Work related etiology of amyotrophic lateral sclerosis (ALS): a meta-analysis.

The aim of this meta-analysis was to evaluate the association between ALS and occupational exposure to physical (ELF-EMF) and chemicals (solvents, heavy metals and pesticides) agents.We considered articles published from 1980 up to April 2013; in total, 750 publications were evaluated. The studies had to satisfy the following criteria: 1) cohort or case-control studies; 2) the presence of individual exposures; 3) clinical diagnosis of sporadic ALS or sporadic ALS on the death certificate. We followed the evaluation of quality in two steps. The first step classified studies according to a rating system based on a mix of criteria developed by scientific organizations, especially developed for studies of risk factors for ALS. The ratings obtained range from I (highest) to V (lowest). The data on risk factors derived from studies with Armon ratings of I, II, and III can reach levels of evidence A (established risk factor), B (likely risk factor), or C (possible risk factor). The second step evaluated the exposure and a score from 1 to 4 was assigned to each item; an exposure with a score of 3 or 4 was considered sufficient. Different analyses were performed on ALS and exposure to metals, solvents, pesticides and electromagnetic fields. In our study the heterogeneity was assessed both by χ2-based Q-tests and through the index of inconsistency I² while the measure RR/OR and CI of 95% to estimate the relationship between ALS and the various considered risk factors was employed.The association between exposure to pesticides and ALS as a whole is weak and not significant. With regard to the results of individual studies the following critical synthesis can be reported: 1) the selected studies showed a low level of association between ALS and electromagnetic fields; 2) as regards the solvents, the association with ALS in some studies is combined with a slightly increased risk, particularly in women, and in others a slight but significant linear association is observed; 3) for the metals, in some cases there was a stronger association in women than in men; for individual metals, there was an association especially with chromium and lead; 4) lastly, with regard to the products of agricultural pesticides in general, there was an association with ALS in men but not in women, with a dose-response relationship.The lack of statistically significant association between occupational exposure and ALS is mainly due to the methodological diversity of the studies and the lack of prospective studies at the workplace.