A new study is prompting doctors to rethink the definition of ADHD. Researchers have discovered impairments in three unique brain systems, suggesting that Attention Deficit Hyperactive Disorder may, in fact, be three distinct types of ADHD disorders, with no commonality, instead of a single diagnosis.

News of the findings has the potential to radically reframe how the medical, psychiatric and educational communities think about and treat ADHD.

“This study found evidence that clearly supports the idea that ADHD-diagnosed adolescents are not all the same neurobiologically,” said the study’s lead author, Dr. Michael Stevens, of the Olin Neuropsychiatry Research Center and Yale University. Rather than a single disorder with small variations, the findings by Stevens’s team suggest an ADHD diagnosis instead encompasses a “constellation” of completely different brain functions.

The researchers tested the impulsive behavior of 117 adolescents with ADHD, all diagnosed according to the same clinical definition of the disorder. Impulsive behavior is a typical feature of the disorder, but it’s a broad term; the team tested different types of impulsivity, and three distinct groups emerged based on the participants’ performance. One group demonstrated impulsive motor responses during fast-moving visual tasks (a measure of executive function, that is, the set of mental skills that allow you get things done — e.g. planning, paying attention, managing time, etc); one group showed a preference for immediate reward; and the third group performed relatively normal on both tasks, compared to 134 non-ADHD adolescents.

“These three ADHD subgroups were otherwise clinically indistinguishable for the most part,” Dr. Stevens said. “Without the specialized cognitive testing, a clinician would have had no way to tell apart the ADHD patients in one subgroup versus another.”

Dr. Stevens and colleagues then used functional magnetic resonance imaging (fMRI), a technique that allows researchers to make connections between behavior and brain function, to investigate how these different types of impulsive behavior related to brain dysfunction. They found three distinct patterns of brain function related to the three types of impulsivity — but no common brain function that could tie them all together as one disorder.

“Far from having a core ADHD profile of brain dysfunction, there was not a single fMRI-measured abnormality that could be found in all three ADHD subgroups,” said Dr. Stevens. Instead, each subgroup had dysfunction in different brain regions related to their specific type of behavioral impairment.

“The results of this study highlight that there are different neural systems related to executive functions and reward processing that may contribute independently to the development of ADHD symptoms,” said Dr. Cameron Carter, editor of Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, the journal in which the study published.

It will take more research to prove that Attention Deficit Hyperactive Disorder is actually a collection of completely different disorders, but this study opens up a direction of study that could lead to better diagnosis and more effective treatment. For example, it could help explain why medications that may not work well in some patients, may be effective for others.

“Ultimately, by being open to the idea that psychiatric disorders like ADHD might be caused by more than one factor, it might be possible to advance our understanding of causes and treatments more rapidly,” said Dr. Stevens.