Lead and Attention-Deficit/Hyperactivity Disorder: A Meta-Analysis

Attention-Deficit/Hyperactivity Disorder (ADHD) is thought to have a significant neurological component, and several brain structures have been implicated. Environmental variables like lead have been shown to affect brain structures, which in turn impacts cognitive development and behavior. Some studies have begun to associate environmental variables like lead with the inattentive and hyperactive-impulsive symptoms of ADHD. This meta-analysis examined the association between different components of ADHD (including attention problems and hyperactivity/impulsivity) and level of lead exposure in children and adolescents. Articles focusing on the association between lead and inattention or hyperactivity-impulsivity symptoms were gathered from the online databases PsycINFO and Medline. These articles were then coded for content, including the methods used to assess lead exposure, the type of ADHD symptoms examined, the methods used to assess ADHD, gender of participants, average age of participants, year of publication, and sample size. These variables were analyzed using meta-analytic procedures. It was predicted that a medium sized association exists between lead burden and the ADHD symptom sets of inattention and hyperactivity impulsivity. Furthermore, it was predicted that the method of lead burden assessment, particularly the use of hair samples (as opposed to the use of blood, bone, or teeth samples) would moderate the association by causing the effect to appear larger than that of the other methods of assessment. It was also predicted that the year of publication would moderate the association between lead burden and components of ADHD, as mean lead burden has decreased over the last few decades. The meta-analysis of 29 studies with a total N of 12,322 subjects and published between 1974 and 2010 revealed that a moderate association (r = .19) exists between lead burden and inattention as well as lead burden and hyperactivity-impulsivity. Neither method of lead burden assessment nor year of publication moderated the lead and inattention or lead and hyperactivity-impulsivity association. Sample size moderated the association between lead and total ADHD ratings, but this effect vanished following the removal of three outliers. Although this metaanalysis contained several limitations, it provided important information regarding the etiology, treatment, and prevention of ADHD and can be used to guide future research of the lead-ADHD symptomatology association. Doctoral dissertation: http://aquila.usm.edu/dissertations/765