What can cause sudden cardiac death in young people?
Hypertrophic cardiomyopathy (HCM). This is a disease in which the heart muscle (myocardium) becomes abnormally thick, making it harder for the heart to pump blood.
Hypertrophic cardiomyopathy, while usually not fatal in most people, is the most common cause of heart-related sudden death in people under 30. It's the most common cause of sudden death in athletes. HCM often goes undetected.
Coronary artery abnormalities. Sometimes people are born with heart arteries (coronary arteries) that are connected abnormally. The arteries can become compressed during exercise and may not provide proper blood flow to the heart.
Long QT syndrome. Long QT syndrome is an inherited heart rhythm disorder that can cause fast, chaotic heartbeats. The rapid heartbeats, caused by changes in the part of your heart that causes it to beat, may lead to fainting. These irregular heartbeats can be life-threatening.
In some cases, your heart's rhythm may be so erratic that it can cause sudden death. Young people with long QT syndrome have an increased risk of sudden death. 
- Mayo Clinic website
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