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Attention-deficit hyperactivity disorder (ADHD) is one of the most common psychiatric disorders of childhood, affecting 3% to 6 % of school-age children. In addition, it is associated with a higher risk of academic problems; with significant impairment in family and peer relationships; and with high rates of comorbidity with anxiety, depression, conduct problems, and delinquency (1). Moreover, impairments seem to continue through adolescence and adulthood (2,3). ADHD is associated with neuropsychological difficulties that interfere with the adequate functioning of affected subjects and with their adaptation to the demands of the social milieu (2). There are controversies about the ability of neuropsychological measures to provide a better understanding of brain mechanisms involved in the disorder (4). Nonetheless, these measures can document clinical differences found among ADHD subjects at a neuropsychological level. Several investigations have demonstrated more cognitive impairments on neuropsychological tests in subjects with ADHD than in either control subjects or patients without ADHD (5–8). Seidman and others found that subjects with ADHD demonstrated significantly more impairment on both the Wisconsin Card-Sorting Test (WCST) and the Stroop Test (ST) than did control subjects, regardless of various psychiatric comorbidities (9). However, previous neuropsychological studies have important shortcomings. First, several investigations have shown modifications of the cognitive performance in patients with ADHD following pharmacologic treatment (10–14). As a result, the cognitive assessment in medicated subjects with ADHD has been a significant limitation in the previous literature (9,12,15,16). Second, because most studies were conducted with referred samples (5,17,18), referral bias limits the degree to which findings can be generalized (nonreferred samples represent more fully the distribution of symptom severity). Third, previous studies included mixed samples of children and adolescents (7,18). Because differences in cognitive functioning may occur according to the developmental stage, studies with homogeneous age samples are needed. Finally, investigations of neuropsychological performance in children and adolescents with ADHD from cultures outside the US or other developed countries are scarce. Considering that cultural factors may modulate the clinical manifestation of disruptive behaviour disorders (19,20), differences in neuropsychological performance among subjects from different cultures should also be evaluated. Successive revisions of the DSM have taken several approaches to the clinical heterogeneity of ADHD. The latest edition (DSM-IV) (21) describes 3 subtypes of ADHD: inattentive (ADHD-I), hyperactive-impulsive (ADHD-HI), and combined (ADHD-C). Although these subtypes were developed empirically from the DSM-IV field trials (22), little is known about their diagnostic validity for use in either clinical or research settings. The primary motivation for identifying the DSM-IV subtypes was factor-analysis literature consistently showing that only 2 dimensions were needed to explain the covariation of ADHD symptoms (that is, inattention and hyperactivity-impulsivity) (22–24). Several studies have demonstrated that the subtypes ADHD-C and ADHD-I, which display inattention symptoms as a significant part of the clinical presentation, seem to be associated with more academic problems than are found in either the ADHD-HI subtype or in control subjects without ADHD (18,25,26). For example, Baumgaertel and others surveyed 1077 elementary school students, using behaviour ratings from teachers (27). Among the children with ADHD, those with the inattentive subtype were more likely to be rated as below average or failing in school (63%) than were children with the combined (50%) or hyperactive-impulsive (12%) subtypes. This clinical evidence suggesting differences among ADHD subtypes indicates the relevance of exploring possible differences in their cognitive profiles. In this regard, Barkley suggests that even inattention is qualitatively different among ADHD subtypes (28). He proposes that the poor sustained attention which apparently characterizes those with ADHD-C probably represents impaired goal- or task-directed persistence arising from poor inhibition, which takes a toll on self-regulation and disrupts broad executive functions. Conversely, ADHD-I (typically characterized as daydreaming, spacey, easily confused, in a fog, or staring) is most likely to reflect deficits in speed of information processing and in selective attention. Thus, according to Barkley’s model, 2 qualitatively different disorders are currently classified under the general construct of ADHD. Despite the clinical evidence documenting differences among ADHD subtypes and the existence of a theoretical model proposing that the neuropsychological mechanisms associated with inattention might be different for ADHD-C and ADHD-I, few studies have examined differences in the neuropsychological profiles of the ADHD subtypes. Before DSM-IV (21), only 3 controlled studies evaluated differences among ADHD subtypes according to specific neuropsychological tests. One of these investigations found no evidence to support different cognitive profiles among the subtypes (29). The other 2 showed significant differences in a few variables only—too few to indicate that ADHD subtype strongly influenced neuropsychological performance (30,31). After publication of the DSM-IV, Faraone and others assessed neuropsychological performance to verify differences among DSM-IV subtypes in children and adolescents (18). On psychometric measures of intellectual functioning and academic achievement, subjects with each subtype showed more impairment than did subjects without ADHD. These differences were significant for ADHD-C and ADHD-I. In measures of neuropsychological functioning among the 3 ADHD subtypes, however, they found no differences. Recently, Klorman and others compared patients with ADHD-I and ADHD-C for performance in executive functions (7). They found that the performance of patients with ADHD-C was significantly worse. However, Gadow and others did not find significant differences when they used a battery of 6 measures to test neuropsychological performance among DSM-IV subtypes in a sample of young adolescents with ADHD (32). Therefore, possible differences in neuropsychological performance among ADHD subtypes are still unclear. In this exploratory study, we assess neuropsychological performance in a sample of untreated Brazilian adolescents with ADHD. Based on the literature, we hypothesized that 1) the group with ADHD-HI would not present significantly more impairments on neuropsychological measures of executive function and selective attention, compared with control subjects (following the clinical literature); 2) the ADHD groups with clinically significant inattention (ADHD-I and ADHD- C) would present significantly more deficits on those neuropsychological measures, compared with control subjects; 3) the group with ADHD-I would show the worst performance on neuropsychological measures evaluating selective attention, compared with the other groups (following Barkley’s model). MethodSubjects Psychiatric Assessment
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