Is There a Stroke Belt in China and Why?
Background and Purpose—In 1960s, a stroke belt with high stroke mortality was discovered in the southeast United States. In China, where stroke is the leading cause of death, we aimed to determine whether a focal region of high stroke incidence (stroke belt) exits and, if so, the possible causal and modifiable factors.
Methods—We systematically reviewed all studies of stroke incidence in China between 1980 and 2010, and included those which met our criteria for a high-quality study. Criteria for a provincial region of high stroke incidence were ranking in the top one third of all provinces for stroke incidence and ranking of more than one third of prefectural regions within the province in the top two sevenths of all prefectural regions for stroke incidence. We also reviewed regional distribution of major vascular risk factors, socioeconomic status, and demographic profiles in China.
Results—Nine eligible studies provided data on the incidence of stroke in 32 of 34 provincial regions of China (with Hong Kong and Macao as exceptions) and 52% of the 347 prefectural regions. Nine provincial regions (Heilongjiang, Tibet, Jilin, Liaoning, Xinjiang, Hebei, Inner Mongolia, Beijing, and Ningxia) met our criteria for a region of high stroke incidence and constitute a stroke belt in north and west China. The incidence of stroke in the stroke belt was 236.2 per 100 000 population compared with 109.7 in regions outside the belt (rate ratio, 2.16; 95% confidence interval, 2.10–2.22). The mean population prevalence of hypertension and overweight (body mass index, >25) was greater in the stroke belt than that in other regions (15.3% versus 10.3%, P<0.001; 21.1% versus 12.3%, P=0.013, respectively). The prevalence of hypertension and overweight also correlated significantly with regional stroke incidence (R=0.642, P<0.001; R=0.438, P=0.014, respectively, by Spearman rank correlation).
Conclusions—A stroke belt of high stroke incidence exists in 9 provincial regions of north and west China. The stroke belt may be caused, at least in part, by a higher population prevalence of hypertension and excess body weight. Lowering blood pressure and body weight in the stroke belt may reduce the geographic disparity in stroke risk and incidence in China.
In contrast to Western populations, stroke is the leading cause of death in China, where it accounted for 22.5% of deaths (about 2 million) in 2005.1 Stroke mortality in China is still increasing and is projected to double during the next 2 decades, from 2 to 4 million deaths.2 For survivors of stroke in China, the socioeconomic burden is substantial.3 Health policies to circumvent the epidemic of stroke in China require an accurate understanding of its nature and its causes.4
In the 1960s, a stroke belt or region was identified in the southeastern United States, where the rates of stroke mortality among its residents were ≈50% higher than the remainder of the United States.5 Subsequent studies suggested that geographic and racial variations in stroke mortality relate to differences in incidence, not case fatality, and that geographic disparities in stroke risk factors, such as hypertension and diabetes mellitus, may underlie the observed differences in stroke mortality.6 If these findings are valid, interventions to reduce geographic disparities in stroke factors may reduce geographic disparities in stroke incidence (and mortality).
We hypothesized that a similar phenomenon of geographic variation in stroke incidence and mortality could exist in China, which has been observed and discussed in individual studies,7 but has not been study systemically.
As mortality is determined by both incidence and case fatality, and as a major priority is the prevention of incident stroke (to complement effective treatments that reduce case fatality),8 we undertook a systematic review of all studies of the incidence of stroke in China. The aim was to determine whether stroke incidence rates vary geographically throughout the provincial and prefectural regions of China, whether a region with a consistently high stroke incidence (stroke belt) exists, and, if so, whether there may be major contributing factors to the stroke belt that could be modifiable.
We searched the international databases PubMed, Scopus, ISI Web of Science, and Science Direct, and the Chinese databases China Hospital Knowledge Database and China National Knowledge Infrastructure, from January 1980 to December 2010 with the words stroke OR cerebrovascular diseases AND incidence OR prevalence AND China OR Chinese. We further checked other literature sources, such as Xinhua Booklist, Amazon online, Google Scholar, and the website of the Ministry of Health of China for extra information concerning the included studies. Some data were obtained or confirmed by contacting the authors of the publications.
Eligibility criteria were as follows: complete population-based case ascertainment in China based on multiple overlapping sources of information (hospitals, outpatient clinics, and death certificates), standardized World Health Organization definition of stroke,9 incident stroke cases reported, data collection over whole years, stroke incidence estimates were age-adjusted or the age constitution ratios of the sample population were provided, prospective study design. Pathological subtypes of stroke included ischemic stroke, intracerebral hemorrhage, and subarachnoid hemorrhage. Transient ischemic attack was not included. Only studies published in English or Chinese were evaluated.
Classification of Geographical Regions
There are 5 levels of regions in China: province, prefecture, county, township, and village. Since the 17th century, provincial and prefectural boundaries in China have remained relatively static. The provinces serve an important cultural role in China, as people tend to identify with their native province. At present, there are 34 provincial, 333 prefectural, 2862 county, or 41 636 township level regions in China.10 There are 4 types of provincial regions, which included 23 provinces (include Taiwan), 5 autonomous regions, 4 municipalities, and 2 special administrative regions (Hong Kong and Macao). There are 23 counties directly affiliated to the corresponding provinces. To match the territory and population sizes, some neighboring counties of this kind were merged as presumed prefectural regions. Each of the 4 municipalities (Beijing, Shanghai, Tianjin, and Chongqing), which has a comparable territory of prefectural regions but with a larger population, was treated as a whole when assessing prefectural stoke risk. These arrangements resulted in 347 factual or presumed prefectural regions.
Sources of Data for Regional Stroke Incidences
If data for the incidence of stroke in a region were available from only 1 study, the single-source was accepted. However, if data for the incidence of stroke in a region were available from more than 1 eligible study, the incidence rates from all sources were combined to produce an average that was weighted according to their sample sizes. If data for the incidence of stroke in a region were not available, the regional incidence was calculated on the basis of available incidence data from the surrounding regions. These calculations used a strategy reported in previous studies for determining the stroke belt in the Unites States.11 Briefly, the stroke incidence for a particular region without observed incidence was estimated as a weighted average of all observed estimates of stroke incidence in the surrounding regions. The weighting incorporated the size of the population sampled and (the reciprocal of) the distance to the center of each of the surrounding regions.
Geographical Classification of Stroke Incidence and Definition of the Stroke Belt
Stroke incidence estimates in provincial and prefectural regions were ranked ordinally. Provincial regions with a stroke incidence in the top third and prefectural regions with a stroke incidence in the top two sevenths were arbitrarily classified as regions with a high stroke incidence. The stroke belt was defined as a region containing provinces that met the following 2 criteria for a region of high stroke incidence: (1) the province was ranked in the top one third of provinces for stroke incidence (ie, met the above criteria for high stroke incidence) and (2) more than one third (>33.3%) of the prefectural regions within the province were ranked in the top two sevenths of prefectural regions for stroke incidence (ie, met the above criteria for high stroke incidence). The rationale for using 2 criteria for determining the stroke belt was to exclude provinces with extremely high incidence in only 1 or 2 prefectural regions.
Exploring Possible Causes of the Stroke Belt
To explore the possible causes of the stroke belt, we sought and analyzed several sources of data. We systematically reviewed Chinese nationwide epidemiological surveys to obtain the prevalence of risk factors for cardiovascular diseases, such as hypertension,12 diabetes mellitus,13 smoking,14 and overweight15 in each province. The methods of diagnosing and ascertaining cases with these risk factors are described in the original publications.12–14 We also reviewed 2 Chinese nationwide nutrition surveys15,16 and the China Yearbook of Statistics10 to ascertain per capita consumption of classified foods. Population morphology, socioeconomic indices, environmental and geographic parameters were retrieved from China Yearbook of Statistics and China Yearbook of Health Statistics.10,17 To increase homogeneity, we did not combine the multisource data, but those from the most representative survey were selected. If only 1 survey was available for the distribution of a given factor, the data from this single-source survey were accepted. If multiple surveys were available for the distribution of a given risk factor, the data from the survey with larger sample size and more provincial regions involved were selected. When surveys with similar sample size and number of provincial regions were available for multiple years, those of, or nearest to 2000 were accepted.
Stroke incidence in the 31 provincial regions of Mainland China and Taiwan was analyzed. Crude stroke incidence rates were reported as the number of new cases of stroke in a whole year per 100 000 individuals. Age-standardized stroke incidence according to the Segi’s world population18 or US 1960 population was retrieved from the enrolled studies. For studies in which stroke incidence was reported in a restricted age range (eg, aged >35 years in the Taiwan study),19 we used imputed estimates of stroke incidence for the missing ages from those obtained in a large survey of stroke incidence among all ages.20 The impact of this transformation was minimal because the stroke incidence in population aged ≤35 years was extremely low (1.2 per 100 000 compared with 301.7 in population aged >35 years).20
After determining the stroke belt, the overall stroke incidence in the stroke belt was calculated from the corresponding provincial incidence rates by weighting the provincial population size. Distribution of risk factors for stroke, such as hypertension, smoking, diabetes mellitus, and overweight in provincial regions, was obtained from nationwide epidemiological surveys. Using these provincial data, distribution of risk factors in stroke belt and regions outside was calculated by weighting the provincial population size. Provincial stroke incidences and possible influential factors in population were ranked, with their relationship analyzed by Spearman rank correlation. Differences in risk factor distribution between stroke belt and other regions were compared using 1-way ANOVA, with provincial data treated as unit of analysis. A P<0.05 was deemed as statistical significance.
Results of Literature Searching and Quality of the Included Studies
The database search for studies on stroke incidence in China yielded 3776 articles. The titles and abstracts of these were screened and 3652 did not meet the eligibility criteria. After reading the full text of the remaining 124 papers, and tracking any relevant references, an additional 31 publications were identified. Two reviewers (G.X. and M.M.) appraised these 151 potentially relevant publications independently, and 9 studies (12 publications) met the selection criteria for eligibility and analysis.7,19–29 Table 1 shows the characteristics of these included studies. Of these 9 studies, the 2 Monitoring Trends and Determinants in Cardiovascular Disease (MONICA) studies26,27 had a large proportion of overlapped sample population. We, therefore, only included the one26 with larger sample in data analysis. The 9 eligible studies provided data for the incidence of stroke in 32 (94.1%, with Hong and Macao as exceptions) of the 34 provincial regions and 182 (52.4%) of 347 prefectural regions in total.
All 9 included studies used the door-to-door survey method in all or part of their sampling populations. The sample population was initially screened by means of a questionnaire which identified cases of suspected stroke, who were then examined by a specialist team. The diagnosis of stroke was confirmed or rejected by evaluating the results of medical records, physical examinations, and laboratory and imaging tests. Although first-ever stroke cases in Sino-MONICA study26,27 were diagnosed on the basis of dynamic reports of grassroots health station personnel and hospital records, the diagnosis of stroke was confirmed retrospectively by door-to-door survey in a randomly selected 10% of the study population. Fatal cases were confirmed by the local death registry system and medical records. For stroke diagnostic criteria, 5 of 9 studies used World Health Organization criteria, and the other 4 studies used similar criteria (Table 1). The distribution of the all sampling sites from the enrolled 9 studies is shown in Figure 1. The sampling sites were disproportionally distributed across the country, with more sampling sites in east coast regions and fewer in west regions. There was only 1 sampling site in the southwest region (Tibet; Table I in the online-only Data Supplement).
Stroke Incidence in Provincial and Prefectural Regions
Table 2 shows the stroke incidence in the 31 provincial regions of Mainland China and Taiwan in rank order. The incidence of stroke was highest in Heilongjiang (466.9 per 100 000 per year) followed by Tibet (450.4), and lowest in Guangxi (73.1) followed by Zhejiang (75.5). The difference in stroke incidence between the highest and the lowest provincial regions was more than 6 times.
Figure 2 is a color-coded map of China showing the incidence of stroke in the 347 prefectural regions with total population. Because most studies provided data for the incidence of stroke in women and men separately, in addition to the overall population, it was possible to generate separate color-coded maps of stroke incidence according to prefectural regions among men (Figure I in the online-only Data Supplement) and women (Figure II in the online-only Data Supplement).
The Stroke Belt
The 9 provincial regions that met both criteria for the stroke belt included Heilongjiang, Tibet, Jilin, Liaoning, Xinjiang, Hebei, Inner Mongolia, Beijing, and Ningxia (Table 2; Figure 2). Although Henan had a high stroke incidence of 174.0/100 000, among the top third, only 1 of its 18 (5.6%) prefectural regions had a high stroke incidence. Similarly, Hunan had a stroke incidence of 160.0/100 000, among the top third, only 2 of its 14 (14.2%) prefectural regions had a high stroke incidence. Both Henan and Hunan, therefore, did not meet our criteria for inclusion in the stroke belt.
The small map in Figure 2 shows that the 9 provincial regions that met the criteria for the stroke belt of high stroke incidence form a near continuous belt. The belt ranges along the boundary of China from southwest (Tibet) to northwest (Xinjiang and Ningxia), and then to north (Inner Mongolia, Beijing, and Hebei) and northeast (Heilongjiang, Jilin, and Liaoning). This C-shaped belt is interrupted by a very slim area of reported lower stroke incidence between Xinjiang and Inner Mongolia.
According to the 2001 China Census, the national population (excluding Taiwan, Hong Kong, and Macao) was 1.262 billion, of whom 239 million (18.9%) resided in the stroke belt, and 1023 million (81.1%) outside the belt.10 The stroke belt covered 53.6% (515/960 million km2) of China’s territory. The overall incidence of stroke in Mainland China was 135.6/100 000, but the incidence of stroke in the stroke belt was 236.2 per 100 000 compared with 109.7 per 100 000 in other regions; P<0.001). The incidence of stroke in the belt was more than 2 times (rate ratio, 2.16; 95% confidence interval, 2.10–2.22) of that in other regions. Similar patterns were apparent for both men and women; detailed information can be found in the Table II in the online-only Data Supplement.
Prevalence of Vascular Risk Factors Within the Stroke Belt
Table 2 shows that the 9 provincial regions within the stroke belt were among the top 12 ranked regions for the prevalence of hypertension—the major causal risk factor for stroke.12 The prevalence of hypertension also correlated very significantly with regional stroke incidence (R=0.642; P<0.001, by Spearman rank correlation). Table 3 shows that the mean population prevalence of hypertension was greater in the stroke belt than that in other regions (15.3% versus 10.3%; P<0.001).
Six of the 9 provincial regions within the stroke belt were also among the top 9 ranked regions for body mass index >25 (overweight).13 The prevalence of overweight also correlated significantly with regional stroke incidence (R=0.438; P=0.014). Overweight prevalence also correlated significantly with hypertension prevalence (R=0.664; P<0.001; Table 2). The mean population prevalence of overweight (body mass index, >25) was greater in the stroke belt than that in other regions (21.1% versus 12.3%; P=0.013; Table 3).
Table 3 shows that, compared with other regions, the stroke belt was also characterized by a higher proportion of minority (non-Han) ethnicities (14.80% versus 6.98%; P=0.038), less housing area per population (15.70 versus 20.72 m2/population; P=0.005), more patient beds per 1000 individuals (3.34 versus 2.33; P=0.005), more doctors per 1000 individuals (2.16 versus 1.52; P=0.009), and higher expenditure on health care (8.08% versus 5.62%; P=0.002). Capital cities within the stroke belt are colder (annual average temperature 8.67°C versus 16.26°C; P<0.001), drier (annual average humidity 57.2% versus 72.1%, P<0.001; annual rainfall 409.5 versus 1027.9 mm, P=0.001), and with more sunshine hours (2555.9 versus 1728.1 hours of sunshine per year; P<0.001).
We have identified a stroke belt of high stroke incidence in China. The belt stretches across 9 provincial regions located in north and west China, where the incidence of stroke was 236.2 compared with 109.7/100 000 in regions outside the belt (rate ratio, 2.155; 95% confidence interval, 2.097–2.215). The population prevalence of hypertension and overweight correlated significantly with regional stroke incidence.
A strength of this study is the use of stroke incidence, rather than mortality (as used in identifying the US stroke belt) in evaluating geographic variation in stroke incidence and risk, as it avoids biases because of variation in treatment patterns among different regions.
Several limitations should be emphasized when interpreting the results of this study. Retrieving age-adjusted stroke incidence rates from multiple epidemiological surveys may generate bias because of discrepancies in sampling and other methodology. The geographical distribution of the sampling sites of the 9 enrolled studies was uneven. Although the population density in China is uneven, and much denser in east China than west China (eg, >700/km2 in Jiangsu versus <10/km2 in Tibet), there were fewer sampling sites in the less populated regions of western China than in eastern China and, in particular, there was only 1 sampling site in southwest China (Tibet; Table III in the online-only Data Supplement). We have made an assumption that the estimates of a high stroke incidence in Tibet are similar in neighboring regions of southwestern China, where there are no data about stroke incidence. We have included Tibet and surrounding regions in the stroke belt, on the basis of this assumption, but acknowledge that there are limited data for this region and our assumption may be incorrect. The 9 enrolled studies used different reference populations for evaluating age-adjusted stroke incidence (US 1960 population in 4 study and Segi’s world population in 5 studies). However, the effect of using 2 different reference populations in evaluating age-adjusted stroke incidence seems mild and acceptable in the light of the 2.2 times difference in stroke incidence observed between stroke belt and other regions (Table III in the online-only Data Supplement). The enrolled studies were conducted during 1983 and 1992. We cannot evaluate the temporal change of geographic distribution of stroke incidence because of the absence of recent data. However, stroke risk may change over time in a given region, as observed in some local longitudinal studies.29
Our data suggest that some of the variation in stroke incidence in China may be attributable to different prevalences of hypertension and obesity among the provinces. The overall prevalence of hypertension in adult Chinese (≥18 years) was estimated to be 18.8% in 2002,30 a 31% increase compared with 1991.12 Unlike in United States, where only minor geographic variation in blood pressure and hypertension prevalence was observed,31 4 nationwide hypertension surveys in China conducted in 1959,32 1980,33 1991,12 and 200230; all observed marked regional discrepancies in blood pressure and the prevalence of hypertension. In the 1991 survey,12 the prevalence of hypertension was greatest in Tibet (19.54%) and lowest in Hainan (6.75%). The prevalence of hypertension in China follows a striking north-south gradient, highest in the north and lowest in the south, which is in concordance with the north-south gradient of stroke incidence observed in other studies.7,20,23,25–27
Why is hypertension more prevalent in the north than in the south? Daily salt intake has long been proposed as a cause.34,35 People in a province usually share similar cooking habits and dietary preferences. There are 8 major cuisines in China: Hui (Anhui), Yue (Cantonese), Min (Fujian), Xiang (Hunan), Su (Jiangsu), Chuan (Sichuan), Zhe (Zhejiang), and Lu (Shandong). One distinguishing feature of these cuisines is the preference of salt and other spices. For example, the Lu cuisine, which is popular in north China, favors salty savor.36 The influences of these regional dietary preferences (eg, salty food) on blood pressure and hypertension risk have been confirmed by nutrition surveys and urinary sodium measurements.34,35,37 However, in this study no direct association between annual salt consumption and regional stroke incidence was observed. This may reflect the fact that in this study the annual salt and spice consumption per capita retrieved from the nationwide sampling survey was based on the resident’s recall of total salt consumption. Therefore, the amount may not reflect the real salt intake because salt may be used for purposes other than flavoring or processing foods, as evidenced by the relatively higher salt and spices consumption (≈11 kg annually, or 30 g daily) per capita.
The cold weather in north China may limit outdoor physical activity in the long winter, enhance fat reservation,38 and consequently increase the northerners’ risk of obesity and related metabolic abnormalities.39 This study observed overweight (body mass index, >25; as defined in the Third National Nutrition Survey)16 was more prevalent in the stroke belt (north China), and the distribution of overweight correlated significantly with regional hypertension prevalence.
Although geographically located in south China, Tibet has a high stroke incidence. Preference for meats (beef and mutton) and salty butter tea (su-you-cha) increased the risk for hypertension and hyperlipidemia in Tibetans.40,41 The hypobaric hypoxia related to high altitude in Tibet can reactively increase the number of red blood cells (high altitude polycythemia), hematocrit and blood viscosity, which consequently increases the risk of hypertension and stroke.42 Hypoxic vasoconstriction encountered at altitude may raise pulmonary vascular resistance and arterial pressure and increase the likelihood of right to left shunting, and consequently increase the risk of cryptogenic stroke because of paradoxical embolism.43 Although multiple data sources have indicated Tibet as a region with high stroke incidence and risk, this presumption cannot be confirmed by the limited data on stroke incidence in this region at present. Future studies of stroke incidence should include more sampling sites in this less populated region.
In conclusion, we have identified a stroke belt of high stroke incidence across 9 provincial regions located in west and north China. Higher regional prevalence of hypertension and excess body weight may be responsible and appropriate targets for population-based stroke prevention.
We thank Professor Stephen M. Davis in Melbourne Brain Center, Royal Melbourne Hospital, Parkville, VIC, Australia for his help in formulating the initial hypothesis of a stroke belt in China and planning the study.
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.113.001238/-/DC1.
- Received February 20, 2013.
- Revision received April 2, 2013.
- Accepted April 4, 2013.
- © 2013 American Heart Association, Inc.
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