Academic journal article Environmental Health Perspectives

Attention Deficit/hyperactivity Disorder: A Focused Overview for Children's Environmental Health Researchers

Academic journal article Environmental Health Perspectives

Attention Deficit/hyperactivity Disorder: A Focused Overview for Children's Environmental Health Researchers

Article excerpt

OBJECTIVES: Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed childhood neurobehavioral disorder. Much research has been done to identify genetic, environmental, and social risk factors for ADHD; however, we are still far from fully understanding its etiology. In this review we provide an overview of diagnostic criteria for ADHD and what is known about its biological basis. We also review the neuropsychological functions that are affected in ADHD. The goal is to familiarize the reader with the behavioral deficits that are hallmarks of ADHD and to facilitate comparisons with neurobehavioral deficits associated with environmental chemical exposures.

DATA SOURCES: Relevant literature on ADHD is reviewed, focusing in particular on meta-analyses conducted between 2004 and the present that evaluated associations between measures of neuropsychological function and ADHD in children. Meta-analyses were obtained through searches of the PubMed electronic database using the terms "ADHD," "meta-analysis," "attention," "executive," and "neuropsychological functions" Although meta-analyses are emphasized, nonquantitative reviews are included for particular neuropsychological functions where no meta-analyses were available.

DATA SYNTHESIS: The meta-analyses indicate that vigilance (sustained attention), response inhibition, and working memory are impaired in children diagnosed with ADHD. Similar but somewhat less consistent meta-analytic findings have been reported for impairments in alertness, cognitive flexibility, and planning. Additionally, the literature suggests deficits in temporal information processing and altered responses to reinforcement in children diagnosed with ADHD. Findings from brain imagining and neurochemistry studies support the behavioral findings.

CONCLUSIONS: Behavioral, neuroanatomical, and neurochemical data indicate substantial differences in attention and executive functions between children diagnosed with ADHD and non-ADHD controls. Comparisons of the neurobehavioral deficits associated with ADHD and those associated with exposures to environmental chemicals may help to identify possible environmental risk factors for ADHD and/or reveal common underlying biological mechanisms.

KEY WORDS: ADHD, attention, executive function. Environ Health Perspect 118:1646-1653 (2010). doi:10.1289/ehp.1002326 [Online 9 September 2010]

doi:10.1289/ehp.1002326

In recent years, there has been increasing awareness of the role of environmental factors in neurodevelopmental disorders, including attention deficit/hyperactivity disorder (ADHD) (e.g., Banerjee et al. 2007; Nigg 2006b; Swanson et al. 2007). In this review we provide a focused overview of ADHD for researchers who are interested in the association between environmental exposures and ADHD risk but have little familiarity with the disorder's diagnosis and prevalence, the functional domains that are impaired, or the underlying changes in brain structure and function. A second goal is to summarize behavioral deficits that are hallmarks of ADHD in order to facilitate comparisons with behavioral deficits associated with widely dispersed environmental chemicals--specifically lead and polychlorinated biphenyls (PCBs), which are reviewed in the companion paper by Eubig et al. (2010). At present, there is compelling evidence suggesting that several key brain functions are implicated in ADHD--attention, executive functions, processing of temporal information, and responses to reinforcement (Nigg and Nikolas 2008)--all of which are critical for modulating behavior (Barkley 1997; Nigg and Casey 2005). We review several meta-analyses published since 2004 that compare the performance of children and adolescents diagnosed with ADHD against non-ADHD controls on neuropsychological tasks measuring attention and executive functions. Additionally, we summarize the performance of ADHD children and adolescents on tests of temporal information processing and responses to reinforcement, which have not been evaluated in meta-analyses to date. …

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