Sports and trauma in amyotrophic lateral sclerosis revisited.

An evidence-based review was undertaken of the literature published between 2002 and 2006 about sports, trauma and ALS in order to see if there were new data to modify the conclusions of a previous review (2003). The new data support the previous conclusions that physical activity and trauma are probably ("more likely than not") not risk factors for ALS (Level II conclusions). This review concludes also that the reports of an apparent excess of ALS in Italian soccer players likely reflect incorrect analysis of the data. The appearance of excess relies on accepting as valid estimation methods resulting in improbably low numbers of expected cases. A different method is proposed: it generates more plausible numbers of expected cases, compared to which there is no excess of total cases (Level C conclusion). A theoretical framework is developed to analyze the possible influence of a "healthy worker effect" on incidence of neurodegenerative diseases in cohorts of employed or formerly employed individuals. In lieu of theoretical speculations, data are needed to measure this effect, while controlling for known lifestyle factors and accounting for the effect of loss of competing causes of mortality.