Mortality from motor neuron disease in Japan, 1950-1990: association with radioactive fallout from atmospheric weapons testing.

Motor neuron disease (MND) is a progressive and invariably fatal disease affecting the nuclei of the pyramidal tract and anterior horn cells. Despite intensive research into environmental agents associated with the onset or course of the disease, there is no single factor that can be confidently linked over time with regional, national or international variations in mortality rates. However, unusual variations in MND mortality rate in Japan from 1950-1990 were found to correlate highly significantly with variations in radioactive fallout released by atmospheric weapons testing in the Pacific. This association could be explained by the ingestion of alpha-emitting radionuclides acting upon a pre-existing susceptible subpopulation, a hypothesis which is consistent with recent research on the epidemiology and pathology of MND. However, it is likely that radiation is only one of many factors that act singly or in combination to accelerate the condition in subpopulations susceptible to MND.