Joyful events - the birth of a child, a big win by your team - can trigger a dangerous condition called the "broken heart syndrome", doctors and researchers reported on Thursday.

Takotsubo syndrome, as it is also known, involves the sudden weakening of heart muscles, causing the left ventricle - the chamber which pushes oxygen-rich blood through the body - to balloon out abnormally at the bottom.

Besides acute chest pain and shortness of breath, the condition can lead to heart attacks and death.

It has long been known that an unexpected emotional shock - typically something unpleasant, such as the death of a spouse, or a violent argument - can provoke an attack.

But statistics were lacking, and no one had ever investigated whether an intensely happy event could give the same result.

In 2011, a pair of researchers in Switzerland - Christian Templin and Jelena Ghadri, both of University Hospital Zurich - set up a global registry to track cases of the syndrome, which is fairly rare.

Five years later, the network of 25 hospitals spread across nine countries had collected data on statistically significant 1,750 cases of the Takotsubo syndrome (TTS).

For the study, Templin and Ghadri, leading a team of 16 researchers, determined that emotional jolts were responsible for 485 of those cases.

And within that group, 4% - a total of 20 individuals - could be said to have suffered from "happy heart syndrome."

Birth of a grandchild  

"We have shown that the triggers for TTS can be more varied than previously thought," said Ghadri. "The disease can be preceded by positive emotions too."

The 20 cases set off by joyful events included a birthday party, a wedding, a surprise farewell celebration, a favourite rugby team winning a game, and the birth of a grandchild.

None of them proved fatal.

Emergency room doctors should be aware of the fact that patients with signs of heart attack could be suffering from TTS, sparked by either positive or negative experiences.

For reasons the researchers do not understand, 95% of the patients in both the "broken heart" and "happy heart" groups were women, mostly in their mid-to-late 60s.

"We still do not know why women are predominately affected by the Takotsubo syndrome," Ghadri told AFP.

"We can only speculate that the hormonal state - namely, oestrogen - might play a role in the disease mechanism."

Women have much higher levels of oestrogen than men, in whom testosterone is the dominant hormone.

The skew towards women is even more perplexing, she said, because heart attacks are more common among men.

Further studies are also needed to figure out if both happy and sad life events, while obviously different, ultimately share the same pathway in the central nervous system for triggering the syndrome.  

The condition - discovered by Japanese researchers - gets its name from a traditional Japanese octopus trap, which is said to resemble the distended heart chamber.