ebook_ADHD2019_engl.

The World Federation of ADHD Guide 13 impairments becomes more likely. At lower levels of vulnerability, other related conditions may be expressed (e.g., neuropsychological impairments, learning di- sabilities, emotional dysregulation). The multifactorial model posits that no single factor is required for ADHD to occur. Although under the multifactorial model, all cases of ADHD arise from the same set of risk factors, the set of risk factors impacting specific patients might be quite different. For example, if there are 100 risk factors for ADHD and 50 are needed to develop ADHD, then two patients could have an entirely different set of risks causing their ADHD. Given that ADHD has been shown to be polygenic and that many environmen- tal risk factors have been discovered, the multifactorial model of ADHD seems more consistent with the data than an etiologic heterogeneity model. Apart from rare cases caused by gross abnormalities of chromosomes, CNVs or SNVs, we do not expect ADHD to be easily subdivided into separate etiologic entities. Figure 1.3 provides a schematic view of how genes and environment combined to produce persistent ADHD, remitting ADHD and subthreshold forms of ADHD. It seems likely that this view of ADHD is a good guide to the disorder’s true etiology, with the understanding that future work will clarify the number of discrete cases due to rare variants and the degree to which gene by environment interaction accounts for the etiology of the disorder. Figure 1.3 Model of the etiology of ADHD. Persistent ADHD Environmental risk Remitting ADHD Genetic predisposition (common and rare variants) Subthreshold cases: Inattentiveness Overactivity Impulsivity Novelty seeking Accidents Emotional dysregulation