Longitudinal analysis of amyotrophic lateral sclerosis mortality in Norway, 1966-1989: evidence for a susceptible subpopulation.

Mortality from amyotrophic lateral sclerosis (ALS) in Norway has increased substantially over the last two or three decades, as it has in other Western nations. The reasons for this general increase are not clear, but the rise has been attributed to increasing exposure to a broad range of environmental agents. Our previous research has indicated that another explanation for the substantial rise in ALS mortality is more likely to be valid. In particular, the mortality rate curve for ALS is consistent with deaths being confined to an inherently susceptible subpopulation. The size of susceptible subpopulations and mortality rates within them have been shown to be consistent in England and Wales, Japan and the United States. The same methods have been used to analyse Norwegian mortality data. The analysis demonstrates that the rise in ALS mortality is real and that the cause of the increase is almost entirely attributable to the increasing life expectancy of the Norwegian population. Increased life expectancy, a consequence of decreasing general mortality, allows ALS susceptible individuals to survive to the ages at which ALS is expressed. As in other national populations, it is expected that mortality from the disease will continue to increase with continuing increases in life expectancy.