Amyotrophic lateral sclerosis, heterogeneous susceptibility, trauma, and epidemiology

J. E. Riggs
Department of Neurology, West Virginia University School of Medicine, Morgantown, USA.

Epidemiological studies relating antecedent trauma and amyotrophic lateral sclerosis (ALS) demonstrate a contradiction: positive (but poorly structured) retrospective case-control studies and negative (but uninterpretably small) prospective cohort studies. In this report, the equations for the case-control odds ratio and cohort relative risk in populations with heterogeneous susceptibility to ALS are analyzed. The case-control odds ratio and cohort relative risk converge as the proportion of ALS-nonsusceptible individuals in a population increases and the rate of ALS in nonsusceptible individuals decreases. Cohort studies of antecedent trauma and ALS have no significant advantage over case-control studies in populations in which most individuals are relatively nonsusceptible to ALS. Accordingly, the relationship between antecedent trauma and ALS can be addressed by carefully defined case-control studies.