The association between mercury levels and autism spectrum disorders: A systematic review and meta-analysis

Background & aims: The relationship between mercury and autism spectrum disorders (ASD) has always been a topic of controversy among researchers. This study aimed to assess the relationship between ASD and mercury levels in hair, urine, blood, red blood cells (RBC), and brain through a meta-analysis. Methods: A systematic search was performed in several databases including PubMed, ISI Web of Science, Cochrane register of controlled trials, Google Scholar, Scopus, and Maglran until June 2017. Case-control studies evaluating concentration of total mercury in different tissues of ASD patients and comparing them to the healthy subjects (control group) were identified. Necessary data were extracted and random effects model was used to calculate overall effect and its 95% corresponding confidence interval (CI) from the effect sizes. Results: A total of 44 studies were identified that met the necessary criteria for meta-analysis. The mercury level in whole blood (Hedges = 0.43, 95% CI: 0.12, 0.74, P = 0.007), RBC (Hedges = 1.61, 95% CI: 0.83, 2.38, P < 0.001), and brain (0.61 ng/g, 95% CI, 0.02, 1.19, P = 0.043) was significantly higher in ASD patients than healthy subjects, whereas mercury level in hair ( 0.14 mg/g, 95% CI: 0.28, 0.01, P = 0.039) was significantly lower in ASD patients than healthy subjects. The mercury level in urine was not significantly different between ASD patients and healthy subjects (0.51 mg/g creatinine, 95% CI: 0.14, 1.16, P = 0.121). Conclusions: Results of the current meta-analysis revealed that mercury is an important causal factor in the etiology of ASD. It seems that the detoxification and excretory mechanisms are impaired in ASD patients which lead to accumulation of mercury in the body. Future additional studies on mercury levels in different tissues of ASD patients should be undertaken.