Lifetime risk of stroke and impact of hypertension: estimates from the adult health study in Hiroshima and Nagasaki

Lifetime Risk of Incident Coronary Heart Disease, Stroke, and Cardiovascular Disease: The Japan Public Health Center-Based Prospective Study

AIM

The constellation of cardiovascular disease (CVD) risk factors greatly impacts the lifetime risk (LTR) of incident CVD, but the LTR has not been thoroughly evaluated in the Japanese population.

METHODS

We conducted a prospective study involving a total of 25,896 individuals 40-69 years old without a history of CVD in 1995 (Cohort I) and 1993-1994 (Cohort II) in Japan. CVD risk factors (blood pressure, non-high-density lipoprotein [HDL] cholesterol levels, smoking status, and glucose concentrations) were used to stratify them by risk. The sex-specific LTR of incident coronary heart disease, stroke, atherosclerotic CVD, and total CVD were estimated for participants 45 years old in the 4 risk categories with the cumulative incidence rate, adjusting for the competing risk of death.

RESULTS

We found apparent differences in the LTR of total CVD according to the risk stratification. Individuals with ≥ 2 of the risk factors of blood pressure ≥ 140/90 mmHg or treated, non-HDL cholesterol level ≥ 170 mg/dL or treated, current smoker, and diabetes had substantially higher adjusted LTRs of CVD than those in other groups, with a LTR of 26.5% (95% confidence interval, 24.0%-29.0%) for men and 15.3% (13.1%-17.5%) for women at 45 years. The LTR of incident stroke was the highest among CVDs, and the presence of hypertension and diabetes mellitus strongly influenced the LTR of total CVD.

CONCLUSION

The impact of risk accumulation on LTR of CVD was greater in men, and 1 in 4 men with ≥ 2 major risk factors at 45 years of age developed CVD in their lifetime.

Time to benefit for stroke reduction after blood pressure treatment in older adults: A meta-analysis

Background

Hypertension treatment in older adults can decrease mortality, cardiovascular events, including heart failure, cognitive impairment, and stroke risk, but may also lead to harms such as syncope and falls. Guidelines recommend targeting preventive interventions with immediate harms and delayed benefits to patients whose life expectancy exceeds the intervention's time to benefit (TTB). Our objective was to estimate a meta‐analyzed TTB for stroke prevention after initiation of more intensive hypertension treatment in adults aged ≥65 years.

Methods

Studies were identified from two Cochrane systematic reviews and a search of MEDLINE and Google Scholar for subsequent publications until August 31, 2021. We abstracted data from randomized controlled trials comparing standard (untreated, placebo, or less intensive treatment) to more intensive treatment groups in older adults (mean age ≥ 65 years). We fit Weibull survival curves and used a random‐effects model to estimate the pooled annual absolute risk reduction (ARR) between control and intervention groups. We applied Markov chain Monte Carlo methods to determine the time to ARR thresholds (0.002, 0.005, and 0.01) for a first stroke.

Results

Nine trials (n = 38,779) were identified. The mean age ranged from 66 to 84 years and study follow‐up times ranged from 2.0 to 5.8 years. We determined that 1.7 (95%CI: 1.0–2.9) years were required to prevent 1 stroke for 200 persons (ARR = 0.005) receiving more intensive hypertensive treatment. Heterogeneity was found across studies, with those focusing on tighter systolic blood pressure control (SBP < 150 mmHg) showing longer TTB. For example, in the SPRINT study (baseline SBP = 140 mmHg, achieved SBP = 121 mmHg), the TTB to avoid 1 stroke for 200 patients treated was 5.9 years (95%CI: 2.2–13.0).

Conclusions

More intensive hypertension treatment in 200 older adults prevents 1 stroke after 1.7 years. Given the heterogeneity across studies, the TTB estimates from individual studies may be more relevant for clinical decision‐making than our summary estimate.

See related Editorial by Mark A. Supiano in this issue.

Risk Classification for Metabolic Syndrome and the Incidence of Cardiovascular Disease in Japan With Low Prevalence of Obesity: A Pooled Analysis of 10 Prospective Cohort Studies

Background

It is uncertain whether risk classification under the nationwide program on screening and lifestyle modification for metabolic syndrome captures well high‐risk individuals who could benefit from lifestyle interventions. We examined the validity of risk classification by linking the incidence of cardiovascular disease (CVD).

Methods and Results

Individual‐level data of 29 288 Japanese individuals aged 40 to 74 years without a history of CVD from 10 prospective cohort studies were used. Metabolic syndrome was defined as the presence of high abdominal obesity and/or overweight plus risk factors such as high blood pressure, high triglyceride or low high‐density lipoprotein cholesterol levels, and high blood glucose levels. The risk categories for lifestyle intervention were information supply only, motivation‐support intervention, and intensive support intervention. Sex‐ and age‐specific hazard ratios and population attributable fractions of CVD, which were also further adjusted to consider non–high density lipoprotein cholesterol levels, were estimated with reference to nonobese/overweight individuals, using Cox proportional hazard regression. Since the reference category included those with risk factors, we set a supernormal group (nonobese/overweight with no risk factor) as another reference. We documented 1023 incident CVD cases (565 men and 458 women). The adjusted CVD risk was 60% to 70% higher in men and women aged 40 to 64 years receiving an intensive support intervention, and 30% higher in women aged 65 to 74 years receiving a motivation‐support intervention, compared with nonobese/overweight individuals. The population attributable fractions in men and women aged 40 to 64 years receiving an intensive support intervention were 17.7% and 6.6%, respectively, while that in women aged 65 to 74 years receiving a motivation‐support intervention was 9.4%. Compared with the supernormal group, nonobese/overweight individuals with risk factors had similar hazard ratios and population attributable fractions as individuals with metabolic syndrome.

Conclusions

Similar CVD excess and attributable risks among individuals with metabolic syndrome components in the absence and presence of obesity/overweight imply the need for lifestyle modification in both high‐risk groups.

Lifetime risk of stroke stratified by chronic kidney disease and hypertension in the general Asian population: the Ohasama study

Lifetime risk (LTR) evaluates the absolute risk of developing a disease during the remainder of one's life. It can be a useful tool, enabling the general public to easily understand their risk of stroke. No study has been performed to determine the LTR of cardiovascular disease in patients with chronic kidney disease (CKD) with or without hypertension; therefore, we performed this study in an Asian population. We followed 1525 participants (66.0% women; age 63.1 years) in the general population of Ohasama, Japan. We defined CKD as an estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m² and/or proteinuria. Hypertension was defined as a systolic/diastolic blood pressure ≥140/≥90 mmHg and/or the use of antihypertensive medication. We calculated the sex-specific LTR of stroke adjusted for the competing risk of death. During the mean follow-up period of 16.5 years, a first stroke occurred in 238 participants. The 10-year risk of stroke at the age of 45 years was 0.0% for men and women. The LTRs of stroke at the index age of 45 years (men/women) were 20.9%/14.5% for participants without CKD and hypertension, 34.1%/29.8% for those with CKD but not hypertension, 37.9%/27.3% for those with hypertension but not CKD, and 38.4%/36.4% for those with CKD and hypertension. The LTRs of stroke tended to be higher in younger participants than in older participants with CKD and/or hypertension. CKD contributed to the LTR of stroke, as did hypertension. The prevention of CKD and hypertension can reduce the LTR of stroke, especially in young populations.

A Combination of Blood Pressure and Total Cholesterol Increases the Lifetime Risk of Coronary Heart Disease Mortality: EPOCH-JAPAN

AIM

Lifetime risk (LTR) indicates the absolute risk of disease during the remainder of an individual's lifetime. We aimed to assess the LTRs for coronary heart disease (CHD) mortality associated with blood pressure (BP) and total cholesterol levels in an Asian population using a meta-analysis of individual participant data because no previous studies have assessed this risk.

METHODS

We analyzed data from 105,432 Japanese participants in 13 cohorts. Apart from grade 1 and 2-3 hypertension groups, we defined "normal BP" as systolic/diastolic BP ＜130/＜80 mmHg and "high BP" as 130-139/80-89 mmHg. The sex-specific LTR was estimated while considering the competing risk of death.

RESULTS

During the mean follow-up period of 15 years (1,553,735 person-years), 889 CHD deaths were recorded. The 10-year risk of CHD mortality at index age 35 years was ≤ 0.11%, but the corresponding LTR was ≥ 1.84%. The LTR of CHD at index age 35 years steeply increased with an increase in BP of participants with high total cholesterol levels [≥ 5.7 mmol/L (220 mg/dL)]. This risk was 7.73%/5.77% (95% confidence interval: 3.53%-10.28%/3.83%-7.25%) in men/women with grade 2-3 hypertension and high total cholesterol levels. In normal and high BP groups, the absolute differences in LTRs between the low and high total cholesterol groups were ≤ 0.25% in men and ≤ 0.40% in women.

CONCLUSIONS

High total cholesterol levels contributed to an elevated LTR of CHD mortality in hypertensive individuals. These findings could help guide high-risk young individuals toward initiating lifestyle changes or treatments.

Association between low doses of ionizing radiation, administered acutely or chronically, and time to onset of stroke in a rat model

Exposure to high-doses of ionizing radiation has been reported to be associated with the risk of stroke. However, risks associated with lower dose exposures remain unclear, and there is little information available for the risk modification according to the dose-rate. There are few studies using animal models which might be able to provide complementary information on this association. In this study, the male stroke-prone spontaneously hypertensive rat (SHRSP) was used as a model animal. The rats were acutely irradiated with doses between 0 and 1.0 Gy or chronically irradiated with a cumulative dose of 0.5 or 1.0 Gy (at a dose rate of 0.05 or 0.1 Gy/day, respectively). The onset time of stroke related symptoms in SHRSP was used as an endpoint for evaluating the effects of low dose and the low dose-rate gamma-ray exposures. With respect to acute exposure, the time to the onset of stroke in the irradiated rats suggested the presence of a threshold around 0.1 Gy. For the low dose-rate chronically exposed, no significant increase in stroke symptom was observed. These findings are novel and demonstrate that the SHRSP system can be used to determine the association between the risk of stroke and radiation exposure with high sensitivity. Moreover, these studies provide important information regarding the association between the low dose and low dose-rate radiation exposure and circulatory diseases, especially stroke.

Impact of hypertension stratified by diabetes on the lifetime risk of cardiovascular disease mortality in Japan: a pooled analysis of data from the Evidence for Cardiovascular Prevention from Observational Cohorts in Japan study

Lifetime risk is an informative estimate to motivate people to change lifestyle behaviors, especially from a younger age, in public health education. The impact of the combination of hypertension and diabetes on the lifetime risk of cardiovascular mortality has not been investigated in Asian populations. A pooled analysis of individual data from nine cohorts was performed. A total of 57,339 Japanese men and women were eligible for the analysis. We used the modified Kaplan-Meier approach and estimated the remaining lifetime risk of cardiovascular mortality starting from the index age of 35 years. Participants were classified into four categories defined by hypertension and diabetes. The lifetime risk was increased in the order of those without either risk, those without hypertension but with diabetes, those with hypertension but without diabetes, and those with both risks. The lifetime risk of cardiovascular mortality at the 35-year index age was as follows: 7.8% in men and 6.2% in women for those without either hypertension or diabetes, 13.2% in men and 9.5% in women for those without hypertension but with diabetes, 17.2% in men and 11.7% in women for those with hypertension but without diabetes, and 19.4% in men and 15% in women for those with both risks. These findings reinforce the need for a life-course perspective in the management of hypertension and diabetes from a younger age.

Dynamic cerebral autoregulation in anterior and posterior cerebral circulation during cold pressor test

We hypothesized that cerebral blood flow (CBF) regulation in the posterior circulation differs from that of the anterior circulation during a cold pressor test (CPT) and is accompanied by elevations in arterial blood pressure (ABP) and sympathetic nervous activity (SNA). To test this, dynamic cerebral autoregulation (dCA) in the middle and posterior cerebral arteries (MCA and PCA) were measured at three different conditions: control, early phase of the CPT, and the late phase of the CPT. The dCA was examined using a thigh cuff occlusion and release technique. The MCA and PCA blood velocities were unchanged at CPT compared with the control conditions despite an elevation in the ABP. The dCA in both the MCA and PCA remained unaltered at CPT. These findings suggest that CPT-induced elevations in the ABP and SNA did not cause changes in the CBF regulation in the posterior circulation compared with the anterior circulation.

Anthology of stroke epidemiology in the 20th and 21st centuries: Assessing the past, the present, and envisioning the future

This narrative overview of stroke epidemiology shows dramatic changes in stroke incidence, prevalence, mortality, disability, and the understanding of risk factors and primary stroke prevention strategies over the last few decades. Likely future directions of stroke epidemiology and prevention are outlined.

Lifetime Risk of Stroke and Coronary Heart Disease Deaths According to Blood Pressure Level

Lifetime risk (LTR) provides an absolute risk assessment during the remainder of one’s life. Few studies have focused on the LTRs of stroke and coronary heart disease (CHD), categorized by fine blood pressure in Asian populations. We aimed to assess it using a large database of a meta-analysis with the individual participant data. The present meta-analysis included 107 737 Japanese (42.4% men; mean age, 55.1 years) from 13 cohorts. During the mean follow-up of 15.2±5.3 years (1 559 136 person-years), 1922 died from stroke and 913 from CHD. We estimated risks after adjusting for competing risk of death other than the outcome of interest. The 10-year risk of stroke and CHD deaths at index age of 35 years was ≤1.9% and ≤0.3%, respectively. The LTRs of stroke death at the index age of 35 years (men/women) were 6.1%/4.8% for optimal, 5.7%/6.3% for normal, and 6.6%/6.0% for high-normal blood pressure groups, and 9.1%/7.9% for grade 1, 14.5%/10.3% for grade 2, and 14.6%/14.3% for grade 3 hypertension groups. The LTRs of CHD death similarly elevated with an increase in blood pressure but were lower (≤7.2%) than those of stroke death. In conclusion, blood pressure was clearly associated with an elevated LTR of stroke or CHD death, although the LTR of CHD death was one-half of that of stroke death in an Asian population. These results would help young people with hypertension to adopt a healthy lifestyle or start antihypertensive therapy early.

Global, Regional, and Country-Specific Lifetime Risks of Stroke, 1990 and 2016

BACKGROUND

The lifetime risk of stroke has been calculated in a limited number of selected populations. We sought to estimate the lifetime risk of stroke at the regional, country, and global level using data from a comprehensive study of the prevalence of major diseases.

METHODS

We used the Global Burden of Disease (GBD) Study 2016 estimates of stroke incidence and the competing risks of death from any cause other than stroke to calculate the cumulative lifetime risks of first stroke, ischemic stroke, or hemorrhagic stroke among adults 25 years of age or older. Estimates of the lifetime risks in the years 1990 and 2016 were compared. Countries were categorized into quintiles of the sociodemographic index (SDI) used in the GBD Study, and the risks were compared across quintiles. Comparisons were made with the use of point estimates and uncertainty intervals representing the 2.5th and 97.5th percentiles around the estimate.

RESULTS

The estimated global lifetime risk of stroke from the age of 25 years onward was 24.9% (95% uncertainty interval, 23.5 to 26.2); the risk among men was 24.7% (95% uncertainty interval, 23.3 to 26.0), and the risk among women was 25.1% (95% uncertainty interval, 23.7 to 26.5). The risk of ischemic stroke was 18.3%, and the risk of hemorrhagic stroke was 8.2%. In high-SDI, high-middle-SDI, and low-SDI countries, the estimated lifetime risk of stroke was 23.5%, 31.1% (highest risk), and 13.2% (lowest risk), respectively; the 95% uncertainty intervals did not overlap between these categories. The highest estimated lifetime risks of stroke according to GBD region were in East Asia (38.8%), Central Europe (31.7%), and Eastern Europe (31.6%), and the lowest risk was in eastern sub-Saharan Africa (11.8%). The mean global lifetime risk of stroke increased from 22.8% in 1990 to 24.9% in 2016, a relative increase of 8.9% (95% uncertainty interval, 6.2 to 11.5); the competing risk of death from any cause other than stroke was considered in this calculation.

CONCLUSIONS

In 2016, the global lifetime risk of stroke from the age of 25 years onward was approximately 25% among both men and women. There was geographic variation in the lifetime risk of stroke, with the highest risks in East Asia, Central Europe, and Eastern Europe. (Funded by the Bill and Melinda Gates Foundation.).

Simultaneous Imaging of Cerebrovascular Structure and Function in Hypertensive Rats Using Synchrotron Radiation Angiography

Hypertension has a profound influence on the structure and function of blood vessels. Cerebral vessels undergo both structural and functional changes in hypertensive animals. However, dynamic changes of cerebrovasculature and the factors involved in this process are largely unknown. In this study, we explored the dynamic changes of vascular structure in hypertensive rats using novel synchrotron radiation angiography. Twenty-four spontaneously hypertensive rats (SHR) and 24 Sprague–Dawley (SD) rats underwent synchrotron radiation (SR) angiography. Each group had 8 animals. We studied the cerebral vascular changes in SHR over a time period of 3–12-month and performed quantitative analysis. No vascular morphology differences between SHR and SD rats were observed in the early stage of hypertension. The number of twisted blood vessels in the front brain significantly increased at the 9- and 12-month observation time-points in the SHR compared to the SD rats (p < 0.01). The vessel density of the cortex and the striatum in SHR was consistently higher than that in SD rats at time points of 3-, 9-, and 12-month (p < 0.001). Vascular elasticity decreased both in SHR and SD rats with aging. There were statistically significant differences in the relative vascular elasticity of extracranial/intracranial internal carotid artery, middle cerebral artery, posterior cerebral artery and anterior cerebral artery between SHR and SD rats at 12-month (p < 0.01). We concluded that the dynamic vascular alterations detected by SR angiography provided novel imaging data for the study of hypertension in vivo. The longer the course of hypertension was, the more obvious the vascular differences between the SHR and the SD rats became.

Diabetes and lifetime risk of stroke and subtypes in an urban middle-aged population

AIMS

Lifetime risk (LTR) is defined as the cumulative probability of developing a disease in one's remaining lifetime from a given index age. The impact of diabetes on the LTR of stroke events in Asians, where stroke incidence is higher than for Westerners, has not been estimated yet. These estimates can be useful for diabetes knowledge translation activities.

METHODS

All participants who were stroke-free at baseline in the Suita Study, a cohort study of cardiovascular diseases in Japan, were included in the study sample. Age, in years, was used as the time-scale. Age-specific incidence rates were calculated using the person-years method within five-year bands. We estimated the sex- and index-age-specific LTR of first-ever stroke accounting for the competing risk of death.

RESULTS

In this cohort study, we followed 5515 participants from 1989 to 2007 for 71,374.23 person-years. At age 40, the LTRs, adjusted for competing risk of death, for all strokes were 15.98% for men without diabetes and 26.64% for men with diabetes. The LTR for stroke was 10.66% higher for men with diabetes than men without diabetes. For women of same index age, the LTR of stroke was 17.29% and 30.72% with diabetes and without diabetes, respectively. The difference in LTR between persons with diabetes and without diabetes was 13.43%. This increased LTR of strokes for persons with diabetes was observed among both men and women across all index ages. Similar results were observed for cerebral infarction stroke subtype.

CONCLUSIONS

In this urban community-based population we observed that diabetes has a significant effect on the residual LTR of stroke for both men and women of middle age. This knowledge can be used to inform public health education and planning.

Hypertension and lifetime risk of stroke

BACKGROUND

The lifetime risk (LTR) articulates the probability of disease in the residual lifetime for an index age. These estimates can be useful for general audience-targeted knowledge translation activities against hypertension. There are only a few reports on lifetime of impact of hypertension on stroke events in Asians in whom stroke incidence is higher than Westerners.

METHODS

The Suita Study, a cohort study of cardiovascular diseases in Japan, was established in 1989. We included all participants who were stroke free at baseline. Age (in years) was used as the time scale. Age-specific incidence rates were calculated with person-year method within 10-year bands. We estimated the sex and index-age specific LTR of first-ever stroke with taking the competing risk of death into account.

RESULTS

We followed 5783 men and women during 1989-2007 for 74 933 person-years. During the follow-up period, 276 (149 men and 127 women) participants had incident stroke. Of them, majority were cerebral infarction; 166 (102 men and 64 women). The LTR of stroke, accounted for competing risk of death, at 45 years of age for men without hypertension was 17.21% and it was 32.79% for hypertensive men. Among the hypertensive patients, participants with stage 2 or greater hypertension had higher LTR of stroke than the participants with stage 1 hypertension. This increased LTR of stroke for hypertensive patients were also observed among women and across all index ages for stroke.

CONCLUSION

In this urban community-based population, we observed that hypertension has significant effect on the residual LTR of stroke among both men and women of middle age, specifically for ischemic stroke.

Mitochondrial DNA association study of type 2 diabetes with or without ischemic stroke in Taiwan

Background

The importance of mitochondrial DNA (mtDNA) polymorphism in the prediction of type 2 diabetes (T2D) in men and women is not well understood. We questioned whether mtDNA polymorphism, mitochondrial functions, age and gender influenced the occurrence of T2D with or without ischemic stroke (IS).

Methods

We first designed a matched case–control study of 373 T2D patients and 327 healthy unrelated individuals without history of IS. MtDNA haplogroups were determined on all participants using sequencing of the control region and relevant SNPs from the coding region. Mitochondria functional tests, systemic biochemical measurements and complete genomic mtDNA sequencing were further determined on 239 participants (73 healthy controls, 33 T2D with IS, 70 T2D only and 63 IS patients without T2D).

Results

MtDNA haplogroups B4a1a, and E2b1 showed significant association with T2D (P <0.05), and haplogroup D4 indicated resistance (P <0.05). Mitochondrial and systemic functional tests showed significantly less variance within groups bearing the same mtDNA haplotypes. There was a pronounced male excess among all T2D patients and prevalence of IS was seen only in the older population. Finally, nucleotide variant np 15746, a determinant of haplogroup G3 seen in Japanese and of B4a1a prevalent in Taiwanese was associated with T2D in both populations.

Conclusions

Men appeared more susceptible to T2D than women. Although the significant association of B4a1a and E2b1 with T2D ceased when corrected for multiple testings, these haplogroups are seen only among Taiwan Aborigines, Southeast Asian and the Pacific Ocean islanders where T2D is predominant. The data further suggested that physiological and biochemical measurements were influenced by the mtDNA genetic profile of the individual. More understanding of the function of the mitochondrion in the development of T2D might indicate ways of influencing the early course of the disease.

Systematic review and meta-analysis of circulatory disease from exposure to low-level ionizing radiation and estimates of potential population mortality risks

BACKGROUND

Although high doses of ionizing radiation have long been linked to circulatory disease, evidence for an association at lower exposures remains controversial. However, recent analyses suggest excess relative risks at occupational exposure levels.

OBJECTIVES

We performed a systematic review and meta-analysis to summarize information on circulatory disease risks associated with moderate- and low-level whole-body ionizing radiation exposures.

METHODS

We conducted PubMed/ISI Thomson searches of peer-reviewed papers published since 1990 using the terms "radiation" AND "heart" AND "disease," OR "radiation" AND "stroke," OR "radiation" AND "circulatory" AND "disease." Radiation exposures had to be whole-body, with a cumulative mean dose of < 0.5 Sv, or at a low dose rate (< 10 mSv/day). We estimated population risks of circulatory disease from low-level radiation exposure using excess relative risk estimates from this meta-analysis and current mortality rates for nine major developed countries.

RESULTS

Estimated excess population risks for all circulatory diseases combined ranged from 2.5%/Sv [95% confidence interval (CI): 0.8, 4.2] for France to 8.5%/Sv (95% CI: 4.0, 13.0) for Russia.

CONCLUSIONS

Our review supports an association between circulatory disease mortality and low and moderate doses of ionizing radiation. Our analysis was limited by heterogeneity among studies (particularly for noncardiac end points), the possibility of uncontrolled confounding in some occupational groups by lifestyle factors, and higher dose groups (> 0.5 Sv) generally driving the observed trends. If confirmed, our findings suggest that overall radiation-related mortality is about twice that currently estimated based on estimates for cancer end points alone (which range from 4.2% to 5.6%/Sv for these populations).

A prospective follow-up study of the association of radiation exposure with fatal and non-fatal stroke among atomic bomb survivors in Hiroshima and Nagasaki (1980-2003)

OBJECTIVE

Use of medical radiotherapy has increased markedly in recent decades. Whether the consequence includes an increased risk of cardiovascular disease remains to be determined. The purpose of this study was to examine the association between radiation exposure and the incidence of stroke among Japanese atomic bomb survivors.

DESIGN

A prospective follow-up study.

SETTING AND PARTICIPANTS

Radiation exposure from the atomic bombing was assessed in 9515 subjects (34.8% men) with 24-year follow-up from 1980. Subjects were free of prevalent stroke when follow-up began.

OUTCOME MEASURES

Stroke events and the underlying cause of death were reviewed to confirm the first-ever stroke. Subtypes (ischaemic and haemorrhagic events) were categorised based on established criteria according to the definitions of typical/atypical stroke symptoms.

RESULTS

Overall mean radiation dose (±SD) in units of gray (Gy) was 0.38±0.58 (range: 0-3.5). During the study period, 235 haemorrhagic and 607 ischaemic events were identified. For men, after adjusting for age and concomitant risk factors, the risk of haemorrhagic stroke rose consistently from 11.6 to 29.1 per 10 000 person-years as doses increased from <0.05 to ≥2 Gy (p=0.009). Incidence also rose within the dose range <1 Gy (p=0.004) with no dose threshold. In women, the risk of haemorrhagic stroke rose with increasing radiation exposure but not until doses reached a threshold of 1.3 Gy (95% CI 0.5 to 2.3). Among women, for doses <1.3 Gy, differences in stroke risk were modest (13.5 per 10 000 person-years), while it increased to 20.3 per 10 000 person-years for doses that ranged from 1.3 to <2.2 Gy and to 48.6 per 10 000 person-years for doses that were higher (p=0.002). In both sexes, dose was unrelated to ischaemic stroke.

CONCLUSION

While the risk of haemorrhagic stroke increases with rising radiation exposure for both sexes, effects in women are less apparent until doses exceed a threshold at 1.3 Gy.