Does Your Ethnicity Affect Your Chance of Getting a Stroke?

I. INTRODUCTION

Accounting for nearly 10% of all deaths, stroke mortality rates are second only to coronary heart disease (13%) and cancer (12%). Have you ever wondered how prone you are to strokes? In fact, there are differences in the occurrence of strokes between ethnic groups. In this article, the differences in the occurrence of strokes in Caucasians and Asians, as well as the underlying reasons, will be explored. However, to have a better understanding of the trends, we have to first understand the mechanisms of the two major types of strokes: ischemic strokes and hemorrhagic strokes.

II. CLASSIFICATION OF STROKES

Ischemic strokes are the most common type of strokes, amounting to 87% of strokes (Nall, 2018, para. 3). It is caused when blood is blocked from flowing to the area(s) of your brain because of a blood clot in a cerebral artery (e.g. internal carotid artery). When brain tissues cannot obtain enough oxygen and nutrients, infarction (cellular death) occurs. Certain brain function is then diminished or hindered. This process is irreversible since neurons cannot repair themselves. Ischemic strokes can be further categorized into thrombotic strokes, lacunar strokes and embolic strokes. Thrombotic stroke is due to atherosclerosis, which is a buildup of fatty deposits on the inner lining of a cerebral artery (Nall, 2018, para. 5). At damaged areas of an atheroscleoritic plaque, blood clots form, which block blood flow. Brain tissues, obtaining oxygen and nutrients from this artery, then die. Lacunar stroke is similar to thrombotic stroke, but occurs in a smaller artery. It occurs when blood flow to one of the small arteries deep within the brain becomes blocked (Pietrangelo, 2018, para. 1). On the other hand, embolic ischemic strokes occur when the blood clots or debris come from elsewhere in the body, typically the heart valves, through the circulatory system (Pathophysiology of stroke, n.d., para. 5). In other words, you can consider the cause of thrombotic strokes and lacunar stroke as a rather localized one, where blood clots are formed in an artery near the brain directly. On the contrary, embolic ischemic strokes, where blood clots from anywhere in the body get stuck in an artery supplying blood to the brain, are not localized.

Hemorrhagic strokes occur when a blood vessel in your brain ruptures or breaks, with blood spilling into the surrounding tissues (Nall, 2018, para. 9). As a result, the brain tissue, compressed from an expanding haematoma, is distorted and injured (Pathophysiology of stroke, n.d., para. 14). Apart from direct compression, a brain hemorrhage may have direct toxic effects on the brain tissue and vasculature (the arrangement of blood vessels) as well (Pathophysiology of stroke, n.d., para. 14). When there is extravasated blood in brain tissues, the breakdown of haemoglobin proteins leads to the build-up of heme molecules and iron. (Aronowski & Zhao, 2011). Heme molecules and iron are able to interact in redox reactions, creating oxidative stress to surrounding brain cells. This could lead to cell death. Moving on to the categorization of hemorrhagic strokes. Again, hemorrhagic strokes are further categorized into intracranial hemorrhage and subarachnoid hemorrhage. For intracranial hemorrhage, a leakage of blood is caused by rupture of a blood vessel in the brain. The rupture may be secondary to pre-existing conditions such as chronic hypertension or vascular malformations (Pathophysiology of stroke, n.d., para. 14). Subarachnoid hemorrhage is typically caused by trauma to the head or rupture of a cerebral aneurysm (Pathophysiology of stroke, n.d., para. 14). Therefore, there is a gradual collection of blood in the subarachnoid space, the space between the innermost meninges of the brain and the pia mater of the brain. The increase in volume of blood compresses on the cerebral cortex (surface of the brain). Then, brain tissue is damaged.

III. OCCURRENCE OF STROKES IN CAUCASIANS AND ASIANS

In a relatively older study, it was found that lacunar infarction and intracerebral hemorrhage were more common in Malaysians, while cardioembolic strokes were more common in Australians. (Ng et al., 1998) Similarly, a more recent systematic review found that in general, there was a slightly higher overall stroke incidence and higher proportion of intracerebral hemorrhage in Chinese than Caucasians. In addition, they also identified a higher occurrence of lacunar strokes in Chinese. (Tsai et al., 2013) However, their study did not show any clear evidence for different distributions of ischemic stroke subtypes (i.e. thrombotic stroke, embolic stroke and lacunar stroke) due to unrepresentative data.

Obviously, these studies, which have been conducted to explore the relationship between the occurrence of different types of strokes and ethnic groups, may not be fully representative. For the study conducted by Tsai et al. (2013), only hospital-based studies from Chinese populations were available. Different hospital admission rates could contribute to the variation among Chinese studies as well as between Chinese studies and community-based studies in white populations.

The two studies have similar results after analyses, and therefore, in general, it can be inferred that the occurrence of lacunar infarction and intracerebral hemorrhage are more common in Asians while cardioembolic strokes are more common in Caucasians.

The risk factors behind the occurrences of strokes in different ethnic groups are complex, with no clear, universal correlation yet established. To explain the trend, Tsai et al. (2015) conducted a systematic review to identify particular risk factors. There was a lower prevalence of atrial fibrillation (irregular beating of the atria) in Chinese. (Chinese: 11% vs. Caucasians: 27%) Similarly, Chinese have a lower prevalence of hypercholesterolemia (high cholesterol) (Chinese: 9% vs. Caucasians: 30%). A same trend can be observed for ischemic heart disease (blockage of coronary arteries) as well. (Chinese: 11% vs. Caucasians: 20%) The prevalence of other risk factors, such as hypertension, smoking and consumption of alcohol, were insignificant to trends.

These risk factors are consistent with the trends of occurrences mentioned above. Caucasians have a higher prevalence of atrial fibrillation, hypercholesterolemia and ischemic heart disease. All of these risk factors point toward the occurrence of cardioembolic stroke. Atrial fibrillation causes stagnant blood flow in the atria (especially in the left atrium), causing blood clots to form, which flow towards a cerebral artery and clogs it. Hypercholesterolemia increases the deposition of lipids and increases the formation of plaques in the coronary arteries, causing ischemic heart disease. As for why Caucasians have higher risks of developing atrial fibrillation, hypercholesterolemia and ischemic disease, this is not the focus of this article, but it may be explained by different lifestyle choices (e.g. dietary choices) and genetics.

However, as for why Asians have a higher risk of intracranial hemorrhage and lacunar stroke, there are few studies that can accurately explain this trend. There are so many factors that may explain the higher risk of intracranial hemorrhage, such as hypertension differences between Asians and Caucasians, genetic differences, dietary habits and even climate (Tsai, 2013).

With that in mind, to further illustrate the complexity of the factors that come into play in hemorrhagic stroke occurrences, we can look at an interesting study on a unique risk factor exclusive to South Asians - the effect of squatting and straining at stools in Indians. The relationship of the clinical observations regarding stroke onset with the blood pressure (BP) changes noted on squatting in healthy as well as hypertensive subjects appears to be more than fortuitous. Squatting induced rise in BP appears to be an important triggering factor for stroke onset in subjects at risk in India. In particular, more than half of the haemorrhagic strokes occurred while the subjects were in squatting position. (Chakrabarti, 2002) This is another example of how lifestyle and cultural factors come into play when it comes to the occurrence of hemorrhagic strokes. It becomes apparent now, that there is no distinct feature or factor that can distinguish hemorrhagic stroke occurrences in Asian and Caucasians. Rather, it is a combination of many.

IV. TREATMENT DIFFERENCES IN ASIANS AND CAUCASIANS

Currently, there are not many studies discussing the treatment differences in Asians and Causcasians. One noteworthy research, however, found that Asian-Americans suffering from ischemic strokes tend to receive clot busting stroke treatment less frequently than whites. (Sarah, 2019) This finding was presented at the American Heart Association International Stroke Conference 2018. The clot busting stroke treatment cited in the study was the intravenous (directly administered into a person’s veins) clot-busting drug tissue plasminogen activator (tPA). Tissue plasminogen activator (tPA) is classified as a serine protease (enzymes that cleave peptide bonds in proteins). It is one of the essential components of the dissolution of blood clots. Its primary function includes catalyzing the conversion of plasminogen to plasmin, the primary enzyme involved in dissolving blood clots. (Talha, 2021) Despite this, Asian Americans had more symptomatic hemorrhage after tPA and overall post-tPA complications. “Further research is necessary to better define interventions that can improve the disparities that Asian American patients experience in acute ischemic stroke,” the researchers wrote.

V. CONCLUSION

In this article, we have explained the classification of different strokes. We then moved on to describe the trends of the occurrences of strokes in Caucasians and Asians. “So how prone are we to strokes?” From the studies above, we can conclude that Asians are more prone to strokes in general, especially lacunar strokes and intracranial hemorrhages while Caucasians are more vulnerable to cardioembolic strokes. However, it is nearly impossible to explain why certain strokes are more prevalent in ethnic groups. Even if large sets of data are collected and analyzed, researchers may not be able to explain the trends. The occurrence of strokes between Asians and Caucasians may differ due to a combination of risk factors, such as the physiological lifestyle differences as highlighted in this article. Finally, in the future, it is hoped that we can work towards lowering the prevalences of different kinds of strokes, having identified the difference in particular risk factors between Caucasians and Asians.

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