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The differentiation of attention-deficit/hyperactivity disorder (ADHD) into subtypes is supported by a considerable body of research, most of which has relied upon the use of factor analytic and cluster analytic methods to delineate correlational patterns among ADHD symptoms (e.g., Bauermeister et al., 1995; DuPaul et al., 1997; Lahey et al., 1988). Recently, several concerns have been raised about the criteria used to make subtype distinctions among individuals with ADHD (see Carlson & Mann, 2000; McBurnett, Pfiffner, & Frick, 2001; Todd et al., 2001). One important concern is that the diagnostic criteria may fail to distinguish ADHD subtypes that are clearly distinct from each other with regard to clinical correlates.
The two most prevalent and commonly researched subtypes are ADHD, Inattentive type (ADHD/I) and ADHD, Combined type (ADHD/COM; Barkley, 1998). These subtypes consistently have been found to differ on categorical (structured interviews) and dimensional (rating scales) measures of aggression, oppositionality, and conduct problems (e.g., Eiraldi, Power, & Nezu, 1997; Faraone, Biederman, Weber, & Russell, 1998; Gaub & Carlson, 1997). Subtype differences have also been found on neuropsychological measures, leading Barkley (1997) and others (e.g., Mirsky, 1996; Posner & Raichle, 1994) to hypothesize that differences between the groups are neurologically based. According to Barkley, individuals with ADHD/COM may have a fundamental deficit related to disinhibition, leading to sustained attention problems, disruptive behavior, and social rejection. In contrast, children with ADHD/I have a fundamental deficit with the focusing of attention, resulting in selective attention problems, passivity, and social withdrawal (Barkley, 1998).
The research literature presents a mixed picture with regard to ADHD subtype differences in internalizing symptoms, including anxiety and depression. The work of Lahey and colleagues in the 1980s identified differences between the ADHD subtypes in internalizing symptoms, with children who had attention deficits without hyperactivity (ADD/WO) showing more internalizing problems than those having attention deficits with hyperactivity (ADD/H; see Lahey & Carlson, 1992). In addition, Barkley, DuPaul, and McMurray (1990) found differential patterns of family history among children with these two subtypes. Children with ADD/WO were more likely to have biological parents with internalizing problems than those with ADD/H. These studies, among others, led Hinshaw (1994, p. 74) to remark that "a prevalent belief is that ADD/WO actually resembles an internalizing more than an externalizing disorder."
More recent studies have questioned this pattern of findings. For example, Faraone et al. (1998) found that children with ADHD/COM did not differ from those with ADHD/I on structured interviews and rating scales assessing internalizing problems. Other researchers have also failed to detect differences between the subtypes on parent and teacher ratings of internalizing symptoms (Eiraldi et al., 1997; Morgan, Hynd, Riccio, & Hall, 1996). In these studies, ratings of internalizing symptoms often have been higher for children with ADHD/COM compared to those with ADHD/I, with effect sizes in the moderate to large range, but the differences generally failed to reach statistical significance.
The discrepancy in findings between the Lahey et al. group (Lahey et al., 1987, 1988) and other research teams with regard to subtype differences in internalizing symptoms may be related to several factors. First, most of the research suggesting subtype differences in internalizing functioning used criteria from the third edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III; American Psychiatric Association, 1980) to classify children into subtypes. Recent studies using criteria from the fourth edition of DSM (DSM-IV; American Psychiatric Association, 1994) generally have failed to detect differences between the subtypes with regard to internalizing functioning. One obvious difference between the earlier and later versions of DSM pertains to the conceptualization of symptoms of impulsivity. In the DSM-III, impulsivity is linked with inattention and distinguished from hyperactivity, whereas in DSM-IV, impulsivity is linked with hyperactivity and distinguished from inattention. Also, some of the inattention items from the DSM-III have been changed in DSM-IV and several items have been added.
Second, inconsistencies in findings may be due in part to variations in the informants used to diagnose ADHD and its subtypes. For example, in the Lahey et al. (1988) study, children were diagnosed with ADHD based on clinician judgment after reviewing data from structured interviews and behavior rating scales administered to parents, teachers, and children. In the Faraone et al. (1998) study, ADHD diagnosis was based on a structured interview conducted with children. Further, in a study examining subtypes conducted by Wolraich, Hannah, Pinnock, Baumgaertel, and Brown (1996), children were categorized on the basis of teacher reports on ratings scales. Given that informants often differ markedly in their ratings of ADHD symptoms (Mannuzza, Klein, & Moulton, 2002; Power, Costigan, Leff, Eiraldi, & Landau, 2001), variations in informant can be expected to lead to meaningful differences in the formulation of diagnostic groupings.
Third, in most studies involving the subtyping of ADHD, distinctions between the subgroups have not been clearly demarcated. For example, children may be classified as having ADHD/I even if they demonstrate relatively high, yet subthreshold, levels of hyperactivity/impulsivity, such as when there are six or more symptoms of inattention but only five definitive symptoms of hyperactivity/impulsivity. The decision to assign a case like this to the ADHD/I subtype as opposed to the ADHD/COM group may depend on the failure to obtain an endorsement of one symptom. Similarly, children may be classified as having ADHD/I if they meet criteria for this subtype on structured interviews yet there is no check to make sure they also have relatively low ratings for Hyperactivity--Impulsivity on rating scales. The presence of these borderline cases in ADHD studies may serve to minimize differences existing between groups.
Fourth, discrepancies in research findings also may be due to variations in sample size and statistical power to detect differences between groups. In most studies, the sample sizes have been relatively small making it impossible to detect moderate or even large effect sizes. Further, even when sample sizes have been adequate, very few studies have reported effect sizes for the magnitude of the difference between groups.
Fifth, studies comparing ADHD subtypes with regard to level of internalizing problems have typically failed to account for subtype differences in the severity of aggression and conduct problems. Given the relatively high correlation between internalizing and externalizing functioning (Achenbach, McConaughy, & Howell, 1987) and the high rates of comorbidity between depression and externalizing disorders, particularly conduct disorders (Loeber, Burke, Lahey, Winters, & Zera, 2000), it is important to control for subtype differences related to externalizing problems.
This study was designed to investigate whether ADHD/COM and ADHD/I can be distinguished from each other with regard to their associations with internalizing symptoms. In this study, we systematically evaluated whether there are differences between the ADHD subtypes with regard to the severity of anxiety and depression. Diagnostic criteria from the DSM-IV were used to differentiate children into ADHD subtypes. A rigorous multi-informant, multimethod strategy, involving parents and teachers as informants and structured interviews and rating scales as methods, was employed to classify children into diagnostic groups. Stringent criteria were applied to differentiate children into ADHD subtypes to insure accurate classification and to minimize problems that can arise with cases that are on the borderline between subtypes. This is the first study investigating subtype differences in level of internalizing problems to control for …