HONOLULU – Traditional risk factors and socioeconomic status account for less than half of the underlying causes of differences in the incidence of stroke between blacks and whites, results from a large ongoing study demonstrated.
"We’re halfway there in understanding why these disparities might exist," George Howard, Dr.PH., said at the annual meeting of the American Academy of Neurology.
Dr. George Howard
The magnitude of the racial disparities in stroke "are nothing short of striking," said Dr. Howard, professor and chair of biostatistics at the University of Alabama at Birmingham. "For African Americans below age 65, the black to white mortality ratio is two to three times greater than their white counterparts. With increasing age, this disparity diminishes so that at age 85 there are no racial disparities in stroke."
Data from the Greater Cincinnati/Northern Kentucky Stroke Study suggest that the excess stroke mortality among blacks in the United States is a product of higher incidence, not higher case fatality. "Most people jump to the conclusion that that’s because of a higher prevalence of hypertension and diabetes among blacks and a lower socioeconomic status among blacks," Dr. Howard said. "While these disparities have existed for more than 60 years, there are few data to assess whether these differences explain the disparity."
During 2003-2007, he and his colleagues enrolled 30,239 black and white adults aged 45 years and older in the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, a national, population-based, longitudinal study to provide insights into the excess stroke mortality among African Americans and Southerners. They conducted follow-up visits with the participants at 6-month intervals. To date, there have been 489 physician-adjudicated incident stroke events in the cohort over an average follow-up of 4.5 years.
At the meeting, Dr. Howard, the principal investigator, presented results on 25,727 study participants who were stroke free at baseline and for whom he and his associates had full data. The analysis included the estimated stroke risk among whites vs. blacks and an assessment of the proportion of risk attributable to traditional risk factors and socioeconomic status (SES).
The average age of whites and blacks was 65 years and 64 years. There were significant differences in the prevalence of many traditional risk factors between the groups. For example, 29% of blacks had diabetes compared with 15% of whites. Blacks also differed from whites in terms of having a higher mean systolic blood pressure (131 mm Hg vs. 125 mm Hg), and were more likely to be on antihypertensive medications (62% vs. 42%), to have left ventricular hypertrophy (15% vs. 6%), and to be a current smoker (17% vs. 12%).
Some SES factors also were significantly different between white and black individuals. A higher proportion of whites than blacks completed college (42% vs. 27%) and reported a household income of more than $75,000 per year (22% vs. 10%).
Dr. Howard reported that blacks were 2.84 times more likely than whites to have an incident stroke at age 45 and 1.67 times more likely than whites to have an incident stroke at age 65. "This disparity diminished by age 85," he said.
Adjustment for traditional risk factors attenuated these excesses by 38% at age 45 and 45% at age 65, resulting in black vs. white relative stroke risks of 2.14 and 1.37. "Approximately one-half of this mediation by risk factors is attributable to racial differences in systolic blood pressure," Dr. Howard said.
Further adjustment for SES factors attenuated these excesses by 46% at age 45 and 54% at age 65, resulting in black vs. white relative stroke risks of 2.00 and 1.31.
"We can explain almost half of the reasons for the disparity in stroke incidence between blacks and whites," Dr. Howard concluded. About half of this is attributable to racial differences in systolic blood pressure, he said, while much of the remaining half of the mediation is attributable to the use of antihypertensives, to having diabetes, and to SES.
"What can be done to address the hall-full portion that we understand?" Dr. Howard asked. "For most risk factors we are examining prevalence, not effectiveness of treatment. This implies that risk factor treatment is not the key, but rather risk factor prevention is."
This suggests, he continued, "that instead of focusing on racial disparities in stroke, perhaps we should be focusing on racial disparities in the development of stroke risk factors. The exception is controlled systolic blood pressure, where control is important. We think that differential susceptibility to blood pressure – particularly systolic blood pressure – could be a promising route to try to reduce these disparities."