What are the causes of SCA?
Coronary heart disease:
Coronary heart disease is the leading cause of overall death in adults in industrialized countries. It accounts for about 1/3 of all deaths in subjects over age 35 in the United Sates. As mentioned above, as much as 70% of the SCAs are secondary to blockages in the arteries of the heart (acute myocardial infarction) or to the ensuing damage that affects the heart muscle (ischemic cardiomyopathy). SCA is the initial manifestation in 15% of the patients with coronary heart disease (CHD).
Other forms of structural heart disease:
Structural heart disease not related to blockages in the coronary arteries supplying the heart is responsible for about 10% of SCA occurring outside hospitals. Such disorders can affect the heart muscle itself, the heart valves or the main artery connected to the heart (Aorta). Examples of such conditions include the following:
• Dilated Cardiomyopathy: a condition in which the heart muscle weakens and one or more of the heart chambers gets bigger. The heart becomes unable to pump blood as well as it should, leading to a condition called “heart failure”. SCA is responsible for about 1/3 of deaths in this condition.
• Hypertrophic cardiomyopathy (HCM): In this usually inherited condition, the walls of the heart muscle thicken. The thickened muscle can disrupt the heart’s electrical system, leading to fast or irregular heartbeats (arrhythmias), which can lead to sudden cardiac death. The annual incidence of SCA in this condition is about 1%. HCM is one of the leading causes of SCA in athletes and can often go undetected.
• Myocarditis: it is an inflammation of the heart often caused by a virus or autoimmune disease (body’s infection-fighting system attacks healthy tissue instead of infections). Different series of patients with unexpected sudden death showed an association with myocarditis varying from 6% to 20%.
• Arrhythmogenic right ventricular cardiomyopathy (ARVC): this entity is classically characterized by enlargement and structural anomalies affecting the right lower chamber of the heart (right ventricle) associated with abnormal heart rhythms originating from this cavity. In a review from France 10.4% of sudden cardiac deaths were associated with ARVC.
• Congenital coronary artery abnormalities: Sometimes people are born with heart arteries (coronary arteries) that are connected abnormally. The arteries can become compressed during exercise and not provide proper blood flow to the heart. Anomalous origin of a coronary artery was found in 12% to 33% of young athletes with sudden cardiac death.
Absence of structural heart disease:
About 10-12% of cases of SCA occurring in subjects under age 45 occur in the absence of apparent structural heart disease. The structure of the heart muscle and valves appears normal, however the electrical function of the heart can be impaired predisposing to SCA. This can occur in a variety of settings:
• Brugada Syndrom, idiopathic Ventricular Fibrillation, Congenital long QT syndrome, Cathecholaminergic Polymorphic Ventricular Tachycardia: those different syndromes are usually due to a genetic abnormality in the channels that handle the transport of electrical ions within the heart cells (congenital cardiac channelopathies). Abnormalities in electrical ion handling can lead to the sudden generation of abnormal heart rhythms (Ventricular Tachycardia or Ventricular Fibrillation) responsible for SCAs.
• Wolff-Parkinson-White Syndrome: patients affected by this entity are born with an abnormal additional electrical connection (Accessory Pathway) between the upper chambers of the heart (Atria) and the lower chambers (Ventricles). Rarely patients affected with this condition can suffer from abnormal rapid heart rhythms in the upper chambers of their heart generating abnormal rapid heart rhythms in the lower chambers of the heart (ventricular fibrillation) and leading to SCA.