Using data from the Rotterdam study, Michiel Bos and colleagues estimate the proportion of strokes that are attributable to established modifiable etiological factors for stroke.
Please see later in the article for the Editors' Summary
Stroke prevention requires effective treatment of its causes. Many etiological factors for stroke have been identified, but the potential gain of effective intervention on these factors in terms of numbers of actually prevented strokes remains unclear because of the lack of data from cohort studies. We assessed the impact of currently known potentially modifiable etiological factors on the occurrence of stroke.
Methods and Findings
This population-based cohort study was based on 6,844 participants of the Rotterdam Study who were aged ≥55 y and free from stroke at baseline (1990–1993). We computed population attributable risks (PARs) for individual risk factors and for risk factors in combination to estimate the proportion of strokes that could theoretically be prevented by the elimination of etiological factors from the population.
The mean age at baseline was 69.4 y (standard deviation 6.3 y). During follow-up (mean follow-up 12.9 y, standard deviation 6.3 y), 1,020 strokes occurred. The age- and sex-adjusted combined PAR of prehypertension/hypertension, smoking, diabetes mellitus, atrial fibrillation, coronary disease, and overweight/obesity was 0.51 (95% CI 0.41–0.62) for any stroke; hypertension and smoking were the most important etiological factors. C-reactive protein, fruit and vegetable consumption, and carotid intima-media thickness in combination raised the total PAR by 0.06. The PAR was 0.55 (95% CI 0.41–0.68) for ischemic stroke and 0.70 (95% CI 0.45–0.87) for hemorrhagic stroke.
The main limitations of our study are that our study population comprises almost exclusively Caucasians who live in a middle and high income area, and that risk factor awareness is higher in a study cohort than in the general population.
About half of all strokes are attributable to established causal and modifiable factors. This finding encourages not only intervention on established etiological factors, but also further study of less well established factors.
Please see later in the article for the Editors' Summary
Every year, 15 million people worldwide have a stroke. About 6 million of these people die within hours, and another 5 million are left disabled. Stroke occurs when the brain's blood supply is suddenly interrupted by a blood vessel in the brain being blocked by a blood clot (ischemic stroke) or bursting (hemorrhagic stroke). Deprived of the oxygen normally carried to them by the blood, the brain cells near the blockage die. The symptoms of stroke depend on which part of the brain is damaged but include sudden weakness or paralysis along one side of the body, vision loss in one or both eyes, and trouble speaking or understanding speech. Anyone experiencing these symptoms should seek immediate medical attention because prompt treatment can limit the damage to the brain. In the longer term, post-stroke rehabilitation can help overcome the disabilities caused by stroke, and various drugs alongside behavioral counselling can reduce the risk of a second stroke.
Why Was This Study Done?
Fifty years ago, it was discovered that treatment of high blood pressure (hypertension) reduces the risk of stroke among people with severe hypertension. This discovery led researchers to search for other potentially modifiable etiological factors for stroke (risk factors that cause stroke). The list of established etiological factors now includes smoking, diabetes, atrial fibrillation (an irregular heartbeat), heart disease, and overweight/obesity, in addition to hypertension. But how many strokes would modification of these causal risk factors prevent? In this population-based cohort study, the researchers calculate the individual and combined population attributable risks (PARs) for these established etiological factors to provide an estimate of what proportion of strokes could theoretically be prevented by optimal treatment of known etiological factors. A population-based cohort study enrolls a group of people, determines their characteristics at baseline, and follows them to see whether specific characteristics are associated with specific outcomes. A PAR of an etiological factor for a disease indicates the proportion of that disease in the population that would not occur in the absence of the risk factor.
What Did the Researchers Do and Find?
The researchers used data from 6,844 participants in the Rotterdam Study, which was designed to investigate the causes and consequences of long-term and disabling diseases in the elderly. At baseline, all of the participants were over 55 years old and free from stroke. During follow-up, 1,020 strokes occurred among the participants. Using data on exposure at baseline to various etiological factors for stroke, the researchers calculated PARs for individual factors and used a special statistical technique to calculate PARs for the factors in combination. The combined PAR of prehypertension/hypertension, smoking, diabetes, atrial fibrillation, heart disease, and overweight/obesity was 0.51 for any stroke. That is, about half of the strokes in the study population were attributable to this combination of etiological factors. Hypertension and smoking were the most important individual factors (PARs of 0.36 and 0.16, respectively). Notably, the inclusion of several less well established etiological factors (increased blood levels of C-reactive protein, low fruit and vegetable consumption, and thickening of the lining of arteries) only raised the total PAR for any stroke by 0.06.
What Do These Findings Mean?
These findings indicate that about half of the strokes in the study cohort were attributable to established modifiable etiological factors and could theoretically be prevented by eliminating these risk factors from the population. Previous studies have estimated that a larger proportion of strokes could be prevented by eliminating known etiological factors. The researchers acknowledge that some aspects of their study may have led to an underestimation of the proportion of stroke attributable to established etiological factors and note that their findings may not be generalizable to underprivileged or racially diverse populations. Nevertheless, they argue that previous studies are likely to have overestimated the PARs for stroke because they were based on case–control studies (in which exposure to etiological factors was assessed after a stroke had occurred in cases and control individuals, rather than before a stroke as in a population-based cohort study) and often did not use optimal statistical techniques to calculate the total PAR. Importantly, these new findings underscore the importance of interventions targeted at reducing smoking and hypertension and support the search for additional etiological factors for stroke.
Please access these websites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001634.
The US National Institute of Neurological Disorders and Stroke provides information about all aspects of stroke (in English and Spanish); its Know Stroke site provides educational materials about stroke prevention, treatment, and rehabilitation including personal stories (in English and Spanish); the US National Institutes of Health SeniorHealth website has additional information about stroke
The Internet Stroke Center provides detailed information about stroke for patients, families, and health professionals (in English and Spanish)
The UK National Health Service Choices website also provides information about stroke for patients and their families, including personal stories
MedlinePlus has links to additional resources about stroke (in English and Spanish)
Information about the Rotterdam Study is available
The UK not-for-profit website Healthtalkonline provides personal stories about stroke