Doctors have discovered another hereditary heart fault which could trigger a fatal heart attack in a young, apparently healthy, patient.
These rare conditions can cause heart attacks
But how common are such illnesses - and how can doctors treat them?
Many thousands of people in the UK suffer from heart arrhythmias - and the vast majority of these are harmless.
But doctors still want to check them all out - because a small number can be dangerous.
Even worse, some of the most deadly can kill without ever giving a hint as to their presence.
A condition called "Long QT Syndrome" is known to run in families.
It is caused by a problem in one particular sequence of the pattern of electrical impulses that drives the heart beat, and can trigger an irregular heartbeat which could lead to a heart attack.
All this can happen with little or no warning, often to very young adults during quite normal exertion, such as a game of football or tennis.
As understanding of Long QT has increased, the UK has set up special clinics designed to detect those at greatest risk and treat them either with drugs, or in extreme circumstances, an implantable defibrillator which will shock their hearts into action should they suffer cardiac arrest.
Short but deadly?
But a related condition called Short QT is only just coming to light, which, doctors believe, could be just as deadly.
As the name suggests, it also concerns a fault with a particular electrical heart rhythm.
It is also symptomless, and can be fatal.
Researchers in Germany and Italy found one family in which six members had suffered sudden cardiac death - and many surviving members showed signs of this fault.
In the US, many members of a single family from Illinois have been diagnosed with the condition.
Three members of the Hills family, from Collinsville, are undergoing surgery to implant defibrillators.
Mary Hill said: "I'm just glad this was discovered before anything more happened in our family - it's odd that we're one of the first families in the world to be diagnosed with this condition."
The specialist who spotted the problem, Dr Preben Bjerregaard, from Saint Louis University Hospital, said that doctors should be on the lookout for both forms.
"We've known for years that a long QT interval is a hereditary condition and is dangerous, but the short QT interval also appears to be hereditary and often exhibits no symptoms at all."
Professor John Camm, from St George's Hospital in south London, runs a clinic which specialises in detecting and treating potentially fatal heart problems such as these.
He says that one of the researchers who discovered "Short QT" tells him he believes that there may be more people with this form of dangerous arrhythmia than with the better-known "Long QT".
He said: "We have noticed this short QT interval in some of our patients in the past, but not really taken much notice of it.
"It is still early days in terms of finding out what it is and learning a way to manage the condition."
He said implanting defibrillators was a "blunderbuss" approach - he hoped that drug treatments might offer a solution.
"We don't like to use these devices unless it's absolutely necessary - for a start, it can be very debillitating to the patient knowing they have an implanted defibrillator.
"It can be set off by extreme exertion if the heart rate gets too high - it's like being given a powerful kick in the chest."
But he urged people with arrhythmias not to worry they might be at risk of sudden death.
"The vast majority of these conditions are completely harmless."