Response to Letter by Xu and Liu
First of all, we would like to thank Drs Xu and Liu for their interest in our recently published article and their comments and discussions on the proper explanations of the trends of subtype stroke from 1984 to 2004 in Sino-MONICA Beijing population. Dr Xu addressed that the population aging might be an explanation for the shift of stroke subtype from hemorrhagic to ischemic. We think there should be discussion that helps to further clarify whether we should consider population aging as a contributor for the increasing in incidence rate of ischemic stroke and decreasing in hemorrhagic stroke.
We mentioned in the Discussion section of our article that the proportion of the population aged over 65 years increased from 7% in 1984 to 15% in 2004 in the Sino-MONICA Beijing population, an increase that was addressed as a main explanation for the increase in mean onset age of acute stroke events. This significant rise in proportion of aging population in later years of Sino-MONICA surveillance study would undoubtedly result in an increase in crude incidence rates of stroke. That is exactly the reason why we used age-standardized rate for all comparisons in our study. All the proposed interpretations regarded the age-standardized incidence rate of stroke. In other words, the observed trends of age-standardized incidence rates of ischemic stroke and hemorrhagic stroke may be explained by any other factors except population aging. We think it is very important for us to further estimate the attribute of population aging on crude incidence rates in the future.
Drs Xu and Liu mentioned in their letter that hemorrhagic stroke reached a peak at about 55 to 65 age group and began to decrease slightly thereafter, whereas the incidence of ischemic stroke increased continuously with the advancing of age. This conclusion is obviously different from our current knowledge and published evidence.1–2 The data in Table 1 showed age group specific incidence rates of hemorrhagic stroke from 1984 to 2004 in men and women of Sino- MONICA Beijing population. The incidence rate of hemorrhagic stroke increased with the advancing age from 25 to 74. In our study population, we did not find that population in 55 to 65 age group in any given observational year had higher incidence rate of hemorrhagic stroke than that in 65 to 74 age group.
It is important to indicate that hypertension is a very important risk factors for both ischemic stroke and hemorrhagic stroke in Chinese people.3–5 But hypertension is much more closely associated with the risk of hemorrhagic stroke than with the risk of ischemic stroke. Table 2 showed the relative risk between blood pressure levels and risks of subtype stroke based on a large cohort study in China. After the effects of other risk factors were adjusted, the relative risk was 13.1 for ten-year risk of hemorrhagic stroke and 6.4 for ten-year risk of ischemic stroke in people with systolic blood pressure over 160 mm Hg or diastolic blood pressure over 100 mm Hg whereas the people with normal blood pressure level was taken as compared group.5 Therefore, it is a logical and reasonable generalization that the significant increase in control rates of hypertension observed in the same population may be one of the explanations for the significant decrease of age-standardized incidence rate of hemorrhagic stroke. We believe there are many other contributors for the observed trend of subtype stroke. Additional model studies are needed for an interpreted evaluation of contribution of different risk factors to the observed trend of stroke in Chinese population, included population aging.
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