Jeremy Laurance: A hidden condition that snuffs out 12 young British lives every week

Twelve people aged under 35 die each week in Britain from undiagnosed heart conditions, now thought to be the most likely cause of Stephen Gately's death.

The finding of pulmonary oedema (accumulation of fluid in the lungs) at post mortem is not definitive but, in the absence of other factors, it points to a defective heart. When the heart fails to beat properly, pushing blood around the body, fluid leaks out of tiny blood vessels called pulmonary capillaries into the lungs, making it progressively harder for the person to breathe.

Officials at the court hearing ruled out earlier suggestions that Gately may have died from inhaling his own vomit after consuming excessive drink or drugs. The position in which his body was found – kneeling and slumped forward – also eliminates the choking theory, which could only have happened if he had fallen asleep on his back.

It is possible, but unlikely, that he suffered a heart attack. More likely is the theory that he had an undetected arrhythmia, a disorder of the heart's electrical activity which causes it to beat very fast until it goes into spasm and ceases to be capable of pumping blood around the body.

The charity Sudden Arrythmic Death Syndrome said the account of Gately's death fitted with the description of the condition, which a standard post mortem would not pick up because electrical activity in the heart cannot be tested after death. One in every 20 cases of sudden cardiac death shows no cause – about 500 a year in Britain.

Alternatively, he may have suffered from undetected cardiomyopathy, an inherited condition that causes thickening of the heart muscle leading to an enlarged heart, which finally loses the capacity to pump blood.

Cardiomyopathy often affects young people who are physically active and very fit. Although its symptoms include increasing fatigue, fit young people may fail to read the signs, concluding that they must be unfit and need to work harder.

A spokeswoman for the British Heart Foundation said: "About a million people are currently living with heart failure in the UK. Most are older people who have had a previous heart attack or have chronic high blood pressure. But there are about 12 deaths a week in people under 35 with undiagnosed heart conditions.

"An acute attack of pulmonary oedema can come on without warning, very suddenly, causing breathlessness. What has happened to Stephen Gately is very tragic."