The prevalence of mental distress before the Great East Japan Earthquake and the associated impact of an aged society: An ecological study

Various studies have determined that the Great East Japan Earthquake (GEJE) caused mental distress among residents in affected areas. However, previous studies had not considered the prevalence of mental distress before the GEJE, and ignored the impact of an aged society on mental distress. Therefore, we aimed to describe the prevalence of mental distress before the GEJE in Miyagi Prefecture, Japan and elucidate the effect of an aged society on mental distress. We conducted an ecological study, using municipality in Miyagi Prefecture as the study unit. We used the cross-sectional mail survey data conducted in February 2011. We performed a correlation analysis in each of the 39 municipalities in Miyagi Prefecture. The prevalence of serious mental distress was 9.1%. The proportion of the population aged 65 years or older was related to the prevalence of serious mental distress in municipalities with a low proportion of all workers engaged in primary industry and with a high estimated number of inpatients with mental illness. We found that residents in Miyagi Prefecture suffered from poor mental health before the GEJE. Aged society was related to serious mental distress in the areas with advanced industrial structure and more patients with mental illness. We should approach mental health problems in the context of social structure, particularly in an aged society, based on facts about mental distress before the GEJE.


Introduction
The Great East Japan Earthquake (GEJE) occurred on March 11, 2011 on the coast of northeast Japan, resulting in the death or disappearance of approximately 20,000 people [1]. The GEJE mainly affected the Iwate, Miyagi, and Fukushima Prefectures, in the Tohoku region of Japan (Fig 1). Most of the damage to the houses or harm to humans was however, concentrated in

Study design
We conducted an ecological study, using municipalities in Miyagi Prefecture as the study unit (N = 39). Miyagi Prefecture is located approximately 300 km north of Tokyo, and consists of 39 municipalities (Fig 2). The population of Miyagi Prefecture is about 1.9 million.

Data
Mental distress data by social survey. We used the cross-sectional mail survey data to determine the prevalence of mental distress. We conducted the survey in February 2011. Subjects were 2,500 randomly selected residents of Miyagi Prefecture, aged 20−74 years. With an estimation error of 2.5%, we needed to extract 1,500 samples to estimate the precise prevalence in the population of approximately 1.9 million. Therefore, assuming a response rate of 60%, we required 2,500 samples. We chose the sample according to the population size of each municipality. The sample size from each municipality was determined according to population size, with at least approximately 30 samples from each location ( Table 1). The survey was selfadministered by those willing to participate in the study. The survey received 1,543 responses (response rate: 62.5%). This study was approved by the Tohoku University Medical Sciences' ethical review board (No. 2010-245, September 15, 2010). We interpreted the voluntary return of the self-administered questionnaire in the postal survey as informed consent. This consent procedure was also approved by the ethical review board above. The data were analyzed anonymously. The data are available from a supporting information file (S1 Dataset).
We used the Japanese version of the Kessler Psychological Distress Scale (K6) [22,23] to measure mental distress in the survey. The validity of the use of the Japanese K6 among the Japanese population has been confirmed as detailed in a previous publication [24]. The K6 is used to assess non-specific psychological distress. The Japanese version consists of six items which ask how frequently respondents have experienced the following symptoms during the past 30 days: "feeling so sad that nothing could cheer you up," "feeling nervous," "hopeless," "restless or fidgety," "feeling that everything was an effort," and "feeling worthless." Each item was scored according to a five-point scale; responses of "none of the time" were allocated a score of 0 while responses of "all of the time" were allocated a score of four. The sum of scores for the six items (ranging from 0 to 24) was used to indicate severe mental disorders [25], high scores indicated a poor mental health status. A score of 13 or more was defined as serious mental distress as per the international standards for the classification of serious mental distress [26]. Excluding the participants who did not respond to the questions regarding K6, we analyzed the remaining responses (n = 1,487, final response rate: 59.6%) and calculated the serious mental distress rate for each of the municipalities.
Social structural data. We determined the proportion of the population aged 65 years or older (continuous) to measure the aged society for each municipality. Additionally, we employed social structural characteristics from the following perspectives of urbanization, economic, industrial, and health determinants to examine the effect of the proportion of the population aged 65 years or older on the prevalence of serious mental distress. We used the population density of the residential area (number of people living per km 2 ) as an urbanization determinant of social structure to measure the degree of urbanization for each municipality. The unemployment rate was used as an economic determinant of social structure to measure the magnitude of unemployment in each municipality. We used the proportion of all workers engaged in primary industry as an industrial determinant of social structure to measure the industrial structure in each municipality. Suicide rate (per 100,000 population) and the estimated number of inpatients with mental illness (per day) were used as health determinants of social structure, to measure suicide and mental illness in each municipality, respectively.
We used the national census data from Miyagi Prefecture conducted in 2010, which was published by the National Statistics Center [27], for information on the proportion of the population aged 65years or older, unemployment rate, and the proportion of all workers engaged in primary industry. We used data from "the statistical observations of Shi, Ku, Machi, Mura 2012," published by Statistics Japan [28], for information regarding the population density of residential areas as of 2010. Statistics on population and residential area are required to calculate the population density of residential areas. Since these statistics of 2010 are listed in the dataset of "the statistical observations of Shi, Ku, Machi, Mura 2012," we used this dataset. We used the national data on suicide of 2009 and 2010, which was published by the Ministry of Health, Labor and Welfare [29], and determined the average value of both years as the suicide rate, as the suicide rate fluctuates greatly even if the number of suicides is small in municipalities with little population. However, because the suicide counting method before 2008 was different from the current one, we adopted only two years, from 2009 to 2010. Patient survey data from 2008, published by the Ministry of Health, Labor and Welfare [30], was used to obtain the estimated number of inpatients with mental illness (per day). Patient surveys were conducted once every three years. We used the 2008 survey data as the 2011 data would have been influenced by the GEJE, and would not have provided an accurate representation of mental illness before the GEJE. The estimated number of inpatients with mental illness was calculated not in a municipal unit but in a secondary medical care area unit. The secondary medical care area is a regional zone, composed of several municipalities. Table 2 presents a brief summary of these social structure data. These data are available from a supporting information file (S2 Dataset).

Statistical analysis
We performed an ecological correlation analysis in each of the 39 municipalities in Miyagi Prefecture. Firstly, we calculated simple correlation coefficients (Spearman's rho) between the proportion of the population aged 65 years or older, population density of the residential area, unemployment rates, the proportion of all workers engaged in primary industry, suicide rate, the estimated number of inpatients with mental illness, and the prevalence of serious mental distress. Secondly, we categorized the population density of residential areas, unemployment rate, the proportion of all workers engaged in primary industry, suicide rate, and the estimated number of inpatients with mental illness into binary variables (high or low), in order to examine the relationship between the proportion of the population aged 65 years or older and the prevalence of mental distress. If population density in residential areas was below 1,000 it was defined as low; conversely a density of 1,000 or greater was defined as high. If the estimated number of inpatients with mental illness was less than one, it was defined as low; an estimated number of at least one was defined as high. Other variables were categorized based on the median value. Finally, we calculated simple correlation coefficients between the proportion of the population aged 65 years or older and the prevalence of serious mental distress, for each binary variable category. P-values <0.05 were considered statistically significant.

Results
Of the 1,487 surveyed participants included from Miyagi Prefecture, 9.1% (n = 135) had serious mental distress (Fig 3). The highest and lowest serious mental distress prevalence rates were 20% in Tome City and 0% in Onagawa and Tomiya Towns, respectively. The serious mental distress rate was distributed without bias in the specific areas (Fig 4). Table 3 shows the descriptive statistics for mental distress and social structure in Miyagi Prefecture. The mean proportion of the population aged 65 years or older was 25.4%. The mean population density of the residential area, unemployment rate, the proportion of all workers in primary industry, suicide rate, and the estimated number of inpatients with mental illness were 1067.2, 7.7%, 8.6%, 26.6, and 1.3, respectively. We found the prevalence of serious mental distress was not statistically significantly correlated to the proportion of the population aged 65 years or older or any other social structural variables (Table 4). We found a statistically significant correlation between the proportion of the population aged 65 years or older and population density of the residential area (r s = −0.662, P <0.001), the proportion of all workers engaged in primary industry (r s = 0.798, P <0.001), and the estimated number of inpatients with mental illness (r s = −0.584, P <0.001). Table 5 shows the correlation coefficients between the proportion of the population aged 65 years or older and the prevalence of serious mental distress, for each binary variable category. Even in municipalities with low or high population density, the correlation between the proportion of the population aged 65 years or older and the prevalence of serious mental distress was not statistically significant. Similar results were found for the rate for unemployment and suicide. Regarding the municipalities with a low proportion of all workers engaged in primary industry (Fig 5), we found a statistically significant positive correlation between the proportion of the population aged 65 years or older and the prevalence of serious mental distress (r = 0.648, P = 0.003). Similarly, we found a statistically significant positive correlation between the proportion of the population aged 65 years or older and the prevalence of serious mental distress in municipalities with a high estimated number of inpatients with mental illness (r = 0.626, P = 0.005) (Fig 6).

Discussion
We determined that the prevalence of serious mental distress before the GEJE in Miyagi Prefecture was 9.1%. Since this study showed the results of a randomly sampled social survey, taking into account the population size of each municipality, this result of the prevalence of serious mental distress would be reasonable. When compared to a study conducted in 2006, that reported that 3% of general Japanese population had serious mental distress [31], it appears that the prevalence of serious mental distress in this study is high. Further, a report on the prevalence of serious mental distress in the general population using a nationwide internet survey, conducted one year after the GEJE [12], also found the prevalence of serious mental   distress was 8.9%, which was slightly lower than that in our study. Thus, the prevalence of serious mental distress in Miyagi Prefecture was higher than that of Japan before and after the GEJE. The findings suggest that Miyagi Prefecture may have social structural features that affect mental distress.  Our findings suggest that residents in Miyagi Prefecture could have suffered from poor mental health before the GEJE. Therefore, this may question the accuracy of the findings from previous studies that emphasized that the GEJE affected mental health status of residents. Our study, which accurately describes the distribution of serious mental distress before the GEJE, will be useful for post-disaster studies, providing fundamental data from before the GEJE. However, since the subjects from severely damaged areas had a higher prevalence of serious mental distress than that reported in our study [9,10], the GEJE likely affected mental health among the residents in the highly affected areas.
Further, we found a large difference between the maximum and minimum prevalence rates of serious mental distress based on municipality. Moreover, the prevalence of serious mental distress was distributed without bias in the specific areas investigated. Our findings suggest that mental health could be influenced by social structural characteristics of municipalities themselves, because the prevalence of mental distress was not regionally intensive. Thus, it is Mental distress before the Great East Japan Earthquake and an aged society important to clarify the relationship between social structure and mental health to ascertain solutions to mental health problems.
Although the proportion of the population aged 65 years or older was not significantly correlated with the prevalence of serious mental distress directly, we found a significant correlation between the prevalence of serious mental distress and the proportion of population aged 65 years or older among municipalities with a low proportion of all workers engaged in primary industry. These results suggest that mental health problems arise among those with difficulties dealing with social structural problems present in an aged society, in the areas where the proportion of all workers engaged in primary industry was low. Generally, it is considered that areas with a low proportion of all workers engaged in primary industry are urban, not rural. However, the correlation between the prevalence of serious mental distress and the proportion of the population aged 65 years or older was not affected by the degree of population density as an urbanization determinant of social structure. Therefore, industrial determinants of social structure, not urbanization determinants, are a crucial influence on the relationship between an aged society and mental health problems. Therefore, in order to solve mental health issues at a social level, it is desirable to manage industrial problems related to an aged society in areas with advanced industrial structure.
On the other hand, the unemployment rate as an economic determinant of social structure was not directly associated with mental distress. Moreover, the degree of unemployment rate did not affect the correlation between the prevalence of serious mental distress and the proportion of the population aged 65 years or older. Our findings differ from those from previous studies, which indicated that unemployment was associated with mental distress [11,20,21]. This is probably because economic problems are greatly affected by an aged society in Japan. Based on our findings, unemployment rate is unlikely to be the main factor, driving mental distress, and an aged society was one of the factors driving mental distress in areas with advanced industrial structure.
We also found a significant correlation between the prevalence of serious mental distress and the proportion of the population aged 65 years or older among municipalities with a high estimated number of inpatients with mental illness. These results suggest that health determinants of social structure are related to the relationship between mental health and an aged society. Thus, because mental health could be affected by physical and mental changes related to aging, it is desirable to put in place measures to mitigate the various physical and mental problems associated with an aged society in areas with a greater number of mental health patients at a social level.
As previously argued, an aged society has an impact on serious mental distress in areas with advanced industrial structure and with a greater number of patients with mental illness. According to Table 4, there was a strong negative correlation between the proportion of all workers engaged in primary industry and the estimated number of inpatients with mental illness. This result suggests that the development of industrialization might be associated with the onset of mental illness. Therefore, we believe it is important to verify the impact of an aged society on serious mental distress, from the viewpoint of the social structure of industrialization and the number of patients with mental illness. Thus, creating a map of the social structural features of the proportion of all workers engaged in primary industry and the estimated number of inpatients with mental illness will be useful for the materials to be used for the survey after the GEJE. We present the colored map of these two social structural features in a supporting information file for the future studies (S1 Fig). This study was subject to several limitations. We did not take into account demographic characteristics in this ecological study; further studies are needed which should investigate these factors. Furthermore, as the sample size of this survey was not large, care should be taken when interpreting the results. However, since it was scientifically conducted by random sampling, we believe the results are valid and show the distribution of the prevalence of mental distress before the GEJE. Since this was a cross-sectional study of social structure, we did not take longitudinal trends of social structure into account. As the unemployment rate and the proportion of the population aged 65 years or older, in particular, have changed over the last 30 years, it is necessary to examine whether long-term trends are related to mental distress. The K6 evaluated the psychological distress, but it does not directly evaluate mental illness. In the context of population aging in the future, it is necessary to examine the association between K6 and index that could directly evaluate people with psychiatric disorders, such as the elderly population.
We found that residents in Miyagi Prefecture suffered from poor mental health before the GEJE. It is primarily more important to address the social structural problems associated with an aged society in areas with advanced industrial structure and with a greater number of patients with mental illness in order to tackle the mental health issues. Interventions for mental health should not be conducted on a smaller-scale, or targeted at events such as the GEJE, as this will result in turning a blind eye to the present social structural problems of the aged society that are driving mental health problems.
We believe that our findings could greatly contribute to the study of mental distress postdisaster. Following the GEJE, researchers have explored various factors for solving mental problems; however, they have focused on the impact of the GEJE on mental health, disregarding the prevalence of mental distress that might have already been present in the population. We should approach mental health problems in the context of social structure, especially an aged society, taking into consideration facts about mental distress before the GEJE.