Hundreds die in drowning accidents in the UK each year

In the US, drowning is second only to motor vehicle accidents as a leading cause of accidental death. Some 4,000 people drown each year, and no satisfactory explanation can be found for 10% of these cases.

Dr Ackerman examined the case of a 19-year-old woman, a strong swimmer, who died after nearly drowning in four feet of water.

Molecular testing identified the genetic mutation, and testing found that her mother, and a sister had inherited the syndrome.

Dr Ackerman said: "Our study shows that molecular testing can play a vital role in the way we treat and identify individuals who may be at risk from the condition.

"We will be able to diagnose family members who may be at risk even though they've never had symptoms - if we identify the problem, we can intervene and hopefully prevent such tragedies."

The British Heart Foundation says that the most important symptom is fainting or losing consciousness during intense emotional excitement or during physical exertion.

A routine ECG may be able to diagnose, and genetic testing is available.

The BHF estimates that 1 in 10,000 people may have some form of the disease.

Treatment, once diagnosed, is by means of beta blocker drugs. Those at the greatest risk may require the implantation of a pacemaker or a tiny defibrillator, which can shock the heart into restarting should it fall into the wrong rhythm.