Elsevier

Biological Psychiatry

Volume 44, Issue 4, 15 August 1998, Pages 260-268
Biological Psychiatry

Original Articles
Neuropsychological function in adults with attention-deficit hyperactivity disorder

https://doi.org/10.1016/S0006-3223(97)00392-2Get rights and content

Abstract

Background: Recent studies indicate that attention-deficit hyperactivity disorder (ADHD) persists into adulthood, but little is known about the neuropsychological features of adult ADHD. Our objective was to assess neuropsychological functioning in adults with ADHD with a battery of executive function tests.

Methods: Subjects were 64 unmedicated adults, 19–59 years of age, with DSM-III-R ADHD of childhood onset who met criteria for ADHD when referred in adulthood and 73 non-ADHD controls of similar age and gender. Information on neuropsychological performance was obtained in a standardized manner blind to clinical status.

Results: Compared with controls, adults with ADHD were significantly impaired on measures of vigilance, semantic encoding for verbal memory, and written arithmetic, irrespective of age, gender, psychiatric comorbidity, or presence of learning disability. Despite comparable educational level and IQ, ADHD adults had a trend to lower occupational attainment and had significantly more academic problems in school.

Conclusions: These executive, attention, and achievement dysfunctions demonstrated in adults with ADHD provide additional support for the validity of the syndrome in adults.

Introduction

Follow-up studies show that attention-deficit hyperactivity disorder (ADHD) persists into adolescence and young adulthood in 10–60% of cases Biederman et al 1996, Hechtman et al 1984, Mannuzza et al 1991. Despite these findings, the diagnosis of adult ADHD continues to be controversial. Perhaps the most prominent reason fueling the controversy is the retrospective nature of the diagnosis that requires the recollection of distant childhood events that may not be accurate. The degree of controversy about adult ADHD suggests the need for additional studies of referred adults who meet criteria for ADHD. It is important to know if an apparent ADHD diagnosis in an adult carries with it a pattern of risk and outcome that is similar to what has been observed in children with ADHD. The demonstration of such a pattern—or systematic deviations from it—would help to clarify the validity of the adult diagnosis and supplement the clinical picture.

Because cognitive–neuropsychological functions, particularly attentional and executive processes, are frequently impaired in ADHD children (Douglas 1972), the demonstration of such deficits in adults with ADHD would provide additional external validation of the adult syndrome. Cognitive performance measures are useful validating criteria for ADHD because they do not share method variance with other measures. In contrast to neuropsychological measures, assessments of psychiatric symptoms rely on the self-reports of subjects or the reports of parents and teachers that may be influenced by recall biases, halo effects, and other potentially confounding factors. In adults this problem is magnified because an informant is often unavailable to verify behaviors that may have occurred many years earlier. On the other hand neuropsychological measures directly assess performance. Identification of core neuropsychological deficits in adults with ADHD is also important both as an empirical study of performance relevant to adaptive functioning and as a window into hypothesized alterations in brain functioning in frontostriatal systems (Grodzinsky and Diamond 1992).

Neuropsychological deficits, particularly impairments in attention and executive functions, have been fairly well established in ADHD children, primarily in preadolescent boys Barkley et al 1992, Grodzinsky and Diamond 1992, Pennington and Ozonoff 1996, Seidman et al 1997a. They manifest subaverage or relatively weak performance on various tasks of vigilance and sustained attention, motoric inhibition, executive functions (such as organization and complex problem solving), and verbal learning and memory Barkley et al 1992, Pennington and Ozonoff 1996, Seidman et al 1995a. Our work (Seidman et al 1997a) and that of others (Fischer et al 1990) shows these deficits are found not only in children but also in adolescents with ADHD. This pattern of attentional–executive function deficits is similar to that found in adults with acquired frontal lobe damage and thus has generally supported the hypothesis that ADHD may be a brain disorder primarily affecting the frontal cortex or the regions projecting to frontal cortex.

Hypotheses of frontostriatal brain dysfunction in ADHD are gaining support. Recent studies using magnetic resonance imaging (MRI) of the brain indicate that there are reductions in total cerebral volume (Castellanos et al 1996), subtle anomalies in caudate and pallidum Castellanos et al 1994, Castellanos et al 1996, Hynd et al 1993, and corpus callosum size and shape Giedd et al 1994, Semrud-Clikeman et al 1994, and reductions in right frontal area Castellanos et al 1996, Hynd et al 1990 in ADHD. These data are consistent with a positron emission tomography (PET) study that identified abnormalities of cerebral metabolism in the prefrontal and premotor areas of the frontal lobe in ADHD adults who had ADHD children (Zametkin et al 1990). Thus, an emerging neuroimaging literature points to abnormalities in frontal networks in ADHD (frontostriatal dysfunction), and it is these networks that control attention and motor intentional behavior (Voeller 1990), giving support to the notion that neuropsychological dysfunctions would also be present in adults with the disorder.

Despite the importance of understanding neuropsychological dysfunction in adults with ADHD, the empirical research on this issue is scant. In a preliminary study of intellectual performance in adult ADHD, our group found that compared with non-ADHD controls, adults with ADHD had significantly higher rates of repeated grades, tutoring, placement in special classes, and reading disability (Biederman et al 1993). The ADHD adults also had lower estimated Freedom-from-Distractibility IQs on the Wechsler Adult Intelligence Scale–Revised (WAIS-R) (Wechsler 1981). Moreover, they had lower vocabulary, block design, digit symbol, arithmetic, and reading scores. These cognitive deficits are consistent with those seen in ADHD children (Faraone et al 1993). Holdnack et al (1995) also found that ADHD adults demonstrated performance decrements on the WAIS-R including significant impairments on the information, vocabulary, block design, digit span, and digit symbol subtests. Those ADHD adults exhibited significantly slower reaction times to continuous performance test (CPT) targets and slower psychomotor speed, and were also more impaired on verbal list learning. To our knowledge, the Holdnack et al study is the only study of adults with ADHD that used a battery of executive function tests designed to assay frontal lobe functions.

A major concern in the interpretation of abnormal neuropsychological performance in adults with ADHD, however, is the high level of psychiatric comorbidity consistently reported in these subjects. Considering that comorbid psychiatric disorders may be independently associated with cognitive deficits, it is critical to evaluate their role in the neuropsychology of ADHD. Also, learning disabilities (LDs) are commonly comorbid with ADHD Faraone et al 1993, Faraone et al 1996, Semrud-Clikeman et al 1992. Since LDs are typically considered to be neuropsychological disorders, it is crucial to distinguish forms of ADHD with and without LDs in evaluating the neuropsychological correlates of the disorder. These issues have not been addressed by prior studies of adult ADHD.

In this report, we report on the results of a neuropsychological assessment of unmedicated adults with ADHD. Based on the pediatric literature and on previous studies of adult ADHD, we hypothesized that adult ADHD would be associated with executive–attentional dysfunctions irrespective of associated cognitive and psychiatric comorbidities.

Section snippets

Subjects

The subject sampling and diagnostic procedures were previously described Biederman et al 1993, Biederman et al 1994. Subjects were 64 clinically referred adults of both sexes between the ages of 19 and 59 who subsequently met full DSM-III-R diagnostic criteria for ADHD. Each had a diagnosis of childhood-onset and persistent ADHD confirmed by a structured diagnostic interview and clinical interview. They were drawn from a sample of 70 consecutively referred adults with ADHD, 6 of whom were

Sociodemographic and clinical characteristics

Table 1shows that there were small but statistically significant differences in age between adults with ADHD and control subjects. Therefore, age was used as a covariate in all analyses. Compared with controls, adults with ADHD had significantly more mood and anxiety disorders and childhood conduct disorder. The ADHD adults and controls had an identical grade level. Despite the fact that educational level did not differ between the two groups, ADHD adults were significantly more likely to have

Discussion

Unmedicated adults with ADHD performed worse than controls on measures of auditory sustained attention, executive components of verbal learning, and arithmetic despite similar levels of education and IQ. These impairments could not be accounted for by age, learning disabilities, psychiatric comorbidity, or gender. In the absence of learning disabilities, ADHD adults were more likely than controls to have repeated at least one grade in school, to have required placement in special classes, to

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    This work was supported, in part, by USPHS (NIMH) grant R01MH-41314 (J.B.).

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