SUDDEN CARDIAC DEATH (SCD) IN YOUNG

What We Must Know

Sudden cardiac death (SCD) is an unexpected death from cardiac cause occurring in a short time period (generally within one hour of symptom onset) in a person with known or unknown cardiac disease. There is a wrong belief among the public that all SCD is due to myocardial infarction (heart attack). That is not correct — a lot of other cardiac conditions can result in SCD. At present we don't have any large-scale studies regarding SCD either from India or Kerala, so we have to depend on studies from other countries.

This article is based on an important study conducted in Australia on SCD victims. The article is published in the most popular heart journal — Circulation, 2022, Vol. 13, Issue 13 — by the American Heart Association. This study was conducted on 2,006 SCD victims during a 16-year period, from 2000 to 2016. This is a very authenticated study because of the huge number of patients and the long duration of the study. In this study, 95% of victims underwent autopsy to find out the cause of death, and in 97% of victims a toxicological analysis was done to ascertain whether the cause of death was due to poisoning. The victims included were between the ages of 1 year and 35 years.

Causes of Sudden Cardiac Death

Now let us look at the reasons and causes of SCD.

1. Coronary Artery Disease (CAD/ASCVD)

This results from sudden obstruction of blood flow (coronary thrombosis) in the arteries supplying blood to the heart (heart attack), or in patients who had a myocardial infarction developing another acute coronary event (re-infarction), or a fatal arrhythmia like ventricular tachycardia or ventricular fibrillation. SCD also results from mechanical complications like cardiac rupture, or massive pulmonary or cerebral embolism.

2. Hypertrophic Cardiomyopathy (HCM)

This is an abnormality in the cardiac muscle, where the result is excessive thickening — hypertrophy — of the heart muscle, resulting in obstruction of blood flow from the left ventricle to the aorta (sub-aortic obstruction). Patients with this condition can develop SCD as a result of fatal cardiac arrhythmias. A strong genetic basis has been documented in this disease condition.

3. Cardiomyopathies

This is a disease primarily affecting the cardiac muscles. They are of two main types — dilated cardiomyopathy and restrictive cardiomyopathy. The cause of SCD in these conditions is, many a time, due to fatal cardiac arrhythmias.

4. Channelopathies

Ion channelopathies involve abnormalities of sodium and potassium channel permeability to these ions. In this condition the heart is structurally normal; the ECG may show changes sometimes. SCD in these patients results from fatal ventricular arrhythmias. There are different types of channelopathies, described below.

5. Early Repolarisation Abnormality

The ECG in this condition will be abnormal, sometimes mimicking myocardial infarction. There are four types of early repolarisation abnormalities. Here too the heart is structurally normal, but 10% of patients have underlying mitral valve prolapse (MVP).

6. Brugada Syndrome

This has a strong genetic association. Patients with this disease have an abnormal ECG and are classified into various types depending on the ECG changes. They also have a structurally normal heart. SCD occurs due to triggers such as loud sound, a phone ringing, diving into water, or fever.

7. Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy (ARVC)

Patients with this abnormality have ECG changes and an abnormal echocardiogram. Diagnosis is usually confirmed by cardiac MRI. They are prone to lethal ventricular arrhythmia and SCD.

8. QT Syndrome (Long and Short QT Syndromes)

These occur in a structurally normal heart, with high risk for SCD due to fatal arrhythmias. There is a strong genetic association, with autosomal dominant and recessive varieties, where there can be other congenital abnormalities like congenital deafness. Sometimes drugs can produce QT prolongation and lethal arrhythmias, called polymorphic ventricular tachycardia.

Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)

Typically, these patients develop SCD during exercise, where the fatal ventricular arrhythmias are precipitated by exercise resulting in an excess catecholamine surge. The heart is structurally normal.

Coronary Anomalies

This is an abnormality in the origin and course of the coronary arteries — a congenital cardiac anomaly where SCD occurs in certain types of congenital coronary anomaly, where the main coronary artery can be compressed between the aorta and pulmonary artery, especially during exercise (the so-called haemodynamic vise). It is diagnosed by coronary angiography or CCTA.

Incidence of SCD

From European studies, the incidence of SCD in the young (1–35 years) is estimated to be 1.3 to 2.9 per 10,000 population. However, in the present Australian study, it is much lower — reported as low as 0.91 to 1.48 per 100,000 population.

SCD incidence is found to be more in men than in women, and more in non-metropolitan than in metropolitan regions. This difference could be due to the availability of advanced medical facilities, services, and efficient ambulance service systems.

A very important observation in this study is that 34% of SCD victims were found to have obesity, and in 38% of cases SCD occurred during sleep or resting time. Only 7% of SCD occurred during exercise.

The commonest cause of SCD is CAD — about 40% of SCD results from acute coronary syndrome (ACS — acute myocardial infarction or heart attack) or CAD-related causes.

Sudden Arrhythmia Death Syndrome (SADS): After CAD, the commonest cause of SCD is SADS. 10% of SCD is caused by myocarditis, a disease primarily affecting the heart muscle, resulting in reduced myocardial contractility, heart failure, and arrhythmic death. 4% of SCD results from hypertrophic cardiomyopathy (HCM) — this is probably the commonest cause of SCD in young athletes during competitive sports. Acute aortic dissection and dilated cardiomyopathy together account for 5% of SCD.

Important information is available regarding age group and SCD. In the age group between 1 and 15 years, 61% of deaths are caused by myocarditis, and in the same age group 12% of SCD results from SADS. Between 6 and 15 years of age, myocarditis is the culprit in 28% of cases and SADS in 25%. Above the age of 26 and up to 35 years, the majority of SCD occurs due to coronary artery disease.

Interesting information is available regarding the type of activity the person was engaged in at the time of SCD. 20% of SCD developed during exercise, attributable to SADS, and 6% of patients who developed SCD during exercise had HCM. Among those engaged in day-to-day activities, 7% of deaths occurred due to acute aortic dissection.

Heart attack and heart-attack-related SCD were seen more in non-metropolitan areas. Among those who developed SCD during competitive sports, 45% had SADS, whereas 40% had myocardial disease.

Two important risk factors identified for SCD are obesity and smoking. Those with obesity were found to have dilated cardiomyopathy. In obese SCD victims, 12% had left ventricular hypertrophy (LVH), called idiopathic LVH. Obesity results in left ventricular hypertrophy, resulting in focal ventricular muscle disarray and fatty infiltration. These changes result in QT prolongation, which subsequently acts as a precursor for the development of lethal ventricular arrhythmias like ventricular tachycardia and ventricular fibrillation, and SCD.

To some extent, we can prevent SCD. The high-risk population needs evaluation to assess the risk of SCD. The high-risk group includes those with:

•      Family history of SCD, especially at a younger age

•      Family history of CAD at a younger age

•      Family history of HCM or other cardiomyopathies

•      Unexplained syncope

•      History of congenital heart disease in the family

•      Survivors of SCD

All individuals in these categories should undergo detailed cardiac evaluation to assess their SCD risk. Those engaged in competitive sports with these high-risk features should definitely undergo cardiac evaluation before participating in competitive sports. Smokers should quit smoking, and obese people should reduce their weight through regular exercise, diet, and other lifestyle modifications.

In recent times we have witnessed a lot of youngsters collapsing in the gym due to SCD. Those planning to go for exercise training at the gym should always have a cardiac evaluation, especially if they are smokers, obese, or belong to any of the high-risk groups already mentioned.

By taking adequate precautions and preventive measures, one can identify high-risk individuals, thereby preventing SCD to some extent, and will be able to know their heart health in a better way.