KOHMOTO Junko Wakayama Medical College, Research Institute of Applied Medicine, Division of Ne, 応用医学研究所, 助手 (90254557)
KIHIRA Tameko Wakayama Medical College, Research Institute of Applied Medicine, Division of Ne, 応用医学研究所, 講師 (30225015)
The past thirty years, the frequency of amyotrophic lateral sclerosis (ALS) has steadily declined in the Kii Peninsula of Japan as well as Guam. Unlike Guam, the population of Kozagawa, one of the Kii Peninsula foci, has greatly decreased, by about 50%, from 1955 to 1990. It raises a question whether Kozagawa people predisposed to ALS have emigrated from this highrisk area, resulting in "focus extinction".
Previous epidemio-environmental studies (1959-1971) showed that toward the south in the Kii Peninsula, average annual mortality rates increased along with a decrease of Ca and Mg contents in area rivers. However, during the period from 1985-1994, this negative correlation disappeared, corresponding to the disappearance of ALS foci in the Muro district (southernmost Kii).
During the period of 1989-1993, therefore, the incidence rates and migration patterns of patients with ALS in Wakayama Prefecture, including one of the high-incidence Kii Peninsula foci ("Kozagawa focus"), were surveye
d to examine whether the focus truly disappeared or not. During the 5 years, 77 ALS patients were observed with an average age at onset of 61.8 years. Overall, the average annual incidence rate was 1.43 per 100,000 population (2.23 for males and 0.71 for females) ; when age-adjusted to the Japanese 1990 population, it was 1.25 (1.85 and 0.61 respectively by gender). Average annual age- and sex-specific incidence rate was the highest in the 60 to 69-year range, with a peak of 5.31 for both males and females combined ; 8.88 for men and 2.36 for women. The rates varied in the five regions of Wakayama Prefecture from 0.38 to 2.48. Geographically, the areas with high incidence were distributed in the central and southernmost regions ; the highest being in the Kozagawa focus with 9.54 (two ALS cases for the five years ; 4,193 base population, 1990). During the study period, four emigrants from Kozagawa had developed ALS one to four decades after leaving the focus.
Although the remarkable clustering of ALS had been thought to have disappeared, the southern Kii Peninsula remains a high-risk area for ALS,especially if one interprets the data so as to include the emigrants. In general, the age at onset has increased on the past 20 years from 56.5 to 61.7 ; and male predominance is noted. Less