When young people die without physical exertion and without apparent reasons, the cause is usually the same thing that leads to older people dying suddenly - heart trouble. 

For those who are older, this tends to be coronary heart disease, said Dr Alvin Ng, a specialist in cardiology from Raffles Hospital.

That is when plaque blocks the coronary arteries and causes death by cutting off oxygen to the heart.

Heart rhythm disorders are the main reason that young people can die suddenly while asleep or at rest, said senior consultant cardiologist Ching Chi Keong from the National Heart Centre Singapore. The reason for this is not clear.

These disorders include Brugada syndrome or certain types of long QT syndrome. The QT is the interval between the start of the Q wave and the end of the T wave in the heart's electrical cycle, during which the heart contracts and beats, then becomes "charged" up to repeat this.

Such genetic problems exist since birth but for unknown reasons, manifest only when people reach their 20s and 30s.

Questions about sudden death in the young arose when a 24-year-old journalist was found dead next to the toilet bowl in the bathroom of her parents' house last month. She was believed to have died from a fall. It is not known what had triggered the fall.

Sudden deaths in the young are rare, about one in 50,000 to 250,000 compared to one in 15,000 to 18,000 for older adults.

The incidence for Brugada syndrome is about five in 10,000 (about 2,500 people in Singapore) and one in 15,000 for long QT syndrome (about 300 people here).

With such defects, the heart can go into a dangerously chaotic rhythm called ventricular fibrillation. It can then freeze up, causing death in one out of 10 cases.

A few lucky patients may experience warning signs in the form of unexplained fainting or near-fainting spells, said Dr Ching.

They should get themselves screened, especially if they have a parent, sibling or child with a heart rhythm disorder or who died suddenly for unknown reasons, he said.

Sudden deaths in some patients with long QT syndrome can also happen when they are startled by loud noises or devastated by a piece of bad news, said Dr Ching.

There is no local data available, but an oft-cited 1989 study on sudden deaths in young Israeli soldiers found that as many as seven in 10 were not related to exercise.

When picked up, heart rhythm disorders are "immensely treatable", said Dr Ching.

A cardioverter defibrillator can be implanted into the chest to monitor the heartbeat continuously and shock abnormal heart rhythms into place. Long QT syndrome can also be treated with medications called beta blockers to stabilise the heart rhythm and ward off instability. 

Aside from the heart, the other common cause of sudden death in young people can be traced to the brain, in particular subarachnoid haemorrhage or bleeding into the surface of the brain, said Associate Professor Lee Kim En, head of the department of neurology at the National Neuroscience Institute. 

In older people, such a haemorrhage is usually caused by trauma. Said Prof Lee: "This could be a major blow to the head caused by a fall or a road accident which ruptures blood vessels in the brain."

However, if subarachnoid haemorrhage happens to a healthy young adult, it is likely to have been caused by a spontaneous bursting of a brain aneurysm - a weak spot in a blood vessel - or an arteriovenous malformation (AVM), an abnormal tangle of thin-walled blood vessels. The condition is more common in women for unknown reasons.

People often do not know they have a ticking time bomb inside them until it bursts and the resulting build-up of pressure in the brain causes them to experience a sudden severe "thunderclap" headache.

American actress Sharon Stone managed to escape death in 2001 when she had a subarachnoid haemorrhage, but the condition is immediately fatal in one out of three people.

Nobody knows what causes an aneurysm or AVM to rupture.

A recent study published in Stroke, the journal of the American Heart Association, suggested that this can happen when there is a sudden surge in blood pressure during daily activities such as coffee-drinking, nose-blowing, having sex or straining when defecating.

But the findings are not conclusive and doctors remain skeptical.

Dr Lee said: "The brain has an auto-regulating system which can adjust to sudden surges in blood pressure. It is unlikely that stress from such daily activities alone can lead to the rupture of a brain aneurysm."

Another way death could occur in the young is through deep vein thrombosis. Those who spend long hours playing computer games without taking a break could be in danger of dying from a pulmonary embolism if blood clots form and travel from the deep veins in the legs up to the major arteries of the lungs. 

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It is not often that athletes die suddenly while exercising - less than one out of 100,000 or about 20 to 40 Singaporeans a year.

In older people, sudden death can be triggered by exercise if they have an undiagnosed coronary heart disease.

In younger people, it is usually because of hidden congenital problems involving the muscle or electrical system of the heart or the coronary arteries supplying it.

These conditions often do not surface till they are triggered by vigorous exercise in young adulthood, said Dr Ching Chi Keong, the co-director of electrophysiology and pacing at National Heart Centre Singapore. In most cases, death occurs when the heart goes into a chaotic rhythm called ventricular fibrillation. People at risk of sudden deaths from congenital heart conditions can be fitted with a device to correct the abnormal heart rhythms and reduce their risk by avoiding strenuous exercise. Thses congenital heart conditions include:

This is the most common cause of heart-related sudden death in young adults, especially athletes. The heart muscles grow thicker over time, eventually making it difficult for the heart to pump blood. 

This is a rare condition in which fat is deposited in the right ventricle, or the heart's lower right chamber, weakening its ability to pump blood well. The abnormal tissue may cause electrical instability, leading to ventricular fibrillation.

One or more of the coronary arteries which supply blood to the heart originate from an abnormal spot and may go between the major blood vessels - the aorta and the pulmonary arteries.

During physical exertion, the aorta and pulmonary artery expand, squeezing the smaller artery in between. This reduces blood supply to the heart and may cause ventricular fibrillation. A bypass may be done to move the problematic artery to its proper place.

Twelve types of the heart rhythm disorders have been identified so far. Most are triggered by exercise, through a few can happen at rest. The disorder can also cause sudden drowning, when physical stress triggers an episode of ventricular fibrillation.