There are two broad perspectives on the classification of AD-HKD—the dimensional and the categorical approaches. According to the dimensional approach, the constituent behaviours of AD-HKD vary widely in the general population; persons are, therefore, more or less impaired. (29 Accordingly, AD-HKD is best viewed as the extreme of these behaviours, rather than as a discrete entity; affected persons are qualitatively distinct from unaffected persons. The categorical approach embodied in the current diagnostic classification schemas argues that AD-HKD is qualitatively, as well as quantitatively, distinct. The matter of which approach is correct is not easily resolved. It is likely that certain aspects of the syndrome are captured best by the categorical approach, while others are better characterized by the dimensional, or qualitative, perspective.

Even from the categorical perspective, the European approach to the definition and classification of AD-HKD disorders differs from that of North America. The most recent editions of DSM and ICD classifications reflect an effort to bring the definitions of ADHD and HD closer together. DSM-IV and ICD-10 have adopted almost identical criteria for the identification of inattentive, hyperactive, and impulsive symptoms ( Table 1). However, significant differences between the two nosological systems are still evident in their diagnostic algorithms (the number of criteria in each domain required for a diagnosis), the role of inattentiveness, the definition of pervasiveness across situations, and the role of comorbidity.

According to DSM-IV, the diagnosis of ADHD requires the presence of six inattentive or six hyperactive-impulsive symptoms, or both. Three different subtypes of ADHD are permitted: predominantly inattentive (the presence of six or more symptoms of disordered attention and fewer than six symptoms of hyperactivity-impulsivity), predominantly hyperactive-impulsive (the presence of six or more symptoms of hyperactivity-impulsivity and fewer than six symptoms of inattention), and combined (the presence of six or more inattentive and six or more hyperactive-impulsive symptoms). To establish a diagnosis of HD based on ICD-10, at least six inattentive, three hyperactive, and one impulsive symptom must be present. The diagnosis cannot be made in the absence of the symptoms of inattentiveness.

ICD-10 is also more rigorous about cross-situational pervasiveness, requiring that all necessary criteria be present, both at home and at school (or other situations). DSM-IV is more lenient; it demands evidence that criteria be met in at least one situation and that impairment be present in another, without stipulation of the number of symptoms present or their severity in this second situation.

The consequence of the differences in ICD and DSM operational definitions and decision rules is that patients with a diagnosis of HD (as defined by ICD-10) are more severely impaired than those with a diagnosis of ADHD (as defined by DSM-IV).(1. ,29

Perhaps the most salient difference between DSM-IV and ICD-10 classifications is the approach to the diagnosis when more than one disorder coexists with AD-HKD. DSM-IV recognizes all diagnoses that are present, except for schizophrenia, autism, and pervasive developmental disorder. In contrast, ICD-10, in general, discourages multiple diagnoses. In the presence of internalizing disorders such as anxiety and mood disorders, ICD-10 does not recommend the HD diagnosis. The only exception occurs when both HD and conduct disorder coexist. ICD-10 labels this combination the diagnostic subtype hyperkinetic conduct disorder. This new subcategory may deal with the tendency of clinicians in the United Kingdom and Europe to prefer the diagnosis of conduct disorder to HD when both disorders are evident.(39

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