Yousef Bader, MD, and Mark Estes, MD
A 17-year-old woman presents for evaluation of recurrent episodes of abrupt loss of consciousness without a prodrome. She had been diagnosed as having epilepsy in infancy. In early life, she had been having seizures occurring both at rest and during exertion. She was seen by a pediatric neurologist, and despite multiple antiepileptic medications her seizures persisted. Four years prior to her presentation, her seizures began to occur exclusively with exercise. One of her seizures occurred while she was running at a picnic on a Sunday afternoon. A cardiologist who was present at the time witnessed this seizure and took her to the local emergency department to be evaluated. Her vitals were normal; her examination was benign except for a small scalp abrasion. Her complete blood count and basic metabolic panel were normal. An ECG was performed and is shown in Figure 48-1. She was sent home with a 24-hour Holter monitor. The following day she had yet another episode of loss of consciousness while rowing with her high school team. The Holter monitor demonstrated several minutes of torsades de pointes (Figure 48-2).
FIGURE 48-1 ECG showing sinus bradycardia at 48 beats per minute. There is a broad-based, notched T wave with a prolonged QT and T-wave inversions in anterior leads. Note that the T-wave notching can be seen in ≥3 leads.
FIGURE 48-2 This tracing starts in sinus rhythm with a prolonged QT. The T wave is broad-based and notched. Polymorphic VT is initiated after a premature ventricular contraction (PVC). This then self-terminates with a junctional escape rhythm.
Although this patient was diagnosed as having a seizure disorder, it is important to note that these episodes of altered consciousness have been refractory to antiepileptic medications. Recently they occurred during exercise, raising concerns for alternative diagnoses. A seizure can of course be the result of a primary neurologic problem. However, seizure-like activity, or convulsive syncope, can be the result of cerebral hypoperfusion. This can result from common neurocardiogenic syncope or even fatal cardiac arrhythmias. In this case, the typical prodromal features of vasovagal syncope did not precede the loss of consciousness, making that an unlikely diagnosis. Syncope occurring during exercise is a strong predictor that the underlying problem is likely cardiac in origin. The scalp abrasion our patient suffered also supports the diagnosis of a cardiac arrhythmia which would result in a sudden drop in blood pressure and therefore the inability of a patient to protect their head as they fall. (Table 48-1 demonstrates features of cardiac and noncardiac syncope.1)
TABLE 48-1 Features of Cardiac and Noncardiac Syncope
• Exertional syncope is a precursor of sudden cardiac death (SCD). When identified, an appropriate workup should be initiated quickly to achieve a diagnosis and begin managing the patient.
• The differential diagnosis of cardiac syncope in this setting is seen in Table 48-2.
TABLE 48-2 Differential Diagnosis
Benign causes of syncope
• Neurocardiogenic syncope
• Postexertion collapse
Life-threatening cardiac causes of syncope
• Long QT syndrome (LQTS)
• Catecholaminergic polymorphic ventricular tachycardia (CPVT)
• Right ventricular outflow tract ventricular tachycardia (RVOT VT)
• Brugada syndrome
• Arrhythmogenic right ventricular dysplasia (ARVD)
• Hypertrophic cardiomyopathy (HCM)
• Coronary anomalies
• Commotio cordis
• Our patient’s ECG clearly demonstrates a prolonged QT with a broad based, notched T wave. In the absence of QT prolonging medications this is highly suggestive of an inherited long QT syndrome (LQT) type 1. Her Holter also confirmed that her syncope is secondary to torsades de pointes, which is defined as polymorphic ventricular tachycardia (VT) resulting from a prolonged QT.
• Loss of consciousness in athletes, although rare, is devastating, and this is due to our growing understanding of SCD in athletes and its causes. Cardiac causes of syncope can be due to either structural heart disease or a primary arrhythmic condition and account for approximately 1% of all causes of syncope in the athlete. Details of incidence, prevalence, and geographic distribution are seen in Table 48-3.2-4
TABLE 48-3 Causes of Syncope in Athletes
• SCD is two and a half times more common in the competitive athlete than the general population, demonstrating its direct relationship to competitive sports.
• The leading cause of SCD is hypertrophic cardiomyopathy (HCM) accounting for approximately 37% of cases followed by coronary anomalies and ion channelopathies, including long QT syndromes, which account for about 4% (Figure 48-3).
FIGURE 48-3 This pie chart displays causes of sudden cardiac death in athletes and their incidence.
ETIOLOGY AND PATHOPHYSIOLOGY
The majority of cardiac causes of syncope are either congenital or inherited (either autosomal recessive or autosomal dominant).
Long QT Syndrome
• This group of disorders is genetically inherited, affecting cardiac ion channels, which result in prolonged ventricular repolarization, which may lead to polymorphic ventricular tachycardia.
• There are 13 different types of inherited prolonged QT, all of which involve a different genetic mutation. The most common forms of long QT are types 1, 2, and 3 accounting for the majority cases.
LQT1 results from a mutation in the α-subunit of the slow delayed rectifier potassium channel KCNQ1. These mutations result in reducing the amount of repolarizing current, thereby prolonging the action potential. This is the most common form of congenital long QT, making up one-third of cases. LQT1 is often triggered by exercise and has a specific association with swimming.
LQT2 results from a mutation in the α-subunit of the rapid delayed rectifier potassium channel KCNH1. Similar to LQT1, the mutations of LQT2 result in prolongation of the action potential. LQT2 can be triggered by exercise but is more commonly triggered by an emotional event or an auditory stimulus.
LQT3 is result of a mutation in the α-subunit of the sodium channel SCN5A. This leads to failure of the sodium channel to remain inactivated, which results in a prolonged action potential duration. LQT3 is less common than LQT1 and LQT2, but is more lethal.5 As in LQT1 and 2, LQT3 can also be triggered by exercise but more commonly occurs during sleep. Characteristic features of LQT1, LQT2, and LQT3 are seen in Table 48-4.
TABLE 48-4 The Long QT Syndromes
Catecholaminergic Polymorphic Ventricular Tachycardia
• Another genetic disease that may result in polymorphic VT in athletes is catecholaminergic polymorphic ventricular tachycardia (CPVT). It is characterized by cardiac electrical instability due to activation of the sympathetic nervous system. Unlike LQT syndrome, it is not related to prolongation of the QT interval.
It is most commonly inherited in an autosomal dominant pattern resulting with mutations in the RYR2 gene, which encodes for the ryanodine receptor channel. This accounts for about 55% of CPVT.
Autosomal recessive inheritance is by mutations in the CASQ2 gene, which encodes calsequestrin, which is a calcium buffering protein of the sarcoplasmic reticulum and is responsible for about 2% of cases.6
• Mutations in the above receptors lead to calcium loading within myocardial cells and expose these patients to the risk of VT and ventricular fibrillation (VF) associated with acute emotion or exercise. Eighty percent of athletes with this disease have recurrent syncope, and thirty percent fall to sudden cardiac death.
Right Ventricular Outflow Tract Ventricular Tachycardia
• Right ventricular outflow tract ventricular tachycardia (RVOT VT) is the most common idiopathic VT in athletes. It most frequently occurs during emotional stress or exercise. Athletes usually present with palpitations and presyncope; however, it can infrequently present with syncope and exceptionally rarely present with sudden cardiac death.
• Similar to CPVT, the principal mechanism of RVOT VT is due to intracellular calcium overload. One of cyclic adenosine monophosphate’s (cAMP) roles is regulation of intracellular calcium. During exercise, cAMP levels rise, leading to an increase in intracellular calcium. This in turn, by a mechanism of triggered activity, leads to VT.
• There are two main phenotypic types of RVOT VT:
Nonsustained repetitive monomorphic VT.
Paroxysmal exercise-induced sustained RVOT VT, which more commonly affects athletes.7
• Although Brugada syndrome may be acquired, it is most commonly inherited in an autosomal dominant fashion and is due to a loss of function mutation in the SCN5A gene.
• Athletes with Brugada syndrome usually present with unexplained syncope and, less frequently, as sudden cardiac death.
• VF in Brugada patients is usually triggered by hyperthermia and during times of higher vagal tone, often occurring during sleep.8 Because of their extensive training, athletes may have a higher baseline vagal tone, and during exercise rises in temperature may explain why athletes with Brugada are more likely to suffer from ventricular arrhythmias than nonathlete Brugada patients.
Arrhythmogenic Right Ventricular Dysplasia
• Arrhythmogenic right ventricular dysplasia (ARVD) is a genetic disorder with mutations in gene coding for desmosomal proteins, leading to fibrofatty replacement of myocardium. It is most often inherited in an autosomal dominant pattern.
• The right ventricle (RV) is affected in all but 5% of cases. There is left ventricular (LV) involvement in about 50% of cases and in a minority of cases only LV involvement.
• The “triangle of dysplasia” which is between the RV apex, the RV inflow tract, and the RV outflow tract is notoriously involved in ARVD.
• RVOT VT is the most common VT seen in these patients.3
• Hypertrophic cardiomyopathy (HCM) is the most common cause of SCD in athletes and is caused by a number of mutations in genes coding proteins of the cardiac sarcomere most notably β-myosin heavy chain. Its familial form is inherited in an autosomal dominant pattern, but HCM can also be acquired as a de novo mutation.
• It is typically characterized by hypertrophy of the left ventricle in an asymmetric fashion with the anterior LV septum being the most commonly affected region.
• Thickening of the LV septum and systolic motion of the mitral valve can lead to left ventricular outflow tract (LVOT) obstruction, and in some cases a high gradient can lead to syncope.
• Ventricular arrhythmias occurring in patients with HCM are commonly the first clinical presentation of HCM and usually occur during strenuous physical activity.
• Athletes with anomalous coronaries may be completely asymptomatic, suffer from recurrent angina or syncope, and occasionally present with myocardial infarctions or sudden cardiac death.
• The most common coronary anomaly is a left anomalous coronary artery arising from the right coronary sinus, and this is associated with ischemia and sudden cardiac death (Figure 48-4).
FIGURE 48-4 This is a left anterior oblique slightly cranial angiographic view of the right coronary artery with an anomalous left coronary artery arising from the ostium of the right coronary artery.
• The course of an anomalous coronary is occasionally between the main pulmonary artery and the ascending aorta, and at times of exercise, an increase in the cardiac output may lead to compression of the coronary artery between them.
• A large territory of myocardium supplied by the anomalous coronary, the presence of an intramural segment within the aortic wall, and in particular a slit-like orifice of the coronary ostium are all high-risk features associated with increased mortality.
• Commotio cordis is defined as VF resulting secondary from blunt chest injury and is seen in high-impact sports such as baseball, hockey, and boxing.
• For this to occur, the precordial impact must occur at a 10 to 30 ms interval just before the peak of the T wave as seen in Figure 48-5.
FIGURE 48-5 For commotio cordis to occur, the precordial impact must be inflicted by a hard object such as a hockey puck, a lacrosse ball, or a fist. This must occur within a 20 ms interval during the upstroke of the T wave resulting in ventricular fibrillation.
• The mechanism of commotio cordis is believed to be secondary to a series of events following impact of a blunt object. Blunt trauma → increase in left ventricular pressure → the myocardial cell membrane stretches → activation of stretch sensitive K+ATP ion channels → ventricular fibrillation.9
Patients with symptoms and signs suggestive of a cardiac cause for syncope should undergo a thorough, thoughtful, and systematic diagnostic approach, which begins with a detailed history and physical examination. An electrocardiogram (ECG) should be performed on all patients and may be sufficient to provide a diagnosis. Echocardiography (Echo) should be performed in all patients with selective use of magnetic resonance imaging (MRI). Exercise treadmill testing (ETT) is a valuable tool in reproducing a patient’s symptoms and detecting exertional ventricular arrhythmias. If noninvasive testing is diagnostic, electrophysiology studies are of no added value. If noninvasive testing is unfruitful, and there is a high index of suspicion for an arrhythmic cause, an EP study should be performed. The induction of nonsustained ventricular tachycardia, polymorphic ventricular tachycardia, and VF are not useful during an EP study as these findings are nonspecific and can be found in healthy individuals.10 The role of genetic testing in confirming some of the above diagnoses is controversial because genetic mutations do not always have phenotypic consequences. Being aware of the implications of a positive or negative result is crucial.
Long QT Syndrome
• ECG: The ECG changes in long QT syndrome are often dynamic, and therefore their absence does not rule out the diagnosis. If absent, ECG changes can be provoked by drug challenges with β-agonists including epinephrine and isoproterenol. Instead of normal physiologic shortening of the QT interval with these provoking agents, patients with LQT1 and LQT2 experience prolongation of the corrected QT.
• ETT: When tachycardic young adults and children should shorten their QT interval; however, in patients with long QT syndrome, their QT will often fail to shorten or prolong.
LQT1: Diminished shortening of the QT interval, followed by exaggerated lengthening of the QT interval as the heart rate declines during recovery.
LQT2: Marked QT interval shortening followed by exaggerated lengthening of the QT interval as the heart rate declines during late recovery (4 minutes after exercise).
LQT3: Marked decrease in the QT interval.11
Long QT should be <450 ms in males and <470 ms in females.
• Genetic testing: In long QT syndrome, genetic testing will identify a specific mutation in 75% to 80% of cases. Traditionally, a clinical diagnostic criteria and a point system was used to assess probability of the diagnosis. This scoring system has a high specificity but low sensitivity; therefore, clinicians are leaning away from the scoring system and depending more on the length of the QT and genetic testing.12
Catecholaminergic Polymorphic VT
• ECG: The ECG is usually normal at rest. Isoproterenol infusion can be used to trigger VT in patients with CPVT.13 The characteristic VT of CPVT is bidirectional with an alternating 180 degree QRS axis on a beat-to-beat basis as seen in Figure 48-6.
FIGURE 48-6 This ECG displays the typical pattern of bidirectional VT seen in CPVT.
• ETT: Because CPVT is associated with increased sympathetic activity, it is often exposed during ETT, and the severity of the tachycardia is directly related to the level of exertion.
• Genetic testing: This is indicated in patients with a clinical presentation that is highly suspicious for CPVT but is not reproducible by ETT. Mutations in RYR2 are inherited in an autosomal dominant pattern, whereas CASQ2 gene mutations are inherited autosomal recessive.7
Right Ventricular Outflow Tract VT
• ECG: The ECG is usually normal at rest. Patients may have PVCs arising from the RVOT, which would have a left bundle branch block pattern and an inferior axis as seen in Figure 48-7.
FIGURE 48-7 Wide complex tachycardia with an LBBB morphology and an inferior axis most consistent with RVOT VT.
• ETT: Similar to CPVT this may trigger the VT in patients.
• Echo: Normal.
• MRI: Focal thinning and fatty replacement of the right ventricular outflow tract is sometimes seen in patients with RVOT VT, although they are generally described as having structurally normal hearts.14
• ECG: As in long QT, the ECG changes seen in Brugada syndrome are dynamic, so their absence does not rule out the diagnosis. There are three types of repolarization patterns seen in Brugada syndrome mainly affecting right-sided precordial leads (V1-V3). Type 1 Brugada pattern is the only diagnostic pattern of this disease.
Type 1 is characterized by a ≥2-mm coved ST-segment elevation followed by a negative T wave as seen in Figure 48-8.
FIGURE 48-8 This is the typical type 1 Brugada pattern which is the only diagnostic Brugada pattern with >2 mm ST-segment elevation in V2 followed by T-wave inversions.
Type 2 has a saddleback appearance with a downsloping ST segment with a takeoff that is ≥2 mm tapering to ≥1 mm and ending with an upright or biphasic T wave.
Type 3 has either a coved or saddleback ST segment with <1 mm ST elevation.15
• Raising right precordial leads to the second intercostal space can increase the sensitivity of the ECG for detecting a Brugada pattern in some patients. If that fails, flecainide and procainamide can be used to induce a Brugada pattern. Patients must develop a type 1 Brugada pattern for the ECG to be diagnostic.16
• Genetic testing: This involves sequencing SCN5A genes, which are the most common mutations in Brugada; however, only 15% to 30% of Brugada patients have SCN5A mutations, and not all patients withSCN5A mutations have Brugada.17
• Based on the Heart Rhythm Society consensus report, a diagnosis of Brugada syndrome is highly likely based on the presence of a spontaneous or provoked type 1 ECG pattern and one of the following: VF, polymorphic VT, family history of SCD at an age of <45 years, inducible VT with programmed stimulation, unexplained syncope, or nocturnal agonal respiration.
Arrhythmogenic Right Ventricular Dysplasia
• ECG: The most common ECG finding in patients with ARVD is T-wave inversion in leads V1-V3 as seen in Figure 48-9. This is a normal finding in only 4% of women and 1% of men greater than 14 years of age, so it is relatively specific and considered a diagnostic finding. Epsilon waves are seen in 50% of cases of ARVD.18
FIGURE 48-9 This ECG displays the most common findings in ARVD, which are precordial T-wave inversions extending to anterior leads.
• Signal averaged ECGs: (SAECGs) have a diagnostic role in patients lacking typical ECG findings. SAECG is positive if two of the following three parameters are abnormal:
Fltered QRS duration (fQRS) ≥114 ms
Root-mean-square-voltage of the terminal 40 ms of the QRS of ≤20 μV
Duration of the terminal QRS signal <40 μV of ≥38 ms19
• Echo: Echo findings in ARVD include an enlarged hypokinetic RV and a thin RV free wall. Morphologic abnormalities of the RV are not uncommon, and these include heavy trabeculations 55% of the time, a prominent moderator band 35% of the time, and sacculations in about 20% of patients.
• MRI: This is useful when echo is nondiagnostic. It can demonstrate fatty infiltration, thinning of the RV free wall, and regional or global dilation of the RV and late gadolinium enhancement.
• Cardiac catheterization: May be useful in suspected cases without enough diagnostic evidence based on noninvasive approaches. RV angiography would demonstrate a dilated RV with transversely arranged and hypertrophic trabeculae.20 RV biopsy can also be used to aid in the diagnosis; however, it is not commonly performed or recommended. This is because the presence of fat within the RV does not confirm ARVD due to the low specificity, and the absence of fat does not exclude it due to the patchy nature of the disease.
• Genetic testing: This can be used to screen family members of those with ARVD. Although genetic testing will detect 50% of patients with ARVD, the diagnosis is clinical, and therefore the role for it is very limited in this disease.21
• The Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy (ARVD/C) Task Force diagnostic criteria are seen in Table 48-5.19
TABLE 48-5 Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy (ARVD/C) Task Force Criteria for ARVD
• ECG: LVH with ST-segment depression and deep T-wave inversion is seen in 75% of cases. Left axis deviation is seen in about 25% of cases. Abnormal Q waves are seen in II, III, aVF, V5, or V6 in 20% to 25% of cases. Figure 48-10 demonstrates a typical HCM pattern.
FIGURE 48-10 This ECG demonstrates NSR with left ventricular hypertrophy (LVH) criteria and repolarization abnormalities consistent with LVH.
• Echo: Increased LV wall thickness (>15 mm with no other cause for LVH), systolic anterior motion of the mitral valve, and mitral regurgitation are the cardinal findings seen in HCM. Hypertrophy may involve the septum, apex, LV free wall, and even the RV. Echo is sensitive in detecting HCM, but if there are no echocardiographic findings suggestive of HCM but a high index of suspicion for the disease, further testing may needed to rule it out. Figure 48-11demonstrates massive LVH in a patient with HCM.
FIGURE 48-11 (A) Shown is a parasternal long axis echocardiographic view of a patient with HCM demonstrating massive LVH in the anterior septum. (B) Shown is a parasternal short axis view of the same patient quantifying the severity of LVH, which is seen in the anterior septum extending toward the anterior wall. The presence of massive LVH defined as ≥ 3 cm is considered massive and is a risk factor for sudden cardiac death.
• MRI: Because of the superior views of MRI, it is more sensitive in detecting increases in LV wall thickness particularly in the anterolateral left ventricular free wall as well as estimating the degree of hypertrophy in the basal anterolateral free wall. The use of MRIs to evaluate for late myocardial enhancement is gaining an increasing role in HCM by quantifying scar burden and risk for ventricular arrhythmias.2
• Genetic testing: There may be a role for genetic testing in phenotypically negative family members of those with HCM. The proband or affected family member should have genetic testing done first to identify a possible gene mutation. Once that mutation is identified, it may be checked in phenotypically negative family members. This can identify family members who may be at less risk of the disease and those who need closer follow-up.
TABLE 48-6 Management of Cardiac Causes of Syncope
• Computed tomography angiography (CTA): Using multislice CT scanners, patients can be evaluated noninvasively for coronary anomalies with high accuracy. CTA is also useful in mapping the course taken by these anomalous coronaries.23
• Diagnosing commotio cordis is one of exclusion if the clinical scenario is consistent.
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