adults is often ignored entirely or left as a mere footnote.

It’s not surprising that the literature is so sparse—for whatever reason, ADHD seems to be more common in children than adults. According to the Centers for Disease Control and Prevention, roughly 10.2 percent of children are diagnosed with ADHD. Meanwhile, a study conducted in 2009 estimated that only 2.5 percent of adults met the diagnostic criteria for the disorder.

Still, the number of adults living with ADHD is greater than the number of adults who live with obsessive-compulsive disorder and schizophrenia. In fact, recent research shows that some adults with ADHD did not even have symptoms in childhood.

These days, more and more researchers recognize the necessity of studying ADHD beyond childhood and adolescence. If so many children live with ADHD, why do so few adults have the diagnosis? Can ADHD, much like certain forms of epilepsy, be outgrown? Or does the damage associated with ADHD stick around for the long term despite what clinicians once believed?

Unfortunately, research published last August in the scientific journal European Child & Adolescent Psychiatry points toward the latter.

Young adults diagnosed with ADHD as teenagers have a smaller caudate nucleus

The study, conducted by researchers at the University of Cambridge, U.K., and the University of Oulu, Finland, aimed to determine whether or not young adults who had been diagnosed with ADHD as teenagers had significantly different brain structures than their neurologically healthy peers.

The data was based within the 1986 Northern Finland Birth Cohort, a research project that has followed thousands of children born in 1986 from birth to adulthood. The researchers focused on 49 young adults within this cohort who were diagnosed with ADHD at the age of 16 and were now aged from 20 to 24 years. Only one participant had been prescribed medication. These participants were compared to 34 young adults who hadn’t been diagnosed with ADHD or any other developmental disability.

The researchers compared brain scans between the two groups and found that, compared to the group of healthy controls, individuals who had been diagnosed with ADHD had reduced gray matter in the caudate nucleus, a brain region that contributes to a wide variety of cognitive functions including memory.

The surprising thing, however, was that this brain difference was present regardless of whether or not the participant still met the diagnostic criteria of ADHD. In other words, young adults who had been previously diagnosed with ADHD in adolescence but no longer demonstrated clinically significant symptoms still had smaller-than-average caudate nuclei than people without a history of ADHD.

Young adults with a history of ADHD have disrupted memory, brain activity

To determine whether or not these structural differences resulted in cognitive impairment, the researchers took a subset of the participants (21 with a history of ADHD and 23 controls) and had them perform a memory task while in an fMRI scanner. FMRI, or functional magnetic resonance imaging, is a neuroimaging technique that allows researchers to evaluate neural activity in various brain regions by measuring deoxygenated blood.

The results? One-third of the young adults who had been diagnosed with ADHD in the past failed the memory test compared with only one participant in the control group. Even the participants in the ADHD group who managed to pass still performed worse than the controls by an average of 6 percentage points.

Not only did people who had a history of ADHD perform worse on the memory task than controls, but their caudate nucleus was significantly less sensitive. Specifically, when controls were faced with a difficult memory question, their caudate nucleus demonstrated increased activity. When the participants who had been diagnosed with ADHD received more difficult problems, their caudate nucleus maintained the same level of activity as before, seemingly unable to adapt to the more challenging circumstances.

Graham K. Murray, Ph.D., the lead researcher with the department of psychiatry at the University of Cambridge, explained:

“In the controls, when the test got harder, the caudate nucleus went up a gear in its activity, and this is likely to have helped solve the memory problems. But in the group with adolescent ADHD, this region of the brain is smaller and doesn’t seem to be able to respond to increasing memory demands, with the result that memory performance suffers.”

Once again, participants who had “recovered” from ADHD and participants who were still diagnosed with ADHD were no different. Both groups had altered activity in the caudate nucleus, and both groups performed poorly on the memory task.

What does this mean?

Even though it is less common in adults than children, ADHD can still affect adults. Not only can adults still meet the diagnostic criteria for ADHD, but adults who are technically “recovered” from ADHD may struggle with certain cognitive tasks, experience less-than-healthy neural activity and have irregular levels of gray matter in certain brain regions. ADHD doesn’t disappear just because symptoms become less obvious—its effect on the brain lingers.

Of course, this study is only one example of the research that is currently revolutionizing how we view ADHD. Two very recent studies—one led by Jessica Agnew-Blais, Sc.D., at King’s College London and the other led by Arthur Caye at the Universidade Federal do Rio Grande do Sul in Brazil—found evidence that some adults with ADHD never even experienced symptoms as children. In other words, it’s possible that there is such a thing as adult-onset ADHD. Researchers have yet to determine whether or not this form of ADHD is biologically distinct from the childhood form—after all, there are countless ways in which the brain can malfunction and interrupt a person’s ability to pay attention or retain memories.

The results of this study—and studies like it—stress the importance of treating developmental disabilities and mental illnesses not as a series of symptoms, but as physical illnesses with objective markers. We must understand how various illnesses and disorders impact the brain. With this knowledge, our ability to diagnose—and treat—these illnesses will increase tenfold.

Contributed by Courtney Lopresti, M.S.