Nationwide, we found that 9.4% of elderly ischemic stroke patients experience a recurrent ischemic stroke within 1 year; however, there is marked geographic variation in county recurrence rates across the country, with lower rates seen in the northeastern and western regions and high rates in the southern and central USA. These regional differences persisted even after controlling for patient characteristics and vascular risk factors.
The national rate of 1-year recurrent ischemic stroke hospitalization found in this analysis is comparable to the recurrent stroke rates found in other cohort studies (ranging from 5 to 15%) [14
]. At a national level, we found that counties with older populations, more women and larger populations of individuals who were non-White, Black or Hispanic had lower recurrent ischemic stroke rates. An increased prevalence of hypertension was associated with a lower risk of recurrent stroke, potentially due to better control among diagnosed cases [20
]. Consistent with prior work, we also found that counties with a greater comorbidity burden had higher ischemic recurrence rates [15
The geographic patterns identified in this study demonstrate that stroke recurrence rates vary dramatically across the country and corroborate prior work on regional differences in stroke mortality and hospitalizations. Consistent with previous studies of first-ever stroke [5
], we have identified high stroke rates in the southeastern USA. However, the reasons why these areas have a high stroke incidence and recurrence remains unclear. In addition, recurrent stroke rates were also high in areas of the Northwest, locations which have not previously been identified as areas of high stroke incidence. The causes of these geographic disparities remain largely unknown [25
]. High stroke incidence rates and the increased risk of recurrent stroke in the Southeast do not appear to be due to regional differences in population demographics (such as race/ethnicity) or risk factors [24
]. Future research is needed to determine if other potential factors such as genetic variation, environmental factors, healthcare services and infectious disease exposure might be associated with the regional patterns observed in stroke outcomes.
This study has some limitations. The selection of index events, outcomes and comorbidities are based on discharge diagnoses defined by ICD-9-CM codes. Ischemic stroke codes (ICD-9-CM: 433, 434 and 436) have been validated in numerous studies [30
] and have a high sensitivity for primary discharge codes [32
]. Acute myocardial infarction also has a similarly high sensitivity and specificity [34
]; however, the determination of other comorbidities such as smoking and obesity are less robust using administrative data [35
]. Although the present analysis included only recurrent ischemic events, prior studies have found that the majority of recurrences (65–80%) are ischemic and are of the same type as the incident event [18
]. The administrative data upon which these analyses are based do not include cause-specific death. In addition, we cannot account for the estimated 10% of strokes that did not result in a hospitalization [36
]. We were, therefore, unable to determine whether these rates vary by county. Finally, our results for this elderly population may not be generalizable to Medicare patients treated at a health maintenance organization; however, more than 80% of elderly patients in this country are covered by fee-for-service Medicare.
Recurrent ischemic stroke is a common event and a major cause of disability and death. We found significant regional variation in the risk of recurrent ischemic stroke across the USA. These patterns persisted after adjustment for patient demographics and comorbid conditions, with the highest rates observed in the southern and central USA. Future research is needed to understand factors that may contribute to these geographic patterns and target programs that may reduce the burden of ischemic stroke among the elderly.