Vikas Pandey, MD
MorgensternL, Brown D, Smith M, Sanchez, B, Zahuranec D, Garcia N, et al. Loss of theMexican American Survival Advantage After Ischemic Stroke. Stroke. 2014.
The idea of the Hispanic paradox (not to be confused with the Obesity Paradox, Obstructive Sleep Apnea paradox, Statin Paradox, or stroke survivor Kevin Sorbo’s 2010 movie “Paradox”) is the epidemiological mystery that while Hispanic Americans generally have lower socioeconomic status and education level, higher vascular risk factors and less access to healthcare, their mortality risk due to medical illnesses has been found to be equivalent to, or better than (in the case of ischemic stroke) the non-Hispanic white (NHW) population. While there has been much controversy to the reasons for this difference and as to whether a difference even exists, ascertainment of any differences present has value given the potential for providing population-specific stroke prevention and treatment measures.
The published study delved into the difference between Hispanic American (specifically Mexican Americans (MA), 63% of the Hispanic population in the U.S.) and NHW population stroke risk and how the difference has changed over time, specifically from 2000 to 2011. The group in 2006 had previously reported a 42% lower 28-day ischemic stroke case fatality rate and 21% lower all-cause mortality rate following ischemic stroke in MAs compared to NHWs. For this study, they used a population that was part of the Brain Attack Surveillance in Corpus Christi (BASIC) project that takes place in Nueces County, Texas with the majority of residents living in the city of Corpus Christi. The population mainly consisted of stable, non-immigrant persons with very few undocumented residents, making reasons to return to Mexico after stroke unlikely. The group used two-sample statistics to calculate differences in both 30-day mortality and 1 year mortality after an ischemic stroke event. The publishing group adjusted for sex, differences in age quartile distribution, diabetes, coronary artery disease, high cholesterol, history of stroke/TIA, atrial fibrillation, hypertension, smoking status, insurance status, and stroke severity based on NIHSS and found MAs had younger age, more diabetes and hypertension while NHWs had higher prevalence of atrial fibrillation and smoking and were more likely to be insured. The group recorded 4,413 ischemic stroke cases of which 44.7% were in NHW and 55.2% in MA. Across the 11-year time period, the 30 day mortality in NHW decreased from 7.6% to 5.6% (p=0.24) and 1 year mortality from 20.8% to 15.5% (p=0.02). MA mortality rate across same time period stayed the same (5.1% to 5.2%) and slight decline in 1 year mortality rate (17.4% to 15.3%, p=0.26). The ethnic differences between the groups were clear in 2000 (30 day: p=0.01, 1 year: p= 0.06), however by 2011 the differences had diminished (30 day: p=0.63, 1 year: p=0.92).
The authors were able to demonstrate that post-stroke survival advantage of MAs compared to NHWs had diminished over the time frame, but what reasons for this change could there be? The ischemic stroke mortality for all race/ethnic groups has been declining however it appears from the data that this decline is not as pronounced in MAs and other ethnic groups have “caught up”. The other possibility is that there was never any difference to begin with and improvement in record collecting may account for this return to equivalency. The classic explanations for why the Hispanic paradox exists include, for one, that Hispanics tend to have more strokes related to small vessel disease which can be partially explained given the finding in the current group’s data of increased incidence of hypertension and diabetes in MAs. These strokes tend to be smaller and less likely to cause mortality whereas the NHW population with the statistically higher chance of atrial fibrillation may lead to more devastating ICA and MCA occlusions. Proponents of a genetic difference between MA and Mexicans in Mexico would be quick to point out that those Mexicans that are in the U.S. may be more innately capable of withstanding disability from ischemic strokes given that they were able to immigrate into the country and find work to be able to sustain a living. There is also the concept of “Salmon bias” named after the fish which after living out in the sea, return to the river where they were born to spawn and die, applying in this scenario because of the subset of the MA population which may return to Mexico after suffering an ischemic event thus becoming statistically immortal. The authors did an excellent job of making sure that none of these factors were biasing their data by recording no difference in NIHSS severity, and picking a stable population with very little immigration or emigration. They have shown that there may be in fact no difference between the ethnic groups and, on the contrary, the MA population’s mortality rate due to ischemic stroke is not decreasing as much as it should making primary and secondary stroke prevention measures in this subset a point of special emphasis.
Posted by Vikas Pandey (@DrVikasNeuro)