Brain scans of teens with ADHD show researchers the subtle differences between ADHD subtypes.

What does it mean to have attention deficit hyperactivity disorder, or ADHD? There are the obvious physical expressions of the disorder, such as an inability to focus or sit still, but researchers at the University of California, Davis explored the disease on a neurobiological level to get to the heart of the question.

Their work, recently published in the journal Biological Psychiatry, offers new insight into ADHD classification. By observing changes on an electroencephalogram (EEG), a test that records electrical activity in the brain, researchers discovered differences in the brains of teens with inattentive and hyperactive/impulsive forms of ADHD, as well as those who did not have the disorder.

Not only does this study point to a potential biomarker for distinguishing various types of ADHD, it could also provide a solid foundation to determine whether people with the inattentive type of ADHD in fact have a completely different disorder.

“ADHD subtypes appear subjectively very different in the clinical setting, but there are few objective physiological markers that have been able to detect those differences,” said Ali Mazaheri, a guest researcher at the UC Davis Center for Mind and Brain, in a press release. “This study shows that there are changes in brain waves related to visual processing and motor planning that can be used to distinguish ADHD subtypes.”

As it turns out, ADHD is much more complicated and diversified than meets the eye.

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Fifty-seven children ages 12 to 17 participated in the study, conducted between 2009 and 2013 by the UC Davis Center for MIND (Medical Investigation of Neurodevelopmental Disorders) and Brain and the MIND Institute. Twenty-three participants did not have ADHD, while 17 belonged to each of the ADHD subgroups.

The teens’ brain activity was measured with an EEG while they performed computer tasks that involved visual cues. The researchers found that the brain wave patterns of the two groups differed depending on whether they had ADHD, and within the ADHD subtypes.

The inattentive group struggled to process the cues, while the hyperactive/impulsive group had difficulty using the cues to prepare a motor response. Overall, the participants with either form of ADHD paid less attention to the task than the teens without the disorder.

The researchers then examined the participants’ alpha and beta brain waves. They also found differences between the teens with and without ADHD in this part of the test. The alpha waves, associated with wakeful relaxation, showed that teens with inattentive ADHD were unable to take into account the most important information in the test. The beta waves, associated with carrying out motor tasks, showed that teens with the combined ADHD subtype had the most difficulty with the motor task of pushing a button.

This lends support to the idea that no two ADHD diagnoses are alike. “A behavior task is not sensitive enough to detect what is happening upstream in the attention processing system,” says Catherine Fassbender, a research scientist with the MIND Institute. “People are always trying to find an objective way to diagnose these kids because it’s such a subjective diagnosis most of the time.”

In fact, an objective way to diagnose ADHD has long eluded scientists. “The holy grail all along has been to find something measurable and objective between these two groups,”Fassbender told Healthline.

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This study identified some of the specific task-related challenges associated with different types of ADHD, and emphasized important differences between ADHD subtypes. ADHD manifests in different ways, and understanding these differences could lead to improved, more specialized treatments for each ADHD subtype.

And hopefully, Fassbender says, this study will help to affirm the reality of ADHD for those who are skeptical, as well as those who struggle with the disorder.

“We feel that it’s a very controversial diagnosis,” she says. “There’s not a whole lot of advocacy for ADHD, so this is just another study that’s showing this is a real disorder, and people with this disorder face very real challenges in their everyday lives.”