There is growing evidence that the three traditional subtypes of ADHD (Inattentive ADHD, Hyperactive-Impulsive ADHD and the Combined ADHD subtype) may in fact, be more accurately classified as separate disorders altogether. Although the ADHD sub typing method is still likely to persist, new biochemical studies have begun to shine light on some of the physiological differences associated with the three distinct ADHD subtypes.
Significant outward expressional differences among the different subtypes can be seen, such as a more passive, less self-directed behaviors among the predominantly inattentive subtypes and more novelty seeking, stubborn and non-compliant behaviors once the hyperactivity component is added in. Perhaps this is not surprising, given the definition of impulsivity. Nevertheless, differences in accompanying disorders comorbid to ADHD also lend credence to the idea of separating the subtypes out into unique stand-alone disorders.
It has even been posited that the disorder be subdivided further based on accompanying comorbid conditions, but at the moment this sub-classification seems unlikely. Along with comorbid conditions, age and gender differences among the ADHD subtypes have also been postulated.
Although outward behavioral expressions and phenotypes suggest stronger distinctions among the ADHD subtypes, it is the physiological and biochemical differences among these subtypes which may offer some of the most convincing evidence that a further re-classification of the disorder is warranted.
It is possible, for instance, that symptoms such as hyperactivity may predominate more than inattentive behaviors from prior medical problems such as childhood ear infections (which might seem counterintuitive, given that we would expect ear infections to promote hearing loss and compromise the attention side of the disorder more than the hyperactive-impulsive components). However, evidence for the biological differences of ADHD subtypes often goes well beyond earlier exposures to diseases and external stressors.
Getting to the meat of this issue are some recent studies on what is known as the HPA axis of the nervous system and the effects of this. "HPA" stands for hypothalamic-pituitary-adrenal, which include three essential components of the nervous system, which plays an extensive role in the fight-or-flight response in humans. So how does this tie in to ADHD?
One of the key components of this HPA axis is hormonal fluctuation. The chemical cortisol (you may have heard of cortisol from all of those late night TV and radio ads blasting cortisol for its contribution to body fat) is actually a stress-related hormone, meaning that the body produces it in response to internal or external stressors.
The kicker here is that there is now at least some evidence that the production of this cortisol hormone may be variable among the different ADHD subtypes.
It appears that children with the predominantly inattentive component of the disorder are more likely to exhibit a high cortisol response to stress while those with the more hyperactive/impulsive subtypes (just to avoid confusion, the study actually looked at the combined ADHD subtype, which includes the hyperactive component, and not the much rarer hyperactive-impulsive subtype) may have a significantly lower boost in the stressor hormone.
This may not be all surprising, given the tendency and stereotype of the inattentive ADHD kids as being more lazy, overweight couch potatoes, while the hyperactive-impulsive kids are associated with being rail-thin fidgety and bouncing off the walls.
While this study seems to fit the bill and make sense, it is important that we try not to read too much into these results. After all, a number of other studies on the subject found little to no subtype difference with regards to HPA or the cortisol response. However, another recent study did advance this HPA notion a bit further.
This study, done by Maldonado and coworkers, found that ADHD children who exhibited more of the hyperactive-impulsive traits of the disorder had lower cortisol response levels to stressors than did the inattentive symptom dominated groups. It is important to note that the HPA/cortisol/impulsivity association has been studied extensively in the literature.
For example, an earlier study on ADHD children in Korea, the researchers concluded that "the blunted HPA axis response to stress is related to the impulsivity in patients with ADHD", as evidenced by higher error rates on attention-based tasks. To put it another way, a higher HPA axis response (including the secretion of the cortisol hormone) is thought to be advantageous as far as attention symptoms are concerned.
As an interesting side note, this blunted HPA activity subsequent "dulling" of the fight-or-flight response among the ADHD population may, in part, explain the high percentage of ADHD'ers in stressful occupations such as firefighters, EMT's, ER physicians, and combat personnel and the like. In other words, due to the reduced HPA response among most of the ADHD population, ADHD'ers are less likely to be overwhelmed in stressful situations, and may actually be at an advantage in occupations such as these. Remember, ADHD can have its advantages!
Further muddying the waters with respect to cortisol and the HPA axis and ADHD is the presence of comorbid disorders. Another recent publication addressed this issue and found that for boys with ADHD, the presence of a comorbid anxiety disorder was likely to raise the cortisol levels in response to stress for the child, but the presence of an oppositional or disruptive behavioral comorbid disorder showed a tendency to lower the cortisol response to stress in the ADHD child.
These findings show agreement with some of the earlier statements made above, given that comorbid anxiety disorders are often hallmark characteristics of either the inattentive or combined sub-components of ADHD, while oppositional or conduct disorders are seen at higher frequency with the hyperactive/impulsive or combined ADHD subtypes.
Blogger's personal note: The concept of oppositional behaviors in ADHD is somewhat interesting. It appears that there may be much more going on under the surface with regards to ADHD and oppositional/conduct disorders and dysfunction within the nervous system. These behaviors may be associated with seemingly unrelated functions among the ADHD population such as bedwetting. I don't mean to sound like a "conspiracy theorist", but for an interesting read on the subject, this blogger personally recommends an earlier post entitled Bedwetting ADHD Kids and Depressed Dads: Is there a Connection?
Returning to our topic of discussion here, it is important to remember that in the first study mentioned, it was the hyperactive-impulsive children who showed more of a blunted cortisol response to stressors, so these observations from research groups in three different countries all seem to be reaching similar conclusions.
In conclusion, we should take away from these studies that the different ADHD subtypes may exhibit distinct hormonal response differences, as well as neuro-chemical activity differences between the ADHD and the non-ADHD populations. In general the more hyperactivity and/or impulsivity we see, the lesser the HPA-derived cortisol response e would expect to see in reaction to stressful situations.
We can also see that comorbid disorders alongside the ADHD may either further dampen this HPA activity and cortisol response (as in the case of oppositional disorders), or counteract the ADHD response by boosting HPA activity and cortisol levels (as in the example of many anxiety disorders). The take-home message is this: ADHD subtype differences and the presence of comorbid disorders can play a pivotal role in the hormonal fluctuations among the ADHD population.
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