1 Stroke Program, University of Michigan Medical School, Ann Arbor, MI.
2 Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor, MI.
3 Department of Epidemiology, University of Texas School of Public Health, Houston, TX.
4 Stroke Program, University of Texas Medical School, Houston, TX.
5 Corpus Christi Neurology, Corpus Christi, TX.
6 Neurology P.A., Corpus Christi, TX.
7 Department of Biometry, University of Texas School of Public Health, Houston, TX.
Received for publication February 11, 2004; accepted for publication March 17, 2004.
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ABSTRACT |
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cerebral hemorrhage; cerebrovascular accident; ethnic groups; Hispanic Americans; ischemic attack, transient; Mexican Americans; population surveillance; subarachnoid hemorrhage
Abbreviations: Abbreviation: BASIC, Brain Attack Surveillance in Corpus Christi.
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INTRODUCTION |
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Social and biologic risk factors are associated with stroke to a similar extent in Mexican Americans and non-Hispanic Whites (4). The higher prevalence of diabetes mellitus (5), lower socioecononmic status (6), and limited access to quality health care (7) found in Mexican Americans would predict more stroke in Mexican Americans relative to non-Hispanic Whites. However, vital statistics suggest that non-Hispanic Whites actually experience more stroke mortality, particularly at older ages when stroke is most common (8). Vital statistics data may greatly underestimate ethnic-specific stroke mortality as a result of improper coding of ethnicity on the death certificate (9). The most accurate estimates of disease burden are made through population-based ascertainment of stroke cases with reliable definitions of ethnicity and case status.
The Brain Attack Surveillance in Corpus Christi (BASIC) Project is a population-based stroke surveillance effort in a nonimmigrant biethnic community in southeast Texas. We present here cumulative stroke incidence among Mexican Americans and non-Hispanic Whites in this representative US community.
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MATERIALS AND METHODS |
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Abstractors underwent a rigorous training and certification process, and several quality assurance mechanisms were imposed. Cases were screened from emergency department and inpatient sources by manually searching visit and admission logs for a set of previously validated screening diagnostic codes (12). The abstractors routinely canvassed intensive care units and hospital floors searching for in-house strokes or those not ascertained through the screening logs. This hot pursuit was supplemented by review of hospital passive listings of International Classification of Diseases, Ninth Revision, discharge codes for stroke (codes 430438, excluding codes 433.x0 and 434.x0 (x = 19), 437.0, 437.2, 437.3, 437.4, 437.5, 437.7, 437.8, and 438).
Cases that screened positive for potential stroke diagnostic codes were reviewed for eligibility criteria (aged 45 years, Nueces County residence). All eligible cases were fully abstracted and source documentation was copied. Study neurologists validated cases using source documentation, blinded to ethnicity, on the basis of previously published international criteria (13, 14). Mexican-American and non-Hispanic White subjects considered for validation were equally likely to have received head computed tomography or magnetic resonance imaging (Mexican Americans: 94.8 percent; non-Hispanic Whites: 93.7 percent; p = 0.22) (15). Stroke was defined as the acute onset (from minutes to hours) of a focal neurologic deficit specifically attributable to a cerebrovascular distribution that persists for greater than 24 hours (except in cases of sudden death or if the development of symptoms is interrupted by a surgical or interventional procedure) and not attributable to another disease process (seizure, brain tumor, hypoglycemia, metabolic encephalopathy, or hysteria). A diagnosis of transient ischemic attack was the same except that symptoms abated within 24 hours. The diagnosis of intracerebral hemorrhage or subarachnoid hemorhage required the clinical symptoms mentioned above and neuroimaging that demonstrated a spontaneous focal collection of blood in either the parenchyma/ventricle (intracerebral hemorrhage) or subarachnoid space (subarachnoid hemorrhage). A spinal fluid definition of subarachnoid hemorrhage, including xanthochromia and/or greater than 1,000 red blood cells per cubic millimeter without a decrement from first to last tubes of greater than 25 percent (16), was accepted. To determine initial stroke severity, we retrospectively abstracted the National Institutes of Health Stroke Scale from the chart in accordance with the validated method of Willams et al. (17).
Two thirds of the validated cases were randomly selected to participate in a structured patient interview. The interview contained questions regarding risk factors, medications that patients were taking prior to their stroke, access to care, and race/ethnicity. The BASIC Project interview identifies race and ethnicity in a manner similar to that of the 2000 US Census with a question about race and then a separate question about ethnicity. Patients recording their ethnicity "of Hispanic origin" were also asked the country of origin of their parents and their grandparents. Since race/ethnicity data obtained from the interview were available for only a random sample of subjects, race/ethnicity was also obtained from the medical record for all patients. The use of medical record race/ethnicity for this study was validated by the subjects interview data by comparing self-reported and medical record-abstracted race/ethnicity. From a total of 1,194 interviews of 580 Mexican Americans, 546 non-Hispanic Whites, and 68 Blacks, we observed 97 percent agreement (kappa = 0.94) between self-report and medical record for race/ethnicity classification.
An out-of-hospital sampling frame was used to estimate nonhospitalized stroke and transient ischemic attack cases. A random sample from primary care physicians, nursing homes, and neurologists in Nueces County was obtained, with neurologists being oversampled. The following available population of primary care physicians (n = 167), nursing homes (n = 12), and neurologists (n = 11) yielded a sample of 47 primary care physicians, four nursing homes, and 11 neurologists.
This project was approved by the institutional review boards of the University of Michigan and the University of Texas at Houston, the Corpus Christi/Nueces County Health District, and all hospitals.
Regarding statistical analysis, age-specific 36-month cumulative incidence was calculated by ethnicity and stroke type (ischemic stroke, intracerebral hemorrhage, subarachnoid hemorrhage, and transient ischemic attack). Ethnic classification of Mexican American or non-Hispanic White was determined from the medical record for the numerators for the incidence calculations. From the interview data regarding family ancestry, it was determined that only 1 percent of subjects reported ancestry other than Mexican. From this information, we determined that it would be appropriate to classify our Hispanic stroke cases as Mexican American.
The denominators for the incidence calculations were the 2000 US Census estimates using 5- and 15-year age categories (1). The denominators for non-Hispanic Whites were derived from individuals reporting the single race group White and an ethnicity of non-Hispanic origin. This method may have led to an underestimation of the non-Hispanic White population, as individuals who reported two- or three-race combinations were not included; however, these categories represent only 3.7 percent of Texas residents. The denominators for Mexican Americans were derived from individuals who reported an ethnicity of Hispanic origin regardless of race. Less than 1 percent of subjects residing in Nueces County who identified their ethnicity as Hispanic on the 2000 US Census identified their race as other than White. Further, among the Hispanics in Nueces County included in the 2000 US Census, only 1.2 percent identified an ancestral origin from a country other than Mexico (1 percent identified Spain and 0.2 percent identified Central or South America). These data support our designation of the census population data into non-Hispanic White and Mexican American for the population of Nueces County, Texas, and they demonstrate that definitions in the numerators correspond to definitions in the denominators for the incidence calculations.
The numerators were calculated by identifying an individuals first event captured by the BASIC Project. Events were then summed by ethnicity and age categories for each stroke type. The age-specific, 36-month cumulative incidence for first-ever ischemic stroke only was also calculated by ethnicity. Cumulative incidence was first calculated using data from inpatient sources only. Data collected from the out-of-hospital sample during the last 24 months of the 36-month surveillance were then used to estimate the total number of strokes that occurred out of hospital and were used to adjust the number of ischemic strokes and transient ischemic attacks identified by the BASIC Project during 36 months of surveillance.
Risk ratios and 95 percent confidence intervals comparing Mexican Americans and non-Hispanic Whites were calculated to examine ethnic differences in cumulative incidence. To test for interactions between ethnicity and age on risk of stroke, we performed Breslow-Day tests of homogeneity. When there was no significant age-ethnicity interaction, age-adjusted risk ratios and 95 percent confidence intervals comparing Mexican Americans and non-Hispanic Whites were calculated using the Mantel-Haenszel method. The BASIC Project was designed to detect a minimum incidence risk ratio of 1.20 for overall stroke attack rates comparing Mexican Americans and non-Hispanic Whites, assuming 80 percent power and a 0.05 level of significance based on a two-sided hypothesis. The study was also sufficiently powered to study ethnic differences in ischemic stroke incidence. Age categories were prespecified before the study started as 4559, 6074, and 75 or more years.
In a secondary analysis, we compared the proportion of Mexican-American and non-Hispanic White ischemic stroke patients who were taking medication for known personal stroke risk factors. Z tests for binomial proportions were used to test for significant differences between ethnicities. SAS version 8.02 software (SAS Institute, Inc., Cary, North Carolina) was used for analyses.
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RESULTS |
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Table 1 presents the age- and race/ethnic-specific cumulative incidence for completed cerebrovascular events (first-ever and recurrent completed ischemic stroke + intracerebral hemorrhage + subarachnoid hemorrhage). Of the completed cerebrovascular events, 53 percent occurred in Mexican Americans. The risk increased with advancing age and was higher in Mexican Americans compared with non-Hispanic Whites at each age group, although not statistically different for the group aged 75 or more years. There was a significant age-ethnicity interaction for total completed cerebrovascular events. The median National Institutes of Health Stroke Scale for completed cerebrovascular events did not differ by ethnicity (non-Hispanic White: 4 (interquartile range: 29) and Mexican American: 4 (interquartile range: 28)).
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Table 2 shows the cumulative incidence for first-ever and recurrent ischemic stroke. Sixty-eight percent were first-ever ischemic stroke and 32 percent were recurrent ischemic stroke. Mexican Americans had a higher incidence at younger ages, but the risk converges with advancing age. There was a significant age-ethnicity interaction. The age-ethnicity interaction precluded age adjustment. Table 2 also demonstrates cumulative incidence for first-ever ischemic stroke. Mexican Americans between 45 and 59 years of age had almost double the risk of first stroke compared with non-Hispanic Whites. This difference approached zero when ethnic comparisons were made in those aged 75 or more years. This significant age-ethnicity interaction again precluded age adjustment.
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To further evaluate if the lower risk ratios seen in the older age groups were due to the more youthful age distribution of Mexican Americans compared with non-Hispanic Whites, we reanalyzed table 2 (first-ever and recurrent ischemic stroke) by 5-year age groups. This is shown graphically in figure 1. Indeed, the group aged 8084 years still showed an elevated risk for Mexican Americans. The group aged 85 or more years was not reported because of the different age distributions of the two ethnicities after the age of 85 years.
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DISCUSSION |
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Access to care and acculturation are important nonconventional stroke risk factors in Mexican Americans. A recent study found that Hispanics were 12.7 percent less likely to receive the appropriate secondary stroke prevention drug compared with non-Hispanic Whites (19). We did not find similar results in the current study. However, a study in Nueces County found that the general population of Mexican Americans was less knowledgeable about stroke risk factors and felt less empowered to prevent stroke compared with the general population of non-Hispanic Whites (18). Mexican Americans were also less likely to say that they would call 9-1-1 when experiencing an acute stroke, thus limiting access for acute stroke therapy. However, in a case-control study comparing stroke patients with population-based controls, access-to-care variables were equally predictive of stroke in both Mexican Americans and non-Hispanic Whites (4). Although limitations in access to care may be associated with similar risk of stroke for individuals of both ethnic groups, it is possible that stroke-related health disparities are partially explained by the greater relative prevalence of limitations in access to quality health care in Mexican Americans compared with non-Hispanic Whites (7).
The role of genetics as an explanation for the ischemic stroke disparity between Mexican Americans and non-Hispanic Whites has not been addressed. Cavernous malformations, an important cause of intracerebral hemorrhage, were found to have an underlying genetic association with a mutation that clusters in Mexican-American families (20). Ischemic stroke is more likely polygenic and complex, but clues from studies of populations at high risk may be fruitful.
Other minority populations experience an excess stroke risk. African Americans, particularly at younger ages, suffer an excess burden (21, 22). In the predominantly Black Hispanic population of northern Manhattan (New York City), Hispanics are at greater risk of first stroke compared with non-Hispanic Whites. The interaction of age and race/ethnicity demonstrates a higher relative stroke burden for young minority populations compared with older minority populations when comparisons are made with non-Hispanic Whites.
In the current study, a significant interaction of age and ethnicity was demonstrated for ischemic stroke incidence. The etiology of this interaction is poorly understood. Competing mortality does not explain the finding (8), but cohort factors remain to be investigated. An interaction was not seen for intracerebral hemorrhage or subarachnoid hemorrhage, but a trend for an age-ethnicity interaction was seen for intracerebral hemorrhage.
Inclusion of out-of-hospital stroke cases accounted for 5.6 percent of all stroke cases and 14.2 percent of all transient ischemic attacks in the current study, emphasizing the importance of incorporating these measures in estimates of total stroke incidence. We expected higher out-of-hospital totals in non-Hispanic Whites compared with Mexican Americans because of greater access to outpatient services for non-Hispanic Whites. The differences, however, were minimal. These differences may be greater in immigrant communities.
This study provides population-based data from a multiethnic community. We must be cautious about generalizing these results, particularly to immigrant communities. Although every attempt was made to capture all events, there is the possibility of missing data. Direct neurologic examinations and neuroimaging review would have been the ideal way to validate cases, but these were impractical in a large population-based work. It is especially unlikely that the potential sources of bias would affect the direction of risk ratio estimates.
Stroke burden encompasses incidence, stroke severity, and outcome, including disability and mortality. This paper demonstrates clear incidence differences and does not suggest that Mexican Americans have less stroke severity. Additional research is needed to determine if differences in disability and mortality exist.
Mexican Americans are the largest component of the largest minority population in the United States. Their risk of stroke is substantially elevated compared with that of non-Hispanic Whites. As the Mexican-American population ages and continues its fast growth, the medical, social, and economic impacts of stroke will be felt throughout the United States. Efforts to specifically target Mexican Americans for stroke prevention and acute stroke therapy are indicated.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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