Socioeconomic Factors Associated with Geographical Variations in Cerebrovascular Disease in Miyagi Prefecture, Japan

Abstract

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 term "socioeconomic" as used in this research is broadly defined: it includes individual styles of life and behavioral patterns. The specific socioeconomic factors tested are: (1) indoor thermal environment, (2) level of physical work, (3) long absence from home, (4) diet, and (5) health-promoting behavior. 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. Interviews were conducted with the spouses of 35 cases and 35 controls in Shiwahime and the spouses of 34 cases and 34 controls in Karakuwa. 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. Hypertension control has for some time been a prime public health initiative in order to decrease stroke mortality rates in Japan. A campaign which focuses on a reduction in the high salt intake associated with hypertension has received particular attention. This research sheds new light on stroke studies by emphasizing the importance of improving the indoor thermal environment, particularly in harsh climates, as another means to reduce the risk of stroke. The research also delineates the significant features of stroke patterns in the Japanese context and provides data for cross-cultural comparative studies of this disease.