Investigation of a cluster of multiple sclerosis in two elementary school cohorts

The authors investigated a cluster of multiple sclerosis (MS) among people who had attended two elementary schools in El Paso, Texas, from 1948 through 1970. The community was concerned about the possibility of childhood exposure to heavy metals from a large nearby smelter because historical environmental and biological sampling data demonstrated the potential for study cohort members to have been exposed to heavy metals during their pre-adolescent years. One cohort had no reported cases of MS. In the second cohort, 22 members self-reported a diagnosis of MS, and 16 of these cases were confirmed as MS by an independent board-certified neurologist. The crude MS prevalence estimate was 411 per 100,000 (95 percent confidence interval [CI] = 197-603), Prevalence estimates from four different populations were used for calculation of standardized morbidity ratios (SMRs). At the extremes, the study cohort represents a deficit of cases (SMR= 0.9; 95 percent CI = 0.51-1.44) or a four-fold excess (SMR = 4.0; 95 percent Cl = 2.29-6.5).