Research Updates
Gender Differences in ADHD Subtype Comorbidity
Between 50% and 80% of clinically diagnosed children with ADHD also meet diagnostic criteria for disruptive behavior disorders, learning disorders, and communication disorders. A 2005 published study by Australian researchers, Florence Levy, David Hay, Kellie Bennett, and Michael McStephen investigated the comorbidity and gender differences of ADHD symptom subtypes with separation anxiety disorder (SAD), generalized anxiety disorder (GAD), oppositional defiant disorder (OD), conduct disorder (CD), and speech and reading problems.
Past studies have shown some gender differences. Gaub and Carlson (1997) found that girls with ADHD displayed lower levels of inattention, internalizing behavior and peer aggress than boys with ADHD. Abikoff et al., (2002) showed that boys with ADHD engaged in more rule-breaking and externalizing impulsive behavior than did girls with ADHD. Gershon (2002) found that girls with ADHD manifested significantly fewer externalizing problems than boys with ADHD, but significantly more internalizing problems. Levy et al. (1996) reported a stong association between ADHD and reading and speech problems and found that there were more symptoms in boys than girls.
In this current study, Florence Levy and her colleagues examined 1,550 questionnaires returned from a large sample of twins and siblings studied in the Australian Twin ADHD Project. Their analysis of the data suggests that comorbidity does not greatly differ between gneders, but rather relates to the severity of ADHD symptoms (severity being defined as the number of ADHD symptoms shown, therefore, the combined subtype would be considered more severe than the inattentive or hyperactive-impulsive subtypes). While this was clearly so for ODD and CD symptoms, the need for remedial reading was present in boys and girls with the inattentive and combined subtypes suggesting that reading problems and inattention are related.
There did appear to be gneder differences in SAD, which is more apparent in girls with the inattentive subtype, and in boys with the combined subtype, possibly suggesting immaturity in both of these groups.
The results suggest that children in the combined subtype are more likely to show comorbidity and clinicians should be aware of this.
Levy, F., Hay, D.A., Bennett, K.S., & McStephen, M. (2005). Gender differences in ADHD subtype comorbidity. J. Am. Acad. Child. Adolesc. Psychiatry, 44, 368-376
A Comparison of DSM IV Types of ADHD Across Performance Domains
There has been debate in the literature as to whether the inattentive type (IA) ADHD might be a separate disorder from the two other subtypes (hyperactive-impulsive type (HI) and combined type (C)) or whether the subtypes, while somewhat different from one another, nonetheless are similar in treatment response and performance on a range of performance measures.
The February 2005 edition of The ADHD Report (Guilford Publications) contains an artilce by Robin Codding, et al. that reports the findings of a meta-analysis of differences in performance of ADHD subtypes. Codding, et al. examined the literature on the DSM-IV subtypes of ADHD using meta-analysis procedures. They investigated the presence and extent of differences between subtypes with respect to: (a) academic performance; (b) hyperactive and inattentive behavior; (c) social skills; (d) cognitive abilities; (e) comorbid internalizing disorders; (f) comorbid externalizing disorders; and (g) performance on neuropsychological tests.
Their meta-analysis covered data on 1,955 participants between the ages of 3 and 18 who had a diagnosis of ADHD combined type, ADHD-inattentive type, and ADHD hyperactive -impulsive type, or typicals which had no psychiatric or psychological diagnoses. The hyperactive-impulsive type had so few effect sizes across the studies that it was eliminated from the meta-analysis. The primary findings are summarized below:
- IA-type demonstrated a lower Full Scale IQ than typicals and there was a small difference showing lower IQ for IA compared to C-type. Lower scores were found in the IA type on Verbal and Performance IQs than typicals, whereas no such differences were found in the C-type's IQ scores.
- C-type showed lower scores from typicals on both reading ad math. Small to moderate differences from typicals were found in the IA-type. IA and C-types did not differ significantly in achievement measures.
- Poorer performance from typicals was found in both the IA and C-types on neuropsychological measures, but the two types did not differ significantly from one another.
- On measures of CPT, both the IA and C-types showed more errors of ommision from typicals with the C-type showing slightly more errors than the IA-type. Commission errors were more likely in the C-type.
- The IA and C-types both showed higher activity level scores than typicals with C-types being slightly more active than the IA-type.
- Small differences on parent and teacher reports of internalizing problems were found between the C and IA-types. However, both parents and teachers rated C-type as having much more externalizing problems than the IA-type.
- In the domanin os social adjustment, both the IA and C-types had more problems than typicals, with C-type rated as having more problems than IA-type.
Overall, the meta-analysis results suggest that the combined and inattentive types of ADHD are different, but not necessarily distinct. Therre seemed to be more similarities than differences in the two groups with the biggest differences surfacing on measures of externalizing behavior.
Codding, R.S., Eckert, T. L., Lewandowski, L. J., & Fiese, B. H. (2005). Comparing DSM-IV types of ADHD across performance domains: a meta-analysis. The ADHD Report , 13 (1), 9-11.
Working Memory Impairments in Children with ADHD
Working memory (WM) is critical to consious thought because it permits internal representation of information (e.g. rules) to guide desicion making and overt behavior during an activity. in a recent study reported in the Journal of the Merican Academy of Child and Adolescent Psychiatry, Martinussen, et al. (2005) sought to determine if there was evidence for deficits in working memory processes in children and adolescents with ADHD. They concluded that there was evidence of WM impairments in children with ADHD which could compromise academic achievement and social functioning. Many academic activities depend heavily on WM (e.g., arithmetic problem solving, reading comprehension, text generation). It is possible that poor academic progress in children with ADHD may be the result of WM deficiencies rather than a direct consequence of the behavioral symptoms of inattention andh/or hyperactivity-impulsivity.
Martinussen, R., Hayden, J., Hogg-Johnson, S., & Tannock, R. A meta-analysis of working memory impairments in children with attention-deficit/hyperactivity disorder. J. Am. Acad. Child Adolesc. Psychiatry, 44, 377-384.
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