The sudden death of a previously healthy young individual is a rare but tragic event. Every year about one in 100,000 people aged between one and 35 dies suddenly of a natural cause.
The vast majority of sudden death cases in the young are caused by diseases of the heart. In about 70% of cases, an autopsy reveals a structural abnormality of the heart which led to a fatal cardiac arrhythmia.
In young adults and in individuals above the age of 35 the most important cause of sudden cardiac death is a heart attack (myocardial infarction) caused by a blood clot that obstructs the blood and oxygen flowing to a part of the heart. The damage this causes can induce potentially fatal cardiac arrhythmias.
In young adults, heart attacks may be due to inherited abnormalities that lead to high cholesterol levels. And in this group, and also in children, diseases of the heart muscle in which, for example, parts of the heart enlarge (hypertrophic cardiomyopathy), dilate (dilated cardiomyopathy), or in which the muscle tissue is replaced by scar tissue and fat instead of muscle tissue (arrhythmogenic cardiomyopathy), are important causes of sudden cardiac death. Most of these are inherited conditions, so relatives of the dead person may carry the same condition.
Sudden death can also be caused by inflammation of the heart, a condition called myocarditis, a congenital malformation of a coronary artery, or abnormalities of one of the heart valves.
About 15% of sudden death cases in young individuals are caused by conditions outside the heart, in particular bleeding inside the skull (intracranial hemorrhage) and sudden blockage of a lung artery by a blood clot, known as a pulmonary embolism.
Mysterious minority
The final 15% of cases, or thereabouts (there’s a large variation within studies), the autopsy doesn’t reveal the cause of death. These cases are termed sudden arrhythmic death syndrome and are thought to be due to so-called primary inherited arrhythmia syndromes.
These conditions are caused by inherited abnormalities of the ion channels of the heart, which under certain conditions give rise to potentially fatal cardiac arrhythmias. For example, congenital long QT syndrome, the most common primary inherited arrhythmia syndrome, is characterised by a prolonged conduction interval (the QT interval) on a person’s electrocardiogram (ECG).
Under certain conditions, for example during exercise or when suddenly hearing a loud noise, fast, chaotic heartbeats can develop. These rapid heartbeats may trigger a sudden fainting spell or seizure. In some cases, the heart may beat erratically for so long that it can cause sudden death.
These primary inherited arrhythmia syndromes can’t be diagnosed during an autopsy, because they don’t lead to visible abnormalities of the heart. Instead, they can be diagnosed from looking at a previous electrocardiogram of the deceased or by performing genetic testing using collected tissue.
Close relatives of the dead person can carry the same condition, so screening of close relatives in cases where a young person dies from sudden arrhythmic death syndrome victim is strongly recommended.
Preventing sudden death
Sudden death can be prevented in patients with one of the primary inherited arrhythmia syndromes – inherited cardiomyopathies – or other inherited conditions of the heart. This can be done by modifying lifestyle, such as exercise or drug taking, medication or, in some cases, by implanting a pacemaker or defibrillator, which continuously monitors a patient’s heartbeat and delivers extra beats or electrical shocks to restore a normal heart rhythm when necessary.
Some sudden cardiac death experts have advocated screening programs to detect conditions of the heart that may lead to sudden cardiac death, for example in competitive athletes. However, because these conditions are relatively rare and sometimes difficult to diagnose with simple diagnostic tests, this approach has not been universally accepted.
Another important approach may be to thoroughly study the cause of death of young sudden cardiac death victims and screen the victims’ relatives for inherited cardiac conditions to prevent a potential second fatal event within these families. Unfortunately, however, autopsy and family screening aren’t mandatory and so they aren’t frequently performed in most countries.