Quantitative EEG in low-IQ children with attention-deficit/hyperactivity disorder

Abstract

Objective

This study investigated EEG differences between children with attention-deficit/hyperactivity disorder (AD/HD) and matched control subjects, with either normal or low IQs.

Methods

Twenty normal-IQ and 20 low-IQ children with AD/HD, and 40 age-, IQ- and sex-matched control subjects participated in this study. EEG was recorded from 21 sites during an eyes-closed resting condition and Fourier transformed to provide estimates for total power, and absolute and relative power in the delta, theta, alpha and beta bands.

Results

Children with AD/HD had increased theta activity with decreased levels of alpha and beta activity compared to control subjects. IQ was not found to impact on the EEG. No significant differences were found between subjects with normal and low IQs, with the low- and high-IQ AD/HD groups having similar EEG profiles.

Conclusions

Low-IQ children with AD/HD have similar EEG abnormalities to those with normal IQs, and IQ does not appear to impact on EEG power measures.

Significance

This is the first study to investigate EEG differences in low-IQ children with and without AD/HD.

Introduction

Attention-deficit/hyperactivity disorder (AD/HD) is one of the most common psychiatric disorders treated by child and adolescent psychiatrists in the US (Cantwell, 1996), with studies estimating the prevalence rate in children at between 3 and 6% (Lindgren et al., 1990, Pelham et al., 1992). AD/HD is characterized by varying levels of hyperactivity, impulsivity and inattention, which may change with development from preschool through adulthood (APA, 1994). The disorder interferes with many areas of normal development and functioning in a child's life, and if untreated, predisposes a child to social and psychiatric problems in later life.

EEG studies of children with AD/HD have typically found increased theta activity (Clarke et al., 1998, Clarke et al., 2001a, Clarke et al., 2001b, Clarke et al., 2001c, Janzen et al., 1995, Mann et al., 1992, Satterfield et al., 1972), occurring primarily in the frontal regions (Chabot and Serfontein, 1996, Lazzaro et al., 1998), increased posterior delta (Clarke et al., 1998, Clarke et al., 2001a, Clarke et al., 2001b, Matousek et al., 1984) and decreased alpha and beta activity (Callaway et al., 1983, Dykman et al., 1982), also most apparent in the posterior regions (Clarke et al., 1998, Clarke et al., 2001a, Clarke et al., 2001b, Lazzaro et al., 1998); see Barry et al. (2003) for a review. However, most of these studies have used children who are in the normal intelligence range. Studies that have used an IQ level as part of the exclusion criteria typically have a cutoff of between 85 and 90 (Clarke et al., 1998, Clarke et al., 2001a, Clarke et al., 2001b, Clarke et al., 2001c, Dykman et al., 1982, Harmony et al., 1988), above 80 (Capute et al., 1968), or simply report that IQ is in the normal range for children in their study (Mann et al., 1992).

The exclusive use of research subjects in the normal IQ range has resulted primarily from a concern that problems of poor attention, and increased impulsivity and hyperactivity, are considered normal phenomena in many forms of mental retardation. The DSM-III-R states that ‘In mental retardation there may be many of the features of AD/HD because of the generalized delay in intellectual development’ (APA, 1987, p. 52). However, the diagnostic criteria used in most electrophysiological studies of AD/HD do not prohibit diagnosis of this disorder in individuals with low IQs. Both the DSM-IV (APA, 1994) and DSM-III-R (APA, 1987) allow a diagnosis to be made if the symptoms are excessive for the child's mental age. Only the DSM-III (APA, 1980) specifically stated that AD/HD was not to be diagnosed in individuals with severe or profound mental retardation.

To date, only one study in the area has examined the EEG of children with low IQs (Chabot and Serfontein, 1996). That study examined 76 children with an IQ<85, who met the DSM-III (APA, 1980) criteria for attention-deficit disorder with or without hyperactivity, and children with attentional problems. Results were largely similar to those in subjects with normal IQs, with increased theta activity being the most common finding. However, patient results were compared to a normative database constructed using normal children, not allowing determination of how low-IQ children with AD/HD differ from low-IQ children without behavioural problems.

The aim of this study was to investigate the nature of EEG differences between children with and without AD/HD, who also have low IQs, and to determine how their EEGs differ from AD/HD children of normal intelligence.