Adolescents diagnosed with attention deficit hyperactivity disorder (ADHD) seem to have different brain structures, which may be why they don’t perform as well on memory tests, finds a new study published in the journal European Child Adolescent Psychiatry. More importantly, these differences may be proof ADHD doesn't go away.

Some researchers are convinced ADHD goes away once a child's brain develops, with current estimates suggesting that between 10 to 50 percent of children still have ADHD in adulthood. But researchers from the University of Cambridge and Oulu found these studies often base their conclusions on “interview-based assessments,” not so much brain structure and function. So they decided to follow-up with data previously collected for the Northern Finland Birth Cohort in 1986, where thousands of children born in 1986 were studied into adulthood.

Researchers used magnetic resonance imaging (MRI) to observe brain and memory function among 49 adolescents diagnosed with ADHD at age 16 (now in their early 20s); 34 young adults served as the control. They found that those with ADHD had reduced gray matter in the caudate nucleus, a part of the brain associated with memory and cognitive function. Researchers then gave the adults with ADHD a memory test to see if this deficit made a difference.

The results showed that the group diagnosed with ADHD were still experiencing problems "in terms of reduced brain volume and poorer memory function" in their 20s. As far as memory was concerned, one-third of adults with ADHD failed the memory test compared to less than one in 20 of the control group, researchers said. Even if those with ADHD passed their test, however, it was still an average six percent lower than other participants.

Researchers believe it’s the caudate nucleus influencing a person’s memory.

“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,” study author Dr. Graham Murray said in statement. “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 results that memory performance suffers.”

A study such as this one could help provide better access to treatment for those who suffer with ADHD. The study was conducted in Finland, where medication is not often used to treat the condition, hence why only one of the 49 participants was on medication.

The Centers for Disease Control and Prevention reports that approximately 11 percent of children ages 6 to 17 have been diagnosed with ADHD, and year after year the number continues to rise. Measures of the brain, as well as memory function, may be key to understanding the condition.

Murray concluded: "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."

Source: Roman-Urrestarazu A, Lindholm P, Moilanen I, et al. Brain structural deficits and working memory fMRI dysfunction in young adults who were diagnosed with ADHD in adolescence. European Child and Adolescent Psychiatry. 2015.