A large Swedish population study has found strong links between psychiatric conditions that can follow extremely stressful experiences and the risk of several types of cardiovascular disease.

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The link between acute stress and cardiovascular symptoms may be bidirectional, suggests new research.

In addition, the researchers found that the risk of a heart attack and other sudden and severe cardiovascular events is especially high in the 6 months that follow the diagnosis of the stress-related condition.

For other types of cardiovascular disease — such as heart failure, a disease that develops slowly — the risk appears to be highest during the 12 months that follow the psychiatric diagnosis.

For embolism and thrombosis, which are major conditions that develop from blood clots, the risk is likely higher 1 year or more after a diagnosis of stress-induced illness.

In a paper in The BMJ about the study, the authors state that the findings apply “equally to men and women” and do not depend on medical history, family background, or having other psychiatric illnesses.

They also note that the results support those of previous studies on relations between stress-induced conditions and cardiovascular disease.

However, most previous findings have come from research that drew largely on male war veterans or men on active military service, and they also focused almost entirely on PTSD, with symptom data from self-reports.

Anyone who has witnessed or experienced a traumatic event, such as combat, rape, violent assault, or natural disaster can develop PTSD, which affects around 3.5 percent of adults in the United States.

However, witnessing or experiencing a traumatic event does not necessarily lead to PTSD.

When diagnosing PTSD, doctors look for symptoms such as startled reaction to loud noise, flashbacks, and nightmares, together with feelings of detachment, more-than-usual anger, sadness, and irritability that remain intense and do not wane with time.

In some people, the symptoms of PTSD can last for years.

Acute stress disorder is a similar condition to PTSD; it can occur in response to traumatic events and has some of the same symptoms, but it tends to arise within 3–30 days after the traumatic event.

In the U.S., estimates suggest that 13–21 percent of car accident survivors and up to half of those who survive rape, assault, or mass shootings will develop acute stress disorder. Around half of the people with acute stress disorder go on to develop PTSD.

The new investigation used 1987–2013 data from the Swedish National Patient Register on 136,637 patients “with stress-related disorders, including [PTSD], acute stress reaction, adjustment disorder, and other stress reactions.”

The researchers ran comparisons between this “exposed” cohort and two other “unexposed” cohorts, one comprising 171,314 full siblings and the other comprising 1,366,370 matched individuals from the general population. By unexposed, the researchers mean free of stress-related conditions.

The team first calculated the average rate of cardiovascular disease among the three cohorts over the period of the study. This came to 10.5 per 1,000 person-years for the exposed group and 8.4 and 6.9 for the unexposed sibling and matched general population cohorts, respectively.

Further analysis revealed links between psychiatric conditions resulting from trauma or highly stressful life events and a raised risk of developing a number of cardiovascular diseases. These links were particularly marked during the 12 months following the psychiatric diagnosis.

Specifically, those with a stress-related illness were 64 percent more likely to develop a form of cardiovascular disease in the 12 months after a psychiatric diagnosis than their unexposed siblings. Comparisons with unexposed, matched members of the general population yielded a similar result.

The researchers also found a particularly strong link between stress-induced psychiatric conditions and cardiovascular diseases that tend to develop before the age of 50.

The authors point out that, due to the nature of their study design, they cannot conclude that stress-related disorders actually cause cardiovascular diseases.

Simon Bacon, a professor at Concordia University, in Montreal, Canada, takes up this point in a linked editorial.

He raises the possibility of cause being in the other direction. For example, could it be the case that people who already have some degree of cardiovascular disease are more susceptible to developing stress-induced psychiatric conditions?

To counter this, “as evidence” of cause running in the other direction, he points out that the researchers “quite rightly cite” the raised risk that they found of heart attack, stroke, and other acute cardiovascular events 1 year following the psychiatric diagnosis.

However, he also highlights the large effect that the researchers found in the link to a raised risk of heart failure and how this occurred “less than 1 year after diagnosis.” Because heart failure is a disease that progresses slowly, “reverse causation cannot be ruled out entirely.”

Prof. Bacon argues that these questions call for further investigation into the “potential bidirectional nature” of these links.

The researchers suggest that there is a need for doctors to know that cardiovascular disease could be more likely to develop following highly stressful events that lead to diagnoses of stress-related conditions, especially during the first year.

The authors conclude:

“These findings call for enhanced clinical awareness and, if verified, monitoring or early intervention among patients with recently diagnosed stress-related disorders.”