Various subtypes of AD-HKD have been distinguished, based on phenomenology, pervasiveness of symptoms, and patterns of comorbidity. Subgrouping based on the children's response to treatment is less well established, but is suggested by the existence of children who do not respond favourably to methylphenidate (±!> and the apparent role of the genes involved in the dopaminergic and noradrenergic systems. (45>

Some subtypes appear in the current classification schemas. For example, DSM distinguishes among inattentive, hyperactive- impulsive, and combined (both inattentive and hyperactive-impulsive) subtypes. There is weak evidence preferentially linking the hyperactive-impulsive subtype to deficient inhibition, and the inattentive subtype to anxiety, social withdrawal, depression, and mental retardation. (5) The hyperactive-impulsive and combined subtypes are associated with oppositional and conduct disorder, and greater concurrent and lifetime impairment.(49

Both ICD-10 and DSM-IV require evidence of pervasiveness, namely evidence of the disorder in more than a single setting. Yet the criteria for pervasiveness are rather unspecified, particularly in DSM-IV. The requirement for pervasiveness is meant to preclude the diagnosis of AD-HKD for children whose history is provided by a single, potentially unreliable informant or for those who have behavioural problems that arise in a single distressing environment. Reliable differences do exist among children whose symptoms are evident both at home and school, and those who, for the most part, exhibit symptoms at school, but not at home, or at home, but not at school. The pervasive subtype is the least prevalent—possibly 20 per cent of children with AD-HKD have it—but it tends to be the most seriously affecting. Its rate of comorbidity, psychosocial impairment, neurodevelopmental delay, and cognitive impairment is higher. (47) School-only AD-HKD, the most common subtype/48 is typified by learning and cognitive deficits. The existence of a home-only subtype is the most contested. This subtype may be preferentially associated with psychosocial impairment.

Subtypes can also be based on the presence of comorbidity. The basis for this approach to subtyping is the possibility that some disorders (such as a reading disability, or an anxiety or conduct disorder that commonly coexists with AD-HKD) can generate a phenocopy of true AD-HKD.(4 50) According to the phenocopy hypothesis, the AD-HKD symptoms of a child with a reading disability, or an anxiety or conduct disorder may be a non-specific epiphenomenon of the underlying reading, anxiety, or conduct disorder, rather than true AD-HKD. Although these children seem to have AD-HKD, they actually have the comorbid condition. Consequently, AD-HKD arising in the context of these other disorders would not have the same correlates or cause, and may not respond to the same treatment as the AD-HKD that occurs in isolation.

True comorbidity, on the other hand, implies that the comorbid condition represents a combination of each of the individual disorders. True comorbidity arises from the association between the risk factors of each separate disorder, from a common cause for each disorder, or from the process that causes one disorder to increase the risk for the development of the second. (51

Comorbid AD-HKD and conduct disorder, in particular, may represent a distinct disorder. Compared with children with conduct disorder or with AD-HKD, children with the combination of AD-HKD and conduct disorder have more severe cognitive and learning problems, a worse prognosis, a greater incidence of AD-HKD and conduct disorder among relatives, and greater psychosocial adversity. (52>

Neurochemical, treatment-response, and genetic and family-history data partially support the existence of the distinct subtypes of AD-HKD and bipolar disorder, and

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