Neuropsychological Functioning in ADHD Subtypes

When the most recent edition of the diagnostic manual for psychiatric disorders was published in 1994 (DSM-IV), the terminology and diagnostic criteria for ADHD was modified. Previously, the condition had been referred to as Attention Deficit Disorder (ADD), of which there were two varieties -- ADD with hyperactivity and ADD without hyperactivity.

DSM-IV, however, replaced ADD with ADHD (Attention Deficit-Hyperactivity Disorder) and identified three different subtypes: ADHD, Predominantly Inattentive Type (ADHD -IA) ADHD, Combined Type (ADHD-C) and ADHD, Predominantly Hyperactive-Impulsive Type (ADHD-HI). The inattentive subtype is used for individuals with large numbers of inattentive symptoms and comparatively few hyperactive-impulsive symptoms. For the hyperactive-impulsive subtype, the opposite is true. Finally, the combined type diagnosis is used when individuals display both types of symptoms. Thus, although the term ADD is still used quite frequently, it is no longer part of the "official terminology".

Some critics of ADHD as an actual diagnosis have argued that the changing definition of ADHD over the years is evidence that it exists only in the eyes of the beholder, and is not an actual medical condition. The current sub-typing system has been criticized as another attempt to transform typical childhood misbehavior into a medical condition, primarily for the benefit of professionals who are compensated for providing treatment. One way to address these concerns would be to demonstrate that meaningful differences in brain functioning exist between children with and without ADHD, and between the three subtypes of ADHD that are now recognized. The latter is particularly important for establishing the validity of the current subtypes.

One way to accomplish this is to compare measurements of brain functioning in individuals with and without the diagnosis, and between individuals diagnosed with the different ADHD subtypes. Studies of this type have been conducted and report significant differences in brain functioning in individuals with and without ADHD.

Another method is to examine differences in neuropsychological functioning between children with and without ADHD, and between individuals diagnosed with the different ADHD subtypes.

Neuropsychological functioning can be defined as ways in which brain functioning is reflected in behavior. It encompasses a wide range of attributes including the ability to plan ahead, the ability to think before acting (i.e. behavioral inhibition), memory, being able to process information quickly and accurately, etc. When aspects of neuropsychological functioning are assessed, one typically measures observable behavior -- how well an individual performs on a task that requires careful planning-- rather than actual brain functioning. When individuals perform poorly on these tasks, the inference is made that those brain areas involved in successful performance are not functioning as they should be.

Examining neuropsychological functioning in the three subtypes of ADHD was the focus of a recently published study in the Journal of Abnormal Child Psychology (Chabildas, N. et al. (2001). A comparison of the neuropsychological profiles of the DSM-IV subtypes of ADHD. Journal of Abnormal Child Psychology, 29, 529-540.) Participants in this study were 114 eight to eighteen year-old children with ADHD, and 82 comparison children without ADHD. These participants were recruited as part of a larger study of ADHD and reading disability that involved contacting all pairs of twins from 35 separate school districts in Colorado and conducting a telephone screen for ADHD.

Among the participants who met study criteria for ADHD, those in the inattentive (IA), combined (C), and hyperactive-impulsive (HI) subtypes numbered 67, 33, and 14 respectively. Within the three ADHD groups, the ratio of males to females was about 2.5:1. This ratio is consistent with what is generally found in community samples.

Among the comparison children, there were somewhat more girls than boys. (Note: The authors recognize that because they recruited only twins -- the reasons for this had to do with the larger study -- their results are not necessarily representative of the general population of children with ADHD. In addition, their sample of ADHD participants was determined using a screening procedure rather than a comprehensive diagnostic evaluation and may thus have included some children who were misdiagnosed. These factors do not negate the importance of the results reported below, but are factors worthy of consideration.)

All participants were given an individually administered IQ test and achievement test as part of the study. In addition, three different aspects of neuropsychological functioning were assessed. These included two computerized measures designed to assess behavioral inhibition (i.e. the ability to inhibit behavior and refrain from impulsive responding), two measures of processing speed (how quickly they can work with information presented to them), and a single measure of vigilance (sustained attention). The researchers employed widely used and well-validated laboratory measures of these neuropsychological abilities.

The researchers expected that IA children would show deficits in processing speed and vigilance relative to comparison children. Children in the HI group were predicted to show deficits in behavioral inhibition, but to perform similarly to comparison children on the vigilance and processing-speed measures. Because children with the combined subtype of ADHD have both inattentive and hyperactive-impulsive symptoms, they were expected to perform worse than comparison children on all neuropsychological measures administered.

Results

A comparison of IQ and achievement test results between children in the four groups yielded interesting results. Results for the IQ test were as follows: (Note: The average score on this IQ test is 100, and scores between 90 and 109 fall in the average range.)

Inattentive - 98;

Combined - 99;

Hyperactive-impulsive -113;

Comparison -115;

As can be seen, children with the inattentive and combined subtypes of ADHD had significantly lower IQ scores than comparison children and HI children. Children in the latter two groups, in contrast, did not differ significantly from each other.

Similar results were obtained for children's reading achievement. Once again, children in the IA and combined groups had significantly lower reading scores than comparison children or HI children. Unlike the IQ results, however, HI children also scored significantly lower on reading achievement than comparison children.

Results for the neuropsychological measures were somewhat inconsistent with what had been hypothesized. Contrary to expectations, inattentive children scored worse than comparison children on all measures, including the measures of behavioral inhibition in which they were not expected to differ from comparison children. In general, these differences remained significant even after controlling for group differences in IQ and reading ability. The same was true, as had been expected, for children with the combined type of ADHD. Although HI children performed slightly worse than comparison children on measures of inhibition and vigilance, these differences were not statistically significant. In addition, follow-up analysis indicated that differences between the HI children and comparison children were attributable to the higher levels of attention problems among children in the HI group.

After testing for group differences in vigilance, processing speed, and inhibition, the researchers examined how well scores on the inattentive and hyperactive-impulsive symptom rating scales predicted performance on the different neuropsychological measures. Contrary to predictions, every neuropsychological measure was predicted best by symptom scores for inattention. This was true even though IQ and reading ability were also included as predictors. In other words, attention problems predicted poorer performance on measures of neuropsychological functioning even after problems related to low IQ and reading difficulties were taken into account. Hyperactive-impulsive symptom scores, in contrast, did not predict performance on any of the measures.

Summary And Implications

The overall goal of this study was to examine whether inattentive and hyperactive-impulsive symptoms and the associated subtypes of ADHD related to different neuropsychological profiles. Results indicate this is clearly the case. The researchers had expected that IA children would show deficits in processing speed and vigilance, but not in behavioral inhibition. Children with the HI subtype of ADHD were expected to show deficits in inhibition but not vigilance or processing speed. And, children with the combined type of ADHD were expected to show deficits in all three areas.

Contrary to predictions, however, only inattentive symptoms were predictive of neuropsychological deficits. Thus, children with the inattentive and combined forms of ADHD performed worse than comparison children on all measures of neuropsychological functioning. Hyperactive-impulsive children, in contrast, had relatively normal neuropsychological profiles. In addition, children with inattentive symptoms -- whether in isolation or in combination with HI symptoms -- had the poorest IQ and reading achievement results.

These results provide evidence that the current sub typing system for ADHD is related to meaningful differences in important aspects of children's functioning. The fact that significant differences exist between comparison children and children in the ADHD-IA and ADHD-C groups is also consistent with the hypothesis that real differences in brain functioning are present in children with the most commonly diagnosed types of ADHD. The HI subtype is the least common, and whether it is associated with other types of neuropsychological deficits than those assessed in this study is not clear. In this regard, it is noteworthy that, unlike attention problems, for which a high degree of genetic influence has been demonstrated, extreme levels of HI symptoms in the absence of accompanying attention problems are apparently less influenced by genetic factors.

These results are also consistent with prior research indicating that it is the inattentive symptoms of ADHD that are most directly linked to academic problems. Parents of inattentive children thus need to pay especially close attention to academic functioning and the development of core academic competencies in their children. Because children with predominantly inattentive symptoms are often not significant behavior problems, it can be easy to overlook the less obvious struggles they frequently have mastering critical academic skills. As a result, it is sometimes not until late elementary school -- or even middle school -- that academic deficiencies developed over several years finally become evident. By this time, however, a child may have fallen significantly behind his peers and have difficulty catching up. This is certainly consistent with what I have found in some of my own work, in which attention problems, and not hyperactivity, predicted the development of significant reading difficulties during elementary school for many children.

These results also underscore the need to learn more about the HI subtype of ADHD. It appears that underlying brain functioning in these children is quite different from children with the other subtypes of ADHD. The implications of this for the causes, treatment, and long-term outcomes of this ADHD subtype need to be better understood.

Note: This article originally appeared in Attention Research Update, an online newsletter written by Dr. David Rabiner, a Duke University psychologist and former member of CHADD's Professional Advisory Board. You can learn more about Attention Research Update and sign up for a free subscription at www.helpforadd.com.

Reproduced with permission of David Rabiner, Ph.D. - HelpforADD.com