Neuropsychological performance and attention-deficit hyperactivity disorder subtypes and symptom dimensions.

Attention-deficit hyperactivity disorder (ADHD) is a common, costly, and impairing syndrome characterized by age-inappropriate difficulties with overactivity, impulsivity, inattention and disorganization. The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV; APA, 2004) specified three subtypes of ADHD: Primarily Inattentive (ADHD-PI), Primarily Hyperactive-Impulsive (ADHD-PH) and Combined (ADHD-C). While these subtype distinctions may have some clinical utility, the etiological relationships between the subtypes as well as their stability over time remains in question (Lahey et al., 2005; Milich, Balentine, & Lynam, 2001; Willcutt et al, in press). As a result, DSM-5 has proposed to retain these designations but downgrade them to specifiers or presentations, and to add a fourth presentation for restrictive-inattentive. This presentation, as proposed at this writing, would be characterized by the presence of 6 or more inattention symptoms, with 2 or fewer symptoms of hyperactivity-impulsivity (http://www.dsm5.org, accessed November 2011). Neuropsychological approaches can provide a potentially useful method for examining the validity of these designations. In particular, as noted by Willcutt et al. (in press), it may be that subtypes or presentations are essentially severity variations, rather than distinct configurations or etiologies. The configuration of neuropsychological scores can help evaluate that argument (see Goth-Owens et al., 2010). However, outside of meta-analyses and reviews, few new investigations have examined multiple (rather than one or two) domains of neuropsychological functioning as a method for exploring subtype validity. In turn, confirmation of patterns of neuropsychological weakness among youth with ADHD can also provide clues to pathophysiology of the disorder and involvement of particular psychological processes as well as neural systems. Due to the utility of neuropsychological approaches for examining questions of validity such studies of ADHD have proliferated (for one review, see Willcutt et al., 2005). However, a key weakness in the neuropsychological literature concerning ADHD, and particularly regarding performance among the DSM-IV subtypes, has been an excess of single-process hypotheses and studies (Nigg, 2006). Further, failure to consider multi-process hypotheses have thus far prevented more in-depth examination regarding the validity of a categorical versus dimensional view of ADHD. Further validation of the structure of ADHD via measures of neuropsychological functioning may be particularly relevant in light of recent taxometric work highlighting the dimensional structure of ADHD (Marcus & Barry, 2011). If subtypes are essentially arrayed on a severity dimension, then this would undercut further the idea of categorical variation in the disorder. But if subtypes have unique configurations of neuropsychological patterns, this would tend to argue for some degree of categorical variation. Recent studies have begun exploring multi-process hypotheses by examining ADHD and its putative subtypes across multiple domains of neuropsychological functioning, albeit mainly component operations of executive functioning (Guerts et al., 2005; Loo et al., 2007; Martel, Nikolas, & Nigg, 2007; Riccio et al., 2006; Scheres et al., 2004; Solanto et al., 2007; Wodka et al., 2008). Such studies in some instances seem to support the hypothesis of multiple neurocognitive components contributing to ADHD (Sonuga-Barke, Bitsakou, & Thompson, 2010). Most examinations of subtype differences have focused on the DSM-IV Inattentive and Combined subtypes, because prevalence of the Hyperactive-Impulsive subtype is quite low among school-age children and adolescents when full DSM-IV criteria (including impairment) are required. This literature has yielded mixed findings, often with relatively modest sample sizes, but has suggested that ADHD-C has worse performance in most areas than ADHD-PI (Hinshaw et al., 2002; Klorman et al., 1999; Nigg et al., 2002). Few studies have provided evidence of specific patterns of weakness in ADHD-PI relative to ADHD-C, although processing speed may be an exception (Goth-Owens et al., 2010; Nigg et al., 2002; Solanto et al., 2007). As noted previously, vanishingly few studies of subtype effects have considered a broad array of hypothesized neuropsychological functions relevant to ADHD in the same study. Inclusion of multiple domains of neurocognitive functioning appears important for more comprehensive exploration of subtype differences, should they exist, and is crucial for future attempts to examine the validity of the categorical/subtype view of ADHD. Even so, the few studies that have employed multiple measures rarely found effects (Barkley, Grodzinksky, & DuPaul, 1992; Bauermeister et al., 2005; Chhabildas, Pennington, & Willcutt, 2001; Faraone, Biederman, Weber, & Russel, 1998; Guerts et al., 2005; Barkley, Murphy & Bush, 2001), suggesting little validity to the subtype classifications. Again, however, sample sizes were small and studies under-powered to find effects in most instances. Therefore, further clarification of these issues is needed in order to ascertain (1) how many constructs are needed to capture ADHD neuropsychological weakness, and (2) which of many candidate measures or constructs (if any) may be most useful in validating any potential distinctions between the ADHD diagnostic subtypes as well as their relationships to ADHD symptom dimensions (Castellanos et al., 2006; Nigg, 2006; Sonuga-Barke, 2003; Willcutt et al., in press). This is now particularly relevant in light of proposed DSM-5 presentations. While DSM-5 has not yet issued final, published versions, the recent proposal therein for a restrictive inattentive presentation (www.dsm5.org; accessed November 2011), reflects concern that it that heterogeneity within the Inattentive group may obscure potential subtype differences in neuropsychological performance. Although little studied, this hypothesis has received some support from family inheritance studies (Stawicki, von Eye, & Nigg, 2006), as well as from recent evidence of slower processing speed (Goth-Owens et al., 2010) and altered attentional filtering (Carr, Henderson, & Nigg, 2010) in children and adolescents with a “restrictive-Inattentive” presentation. In all, due to small samples, introduction of possible new presentation in DSM-5, and insufficient coverage of hypothesized ADHD neuropsychological deficits, it remains unclear which components of neurocognition make unique contributions ADHD, its subtypes or presentations, or its symptom dimensions. Past work regarding subtype differences has been mixed, and further examination of these issues is needed in order to more thoroughly vet evidence regarding the validity of the categorical (subtype) and dimensional views of the disorder. The current study sought to more definitively address these issues by examining neuropsychological performance in a large clinically-characterized sample of ADHD and control youth with a broad battery of measures designed to capture a wide range hypotheses about ADHD deficits.