ALS and environment: Clues from spatial clustering?

A role for environmental factors in the etiology of amyotrophic lateral sclerocis (ALS) has been suspected for many years. A large body of work has implicated common exposures, conjugal cases, at-risk activities, heavy metals, organic solvents, and electric shocks, among others. One of the most demonstrative relationships between ALS and the environment is spatial clustering. We reviewed the most important and recent spatial clusters in a given area, whatever the geographical size, with either substantial epidemiological approaches or with highly significant associations, and with precise hypotheses. We present a broad, albeit incomplete overview of investigations in different areas, including examples of the difficulties and contradictions of some approaches. Most of the time, the implication of neurotoxins is suspected and, although not always strictly identified, some candidates are emerging: cycasin, MAM, L-BMAA, hydrazine, for example. One other important point is the possibility of interaction among risk/causal factors that increase the complexity of investigation. Additionally, with the exception of Western Pacific ALS, studies of spatial clustering are lacking a major methodological approach, namely a large cohort analysis extended over a long period of time, and probably for decades. Nevertheless, any spatial cluster needs to be identified, described and studied as deeply as possible to illuminate knowledge of the origin of this devastating disorder and to promote primary or secondary disease prevention.