Mortality from cerebrovascular disease (hereafter called stroke) has marked geographical variations in Japan. It is generally lower in urban and fishing areas than in agricultural areas. This research seeks an explanation for the variations. Particular attention is given to associations with socioeconomic factors which have received less attention than dietary and clinical factors related to stroke. The research tests hypotheses that the distribution of stroke is associated with certain socioeconomic factors.
The investigation had two components. First, using data from a mail survey, the relationship between study factors and the frequency of male and female stroke mortality was examined in seven cities and towns which had significantly high or low standardized mortality ratios (SMR) for stroke, in comparison to the SMR of Miyagi as a whole. Second, a detailed investigation of the relationship between socioeconomic factors and stroke mortality was conducted at the individual level, using a matched-pairs case-control design, in two selected towns with different socioeconomic compositions: Shiwahime, an agricultural town with relatively high stroke mortality, and Karakuwa, a fishing town with relatively low stroke mortality.
Hypertension is a major cause of stroke mortality in Japan. However, high blood lipid levels do not appear to play as important a role as they do in western countries. But hypertension does not seem to be the only causal factor, as evidenced by relatively low stroke mortality rates among people in fishing areas where the prevalence of hypertension is not necessarily lower than in agricultural areas. It is concluded that factors such as high salt intake which is associated with hypertension, a lack of regular blood pressure check-ups, and a poor indoor thermal environment contribute to high stroke mortality rates.
It has been demonstrated that exposure to a sudden change in ambient temperature (by going from a heated room to an unheated room) produces a sharp rise in blood pressure in hypertensive people. Thus, contrasting temperatures in indoor environments can possibly increase the risk of stroke by suddenly elevating the level of blood pressure, particularly for those who already have hypertension.