On the occurrence and possible causes of motor neuron disease in Sweden

Sammanfattning: A series of investigations have been performed to study secular trends of motor neuron disease (MND) morbidity and mortality in Sweden, to identify time and/or space clusters, if any, and to uncover risk factors. The occurrence of the disease was estimated from incidence, prevalence and mortality rates, and the strength of various determinants was evaluated by means of the case-control approach, using population controls. The studies comprised different periods of time between 1961 and 1990.The mortality rate from MND in Sweden, for both genders taken together, doubled from 1961 to 1985, reaching 2.5 per 100,000 person-years, and the incidence rates were of a similar magnitude. The age-specific mortality rates peaked at 70 to 75 years of age. Regarding separate birth cohorts the peak was less clear, however, and the mortality rates tended to increase continuously with advancing age. An epidemic of MND was identified in the county of Skaraborg during the period 1973-1984, when three times as many males as expected had onset of MND, especially involving the farm-working population. In the community surveys involving the counties of Viirmland and Skaraborg the 50% survival rate was slightly more than two years.Among males high adjusted odds ratios were obtained for electricity work (0R=6.7, 95% Cl 1.0-32.1), welding (0R=3.7, 95% Cl 1.1-13.0) and exposure to impregnating agents (0R=3.5, 95% Cl 0.9-13.1). Heredity with regard to a neurodegenerative disease and/or thyroid disease yielded OR=2.1, 95% Cl 1.0-4.3. The highest odds ratio was found for the combination of such heredity, exposure to solvents and male gender (0R=l5.6, 95% Cl 2.8-87.0).In sum, an increase in MND seems to have taken place during the last few decades. The regional epidemic especially among males in agricultural work, the high risk estimates regarding some occupational exposures of mainly chemical nature, and the interaction with heredity, indicate that environmental factors might play a role in the pathogenesis of MND, and that a subpopulation might be particularly susceptible.