Sudden adult death syndrome, also known as SADS, occurs when a person under the age of 40 years old suffers a sudden death without a known cause after an autopsy and toxicology screen (drug test). SADS causes a cardiac arrest when the heart rapidly stomps pumping, and the body loses heart function, breathing, and consciousness.

SADS is often referred to as sudden arrhythmic death syndrome. It is also known as sudden death syndrome, though the events themselves are not always the same. 

Most of the time, SADS occurs when the heart develops an abnormal rhythm, otherwise known as an arrhythmia. This can usually cause the heart to beat at an irregular rhythm that is too fast or too slow. If the arrhythmia is not diagnosed and treated, it can lead to cardiac arrest. 

Most scientists think SADS is caused by a heart condition that interferes with the heart’s electrical system. SADS is not the same as a heart attack (also called a myocardial infarction) as it is not caused by a blockage but an arrhythmia.

Source: https://health-desk.org/articles/what-is-sudden-adult-death-syndrome