Motor axonal injury and amyotrophic lateral sclerosis: risk assessment using a reverse probability analysis technique.

A relationship between antecedent trauma and amyotrophic lateral sclerosis (ALS) has been clinically suspected for over a century. Nine cases of ALS that occurred over a 14-year period in young U.S. men (age range, 28-43 years) following trauma that produced motor axonal injury that antedated (range, 5-42 months) the onset of ALS have been reported by a single neurologist. After estimating the number of cases of ALS in 28- to 43-year-old American men that would be expected to occur over a 14-year period, a reverse probability analysis technique was used to determine the expected incidence rates of motor axonal injury in a ideally matched control population that would give probabilities of 0.05, 0.01, and 0.001 of observing nine or more instances of motor axonal injury occurring during the previous 5 to 42 months. This analysis was repeated assuming that this one neurologist had only reported 50 and 10% of all instances of ALS that occurred during this 14-year period in American men ages 28 to 43 years who had experienced trauma that produced motor axonal injury that occurred 5 to 42 months before the onset of ALS. Reverse probability analysis is a technique that can be used in performing risk assessments.