BOSTON - Extreme stress can trigger what seems to be a heart attack, but is really something known as "broken-heart" syndrome, researchers say, cautioning doctors to know the difference.

Unlike conventional heart attacks stemming from blocked arteries, broken-heart syndrome, or stress cardiomyopathy, is caused by an extended surge in adrenalin, such as after a car crash or even learning of the death of a loved one.

It may only require short-term treatment because the heart usually recovers by itself.

In a study published in this week's New England Journal of Medicine, researchers led by Ilan Wittstein of Johns Hopkins University School of Medicine in Baltimore found some people may respond to sudden, overwhelming emotional stress by releasing large amounts of adrenalin and other chemicals into the bloodstream.

These chemicals can be temporarily toxic to the heart, effectively stunning the muscle and producing symptoms similar to those of a heart attack: chest pain, fluid in the lungs, shortness of breath and heart failure.

But closer inspection using blood tests and MRI (magnetic resonance imaging) scans failed to show the typical heart-attack signs, such as irreversible muscle damage and elevated levels of certain enzymes.

Wittstein said being able to recognise broken-heart syndrome in patients had helped his team to avoid "incredibly invasive procedures".

He cited the case of a 27-year-old who developed severe shock and low blood pressure after hearing tragic news. She appeared to be near death and surgeons were prepared to put a mechanical pump in her chest.

But the operation was delayed when doctors recognised broken-heart syndrome.

The woman regained normal heart function without needing an operation.