Stroke : Epidemiology and Pathology

Burden of Stroke

From a population level, the burden of stroke can be measured in three different ways—by measuring mortality, prevalence, or incidence. Each method has its advantages and limitations.

Stroke mortality figures usually include all individuals with stroke recorded as the primary cause of death on their death certificates. Systematic and long-term collection of these data allows assessment of trends over time and comparisons among countries. Mortality figures are subject to limitations, including imprecision in death certification and incomplete assessment of the overall burden of stroke; between 45% and 60% of people with stroke survive beyond 5 years.

Stroke prevalence studies can be used to assess health in survivors and assist with the planning of community health care resources, but may not provide an accurate reflection of the population burden of stroke because of issues such as selection and survival bias. Carefully conducted stroke incidence studies provide the best source of information on the burden of stroke, allowing a better understanding of the empirical relation among incidence, mortality, and survival. For example, changes in stroke mortality may attributable to changes in stroke incidence, case fatality (reflecting changes to stroke severity or poststroke management), or a combination of both.

Comparison between identically conducted stroke incidence studies in the same population will help determine where the changes have occurred. Such repeat incidence studies are expensive and labor-intensive because of the strict ideal criteria required for their conduct. Because of this, most stroke incidence studies in the past decade were undertaken in high-income countries, but there are several now being carried out in low and middle-income countries. Recent comprehensive reviews of the global burden of stroke summarizes many of these studies and shows some marked differences in stroke burden between high-income and low- and middle-income countries.

Stroke Mortality

According to the Global Burden of Diseases Study, stroke and ischemic heart disease collectively contributed to 1279 million deaths in 2010, or one in four deaths worldwide, compared with one in five in 1990. According to the World Health Organization (WHO), stroke is the second most common single cause of death in the world after ischemic heart disease. In 2012, an estimated 6.7 million deaths from stroke occurred worldwide; these deaths comprised approximately 11.9% of all deaths. The contribution of stroke to mortality varies by income level of countries. In 2012, approximately 43% of these deaths occurred in low- to middle-income countries, 55% in upper middle-income countries, and only 22% in high-income countries. The greater number of strokes deaths occurring in low- and middle-income countries than in high-income countries is attributable to their larger population (≈fourfold that of the population in high-income countries).

There are now substantial data on time trends in stroke mortality rates from low-, middle-, and high-income countries. In a comprehensive systematic review, Krishnamurthi and Feigin and colleagues have summarized trends in annual mortality rates by age and country income status from 1990 to 2010. They showed that overall, stroke mortality rates declined over this period, irrespective of a country's income status. The age-adjusted annual mortality rate/100,000 population for ischemic stroke fell significantly from 63.8 (95% confidence interval [CI], 56.5 to 66.0) to 40.3 (95% CI, 38.2 to 43.1) in high-income countries and from 50.1 (95% CI, 42.0 to 64.1) to 43.1 (95% CI, 38.3 to 51.9) in low-income countries. Mortality rates for hemorrhagic stroke also fell from 32.7 (95% CI, 29.9 to 35.7) to 20.3 (95% CI, 18.6 to 22.9) in high-income countries and from 80.4 (95% CI, 63.7 to 96.9) to 61.9 (95% CI, 52.5 to 72.3) in low-income countries.

These declines were observed for all age categories, but were more pronounced in those aged 75 years and older, with up to a 40% reduction in rates in these older age groups ( Table 60-1 ). However, mortality overall from stroke was much greater in those 75 years and older than in those younger than 75 years. In the WHO Monitoring Trends and Determinants in Cardiovascular Disease (WHO MONICA) project, Sarti and associates have provided evidence to suggest that declining case fatality rates may underlie the observed changes in mortality. These observations have been supported by reductions in the mortality incidence rates observed by Krishnamurthi and coworkers from 1990 to 2010 in most countries. The reasons underlying reduced case fatality rates are most likely to be improvements in stroke care, with earlier and more appropriate diagnoses, rapid acute treatments, and increasing presence of organized stroke units. However, it is not yet clear if case fatality rates are higher among those with prestroke frailty, which is common in older adults. In preliminary analyses, a frailty index derived from a combination of prestroke health conditions, function, walking ability, and blood test results was associated with a 16% increased risk of dying in hospital after an acute stroke. These results, although intuitive, need to be supported by more substantive evidence.

Prevalence and Incidence of Stroke and Subtypes

A number of stroke prevalence studies have been conducted around the world. Stroke prevalence (per 100,000 population, standardized to the world population older than 65 years) appears least in rural South Africa (1,539/100,000), United States (4,536/100,000) and New Zealand (4,872/100,000), whereas a greater prevalence was evident in L'Aquila, Italy (6,812/100,000), Newcastle, England (>7,000/100,000), and Singapore (7,337/100,000). Interestingly in Singapore, prevalence rates among Malays (5,396/100,000) appeared less than those of Chinese (7,829/100,000) or Indian (6,871/100,000) descent, although this difference was not statistically different. Differences in environmental or genetic risk factor profiles, poststroke care, or both may influence these geographic variations in prevalence.

Stroke is a heterogeneous condition with two main subtypes, ischemic and hemorrhagic stroke. Depending on the study region, the more common ischemic stroke (IS) accounts for 63% to 84% of all strokes, whereas intracerebral hemorrhage (ICH) accounts for 7% and 20% of all strokes. The proportion of hemorrhagic strokes appears to be greater in nonwhite populations and among those living in low- and middle-income countries compared with white populations in high-income countries. Within the category of ischemic stroke, there are further subtypes that are classified based on clinical signs alone or on actual stroke mechanisms (e.g., large vessel disease, cardioembolism, small vessel disease). The most frequently used classification system in large-scale, population-based epidemiologic studies is based on clinical features alone, as devised by the investigators of the Oxfordshire Community Stroke Project, which differentiates stroke into total or partial anterior infarction, posterior infarction, and lacunar infarction. The advantage of this classification system is that it does not require expensive investigations ,which may be unavailable in low-income countries or less freely available in middle-income countries. The disadvantage of such a system is that the actual subtype of ischemic stroke may be erroneous because at least 10% of those classified as “lacunar” infarctions (implying small vessel disease) will have a proximal source of embolus from large vessels or the heart.

Prevalence studies also provide a measure of the impact of stroke on survivors and the consequent health burden on patients, caregivers, and society at large. Declining stroke case fatality and mortality rates translate into an increased prevalence, resulting in an increased burden of stroke to those communities affected. Importantly, about 50% of stroke survivors are likely to require assistance in everyday activities. Frail older adults who suffer strokes are most at risk of poststroke functional decline, with one preliminary report suggesting an 8% increased risk of major physical disability in those with a higher prestroke score on a frailty index. Stroke and frailty are each also associated with a greater prevalence of cognitive impairment, and hence it is likely that the burden of cognitive impairment will be greater among frail older adults suffering strokes than among others. Thus, frailty may be an important marker of particularly vulnerable stroke patients who are likely to require enhanced health care, rehabilitation, and support systems to maintain their functional status.