ebook_ADHD2019

8 Rohde, Buitelaar, Gerlach & Faraone Most of these variants are fairly common such that everyone carries some genetic risk for ADHD. This creates a polygenic genetic architecture and supports the idea that the risk for ADHD, and its expression in symptoms, is a continuously varying trait in the population. For clinicians, this means that people who come to clinic with subthreshold symptoms may carry some biological risk for ADHD even though they don’t meet full diagnostic criteria for the disorder. ENVIRONMENTAL CAUSES OF ADHD EPIDEMIOLOGIC EVIDENCE FOR ENVIRONMENTAL CAUSES OF ADHD When discussing causation, studies of DNA risk variants have a clear advantage over studies of the environment. Our genome comes into existence prior to our birth. So, when scientists discover an association between ADHD and a DNA va- riant, it is clear that having ADHD cannot “cause” one to have a specific DNA va- riant but that having a DNA variant could logically increase risk for ADHD. Stu- dies of the environment are less clear-cut. For example, if a study documents that poverty is associated with ADHD, that could mean that poor nutrition, stress and other concomitants of poverty increase the risk for ADHD. But it is also possible that having ADHD leads to lower levels of education, poorer job performance and thereby increases the risk for parents with high genetic risk for ADHD to live in poverty. Thus, one must always keep in mind the potential for such ‘reverse causa- tion’ when evaluating environmental risk factors and evaluate whether these have been considered by the relevant studies. That said, keep in mind that when one member of an identical twin pair has ADHD, the risk to the co-twin is only about 50%. Thus, environmental risk factor must contribute to the etiology of ADHD. Some environmental risks are due to exposures to toxins, lack of nutrients or trauma. Many studies have examined the effects of iron and zinc on ADHD be- cause both of these elements are essential for producing norepinephrine and do- pamine in the brain. In a meta-analysis, Scassellati et al. 12 reported that measures of iron deficiency were associated with ADHD. They also found that ADHD was associated with low levels of zinc in the blood. Among the many toxins studied in ADHD patients, the strongest evidence implicates lead contamination. In their meta-analysis, Scassellati et al. 12 found that compared with controls, ADHD cases were more likely to have been exposed to lead. Many studies have tested the idea that pregnancy and delivery complications (PDCs) might cause ADHD by harming the brain at early stages of its develop- ment. Although the literature presents conflicting results, it tends to support the idea that PDCs are risk factors for ADHD. When PDCs have been implicated in ADHD they typically lead to oxygen deprivation and tend to involve chronic exposures to the fetus, not acute events. Among the most investigated PDCs,