Sudden arrhythmic death syndrome (SADS) is the sudden and unexpected death of adolescents and adults caused by cardiac arrest. However, the exact cause of the cardiac arrest, and thus the exact cause of death, is unknown. These deaths occur mainly during sleep or at rest. One type of conduction defect known as Brugada syndrome can be responsible.

The syndrome is rare in most areas around the world but occurs in populations that are culturally and genetically distinct. It was first noted in 1977 among southeast Asian Hmong refugees in the United States and Canada. The syndrome was again noted in Singapore when a retrospective review of records showed that 230 otherwise healthy Thai foreign workers living in Singapore died suddenly of unexplained causes between 1982 and 1990.

Sudden death of a young person can be caused by heart disease (including cardiomyopathy, congenital heart disease, myocarditis, genetic connective tissue disorders) or conduction disease (WPW syndrome, etc.), medication-related causes or other causes. Rare diseases called ion channelopathies may play a role such as long QT syndrome (LQTS), Brugada syndrome (BrS), CPVT (catecholaminergic polymorphic ventricular tachycardia), progressive cardiac conduction defect (PCCD), early repolarization syndrome, mixed sodium channel disease, and short QT syndrome. In 20% of cases, no cause of death can be found, even after extensive examination.

Sudden arrhythmic death syndrome in alcohol abuse is a significant cause of death among heavy drinkers, characterized by older age and severe liver damage, highlighting the need for family screening for heritable channelopathies.

In young people with type 1 diabetes, unexplained deaths could be due to nighttime hypoglycemia triggering abnormal heart rhythms or cardiac autonomic neuropathy, damage to nerves that control the function of the heart.

Medical examiners have taken into account various factors, such as nutrition, toxicology, heart disease, metabolism, and genetics. Although there is no real known definite cause, extensive research showed victims aged 18 or older were found to have had a hypertrophic cardiomyopathy, a condition in which the heart muscle becomes oddly thickened without any obvious cause. This was the most commonly identified abnormality in sudden death of young adults. Where people have died suddenly, it is most commonly found that they had had CAD (coronary artery disease) or ASCAD (atherosclerotic coronary artery disease), or any level of stress. However, studies reveal that people were known to have had symptoms within the week before the terminal event such as chest pain at ~52% of patients, dyspnea at ~22%, and syncope at ~7%. About 19% are not known to have experienced symptoms. Scientists have also associated this syndrome with a mutation of gene SCN5A that affects the function of the heart. A 2011 autopsy-based study found that sudden death was attributed to a cardiac condition in 79.3% of cases, and was unexplained in 20.7%.

In the Philippines, sudden adult death syndrome (or in their term, bangungot) is mainly caused by the Brugada syndrome.

By definition, the diagnosis can only occur post-mortem after other causes are ruled out.