B
POSSIBLE CONCEPTUAL MODELS LINKING MILITARY SERVICE AND AMYOTROPHIC LATERAL SCLEROSIS
Figure B.1 illustrates three possible conceptual models linking military service or other risk factors that might be related to military service with amyotrophic lateral sclerosis (ALS). The models are provided as aids in visualizing the complexity of the possible mechanisms; they are not intended to be exhaustive.
Model A represents a chain of events involving specific environmental exposures. Involvement in military service in any war, some sports, and some occupations may expose people to specific environmental agents. The environmental exposures are the common step in the chain of events. These not-yet-identified exposures (chemical; biologic, including infectious; or physical) might trigger one of several possible mechanisms of neuronal damage leading to the development of ALS.
Model B represents a chain of events involving strenuous physical activity. Involvement in military service in any war, some sports, and some occupations may expose people to strenuous physical activity. Strenuous physical activity is the common step in the chain of events. Strenuous physical activity might trigger one of several possible mechanisms of neuronal damage leading to ALS.
Model C takes into account the possibility of gene-environment interactions. Involvement in military service, some sports, and some occupations may expose people to specific environmental exposures or to strenuous physical activity. These factors might act in conjunction with a genetic susceptibility inherited by some people from their parents. The joint occurrence of those two factors—one genetic and one environmental—is necessary to trigger one of several possible mechanisms of neuronal damage leading to ALS.
