Posted: 00:00 IST
The INTERSTROKE study, published online and in an upcoming publication of medical journal Lancet, shows that a total of 10 risk factors (including high blood pressure, smoking, and waist-to-hip ratio) are associated with 90% of the risk of stroke. The international study, being presented today (Friday, 18 June) at The World Congress of Cardiology, Beijing, is written by Dr Martin O'Donnell of NUI Galway and formerly McMaster University and Dr Salim Yusuf, McMaster University, Hamilton, ON, Canada, and colleagues for the INTERSTROKE investigators.
The contribution of various risk factors to the burden of stroke worldwide is unknown, particularly in countries of low and middle income where the largest burden of stroke occurs. In the first phase of the INTERSTROKE study, the authors aimed to establish the association of known and emerging risk factors with stroke and its primary subtypes, assess the contribution of these risk factors to the burden of stroke, and explore the differences between risk factors for stroke and heart attack.
The authors used data from 6,000 people (3,000 cases of stroke and 3,000 controls) in 22 countries* worldwide, covering the period March, 2007 to April, 2010. Cases were patients with a first acute stroke (within 5 days of symptoms onset and 72 h of hospital admission). Controls had no history of stroke, and were matched with cases for age and sex. All participants completed a structured questionnaire and a physical examination, and most provided blood and urine samples. The authors calculated the increased risk and population-attributable risks (PARs) for the association of all stroke, ischaemic stroke, and intracerebral haemorrhagic stroke with selected risk factors.
The authors found the following 10 risk factors to be significantly associated with stroke: high blood pressure, smoking, waist-to-hip ratio (abdominal obesity), diet, physical activity, lipids (fats), diabetes mellitus, alcohol intake, stress and depression, and heart disorders.
Collectively, these risk factors accounted for 90% of the PAR for all stroke. These risk factors were all significant for ischaemic stroke (caused by a blood clot blocking a blood vessel in the brain), whereas high blood pressure, smoking, waist-to-hip ratio, diet, and alcohol intake were significant risk factors for intracerebral haemorrhagic stroke (bleeding into the brain tissue). The ratio of bad to good blood lipids (apolipoproteins) was an important risk factor for ischemic stroke but not for haemorrhagic stroke.
When looking at individual risk factors, the authors say it is important to note that the individual PARs for risk factors do not add up to the overall PAR for all risk factors combined. This is because many risk factors are inter-related. The study also addresses each risk factor independently, and found that high blood pressure was the most important for stroke, since it was associated with one-third of the risk of all stroke, and increased the risk of stroke more than two-and-a-half fold compared with no history of high blood pressure. Smokers were at double the risk of stroke compared with non-smokers, and smoking was associated with one in five strokes.
Professor Martin O'Donnell, NUI Galway explained: "The INTERSTROKE study is the first large standardised case-control study of risk factors for stroke in which countries of low and middle income were included, and where all cases completed a brain scan (usually a CT scan). Our findings showed that five risk factors accounted for more than 80% of the global risk of all stroke (ischaemic and intracerebral haemorrhagic): hypertension, current smoking, abdominal obesity, diet, and physical activity. With the addition of five other risk factors, including apolipoproteins, the PAR for all stroke rose to 90%."
The authors highlight that nine of ten risk factors (not including cardiac disorders) in INTERSTROKE are the same as in INTERHEART (also led by Dr Yusuf) which looked at risk factors for heart attacks. The relative importance of many of these risk factors is different for stroke and heart attack. For example, blood pressure is the most important risk factor for stroke, while blood lipids (fats) are the most important risk factors for heart attack. However, the nine risk factors in INTERHEART covered 90% of the PAR for heart attacks.
They add that this work proves that a large international epidemiological study of stroke that requires routine neuroimaging is feasible in countries of low and middle income, and conclude: "Targeted population-based interventions that reduce blood pressure and smoking, and promote physical activity and a healthy diet, could substantially reduce the global burden of stroke."
The investigators are currently undertaking Phase 2 of INTERSTROKE, which will include Ireland, aims to include 20,000 participants. This second phase will determine the importance of risk factors within different regions, different ethnic groups, and within ischemic stroke subtypes. In addition, the association between genetics and risk of stroke will be studied—this will require large sample sizes.
In an accompanying Comment, Dr Jack V Tu, Institute for Clinical Evaluative Sciences, Sunnybrook Schulich Heart Centre, University of Toronto, Toronto, ON, Canada, says: "Whilst hypertension is well established as the most important cause of stroke in high-income countries, INTERSTROKE confirms that it is also the most important risk factor for stroke in developing countries. This finding is particularly relevant because it highlights the need for health authorities in these regions to develop strategies to screen the general population for high blood pressure and, if necessary, offer affordable treatment to reduce the burden of stroke. It also provides an impetus to develop population-wide strategies to reduce the salt content in the diet of individuals in these countries."