World history is a court of judgment. Friedrich Hegel
ADHD is not a newly recognized disorder, but it dates back to Hippocrates(460-329 BC), known as the father of medicine, who has been perhaps the first to record ADHD in history when he observed patients who demonstrated “quickened responses to sensory experiences” and went on to describe their inability to stay focused “because the soul moves on quickly to the next impression.” Interestingly an ancient Greek physician recognized the condition both as a cognitive and behavioral nature together and not separate. But ironically, the next 2000 years, we were bouncing back and forth, between the two components of the disorder, hyperactivity and inattention and recognizing one not the other or at last subtyping them to “predominantly inattentive” or “predominantly hyperactive/impulsive” in DSM system, now in the 5th edition! This is despite the clinical observations and research samples that confirm the combined subtype, having both components of inattention and hyperactivity/impulsivity, is almost the rule and “predominantly inattentive” or “predominantly hyperactive/impulsive” are the exceptions!
Unfortunately there are only a few comparison studies between the two above subtypes. Martel and colleagues in comparison between the two subtypes, reported “a composite executive function factor was significantly related to inattentive but not hyperactive-impulsive symptoms.” Nigg and colleagues also have reported that symptoms of inattention-disorganization were uniquely related to executive functioning when hyperactivity-impulsivity controlled. Also Marshal’s group have found academic underachievement in a group of 6-12 years old with ADHD without hyperactivity. In study of the combined ADHD subtype, Nigg and colleagues have reported “considerable heterogeneity with regard to any single cognitive deficit.” So from these studies, it seems that the hyperactive/impulsive subtype does not have any major cognitive deficits but as per other articles here and in the book “ADHD:Revisited”, it is associated with dynamic attentions or “hyper-attention” and high intelligence!
Eisenberg’s group in their pilot study on the genetic risk study of ADHD in 1999 showed “ the impulsive-hyperactive type of ADHD (excluding inattention) was associated with the high enzyme activity COMT val allele.” These frontiers also recognized such genetic association, i.e. COMT val allele with the increases in CNS dopamine (and norepinephrine) clearance, consistent with the use of methylphenidate that increases dopamine (and norepinephrine) turnover, in the treatment of ADHD. This study perhaps was the first one to show the difference between subtypes of ADHD not only clinically and phenomenologically, but also pathophysiologically. Later on, other scientists, e.g. Halleland and colleagues, Paloyelis’ group and Malloy-Diniz and colleagues, have also showed such association between COMT haplotypes and hyperactivity/impulsivity symptoms in both children and adults. Cao and colleagues by using sophisticate diffusion MRI and probabilistic tractography method to examine whole-brain white matter in ADHD subjects showed decreased structural connectivity in the prefrontal-dominant circuitry of the inattentive subtype, but increased connectivity in the orbitofrontal-striatal circuitry in the hyperactivity/impulsivity subtype!
Therefore it seems that the two subtypes of ADHD are not subtypes of one condition, according to DSM, but two quite distinct disorders. From a longitudinal perspective, the illness starts with hyperactivity or lack of inhibition on motor control as early as infancy long before manifestation of any cognitive symptoms, manifesting in the infant’s temperament and Activity Level (AL). This feature or endophenotype (AL) has been shown to be heritable with significant phenotypic and genetic overlap with hyperactivity/impulsivity symptoms later on in life. Therefore if there is any attention-deficit in ADHD hyperactive/impulsive type, it is related to “sustained attention” not other forms of attention, i.e. divided, selective, alternating/switching as these subjects get bored on tasks that are not dynamic, challenging or interesting to them! If there is a “predominantly inattentive” subtype of ADHD, the mechanism of its attention-deficit needs to be dissected and explained as it could not be secondary to hyperactivity/impulsivity as by definition this subtype lacks such features. Moreover “inattentive” subtype could not have any dynamic attentions like the hyperactive/impulsive type, but an almost pure attention-deficit with no other features. Therefore by deduction, the inattentive subtype has no relations with ADHD, hyperactive/impulsive type and is a predominantly cognitive deficit disorder on its own. Again by deduction, the inattentive type could be more associated with other pervasive developmental disorders, e.g. autism and learning disabilities than ADHD, hyperactive/impulsive type that in fact is associated with hyper-attention, dynamic attention, high intelligence and is an evolutionary by-product of our brains.
Read more in the book “ADHD:Revisited” available a Amazon, Kindle books.