A Meta-Analysis of the Neuropsychological Effects of Occupational Exposure to Manganese

Abstract

This article reports a meta-analysis of 25 samples in 20 peer-reviewed published neuropsychological studies of the cognitive, psychological, motor, and sensory/perceptual effects of exposure to manganese. These studies included 1,410 exposed participants and 1,322 controls, for a total N = 2,732. Studies were excluded from this analysis if they were unpublished, had uncodeable data, were based on fewer than four participants, failed to have a comparison group, or reported on manganese effects other than cognitive or sensory/motor (e.g., liver functioning). Because the independent variables defining manganese exposure varied across studies, effect sizes were calculated for exposed versus non-exposed workers. Dose-response relations were considered for measures of manganese levels in air/dust (84% of studies reported), blood (MnB; 76% reported), urine (MnU; 52% reported), and hair samples (4% reported). Level of exposure was also estimated by reported years of exposure (M = 13.1 years). Cohen's d statistic yielded a statistically significant weighted mean effect size of - .17, p < .0001 for manganese exposure. However, an effect this small is typically undetectable when evaluating individuals because it is smaller (about 1/6 SD) than the confidence intervals of most neuropsychological measures. Because the effect is so slight and the overlap so great between exposed and unexposed participants (87%), the error rate would exceed the hit rate if causal conclusions were rendered for occupational exposure to manganese as the source of an individual's cognitive, sensory, or motor impairments based on neuropsychological testing or symptom reports.

Potential neuropsychological profiles in welders occupationally exposed to manganese: An examination of effect size patterns

“…For instance, Lees-Haley et al (2006) found an overall small effect size difference between manganese-exposed workers and controls (d = -0.17) in favor of controls. However, when the various neuropsychological domains were separately examined, effect size differences were fairly similar and ranged from -0.29 for Executive Processing to -0.08 for Perceptual Reasoning.…”

“…This literature is far less extensive than the number of studies that has examined occupational exposure to manganese more broadly. Furthermore, the more recent published research has been criticized for poor design and inappropriate use of statistical procedures (e.g., Lees-Haley et al, 2006). The samples have also typically involved litigants who have financial interests in the conclusions of the researchers (e.g., see Bowler et al, 2003).…”