Age and sex effects in the EEG: differences in two subtypes of attention-deficit/hyperactivity disorder

Abstract

Objectives: This study investigated age-related changes and sex differences in the EEGs of two groups of children with attention-deficit/hyperactivity disorder (ADHD) combined type and ADHD predominantly inattentive type, in comparison with a control group of normal children.

Methods: Forty boys and forty girls were included in each group. The EEG was recorded during an eyes-closed resting condition and Fourier transformed to provide estimates for total power, absolute and relative power in the delta, theta, alpha and beta bands, and for theta/alpha and theta/beta ratios.

Results: Total power, relative alpha, and the theta/alpha and theta/beta ratios were differentiated between all 3 groups. Sex differences between the ADHD subjects and the control group were greater in males than females and matured faster in males. With increasing age, the EEG of the ADHD inattentive group was found to change at a similar rate to the changes found in the normal group, with the differences in power levels remaining constant. In the ADHD combined group, the power was found to change at a greater rate than in the ADHD inattentive group, with power levels of the two ADHD groups becoming similar with age.

Conclusions: These results are supportive of a two-component model of ADHD, with the hyperactive/impulsive component maturing with age and the inattentive component remaining more stable.

Introduction

Even though there have been significant changes made to the diagnostic criteria for attention-deficit/hyperactivity disorder (ADHD), there is still considerable disagreement regarding its aetiology. Two main developmental models of ADHD have been proposed, based primarily on results from electrophysiological studies. The first model proposes that the behavioural symptoms result from a maturational lag in the central nervous system. EEG studies of children with ADHD have typically found an increase in slow wave activity, primarily in the theta band, compared with normal control subjects (Capute et al., 1968, Wikler et al., 1970, Satterfield et al., 1973b, Lazzaro et al., 1998). Mann et al. (1992) found that children with ADHD had an increase in theta activity in frontal and central regions, and a decrease in beta activity in posterior and temporal regions during cognitive tasks. From these results, Mann et al. (1992) concluded that ADHD reflects a maturational delay in the systems that subserve attention. Clarke et al. (1998) investigated EEG differences between children with the combined (ADHDcom), and inattentive (ADHDin) subtypes of ADHD, and control subjects, during an eyes-closed resting condition. The two ADHD groups had increased levels of absolute and relative theta and decreased levels of relative alpha and beta. In posterior regions, relative delta estimates were elevated compared with the control group. All of these results were considered supportive of the maturational lag model.

A second model of ADHD proposes that ADHD represents a deviation from normal development. Chabot and Serfontein (1996) found that children with ADD (using DSM-III criteria) had an increase in absolute and relative theta, with the greatest increase being found in frontal regions and at the midline. From these results, it was concluded that the EEG patterns represented a deviation from normal development, as the obtained results were not typical of a normal child of any age (John et al., 1988, John et al., 1983).

To further clarify the underlying nature of the EEG found in children with ADHD, Clarke et al. (2001b) used ratio coefficients between frequency bands (Matousek and Petersen, 1973, Janzen et al., 1995, Lubar, 1991) and measures of the mean frequency (Matsuura et al., 1993, Katada et al., 1981) within frequency bands. Clarke et al. (2001b) found that the theta/alpha and theta/beta ratios had greater coefficients in the two clinical groups. In the alpha and beta bands, the ADHD groups had a lower mean frequency than controls. In the comparison of the lower frequency bands, the control subjects had lower mean frequencies in the delta band and lower coefficients for the delta/theta ratio. These results suggest that in the ADHD groups, there is a shift in the EEG at both ends of the spectrum, towards the theta frequency range, which does not support the maturational lag model.

The limitation of most of these studies is that they have used measures from children at one age and interpreted their results based on the developmental literature for normal children without investigating changes in the EEG with age. Another area that has not been adequately investigated within the ADHD literature is the existence of sex differences within this population. The ratio of boys to girls with ADHD has been estimated at approximately 4:1 for all 3 DSM-IV subtypes (De Quiros et al., 1994), and this sex difference has been cited as the major reason for the lack of female studies of ADHD within the literature (Arnold, 1996). The results of studies are varied, though they tend to suggest that childhood disorders are generally more prevalent among males, but more severe in females (Eme, 1992). Compared with boys with ADHD, ADHD girls display greater intellectual impairment and lower rates of hyperactivity and other externalizing behaviours (Gaub and Carlson, 1997). Within the EEG literature, no studies have investigated EEG abnormalities in girls with ADHD.

This study aimed to investigate age-related differences in the EEG of children with ADHD and to determine: (1), if changes occur in a similar manner to normal children; and (2), if changes in the EEG of the inattentive and combined subtypes of the disorder are similar. An additional aim was to investigate whether EEG differences exist between boys and girls with ADHD.