ADHD is characterized by a list of 18 symptoms equally divided between the Inattentive and Hyperactive/Impulsive groupings. They yield different presentations: Predominantly Inattentive, Predominantly Hyperactive/Impulsive, or Combined. They used to be called subtypes, but there is so much variability in how ADHD presents across individuals that the groupings have been recognized to be extremely heterogeneous.
Exploring Research-Based Symptoms of Adult ADHD
Within adult ADHD, my chief area of interest,1 studies have identified empirically-based symptoms specific to adult ADHD instead of relying on the same 18 symptoms used for children and adolescents. They’ve yielded different presentation groups, though still relying on symptom questionnaires. Some of the emergent groupings are cognitive inflexibility and disinhibition in one study,2 and ADHD Inattentive and ADHD Emotional Dysregulation in another.3 A third effort produced a single list of nine symptoms that included many executive functioning difficulties that are underrepresented in the current criteria.4
Mapping the Brains of Children for Research-Based Symptoms of ADHD
A recent study of children approached the issue of the heterogeneity of ADHD in an innovative way, which, of course, will have ramifications for adults.5
As part of a large, multi-site study, rather than relying on symptom questionnaires (though they were gathered separately but not used in analyses), structural brain scans (MRIs) from two sets of children with ADHD, 446 in a discovery group and 554 in a follow-up validation group, were compared with controls comprised of children without ADHD.
Complex analyses used metrics that identified targeted features across the brain structures that represent how they function within larger brain networks, namely degree centrality, nodal efficiency, and participation coefficient, which roughly translates into the number of connections, efficiency in signaling, and how well they communicate with other functional brain regions, respectively. These metrics were used to develop morphometric similarity networks (MSNs), identifying individualized networks across structures to examine variations and their similarities, and how they interact. Such analyses can identify distinct “hubs,” particularly influential coupling patterns.
When it came to identifying ADHD-specific patterns, as the study’s authors described it, their analyses allowed them to compare the brain metric results for ADHD participants with the non-ADHD control group’s metric, which they liken to pediatric growth charts used to locate where a child’s height lands in terms of the degree to which it varies from population expectations for other children that age. This individualized, brain data-driven approach provides a different view of ADHD than symptom checklists. The results from the discovery sample children were confirmed in the follow-up validation study.
Emergent Biotypes of ADHD
Three “biotypes” of ADHD emerged and aligned with existing ADHD symptom ratings for children, but in unique ways with nuanced understandings of what’s going on in the brain:
Biotype 1: The severe-combined with emotional dysregulation group, which manifested the more elevated and persistent symptoms, including a greater likelihood of mood disorders, although this likelihood was not statistically significant compared with Biotypes 2 and 3.
Biotype 2: The predominantly hyperactive/impulsive group, with such disinhibited behaviors driven via dysregulated action-mode networks.
Biotype 3: The predominantly inattentive group, with implications that default mode network interference contributes to poor sustained attention.
Each biotype was associated with distinct alternations in different brain structures and neurotransmitter density mapping. It’s too early to understand the implications for diagnosis and treatment, but it’s an interesting avenue for further inquiry.
ADHD Is Still Real and Serious
We are understanding ADHD with ever-increasing nuance, such as provided in this recent brain mapping study. Unfortunately, it still needs to be emphasized that ADHD is a valid and distinct diagnostic syndrome that affects lives and warrants the availability of evidence-supported treatments. It also should not be trivialized, as it often is by cartoonish presentations on social media and “Look, squirrel!” memes.