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Neurobiology and Genetics of ADHD
by Dr. Scott HowardDespite continued controversy, powerful new evidence regarding the biological underpinnings of ADHD have recently emerged. Repeated studies have shown the fact that controlling for confounding variables, the coefficient of heritability for ADHD is .8, or higher than that for Schizophrenia (.7), Intelligence (.5), or Breast Cancer (.3). This information is very compelling evidence for a biological explanation of ADHD, as it basically states that the chance of an identical twin having ADHD, if the other twin meets the diagnostic criteria, is 8 out of 10 times. These studies have been done in Australia, the United Kingdom and Sweden, as well as in various sites throughout the United States.
Dr. Joseph Biederman, one of the leading researchers in the world on ADHD indicates in a recent paper (Attention-deficit hyperactivity disorder, Lancet 2005; 366: 237-248) that candidate genes for ADHD have also been located. Continued research supports the likelihood that ADHD is a genetic disorder with a combination of different genes being responsible for the expression of ADHD. This includes evidence of dysfunction with respect to the cortical networks involving dopamine D4 and D5 receptors, which have been proven to be instrumental in the pathogenesis of ADHD.
Structural abnormalities and imaging studies have also consistently supported the fact that frontal–striatal regions of the brain are very much central to the dysfunction associated with ADHD. This evidence includes neuropsychological deficits involving problems with set shifting, inhibition, working memory, planning, and sustained attention. Imaging studies have showed a volume reduction in many different areas of the brain, but particularly in the frontal and cingulate regions of the cerebrum, which are central to many of the functions and skills associated with attention and executive abilities.
In addition, the specificity and functional effects of the dopaminergic and noradrenergic agents used to treat ADHD are well documented and robust. Recent developments have included longer acting versions of both stimulant and non-stimulant medications, and research is very clear that the therapeutic effects of these medications are strong and unequivocal.
It should be mentioned that Dr. Biederman, in the same article mentioned previously, reports that objections are often raised as to the abuse potential of various stimulant medications. However, research has shown that medicating with stimulants has a protective or prophylactic effect on the development of substance abuse. This is contrary to anecdotal evidence that focuses exclusively on the few individuals who may abuse these substances, rather than scientifically analyzing the relevant data.
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