EEG evidence for a new conceptualisation of attention deficit hyperactivity disorder
Introduction
Attention-deficit/hyperactivity disorder (ADHD) has become one of the most commonly treated disorders of childhood (Cantwell, 1996), with the last decade seeing an approximate 4-fold increase in the number of children diagnosed with this disorder (Brownell and Yogendran, 2001). ADHD has undergone considerable change in its conceptualisation, with debate still continuing over the exact nature of the disorder. The diagnostic and statistical manual of mental disorders (DSM) definition has changed from a single disorder with an emphasis on hyperactivity (American Psychiatric Association, 1968), to a two-dimensional disorder, allowing diagnosis of 3 types (American Psychiatric Association, 1994). In contrast, the tenth version of the International Classification of Diseases (ICD-10; WHO, 1993), widely used in Europe, lists criteria for a similar disability under the title of ‘hyperkinetic disorder’. This differs from the DSM-IV in recognising only one type of the disorder, due to what was considered as a lack of empirical evidence for the hyperactive/impulsive and inattentive types listed in the DSM-IV. However, recent work from our laboratory has provided evidence which argues that the ICD classification needs to recognise the inattentive type as a valid diagnostic category (Clarke et al., 2002b).
The two-dimensional DSM-IV model of ADHD has been questioned on a number of issues. For example, children with the predominantly inattentive type of ADHD (ADHDin) have been found to differ behaviourally from children with the combined type of the disorder (ADHDcom) on more than just their level of hyperactivity (Lahey et al., 1987, Lahey and Carlson, 1991), leading some researchers to suggest that ADHDin may be better categorised as a subtype of a different disorder (Lahey et al., 1985). Another limitation of the present definitional criteria is that diagnosis is based on behaviour alone, without consideration of the cause of the behaviour. It is possible that there could be a number of different underlying causes that result in the behaviours seen in ADHD, and these causes may have different developmental paths and/or require different treatment regimes.
Electroencephalography (EEG) research over the past 30 years has found consistent differences between children with and without ADHD. ADHD children typically have an excess of slow wave activity, primarily in the delta and theta bands, and deficiencies of alpha and beta activities (Clarke et al., 1998, Clarke et al., 2001b, Clarke et al., 2001d, Clarke et al., 2002a, Lazzaro et al., 1998, Chabot and Serfontein, 1996, Janzen et al., 1995, Mann et al., 1992, Dykman et al., 1982, Satterfield et al., 1972). These results have been interpreted as indicating that children with ADHD have a central nervous system (CNS) dysfunction, which has been characterised primarily as either a maturational lag (Mann et al., 1992) or cortical underarousal (Lubar, 1991). In a new approach to the investigation of CNS dysfunction in ADHD, we examined the existence of EEG-defined subtypes within a large sample of ADHDcom children (Clarke et al., 2001c). Despite these children having the same behaviour-based diagnosis, results indicated that there were 3 distinct EEG-defined clusters. These appeared to consist of a hypoaroused group characterised by increased high-amplitude theta activity and decreased delta and beta activities, a maturational-lag group with increased slow wave and deficiencies of fast wave activity, and an over-aroused group with excess beta activity.
The present study advanced that research by investigating the existence of EEG-defined subtypes within children with the inattentive type of ADHD, to determine the level of heterogeneity within this type of ADHD and its relationship to children with other types.
Section snippets
Method
Subject inclusion criteria, testing procedures and statistical analysis in this study are the same as our previously published cluster study (Clarke et al., 2001c) except for the type of ADHD patient included.
Subjects
Subjects in this study consisted of 100 boys with a diagnosis of ADHDin and 40 age-matched male control subjects. All children were between the ages of 8 and 13 years. Subjects had a full-scale WISC-III IQ score of 85 or higher. The children with ADHD were drawn from consecutive new
Study 2
The second phase of this investigation aimed to examine the presence of EEG differences between the hypoaroused and maturationally lagged clusters of children within the DSM-IV types of ADHD.
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