Undetected heart defects affect about one in 500 young people. Most live normal lives unaware that anything is wrong. But in elite sportsmen and women who push their bodies to the limit, a defect may make itself felt suddenly with devastating effect. That may explain Fabrice Muamba's collapse. But with all Premier League players undergoing regular cardiac testing, it does not explain why his problem was not picked up in screening.
If Muamba survives the next few days the worry will be the effect of the minutes he lay on the pitch and in the ambulance while his brain was deprived of oxygen. His chances are improved by the speed with which the paramedics began working on him. But the outcome will depend on how successful they were at keeping his blood circulating while his heart had stopped.
Professor Sanjay Sharma, a cardiologist at St George's Hospital, Tooting, south London, said: "There was a time lag in the ambulance on the way to hospital. If he pulls through, we must hope he does so with minimal brain damage."
The commonest heart defect is cardiomyopathy, an inherited condition that causes thickening of the muscle in the heart wall, compressing the left ventricle and leaving no space for the blood to be pumped.
But cardiomyopathy can be hard to distinguish from "athlete's heart", the enlargement of the heart that occurs as a result of training, which is natural and a sign of fitness. This is especially so in black athletes, who tend to have thickened heart muscles and abnormal ECGs. There is no cure for the condition but early treatment with drugs can prevent it.