A new analysis of mortality from motor neurone disease in Japan, 1950-1990: rise and fall in the postwar years.

Recent studies have demonstrated a worldwide rise in mortality from motor neurone disease (MND). However, in Japan mortality appears to have fallen significantly since the late 1960s, especially amongst women. Studies of the cause of both the worldwide rise, and the unique decline in MND mortality in Japan, have largely failed to substantiate the role of any single factor, or group of factors in these phenomena. Modelling the relationship between age and mortality using gompertzian analysis has already shown that the rise in MND mortality in England and Wales, and the United States, is mainly the result of increased longevity and decreasing competition from other causes of death amongst a susceptible subpopulation. Employing the same techniques on Japanese mortality data from 1950 to 1990 demonstrates that an unusual and accelerated increase of mortality occurred in the 1950s and 1960s, probably caused by an earlier unknown but extremely potent environmental agent or agents. This premature depletion of the susceptible subpopulation resulted subsequently in a lower than expected mortality rate. Mortality is now rising slowly to expected levels as the size of the susceptible subpopulation recovers to reach the ages at which MND is normally expressed. Further substantial rises in mortality are anticipated in future decades.