Why does comorbidity matter? Chapters 1–5 have laid out the clinical description of attention-deficit hyperactivity disorder (ADHD), its epidemiology, and functional impairments. It is well established that in a given sample of patients with ADHD, only about 50–60% of the individuals will be free of any other psychiatric disorder, while the remainder will have one or more psychiatric or learning disorders concurrent with their ADHD . From a clinical perspective, these additional disorders often demand concurrent treatment as they may cause more impairment than the ADHD itself; furthermore the presence of comorbidity complicates studies of the life course and etiology of ADHD. Recently, it has been recognized that “reverse comorbidity” is a reality, that is, patients with “primary” diagnosis of another major psychiatric disorder such as affective disorder or autism spectrum disorder (ASD) also meet criteria for ADHD at rates much higher than in the general population. The term “reverse comorbidity” largely reflects the fact that the comorbidity was first recognized and studied in those with ADHD  – but its importance is now recognized for all psychiatric disorders; and that excess overlap of ADHD exists in certain other psychiatric disorders. The usual paradigm in comorbidity research involves comparing at least three groups: those with ADHD alone, those with ADHD who meet criteria for a comorbid disorder (CM), and a sample of controls; ideally a group of patients who have the CM without ADHD should be studied as well, although few studies do this. The psychiatric diagnosis is made via a structured interview according to DSM-IV criteria and groups are examined for differences on a number of variables. First, care must be taken to rule out artificial comorbidity due to the fact that the DSM criteria for ADHD and some disorders share individual criteria items – for instance poor concentration is a symptom of both ADHD and depressive disorders, while increased activity is common to ADHD and mania. However, most children with comorbidity continue to meet criteria for both disorders even when the overlapping symptoms are subtracted out [3, 4]. Second, while epidemiological studies generally show lower levels of comorbidity than clinical samples, meta-analyses of epidemiological studies clearly show that comorbidity is not purely an artifact of referral bias . Jensen et al. have discussed ways in which the CM associated with ADHD is important to study .
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