Neuropsychological functioning in people with ADHD across the lifespan
Introduction
Although most of our current knowledge about Attention-Deficit/Hyperactivity Disorder (ADHD) developed from clinical observations and research with children, understanding of the disorder in adults is growing rapidly. Children, adolescents, and adults who are diagnosed with ADHD share similar clinical features, comorbidities, and failures in major life domains (e.g., academics and work; (Biederman et al., 2000, Hechtman, 1992, Mannuzza et al., 1993), and possibly brain abnormalities (Seidman, Valera, & Bush, 2004). However, as there is much to learn from different subgroups of persons with ADHD, it is clear that a full understanding of the disorder requires research from a life span perspective — integrating what we know about how it affects both adults and children (Biederman, 1998).
Cross-sectional data suggest that neuropsychological dysfunctions are an important component of the childhood syndrome (Barkley, 1997, Faraone and Biederman, 1998), and a growing literature is suggesting the same for adults (Hervey, Epstein & Curry, 2004). However, are the neuropsychological deficits ubiquitous as suggested by the diagnostic name, “ADHD”? In this article, we review the current state of the literature pertaining to the neuropsychological dysfunctions that are found in children, teenagers and adults with ADHD, focusing particularly on executive functions. Because there is considerable interest in neuropsychological assessment as a clinical tool for persons with ADHD, the nature and validity of clinical neuropsychological assessment for this population will also be addressed.
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Development of the concept of attention-executive and brain dysfunction in ADHD
ADHD, formerly called “hyperactivity,” “hyperkinesis disorder of childhood,” or “minimal brain dysfunction,” was first described 100 years ago as a childhood disorder found mainly in boys (Still, 1902). Revisions in the diagnostic construct have been made a number of times over the past century (Barkley, 1990). The most important shift occurred in the 1970s, when the concept of attention dysfunction was introduced as the defining feature (Douglas, 1972), and the disorder was re-named
Neuropsychological assessment in preschoolers
Although there are more than a hundred studies examining neuropsychological functioning of ADHD in childhood, there are relatively few studies examining such functioning in ADHD preschoolers, ages 3–5 or children just entering school at ages 5–7. Overall, this small body of work is consistent with that observed in older children with the disorder (Valera & Seidman, 2006). Compared to healthy preschoolers, preschoolers with ADHD have been shown to display more inhibitory deficits and be more
Are neuropsychological abnormalities accounted for by comorbidity?
Persons with ADHD frequently have comorbid antisocial, substance abuse, mood, anxiety, or learning disorders (Biederman et al., 1993). Although spurious comorbidity can occur due to referral and screening artifacts, the review by Biederman et al. suggested that these artifacts cannot explain the high levels of psychiatric comorbidity. Family studies of comorbidity by the Biederman research team also dispute the notion that artifacts cause comorbidity (Biederman, Faraone, Keenan, & Tsuang, 1991;
Heterogeneity of ADHD and neuropsychological abnormalities
We have already addressed the fact that ADHD is a heterogeneous clinical disorder with substantial psychiatric and cognitive comorbidity. However, we have not yet investigated the issue of whether neuropsychological deficits are present in all or most individuals with ADHD, or whether the observed group differences can easily be applied to the individual case. To some, this may seem confusing: if ADHD is named “attention deficit (hyperactivity) disorder”, shouldn't all cases with the disorder
Effects of gender on neuropsychological abnormalities in ADHD
Although ADHD affects both genders, most of the research literature, including studies evaluating neuropsychological functioning, has been devoted to males (Berry et al., 1985, Gaub and Carlson, 1997). Gaub and Carlson's review (Gaub & Carlson, 1997) indicated that few studies included sufficient numbers of female subjects to warrant gender-based conclusions. Nevertheless, there are data supporting the presence of a valid syndrome of ADHD in girls (Hinshaw et al., 2002). Recent work by our
Neuropsychological dysfunctions in adult ADHD
Over the past decade, research on clinical neuropsychological dysfunctions in adult ADHD has intensified, and the evidence for such deficits in adults with ADHD is mounting. Recently, a meta-analysis was conducted of neuropsychological deficits in adults with ADHD (Hervey, Epstein, & Curry, 2004). They included only samples with persons 18 years and older, and with a control group. They reviewed 33 published studies and found that neuropsychological deficits are largely consistent with those
Experimental measures of neuropsychological dysfunctions
In contrast to the large number of clinical neuropsychological studies, paradigms from experimental psychology and cognitive neuroscience have been employed more sparingly and yet they offer much potential to illuminate basic, elemental processes impaired in ADHD. These will be reported selectively to illustrate some approaches to this area. For example, experimental investigations of response inhibition or interference control (Bush et al., 1998) have demonstrated excessive sensitivity to
Relating brain structure, function, and neuropsychological dysfunctions
The analysis of attention and EFs into subcomponents, and the mapping of attentional functions onto different brain regions, support the proposition that response inhibition and other executive deficits in ADHD will be associated with structural and functional brain abnormalities in specific regions. However, there is currently limited ADHD research in this area. In children, Casey et al. (1997) found that performance on three response inhibition tasks correlated only with those anatomical
Psychopharmacology of cognitive deficits in ADHD
The mainstay of clinical intervention with children who have ADHD for the past 50 years has been the stimulants (American Psychiatric Association., 2000, Spencer et al., 1996). There is a substantial body of evidence that certain cognitive deficits, such as processing speed, reaction time, vigilance, distractibility, and short-term memory, have been shown to improve with stimulant treatment (Berman et al., 1999, Loiser et al., 1996, Musten et al., 1997, Rapoport et al., 1980). For example, on
Clinical neuropsychological assessment
Although groups of children and adults with ADHD on average perform worse than normal controls on tests of attention and executive function, accumulating data suggest that not all children with ADHD suffer from neuropsychological dysfunction. Therefore, the diagnosis of ADHD should not be ruled in or out using individual neuropsychological test scores or neuropsychological batteries. Using a battery of conventional neuropsychological tests (i.e., Stroop, WCST, CPT, etc), Doyle et al. (2000)
Summary
ADHD is defined by behavioral characteristics similar to neuropsychological disorders of executive dysfunction. This paper reviews the literature of the neurocognitive characteristics of ADHD from early childhood through adulthood. The group data clearly supports the hypothesis that executive dysfunctions are correlates of ADHD regardless of gender and age, and these EF deficits are exacerbated by co-morbidity with learning disabilities such as dyslexia. Persons with ADHD have improved
Future directions for research
Although there is growing information that identifies neuropsychological abnormalities in ADHD in childhood through adulthood, many questions remain. First, there is still relatively little systematic neuropsychological information on ADHD throughout life, particularly in children < age 6, teenagers, and in adults over the age of 40. Second, most of the research is cross-sectional. It would be very important to evaluate a child sample longitudinally to determine whether the neuropsychological
Acknowledgement
This work is supported in part by grants from NIMH (RO1-MH62152) and the March of Dimes Foundation to Larry J. Seidman, Ph.D. and RO1-MH41314 and MH50657 to Joseph Biederman, M.D. Thanks to Joseph Biederman, M.D., Alysa Doyle, Ph.D., Stephen Faraone, Ph.D., Ronna Fried, Ph.D., Michael Monuteaux, Sc.D., and Eve Valera, Ph.D. for their contributions to this work.
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