Silent MIs accounted for more than 45 percent of MIs in a group of nearly 10,000 participants from a community-based cohort study who did not have cardiovascular disease at baseline.

The incidence rate of silent MIs was significantly higher in men than in women, and participants with silent MIs had an increased risk of coronary heart disease death and all-cause mortality compared with those without MIs.

The researchers defined a silent MI as “the presence of pathological Q waves in the absence of a history of typical cardiac symptoms” and said it can often be detected through electrocardiogram screening.

Lead researcher Zhu-Ming Zhang, MD, MPH, of the Wake Forest School of Medicine in Winston-Salem, N.C., and colleagues published their results in Circulation on May 16.

“The outcome of a silent heart attack is as bad as a heart attack that is recognized while it is happening,” senior author Elsayed Z. Soliman, MD, MSc, MS, said in a news release. “And because patients don’t know they have had a silent heart attack, they may not receive the treatment they need to prevent another one.”

The researchers analyzed 9,498 participants from the ARIC (Atherosclerosis Risk in Communities) study, which enrolled participants from 1987 to 1989 in four U.S. communities: Washington County, Md.; the suburbs of Minneapolis, Minn.; Jackson, Miss.; and Forsyth County, N.C.

After enrolling, participants had a telephone interview and clinic visit. They then had four additional examinations between 1990 and 1992, between 1993 and 1995, between 1996 and 1998 and between 2011 and 2013.

At baseline, the mean age was 54 years old, while 56.9 percent of participants were women and 20.3 percent were African American.

During a median follow-up period of 8.9 years, 3.3 percent of participants developed a silent MI and 4.1 percent had a clinically documented MI. During a median follow-up period of 13.2 years, there were 1,833 cases of all-cause mortality, including 189 coronary heart disease deaths.

The incidence rate of silent MI was 5.08 per 1,000-person years in men and 2.93 per 1,000-person years in women, while the incidence rate of clinically documented MI was 7.96 per 1,000-person years in men and 2.25 per 1,000-person years in women. Both differences were statistically significant.

Meanwhile, the incidence rate of silent MI was 4.45 per 1,000-person years in blacks and 3.69 per 1,000-person years in whites, which did not represent a significant difference. The incidence rate of clinically documented MI was 3.24 per 1,000-person years in blacks and 5.04 per 1,000-person years in whites, which was a significant difference.

The researchers also found that silent MIs and clinically documented MIs were associated with an increased risk of coronary heart disease death and all-cause mortality in both men and women. There were no significant differences in mortality rates between races.

“Women with a silent heart attack appear to fare worse than men,” Soliman said. “Our study also suggests that blacks may fare worse than whites, but the number of blacks may have been too small to say that with certainty.”

The researchers cited a few limitations of the study, including that it only enrolled white and black patients, so the results may not be generalizable to other races and ethnicities. They added that the incidence of silent MIs may have been underestimated because Q-waves often disappear after MIs and the use of troponin in the past decade helps detect more clinically documented MIs.