Hypertrophic obstructive cardiomyopathy, nicknamed hocum, is a disorder that makes the heart of young athletes operate like that of an 80-year-old

Fabrice Muamba is not the first to fall so dramatically unwell during a match – since the 19th century more than 80 footballers are known to have died suddenly while playing, all young men, all otherwise apparently fit and healthy. There is no secret why these sportsmen die or collapse: it is called hocum.

Hocum is the innocuous nickname for a deadly disorder: hypertrophic obstructive cardiomyopathy (HOCM). Young, fit athletes have hearts that are a little different from most people. Their well-exercised heart muscle has been reshaped to adapt to the stress of regular, vigorous training.

This remodelling of the heart is useful, as it helps to pump blood more effectively round the body. But in HOCM, a genetic abnormality causes the muscle wall of the heart to grow far thicker than it should. Repeated training makes the problem worse – the muscle wall can become so thick that it stops the normal flow of blood going in and out of the heart. This is where things get dangerous.

A heart that cannot pump blood properly causes problems that usually affect far older and far less healthy people. An athlete with HOCM might seem very fit but the heart is struggling like that of an unwell 80-year-old. Sudden strain, like a football match, demands an overwhelming effort. If he is lucky, shortness of breath or chest pain is a warning sign that something is wrong. But often the first signs of trouble are collapse or what doctors call sudden cardiac death.

The heart's ability to pump has been compromised so severely that it cannot beat in a co-ordinated way any more. If this happens in hospital, CPR and electric shocks from defibrillators might work to start the heart beating properly but it is still a very serious situation. If it happens on a football pitch, the outlook is more difficult but not impossible.

Sports doctors know the risks of HOCM. Only a small proportion – 0.2% — of the population are thought to be at risk of its effects. But the consequences of a missed diagnosis are tragic.

Across Europe there have been studies into sudden death in athletes and in Italy there is already a screening programme under way, where footballers are given simple heart tests to help detect the disease. In most people exercise is key to a long and healthy life.

In a very small minority it can kill. Pinpointing the people at risk is difficult but doctors are working to try to prevent this devastating condition from causing needless future deaths.

Dr Riddington works in the cardiology department at the Friarage hospital in Northallerton, Yorkshire