And, according to a second study published today by the same team, the difference doesn't appear to be related to Mexican-Americans' higher incidence of diabetes, which had been thought to raise their risk of a certain type of stroke.
It's the first time that the stroke gap between the two groups has been demonstrated, and has far-reaching implications for educating Mexican-Americans about preventing strokes and acting quickly when one strikes. It also raises questions about factors that might be contributing to the difference in risk.
The two new studies, published in the Aug. 15 issue of the American Journal of Epidemiology and the Aug. 10 issue of Neurology, come from an in-depth population-based project funded by the National Institutes of Health and called BASIC, for Brain Attack Surveillance in Corpus Christi.
BASIC uses medical records and interviews to study strokes among people in Neuces County, Texas, home to a large population of second- and third-generation Mexican-Americans and many non-Hispanic whites. In all, 2,350 validated strokes and near-strokes occurring in the years 2000 through 2002 were studied.
Lead author Lewis Morgenstern, M.D., who directs the U-M Stroke Program and the BASIC study, calls the results a "wake-up call to the country" about the third-leading cause of death and leading cause of disability in the nation. Mexican-Americans make up two-thirds of Hispanics, the nation's largest minority group.
"This is a best-case scenario population of Mexican-Americans, who have been in this country for several generations and have few cultural and language barriers to getting medical care," he says. "If their risk of stroke is this much higher than that of their non-Hispanic white neighbors, especially in their younger years, this is the tip of the iceberg for the nation's broader Mexican-American community as it grows older."
Morgenstern, an associate professor of neurology at the U-M Medical School, started the BASIC study while he was at the University of Texas at Houston. Several authors on the two new papers are from UT, as well as one from the University of Pittsburgh.
The study shows that Mexican-Americans aged 45 to 59 were twice as likely as their non-Hispanic white counterparts to have any sort of stroke or near-stroke, and those in their 60s and early 70s were about 60 percent more likely to have a stroke or near-stroke, according to the study published in the American Journal of Epidemiology.
The researchers also broke stroke risk down by type: ischemic, in which a blood vessel in the brain is blocked; transient ischemic attack, which is a temporary blockage; and two kinds of hemorrhagic, or bleeding, strokes -- intracerebral and subarachnoid, which occur in different parts of the brain.
The rates of ischemic stroke, adjusted for age according to Nueces County 2000 Census figures, was twice as great for Mexican-Americans aged 45 to 59 as for non-Hispanics, and about 60 percent greater for those aged 60 to 74. There was little difference among those aged 75 and older.
Younger Mexican-Americans also had twice the risk of non-Hispanic whites of suffering a TIA, a warning sign for a future full-blown stroke. The odds were about even for those in their 60s and 70s.
For bleeding strokes, there were differences between the two groups across the board, adjusted for age. In particular, Mexican-Americans in their late 40s through their 50s were three times as likely as non-Hispanic whites in the same age group to suffer an intracerebral hemorrhage, and Mexican-Americans aged 60 to 74 years were more than twice as likely as their white counterparts to have a subarachnoid hemorrhage.
Why all the differences between the two groups? Morgenstern notes that Hispanics are known to have a high rate of diabetes, which increases stroke risk, but they have similar rates of two other major risk factors -- high blood pressure and high cholesterol -- as non-Hispanic whites.
The diabetes difference has prompted many stroke specialists to speculate that Hispanics might have a higher risk of ischemic stroke involving blockages in the smaller blood vessels, which can be brought on by the damage caused by high blood sugar.
But in the new paper in Neurology, Morgenstern and his colleagues report the results of an in-depth analysis of 402 validated ischemic strokes from the BASIC study. They broke the stokes down by subtype, including those caused by blood clots originating in the heart, and those caused by the severe narrowing of large and small blood vessels in the brain. There was no statistically significant difference in any stroke subtype between Mexican-Americans and non-Hispanic whites.
"Obviously, there are other factors contributing to the stroke risk faced by Mexican-Americans that still need to be determined," says Morgenstern. "We think we know all the risk factors for stroke but I don't think we do. And part of the reason it's important to study racial and ethnic differences in disease is that opens our eyes to new risk factors we might not have seen before."
For instance, the authors note in the AJE paper, a genetic predisposition to blood vessel malformations that tends to cluster in Mexican-American families might play a role. But Morgenstern cautions that far more study will be needed to determine what's behind the difference in stroke risk.
BASIC is also looking at factors that might affect Mexican-Americans' behavior regarding stroke prevention, access to preventive care and seeking treatment for acute stroke. A new study within BASIC will look at the roles that spirituality, religiosity, fatalism and depression may play.
Meanwhile, BASIC researchers are studying the effectiveness of different approaches to raising stroke awareness, including a program that teaches hundreds of middle school students about stroke risks and the importance of a rapid response. And, they're continuing to follow current study participants and add new ones, building longer-term data about survival and outcomes.
In the end, Morgenstern says, he hopes the new evidence of a higher stroke risk for Mexican-Americans should prompt more than just the translation of stroke-education materials into Spanish. "Stroke is arguably the most preventable and most treatable of all catastrophic conditions, and while providing bilingual information is a start, we need to help people recognize their higher risk and take control of their destiny, especially as the Hispanic population grows and ages."
In addition to Morgenstern, the authors of the AJE study are Melinda Smith, Lynda Lisabeth, Nelda Garcia and Devin Brown of the U-M Stroke Program; Jan Risser, Olubumi Akuwumi and Lemuel Moye of the UT School of Public Health; Ken Uchino, Fahmi Al-Senani and Areej Al-Wabil of the Stroke Program at the UT Medical School; and Corpus Christi neurologists David McFarling and Paxton Longwell.
Uchino, now at the University of Pittsburgh Medical Center, is lead author of the Neurology paper, on which Morgenstern is senior author and Risser, Smith and Moye are co-authors.
References:
American Journal of Epidemiology, 2004, Vol 160, No. 4, 376–383
Neurology, August 15, 2004, Vol. 63
Journal
American Journal of Epidemiology