Attention deficit/hyperactivity disorder: a focused overview for children's environmental health researchers

Abstract

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.

Figure 1

Neuroanatomical structures and dopaminergic and noradrenergic neuronal projections that have roles in ADHD. The illustrations are of the medial surface of a hemisected human brain. (A) Reductions in prefrontal cortical, caudate nucleus, corpus callosum, and cerebellar volumes are seen in ADHD. Altered functioning of the anterior cingulate and orbitofrontal cortices, the amygdala, and the nucleus accumbens has also been demonstrated in ADHD. The striatum includes the caudate nucleus, the putamen, and the nucleus accumbens. (B) Dopaminergic neurons that are important in ADHD arise in the ventral tegmental area of the midbrain and project to the frontal cortical and limbic structures, where they serve to modulate neurochemical signaling. Other dopaminergic neurons arise from the substantia nigra and project to the striatum, where they participate in controlling voluntary movement. (C) Noradrenergic neurons arise from the locus coeruleus and project to numerous structures including the prefrontal cortex, the limbic system, the thalamus, and the cerebellum. Adapted from Bear et al. (2001).