The symptoms of attention deficit hyperactivity disorder (ADHD) may continue into adulthood, even if current diagnostic methods fail to identify its continued presence, according to new research from the University of Cambridge and the University of Oulu, Finland published in the journal European Child Adolescent Psychiatry.

The research shows that young adults diagnosed with ADHD in adolescence develop different brain structures and perform worse in memory tests compared to their peers.

It has long been believed that as the brain develops into adulthood, children may be able to grow out of ADHD. Until now there has been insufficient research to support this. 

Most previous studies into adolescent ADHD have assessed participants through interviews, meaning that brain structure often goes unexamined, and detailed analyses of processes and functions are impossible.

Current estimates suggest that between 10 and 50 per cent of children diagnosed with ADHD will continue to exhibit some of its symptoms as adults. Detection of the condition in adulthood is currently reliant on the individual demonstrating some of the symptoms set out in checklists, such as the American Psychiatric Association’s Diagnostic and Statistical Manual, which is the most widely used.

In this latest study, researchers traced the development of 49 adolescents diagnosed with ADHD at the age of 16. Comparing the findings to a control group of 34 young adults without ADHD, the team examined changes in brain structure and memory function at 20-24 years old.

The researchers discovered that the group diagnosed with ADHD in adolescence still demonstrated signs of reduced brain volume and poorer memory function as adults, regardless of whether or not they still met diagnostic checklist criteria for ADHD in adulthood.

MRI brain scans revealed that the adolescents with ADHD had less grey matter in the caudate nucleus, a region of the brain known to be responsible for some important cognitive functions, including memory.

To find out how significant these grey matter deficits were, a functional MRI experiment (fMRI) was carried out in order to measure brain activity by putting the working memory of the participants to the test while they were inside the scanner. 

One-third of the adolescents with ADHD failed the memory test, whereas fewer than one in twenty of the control group failed. In this test, a recall accuracy of less than 75 per cent was considered a fail. The adolescents with ADHD who did manage to pass the memory test consistently demonstrated lower scores than the control group. 

Because the study was undertaken in Finland, where medication is rarely used to treat ADHD, only one of the 49 test subjects had been treated with medication. As a result, the researchers were able to confidently rule out the idea that medication may have hampered the development of those diagnosed with ADHD.

“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 does not seem to be able to respond to increasing memory demands, with the result that memory performance suffers,” explained study-leader Dr Graham Murray from the Department of Psychiatry, University of Cambridge.

“We know that good memory function supports a variety of other mental processes, and memory problems can certainly hold people back in terms of success in education and the workplace. The next step in our research will be to examine whether these differences in brain structure and memory function are linked to difficulties in everyday life, and, crucially, see if they respond to treatment.”