Incidence of sudden cardiac death

According to the 36th Bethesda Conference, competitive athletes (in contrast to recreational athletes) are individuals who “participate in an organized team or individual sport that requires regular competition against others as a central component, places a high premium on excellence and achievement, and requires some form of systematic (and usually intense) training.”1 Young athletes are generally considered athletes less than 35 years old. The incidence of SCD in this group is low. The annual incidence of athletic field deaths was 0.5 in 100 000 amongst high school athletes in Minnesota2 3 and 2.3 in 100 000 amongst athletes in Northern Italy.4 This is roughly 1/100 the risk of SCD in the older population. However, athletes may be at increased risk for sudden death. In the Veneto Region of Italy, the incidence of SCD among athletes compared with age-matched non-athletes 12 to 35 years old was 2.8-fold higher.4

Men account for the majority of SCD in young athletes, with a male-to-female ratio of 10:1.4 It is unclear whether this gender difference is due solely to the higher numbers of men participating in competitive athletics or to differences in the incidence of cardiac disorders predisposing to SCD, such as coronary artery disease and hypertrophic cardiomyopathy (HCM).5 Racial differences in the incidence of SCD are also present. Maron et al reported an incidence among white subjects of 0.10%, similar to the reported incidence in Northern Italy of 0.07%.6 However, in African–Americans the incidence nearly doubles to 0.24%.6

Aetiologies of SCD in young athletes

Cardiac causes of SCD amongst young athletes can be divided into structural heart disease (i.e., HCM, arrhythmogenic right ventricular cardiomyopathy (ARVC)), primary electrical abnormalities (i.e., long QT syndrome), and external causes (i.e., commotio cordis). In the United States, HCM has consistently been the most common single cause (fig 1). Autopsy data from the Minneapolis Heart Institute Foundation registry found that 26.4% of SCD in 387 young athletes was due to HCM.7 Commotio cordis was the next most common cause (19.9%), followed by coronary artery anomalies (13.7%). All others disorders were responsible for 5% or less of SCD. Roughly 2% had structurally normal hearts at autopsy.

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Figure 1

Aetiology of SCD in athletes in the US. In the US, HCM is the most common cause, followed by commotio cordis. Data from Maron, N Engl J Med, 2003.7

In contrast, in the Veneto Region of Italy the most common aetiologies were ARVC (12/51 SCD, 23%), coronary artery disease (10/51, 20%), and myocarditis (5/51, 10%). HCM was responsible for only one of 51 SCDs (2%).4 The low prevalence of HCM in the Veneto experience may be related to mandatory preparticipation screening in Italy. Differences in the ethnic and genetic make-up of the populations also likely contribute.

Structural heart diseases

Hypertrophic cardiomyopathy

HCM has an estimated prevalence in the United States general population of 1:500,5 and SCD is most common in individuals under age 30 years.8 HCM is secondary to mutations of various sarcomeric proteins generally inherited via autosomal dominant genes with variable penetrance. Myocardial disarray and hypertrophy accounts for the phenotypical changes observed. The differentiation of HCM from physiological hypertrophy can be difficult.9 10 In these borderline cases or difficult diagnosis, a period of detraining may be necessary.11 Genetic testing is only useful if it is positive (sensitivity of genetic testing for HCM is only 50%); thus it cannot be utilised to rule out the diagnosis.12

The established risk factors for SCD in HCM include a family history of SCD, history of syncope, severe left ventricular hypertrophy, an abnormal blood pressure response to exercise, and non-sustained ventricular tachycardia. Elliot et al demonstrated that patients with two or more risk factors had significantly lower 6 year survival rates than those with zero or one risk factor.13 For this reason, an implantable cardioverter defibrillator (ICD) is generally recommended in patients with two or more risk factors. However, a more recent study suggested that even a single risk factor increases the risk of SCD.14 The 36th Bethesda Conference recommends that all athletes with a probable or unequivocal clinical diagnosis of HCM should be excluded from most competitive sports, except possibly low-intensity activities such as bowling or golfing.8 This recommendation applies regardless of risk factor status or ICD status.

Arrhythmogenic right ventricular cardiomyopathy

ARVC is characterised by replacement of myocardium with adipose and fibrous tissue. The prevalence of ARVC amongst young competitive athletes who suffer SCD demonstrates regional variation. In the US it is relatively uncommon, accounting for only 2.8% of 387 cases in the Minneapolis Heart Institute Foundation registry.7 In contrast, in Northern Italy, ARVC was the most common aetiology in young athletes (24%) and the second most common cause among all youth (12%). Electrocardiographic findings include anterior T wave inversions, epsilon waves and ventricular tachyarrhythmias with a left bundle-branch block morphology. Right ventricular dilatation, segmental wall motion abnormalities, and aneurysms are commonly observed. Risk factors for SCD are not as well established as in HCM. Clearly the strongest risk for SCD is in those with a history of cardiac arrest or ventricular tachycardia with haemodynamic compromise. Additional risk factors may include right ventricular dilatation or dysfunction, younger age, and left ventricular involvement.15 16 17 18 ICD therapy is highly prevalent as appropriate therapy in this population (46% to 78% over 3.25 to 7 year follow-up).15 17 18 The 36th Bethesda Conference recommends that athletes with probable or definite ARVC, similarly to patients with HCM, should be excluded from most competitive sports, with the possible exception of low-intensity sports.19

Coronary artery disease/abnormalities

Abnormalities of the coronary arteries associated with SCD in young athletes include atherosclerotic coronary artery disease, anomalous origin of coronary artery, and myocardial bridging. Exertional chest pain, syncope, or presyncope should alert the clinician to a possible coronary abnormality. Atherosclerotic coronary disease was an uncommon aetiology of SCD in the Minneapolis Heart Institute Foundation registry, accounting for only 2.6% of deaths.7 However, the Italian and Australian experiences demonstrate higher incidence, 18% and 25% respectively.4 20 In both of these studies, the burden of coronary artery disease deaths disproportionately affected those aged 30–35 years old. Coronary artery disease responsible for SCD in this population is most commonly single vessel obstructive disease of the proximal left anterior descending artery.21 In general, patients with evidence of coronary disease should be restricted to low-intensity competitive sports.22

Coronary artery anomalies accounted for 13.7% of SCD in the Minneapolis Heart Institute Foundation registry.7 Corrado et al found coronary abnormalities in nine of 55 (16%) SCDs in young athletes.4 Left main coronary originating from the right sinus of Valsalva with acute angle bend coursing between the pulmonary trunk and aorta is the most common and most serious anomaly.23 Sudden death is thought to result from ischaemia-driven ventricular arrhythmias. The 36th Bethesda Conference recommends that all patients with coronary anomalies of wrong sinus origin with the coronary artery passing between the great arteries should be excluded from competitive sports.24 If ischaemia is present on exercise testing, corrective surgery should be performed.

Myocardial bridging is a rare cause of SCD. The management of patients with isolated myocardial bridging depends on objective evidence of ischaemia (at rest or with exercise) or infarction.22 If neither is present then no prohibition on athletic participation is recommended. If there is evidence of ischaemia or a prior myocardial infarction, activity should be limited to low-intensity competitive sports.

Wolff–Parkinson–White syndrome (WPW)

WPW accounts for a small percentage of athletic deaths. Symptomatic patients with WPW are best treated with radiofrequency ablation. Symptomatic individuals should not participate in sports until they have been cured.19 There is no universal consensus on asymptomatic individuals with WPW. Most agree that there is an increased risk of SCD with competitive athletics and thus some risk assessment of the patient is necessary.25 Others have maintained that all asymptomatic individuals, athletes or not, should undergo invasive electrophysiological testing in order to define high-risk individuals who should receive ablation.26

Primary cardiac electrical abnormalities

Primary electrical abnormalities likely underlie a large proportion of athletes with SCD with structurally normal hearts at autopsy. As noted previously, the incidence of SCD in the absence of structural heart disease is uncommon in the US experience. However, autopsy-negative sudden unexplained death is reported as a more common cause of SCD in other populations.20 27 The emerging practice of postmortem molecular screening of victims of unexplained SCD holds promise in better defining the prevalence of these disorders amongst young competitive athletes.28 29

Long QT syndrome (LQTS)

The LQTS is characterised by a prolonged corrected QT interval on surface ECG, torsade de pointes, and an increased risk of SCD. The estimated prevalence in the general population is 1:5000.30 A rare autosomal recessive form (Jervell and Lange–Neilsen syndrome) is associated with sensineural deafness. The more common form, described by Romano and Ward, demonstrates autosomal dominant inheritance. At least 10 genotypes of LQTS have been defined.31 The three most common genotypes, LQT-1, LQT-2 and LQT-3, account for 50% of cases of LQTS.30 Patients with LQT-1 experience the majority (62%) of cardiac events during exercise (classically swimming) and conditions of elevated heart rate. In LQT-2 patients, cardiac events are associated with arousal or being startled. LQT-3 patients are at highest risk for cardiac events during periods of rest, sleep, or bradycardia.32 Difficult diagnosis or borderline cases of LQTS may be helped by genetic testing33 or epinephrine challenge.34

Standard medical therapy for LQTS is treatment with beta-blockers, which is associated with significant reductions in cardiac events. Not surprisingly, the benefit of beta-blocker therapy is greatest in LQT-1 and LQT-2 and significantly less in LQT-3 patients, who may respond better to sodium channel blockers like flecainide.35 For patients with a history of aborted sudden death, syncope, or recurrent symptoms on beta-blockers, ICD therapy is generally considered. Patients with a history of out-of-hospital cardiac arrest or suspected LQTS-precipitated syncope should be restricted from all competitive athletics except low-intensity sports, regardless of genotype or QTc duration.19 Similarly, asymptomatic patients with prolonged QTc should be restricted to low-intensity competitive sports, although the 36th Bethesda Conference does note that this restriction may be liberalised for patients with genetically proven LQT-3.19

Catecholaminergic polymorphic ventricular tachycardia (CPVT)

CPVT is characterised by ventricular tachycardia or ventricular fibrillation (VF) triggered by sympathetic stimulation. Mutations in the gene encoding the sarcoplasmic reticulum ryanodine receptor cause roughly one-half of cases. Therapy consists of beta-blockers and ICD implantation in symptomatic patients. All patients with CPVT are restricted by the 36th Bethesda Conference from participation in competitive athletics, with the possible exception of low-intensity sports.19

Externally caused SCD

Commotio cordis, sudden death due to innocent non-penetrating chest blows during athletic or recreational activities, is increasingly recognised as a cause of athletic death.7 36 37 It is now regarded as the second leading cause of sudden cardiac death in youth sports in the US.7 Ball impacts in sports such as baseball, softball and lacrosse are the most common causes of commotio cordis. The mean age of individuals experiencing commotio cordis is 14 years.37 Collapse is instantaneous in one-half and within 10 to 20 seconds in the others. Commotio cordis is thought to be secondary to ventricular fibrillation. Although early reports of commotio cordis showed dismal survival, survival is similar to other causes of sudden cardiac death. It has been speculated that resuscitation is more difficult in commotio cordis.38 39 40

A porcine model developed for the study of this syndrome has shed considerable light on commotio cordis, including the documentation of ventricular fibrillation.41 Determinants of ventricular fibrillation following a chest blow include impact delivered directly over the heart,42 and timing within the vulnerable phase of repolarisation (a narrow 10–30 ms window just prior to the T-wave peak, equivalent to only 1–2% of the cardiac cycle)(fig 2).41 43 Furthermore, impact velocity appears to have a Gaussian distribution, with 40 mph the most prone to cause ventricular fibrillation (fig 3).44 In addition, the hardness of the impact object correlated directly with the risk of ventricular fibrillation.41 45

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Figure 2

Electrocardiogram from a 20 kg swine undergoing a 40 mph impact with a lacrosse ball. Two ECG leads are shown along with the left ventricular pressure. Ventricular fibrillation is produced immediately upon impact which occurs during the vulnerable period of repolarisation (10–30 ms prior to the T-wave peak).

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Figure 3

Incidence of ventricular fibrillation (VF) induced by chest wall impacts at the vulnerable period of repolarisation (10–30 ms prior to the peak of the T-wave) with a regulation baseball propelled at velocities ranging from 32 to 113 km/h (20 to 70 mph) in the swine model of commotio cordis. The incidence of VF relative to the velocity of chest impact exhibited a Gaussian distribution with peak incidence at 64 km/h (40 mph) (p<0.001 by logistic regression). From Link et al, J Am Coll Cardiol, 2003.42

Efforts to minimise the risk of SCD due to commotio cordis have centred on the impact object, chest protectors, and changes in rules of the sport. Softer than standard baseballs have been demonstrated to reduce the risk of commotio cordis.41 45 Chest wall protectors intuitively should reduce the risk of commotio cordis. However, in the Commotio Cordis registry approximately one-third of the victims suffering commotio cordis in organised sports were wearing chest protectors, and in the laboratory model chest protectors did not appear to reduce the risk of ventricular fibrillation.46 Ongoing efforts to develop more effective chest protectors may decrease the risk of SCD in vulnerable athletes. The 36th Bethesda Conference promotes the use of age-appropriate soft balls to reduce the risk of commotio cordis, as well as the timely availability of automated external defibrillators (AEDs).47 Return to sport after a commotio cordis event should be on a case-by-case basis. There is concern that patients undergoing a commotio cordis event are more vulnerable to recurrent events, so that return to sport in which chest wall contact occurs should probably be discouraged.

Conclusions

Sudden cardiac death among young athletes is rare. The majority of young athletes with SCD have structural heart disease. Cardiac channelopathies likely account for a significant proportion of SCD without structural heart disease. The majority of athletes with a history of malignant ventricular arrhythmias or conditions placing them at significant risk of these arrhythmias should be restricted from participation in moderate and high-intensity sports. Athletes with commotio cordis can return to sport once normal cardiac function and exercise tolerance are demonstrated, although return to a sport in which chest wall impact is common should probably be discouraged.