Scientists believe they have pinpointed why some people are at risk of sudden adult death syndrome (SADS).
Scientists say a test could pick up heart problems
Researchers say they have discovered a heart abnormality that will help them predict who may be at risk from SADS.
The finding could lead to a test being developed to assess risk.
The condition has proved as difficult to understand as cot death.
It has been estimated that around one in 500 people in the UK suffer from hypertrophic cardiomyopathy, one type of SADS, which is the most common cause of death in young athletes.
This new research identifies a mechanism by which SADS occurs that can be detected in patients at risk
Dr Richard Saumarez, Papworth Hospital
Deaths can happen even when routine tests appear to show the heart working normally.
People with other conditions, such as dilated cardiomyopathy and Long QT syndrome, are also at risk.
In SADS, the heart goes into cardiac arrest. This happens when the heart is unable to effectively pump blood round the body.
It has been difficult to assess people's risk of SADS because there is little understanding of the mechanisms which cause cardiac arrest.
Researchers based at Papworth Hospital near Cambridge, have found it is possible to predict if an individual is at risk by measuring the amount of electrical 'disorganisation' in the heart.
They say this is closely linked to the lethal rhythms that cause sudden death.
If a test based on the discovery could be developed, at-risk patients could be fitted with a device called an internal cardio defibrillator (ICD) which helps prevent SADS by kick-starting the heart.
Dr Richard Saumarez, who led the research, said: "In the past the ways of telling whether someone was at risk from the onset of sudden death were very limited.
"This new research identifies a mechanism by which SADS occurs that can be detected in patients at risk.
"Equipment is under development that will identify people at risk from SADS and, if necessary, implant them with an ICD which will restore the heart back to its normal rhythm if it should go into cardiac arrest."
Professor Sir Charles George, medical director at the British Heart Foundation which funded the research, said: "This research is a significant step in the better understanding of SADS.
"People suffering from SADS may not have any symptoms from the condition and a fatal cardiac arrest may be the first sign that it is present in a family.
"In time this test could be used for such people and their relatives to determine just what their real risk is and help prevent further deaths in the family."