This analysis of the distribution of ischemic stroke subtype by race uniquely employed a diagnostic workup that included diffusion-weighted MR imaging in the vast majority of patients. Unlike prior studies with incomplete workups and/or CT-based imaging, the overwhelming majority of patients included in our analysis had a comprehensive hospital-based diagnostic workup, including echocardiography, telemetry monitoring, vessel imaging, and laboratory testing. We compared results using three approaches to ischemic stroke subtype classification: standard TOAST [11
], SSS TOAST [12
], and imaging-based TOAST [9
]. Employing this comprehensive diagnostic approach, our study found a higher proportion of cardioembolic strokes and a smaller proportion of small vessel strokes in blacks (compared to prior studies) such that both blacks and whites had similar proportions of both subtypes. We also found that whites had a relatively small proportion of large artery strokes, a finding contrary to many previous studies. The main significant difference between groups was that the black cohort had significantly fewer strokes in the “strokes of other etiology” category. There are a number of possible explanations for the noted differences in the racial proportion of ischemic stroke subtypes. As noted above, the extent of the diagnostic workup may have differed. Alternatively, there may be geographic and community-based differences in risk factors and/or subtypes [13
A number of prior studies have reported racial differences in stroke subtype proportions [1
]. In the Northern Manhattan population, White et al. reported similar rates of small-vessel strokes in blacks and whites but a greater rate of the cardioembolic subtype in whites as compared to blacks [2
]. Schneider reported similar findings in the Cincinnati population [1
]. Markus et al. found a significantly greater proportion of lacunes in the British black population as compared to whites offset by a greater proportion of cardioembolic subtype in whites [8
]. This study reports similar proportions of cardioembolic and lacunar strokes in both black and white cohorts employing diffusion-weighted MR imaging in the majority of patients.
An analysis of risk factors in our population showed that the black stroke cohort had significantly greater rates of diabetes and hypertension compared to whites, while the rates of coronary artery disease and hyperlipidemia were similar. In our cohort, the mean age did not differ by race; however there was a significant racial difference in gender between groups with 42% of the black cohort being male compared to 70% of the white cohort. Our population differs in risk factors and baseline characteristics from some prior reports in the fact that age was similar in both groups (whereas in most prior studies blacks have a younger age) and that there were more women in the black cohort.
Compared to the standard TOAST classification, the SSS TOAST classified more patients in the cardioembolic, large artery, small vessel, and other categories and fewer in the undetermined category for both groups, while the imaging TOAST classified more subjects only in the cardioembolic category. In our study, 79 patients (black and white) were found to have strokes of undetermined etiology under standard TOAST criteria. After utilizing, for instance, imaging TOAST criteria, the number of strokes of undetermined etiology decreased to 71, a 10% change. The cardioembolic category had the greatest increase of cases, signaling a shift from an undetermined to a clearer etiology by using DWI MRI. This is likely due to the ability of DWI to detect multiple small acute lesions (not visualized on CT) in different vascular territories [15
]. However, we point out that a substantial number of cases in each method of etiologic TOAST criteria still remained undetermined, despite the comprehensive diagnostic workup that patients received.
Accurate determination of stroke subtype on an individual level is important in order to appropriately tailor secondary prevention strategies. In particular, cardioembolic stroke is often most appropriately treated with anticoagulation rather than antiplatelet agents, particularly in the setting of atrial fibrillation. Recent studies have suggested that cardioembolism from atrial fibrillation may be underdiagnosed in general, but particularly so in the black population [16
]. It is possible that a significant proportion of patients previously diagnosed with stroke of undetermined subtype may actually have had cardioembolism, and particularly in black stroke patients. Our study suggests that more research needs to be done to confirm and explain racial differences in stroke subtype, particularly in the category of “other etiology.” One possibility is a racial or genetic susceptibility to certain types of stroke or hypercoagulable states.
Racial differences in the subcategory of “incomplete workup” using the SSS TOAST system is worth noting, although this difference did not reach statistical significance using the standard TOAST classification. A number of studies [17
], but not all [21
], have found important disparities in the extent of diagnostic evaluation in minority populations with stroke. A study of Michigan hospitals found that African American stroke patients were less likely to receive cardiac monitoring, dysphagia screening, smoking cessation counseling, or a CT within 25 minutes of hospital arrival. However, other aspects of the diagnostic evaluation including imaging of brain, vessels, and heart did not show a racial difference [17
]. Tuhrim et al. reported that black stroke patients were less likely to receive complete diagnostic evaluations and less likely to receive appropriate secondary prevention measures [18
]. These racial disparities in evaluation and treatment may be explained by differences in insurance or socioeconomic status, patient mistrust of the medical system, and/or clinician bias. Since all patients had standard order sets and the stroke team followed a prespecified pathway for diagnostic workup in this study, the underlying reason for disproportionally incomplete evaluations in the black population is unclear.
This study is limited by the fact that it was a hospital-based retrospective chart review rather than a population-based study. As with any hospital-based study, there is the potential for selection bias such that the population is not representative of the general population. However, Washington Hospital Center is the main community hospital for the region and receives over one-half of all strokes in the DC metropolitan area. Furthermore, a hospital-based study has the advantage of incorporating a more comprehensive and consistent diagnostic workup, which is crucial to an accurate determination of stroke subtype. Of note, we were not able to collect data on socioeconomic status or insurance. Future studies should include these variables in analyses, particularly studies evaluating the disparities in diagnostic assessments. Gender differences in our population may also have played a factor in the results we received. We had a much higher proportion of women in the black patient group (58%) compared to the women in the white patient group (30%). Given this was a retrospective study based upon a single hospital's experience, in future studies, a larger, more random sample would give more power to any differences found. We also used three different TOAST criteria to diagnose etiology of ischemic stroke, recognizing that other studies have used other forms of etiologic criteria, such as the modified NINDS scheme.
In conclusion, we found no difference in the frequency of ischemic stroke subtypes when employing a comprehensive diagnostic evaluation including diffusion-weighted MRI. This finding is particularly robust given the findings of high rates of hypertension and diabetes in our population—risk factors specifically associated with small-vessel strokes. In addition, our results suggest that the relatively high rate of cardioembolism may be underappreciated in black stroke patients and that diffusion-weighted MR imaging may be important in confirming this stroke subtype. Further research is required to better understand the racial differences in frequency of “stroke of other etiology” and to explore racial disparities in the extent of diagnostic evaluations.