_{Published December 21, 2021}

In the US, approximately 100–150 athletes each year die from sudden cardiac death (SCD) during competitive events. Though moderate exercise has been shown to be cardioprotective, vigorous exertion during sports can lead to abrupt hemodynamic changes, triggering ventricular tachycardia or fibrillation in the setting of an underlying cardiac abnormality. As a result, SCD is approximately twice as likely in athletes, compared with the general population. In most cases, the underlying cause can be diagnosed by imaging. Radiologists play a pivotal role in helping clear potential athletes before they begin playing sports and in the workup of athletes after a sudden cardiac arrest.

Causes of Cardiac Events and Patient Demographics

The most common causes of SCD in the US are hypertrophic cardiomyopathy (HCM) and anomalous coronary arteries. According to the US National Registry of Sudden Death in Athletes, from 1980 to 2011, HCM constituted 36% of SCD cases and anomalous coronary arteries constituted 19%. A variety of less common causes are responsible for the remainder of cardiac events and include arrhythmogenic cardiomyopathy (AC), noncompaction cardiomyopathy, myocarditis, valvular disease, and coronary artery disease. Additional causes that can be diagnosed without imaging findings include channelopathies, Wolff- Parkinson-White syndrome, and commotio cordis. Similar statistics have been found in series examining sudden cardiac arrest, in addition to SCD. The frequency of cause differs by age. For instance, coronary artery anomalies are more common in middle school athletes. In patients over 35 years old, coronary artery disease is the most common cause.

The vast majority of SCDs occur in male athletes. The reasons for this are not entirely clear, but it has been hypothesized that greater intensity of physical training, more frequent participation in contact sports, such as football and boxing, and undocumented protective metabolic mechanisms in female athletes may contribute [4]. More than half of patients with SCD are football and basketball players. African-American athletes are at higher risk for cardiovascular death than patients of other ethnicities, potentially related to a greater rate of participation in sports and a higher incidence of cardiomyopathy.

Imaging Two Common Causes of Sudden Cardiac Death

Hypertrophic Cardiomyopathy

HCM is the most common cause of death in athletes younger than 35 years old. The underlying cause is usually an inherited abnormality of the sarcomeres. Though HCM can be diagnosed on echocardiography, it is definitively evaluated on cardiac MRI (CMR), allowing complete anatomic and tissue characterization. CMR is also more sensitive for detection of apical aneurysm and thrombus, compared with echocardiography. The main diagnostic criterion for HCM is an end-diastolic wall thickness greater than or equal to 15 mm without left ventricular dilation or evidence of systemic disease to explain the degree of hypertrophy. The location of thickening can vary among patients, most often involving the septum (asymmetric septal hypertrophy) (Fig. 1A). Other types of hypertrophy include concentric (Fig. 1B), midcavity, apical (also known as spade-like) (Fig. 1C), and mass-like.

Left ventricular outflow obstruction in HCM can result in syncope, exercise limitation, or chest pain. MRI typically shows a thickened basal anteroseptum with a dephasing jet secondary to flow acceleration. The mitral valve is pulled into the left ventricular outflow tract as a result of flow acceleration, resulting in further obstruction (Fig. 1D). This systolic anterior motion of the mitral valve is best visualized on the three-chamber view of the left ventricle. Commonly, an associated mitral regurgitation is directed posteriorly because of incomplete coaptation of the mitral valve leaflets. Both 2D phase-contrast and 4D flow imaging techniques have been applied to assess the pressure gradient caused by obstruction, but this is generally better assessed with echocardiography because of improved temporal resolution for observing peak velocity. The criterion for obstruction is a gradient greater than or equal to 30 mm Hg.

Radiologists also have a role in identifying predictors of SCD. Implantable cardioverter-defibrillator (ICD) placement is dictated on the basis of individual risk. Myocardial thickening greater than 30 mm measured at end diastole constitutes massive hypertrophy, and its presence raises consideration for ICD placement. An apical aneurysm (Fig. 1C) also raises the risk for thromboembolism and SCD. LGE is typically hazy and observed in a midmyocardial distribution (Fig. 1E). The presence of LGE greater than or equal to 15% of the myocardial mass doubles the risk of a cardiac event, though current guidelines from the American College of Cardiology Foundation and American Heart Association (AHA) have yet to incorporate LGE. The presence of myocardial delayed enhancement also helps distinguish HCM from athlete’s heart.

Many patients with HCM have abnormal papillary muscles, which may be important to identify for presurgical planning. The identification of a hypertrophied anterolateral papillary muscle inserting directly into the anterior mitral leaflet can lead to greater obstruction. A long anterior mitral leaflet may necessitate mitral valve repair and has been defined as an anterior leaflet length greater than 30 mm. Additional abnormalities include accessory anterolateral papillary muscles and accessory left ventricular muscle bundles that can contribute to outflow obstruction. In such cases, the surgeon will need to adjust the surgical approach, including reduction of the papillary muscles.

Anomalous Coronary Arteries

Anomalous coronary arteries are the second-most common cause of SCD. Normally, the left main coronary artery arises from the left coronary sinus of the aortic root, and the right coronary artery arises from the right coronary sinus. Coronary anomalies consist of deviations of this usual anatomy.

Anomalous coronary arteries are often initially evaluated by echocardiography, followed by coronary CTA for patients with persistent concern for an anomalous coronary artery. In some patients, MRI is performed—such sequences can be performed without contrast material but can be difficult to perform in young patients without anesthesia. In addition, smaller vessels are more difficult to analyze because of lower spatial resolution.

When assessing the coronary arteries, a radiologist should evaluate the origin and course of the left main coronary artery, right coronary artery, left anterior descending artery, and left circumflex artery. Most coronary anomalies are asymptomatic. Examples include a retroaortic course of the left circumflex artery, separate origins of the left anterior descending and left circumflex arteries from the aortic root, and a prepulmonic course of the left anterior descending artery.

The most frequent types of potentially hemodynamic significant coronary artery anomalies are those with an interarterial course. An anomalous origin of the right coronary artery with an interarterial course between the aorta and the main pulmonary artery is the most common in young athletes; if symptomatic, repair is indicated. In the absence of symptoms, institutional policy may vary, but patients generally undergo a stress test to evaluate for inducible ischemia. An anomalous left coronary artery from the right coronary sinus (Fig. 2) has a higher risk of ischemia, and patients with this anomaly are recommended to undergo surgical repair regardless of symptoms. For any interarterial course, a slit-like origin suggests an intramural course with greater risk of SCD.

An important distinction from an interarterial course of the left main coronary artery is a transseptal course in which the anomalous coronary arteries pass through the septum, rather than between the aorta and main pulmonary artery. Unlike an interarterial course, a transseptal course is thought to be benign and rarely requires surgical correction, though an exercise stress test is still required.

An additional benign differential consideration is a highrising coronary artery, which arises 1 cm or more above the level of the sinotubular junction in the ascending aorta. The origin may be slightly rotated clockwise with respect to the right coronary sinus, but this is a benign entity without hemodynamic consequence.

Finally, anomalous left or right coronary arteries arising from the pulmonary arteries generally require surgical treatment in athletes. Because of the lower pressure of the pulmonary arteries, blood flow is actually reversed—this represents a steal phenomenon in which coronary artery blood flow shunts to the pulmonary arteries via collateral vessels.

The author thanks Seth Kligerman for review of this article and assistance with figures.