Cardiac Syncope

Etiology

Syncope is caused by a temporary global failure of cerebral perfusion. When cardiac, the brain is not perfused because the heart is failing to generate enough cardiac output to send its freshly oxygenated blood to the brain. This is typically secondary to either a mechanical or structural cardiac defect or an arrhythmia that alters electrical conduction through the myocardium. Arrhythmias are recognized as the more common mechanism of the two. However, structural and mechanical abnormalities in the heart will often induce such arrhythmias. These processes are therefore often interrelated.

The following is a brief list of the more common etiologies of cardiac syncope. Further discussion regarding how to recognize and manage these etiologies will follow.

Structural

• Ischemic cardiomyopathy (most common structural cardiac etiology of syncope)
• Valvular abnormalities (second most common structural etiology, most commonly aortic stenosis)
• Nonischemic/Dilated cardiomyopathy (third most common structural etiology)
• Hypertrophic obstructive cardiomyopathy
• Aortic dissection
• Cardiac tamponade
• Obstructive cardiac tumors
• Pericardial disease
• Pulmonary hypertension
• Pulmonary emboli
• Arrhythmogenic right ventricular cardiomyopathy[5][3]

Electrical

• Tachyarrhythmia
  • Supraventricular (examples: atrial fibrillation, atrial flutter, paroxysmal supraventricular tachycardia (PSVT), PSVT in the setting of pre-existing accessory conduction pathways)
  • Ventricular (often secondary to mechanical/structural heart disease or channelopathies, such as Brugada)
  • PR interval disorders/accessory conduction pathways (WPW, LGL, Mahaim syndrome, Breijo syndrome)
• Bradyarrhythmia
  • Sinus node dysfunction
  • Atrioventricular conduction block (typically second or third degree)
  • Pacemaker malfunction
• Inherited channelopathies
  • QT interval disorders (Long or short QT)
    • Romano-Ward syndrome: Autosomal dominant congenital long QT syndrome
    • Jervell and Lange-Nielsen syndrome: autosomal recessive long QT syndrome associated with deafness
  • Brugada syndrome
    • An autosomal dominant mutation in the SCN5A gene, which encodes for voltage-gated sodium channels found in the heart
  • Catecholaminergic polymorphic ventricular tachycardia
    • Autosomal dominant mutation of hRyR2 gene, which encodes for ryanodine receptors
    • Autosomal recessive mutation of CASQ2 gene, which encodes for calsequestrin-2

• Drug-Induced: (bradycardias, tachycardias, QT interval prolongation, cardiotoxins, etc.)[3]

Epidemiology

Syncope is a relatively common phenomenon encountered among patients of every age range, race, religion, and socioeconomic status. Cardiovascular disease is the second most common cause of syncope, with arrhythmias being far more common than structural diseases. Ventricular tachycardia alone is responsible for 11% of syncopal events. Cardiovascular syncope overall is far more prevalent in the elderly population than the younger population, with 10% to 30% of syncopal events in patients over 60 having a cardiac origin. Cardiovascular syncope is also more common in men than in women.[6]

Structural cardiac diseases are more prevalent among patients with comorbidities such as diabetes, hypertension, hyperlipidemia, and patients who smoke. Pre-excitation syndromes tend to be more common among women than men.[7]

Brugada syndrome is more common among males and people of Southeast Asian descent.[3]

Pathophysiology

All cases of cardiac syncope have the same general mechanism. When the heart fails to generate adequate cardiac output, the brain is inadequately perfused and temporarily malfunctions, leading to the syncopal event. Bradyarrhythmias lead to this final pathway because the heart is too slow to generate enough flow. Tachyarrhythmias force the heart to pump so fast that it lacks an adequate diastolic phase, leading to ineffective ventricular filling and reduced cardiac output. 

Mechanical obstructions to blood flow and cardiac output will have similar effects but through multiple possible mechanisms. A chronic obstruction to forward blood flow out of the heart will lead to an increase in ventricular size and pressure. The increase in size leads to ventricular myocyte irritability, which can potentially induce arrhythmias. The increase in pressure can stimulate mechanoreceptors and induce a vagal response and secondary hypotension and bradycardia. This is in addition to the primary structural problem of the impeded forward blood flow. Obstructive pathologies, such as aortic stenosis, tumors, tamponade, and congenital hypertrophic cardiomyopathy can all follow this pathway, as can myocardial infarction and ischemia. Infarcted or ischemic ventricular tissue will have impaired contractility. Infarction of valves or chordae tendineae can lead to valve rupture, resulting in subsequent mechanical obstruction to forward flow or induction of retrograde blood flow. When cardiac tissue damage is present along with the conduction system, conduction blocks and other arrhythmias can result. Aortic dissection can induce myocardial infarction when the dissection extends into the coronaries supplying vital portions of the myocardium. A leaking or ruptured aortic dissection can also diminish cardiac output through induction of hypovolemia or tamponade (if aortic contents extend into the pericardium). Pulmonary hypertension and pulmonary emboli will lead to a blockade of flow through the pulmonary artery and a reduction in left-sided preload and subsequent cardiac output in addition to the primary potential symptom of hypoxia.[6]

Histopathology

Hypertrophic cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy are two sources of cardiac syncope that are recognizable by specific histopathologic findings.

Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy is a myocardium disease in which a portion of the left ventricle asymmetrically hypertrophies, often in the septum. This leads to diminished left ventricular filling volume and contractility secondary to the stiffened, overgrown myocardium. The diminished left ventricular filling volume obstructs cardiac output, resulting in diminished blood flow to intramural coronary vessels and the rest of the body. The classic findings on myocardial biopsy of patients with hypertrophic cardiomyopathy are a thickened myocardium (often specifically in the septum) with disorganization of the myocyte fibers, myocardial scarring, and fibrosis along with narrowed vessels and dysplasia of the tunica media cells.

Arrhythmogenic Right Ventricular Cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy can be identified pathologically by fibrous tissue and fat cells replacing myocytes within the right ventricle. Patients with this condition will have dilated right ventricles and sometimes may have ventricular aneurysms. These can be particularly dangerous when located in the posterior basal, apical, or outflow tract portions of the heart.[8]

History and Physical

When evaluating a syncopal patient to assess for a cardiac etiology, clinical history (when available) is one of the most useful diagnostic tools. Clinical history and physical exam alone have been found to identify an accurate diagnosis in 45% of patients who had a primary identifiable etiology of their syncope.[9] Important aspects of the history that will lead to the determination of a cardiac etiology include older age (greater than 60 years), male sex, past medical history significant for any primary cardiac disease, coronary artery disease, valvular disease, or pulmonary emboli/pulmonary emboli risk factors (cancer, immobility, pregnancy, oral contraceptive pill use, etc.). In addition, cardiovascular syncope will more commonly be associated with exertion, occur when supine, involve prodromal palpitations or chest pain, and lack prodromal vagal symptoms. Other key historical clues include a family history of arrhythmias, syncope, or early, unexplained death.[10]

A careful physical exam will assess heart rate and rhythm. Any abnormalities will increase suspicion of cardiac arrhythmia. In addition, an elevated respiratory rate or hypoxia will increase suspicion of a pulmonary embolism. Jugular venous distention and hypotension are suggestive signs of an obstructive mechanical cardiac etiology. Pathological cardiac murmurs, specifically new ones, will clue one into a valvular etiology, hypertrophic cardiomyopathy, or an obstructive intracardiac lesion. Muffled sounds can be heard with pericardial tamponade. Pedal edema or other evidence of deep venous thrombosis increases a patient's risk of pulmonary embolism.[10]

Evaluation

When a syncopal event is identified as cardiac in origin, further workup should involve a 12 lead EKG, rhythm monitoring, and potential further studies such as an echocardiogram, implantable cardiac monitor, or Holter and/or stress test.

The EKG is a helpful tool for identifying arrhythmias. Sometimes it will diagnose active arrhythmias such as conduction block-induced bradycardias to less than 50 beats per minute. Sinus pauses for more than 3 seconds or nonsustained ventricular tachycardias. At other times it will demonstrate suggestive findings such as bifascicular blocks (either a full left bundle branch block or a right bundle branch block couples with a left anterior or left posterior fascicular block), QRS segments prolonged to greater than 120 milliseconds, Mobitz I second degree AV blocks, complete AV blocks, premature ventricular contraction QRS complexes, early repolarization, or Q waves which represent evidence of a preceding ischemic cardiac event resulting myocardial infarction. Electrocardiogram can also identify intrinsic conditions that predispose a patient to intermittent arrhythmias, such as Wolff-Parkinson-White syndrome (delta waves), Brugada syndrome (semi-right-bundle branch block and coved ST-segment elevation greater than 2 mm with inverted T waves in leads V1-V3), Long-QT syndrome (prolonged QT interval), hypertrophic cardiomyopathy (left atrial enlargement, left ventricular hypertrophy criteria possibly with dagger-shaped Q waves in inferolateral leads), and arrhythmogenic right ventricular cardiomyopathy (epsilon waves).[10]

Basic lab tests, such as a complete blood count and basic metabolic panel, are frequently performed on patients presenting to an emergency department with syncope. However, these tests do not assist in predicting cardiac etiology.[10] Troponin, similarly, performs poorly as a diagnostic tool among syncopal patients. However, when measured among syncopal patients, the natriuretic peptide has demonstrated utility in identifying patients with cardiac syncope and patients at risk of short-term major adverse cardiac events (sensitivity, 73%; specificity, 88%).[11][12]

Once cardiac syncope is suspected, if no arrhythmia is identified, echocardiography is perhaps the most useful follow-up diagnostic test as it will directly identify structural abnormalities, as described above. In a retrospective study of 128 patients suspected of having a cardiac syncope, the echocardiogram confirmed diagnosis in 48% of patients and 77% of patients with a specific cardiac diagnosis.[13] CT and MRI are also useful modalities in identifying structural cardiac pathologies, such as pulmonary emboli, cardiac masses, congenital heart disease, sarcoidosis, and arrhythmogenic right ventricular cardiomyopathy.[14][15]

If cardiac syncope is not secondary to a mechanical problem and no arrhythmia is detected on initial EKG, stress testing and longer-term rhythm monitoring can be effective in catching the diagnosis. In cases of exertional syncope, exercise stress testing will often reveal the diagnosis.[16] When symptoms are recurrent and frequent, a Holter monitor is recommended. If syncopal or pre-syncopal symptoms are recurrent but not frequent enough to be noted on the Holter monitor, an external loop recorder or implanted cardiac monitor can be used to generate a diagnosis.[10]

The electrophysiologic study is another method that has been utilized to identify cases of cardiac syncope. This study involves the placement of catheter tips at multiple locations within the heart. Testing begins at the SA node and progresses throughout the remainder of the heart. During the study, the heart is paced. An electric current is injected into various nonconducting endocardium regions and along the conduction pathway to attempt to reproduce an arrhythmia. Slow and/or fast pacing and proarrhythmic drugs may also be utilized for this purpose. When an arrhythmia occurs, this study allows the electrophysiologist to find the exact source of the irregular electrical activity and allows for any misfiring cells' ablation. This procedure can thus be both diagnostic and therapeutic. Theoretically, this sounds like an excellent test. However, in practice, this test has poor sensitivity and specificity. It is only recommended in patients with concurrent ischemic heart disease who would not otherwise receive an implanted cardiac defibrillator or other high-risk patients in whom no other tests could reveal a diagnosis. Electrophysiologic studies are only considered diagnostic if they induce sinus bradycardia with a prolonged sinus node recovery time of more than 525 milliseconds, a bundle branch block with tachycardia or a His-Purkinje block (second or third degree), sustained monomorphic ventricular tachycardia in a patient with preexisting cardiac infarction, or recurrent symptoms in the setting of rapid supraventricular tachycardia.[17]

Treatment / Management

Treatment for cardiac syncope is offered to prevent the patient's morbidity, physical injuries, and monetary costs of recurrent syncopal events and prevent sudden cardiac death. The management of cardiac syncope varies widely and is essentially based on managing the specific condition which precipitated the syncopal event. The following is a general summary of management strategies; however, this is only a limited summary as the topic is vast.

Arrhythmias

Sinus Node Dysfunction 

Sinus node abnormalities will typically lead to bradyarrhythmias. When sinus bradycardia is noted, the first intervention should assess iatrogenic causes, such as medications with bradycardia as an intended or unintended effect. If such medications are in use and unnecessary to the patient, they should be terminated. Otherwise, pacemakers should be the next step sought out in management. Implantable dual-chamber pacemakers are recommended to prevent future syncopal events. Implantation of a pacemaker should theoretically circumvent this outcome.[18] However, there has not been a proven improvement in overall survival from this intervention, and 20% of these patients who receive pacemakers experience recurrent syncopal events.[19](A1)

Atrioventricular Conduction System Dysfunction

In cases of Mobitz type 2 second-degree or third-degree AV block, pacemaker implantation helps prevent recurrence of syncope.[18] Biventricular pacing modes are recommended in patients with coexisting heart failure, depressed left ventricular ejection fraction, or prolonged QRS segments noted on the electrocardiogram.[17](A1)

Fascicular Blocks

Implantable permanent pacemakers are recommended for patients with concerning histories and syncope in the setting of bifascicular or trifascicular bundle branch blocks.[18](A1)

Tachyarrhythmias

Management of tachycardic syncope sources is more complex, as many cases of tachycardic arrhythmias that precipitate syncope are self-limited. Supraventricular tachycardia can occur in patients with alternative atrioventricular nodal reentrant pathways. In such circumstances, catheter ablation of the alternative pathway is the best treatment modality. Medications are often utilized as a bridge to this definitive management if catheter ablation is not immediately possible. Management of atrial fibrillation should be first attempted conservatively with a medication-based rate or rhythm control and then with ablation if such conservative therapy fails. Nonsustained ventricular tachycardia-induced syncope can be managed with beta-blockers or calcium channel blockers. If this is insufficient, antiarrhythmic agents can be employed. Amiodarone is the first-line antiarrhythmic followed by class 1A and 1B antiarrhythmics. If syncopal events are frequent and unresponsive to medications, radiofrequency catheter ablation of the foci from which the tachycardia originates can be performed. In cases of ventricular tachycardia-induced syncope in the setting of underlying structural cardiac disease, patients may benefit from an implanted cardiac defibrillator.[10] Although the placement of a cardiac defibrillator in such patients has been demonstrated to reduce the incidence of sudden cardiac death, these devices do not decrease patients' risk of recurrent syncopal events or the resultant associated physical injuries and expenses.[17](A1)

Acquired QT Interval abnormalities: These are commonly iatrogenic due to medications that prolong the QT interval. In such circumstances, offending drug discontinuation is sufficient therapy.

Inherited Channelopathies: In cases such as long QT syndrome, Brugada syndrome, and other inherited causes of arrhythmogenic syncope, implantable cardiac defibrillators are the definitive management. The decision regarding when to implant these is patient- and circumstance-dependent.[10](A1)

Implanted Pacemaker Failure

A patient with an implanted pacemaker who presents to a hospital following a syncopal event should have the pacemaker interrogated by any available electrophysiologist. Syncope in the setting of pacemaker failure is usually the result of battery depletion, lead oversensing, or lead underpacing. Such problems can be managed with a device or lead replacement.[17]

Structural Syncope

The management of structural causes of syncope is specific to the structural problem. For example, In cases of aortic stenosis, aortic valve replacement is the definitive management. Atrial myxomas or other cardiac tumors should similarly be managed operatively. Heart failure or hypertrophic cardiomyopathy can be managed medically with nitrates and diuretics. Coronary artery disease can be managed with antiplatelet agents and anticoagulants. Acute coronary artery occlusion, when present, can be managed with revascularization techniques such as angioplasty or cardiac bypass surgery. Pulmonary emboli are managed with anticoagulants/thrombolytics. Any patient with an unknown specific etiology of syncope but a known structural cardiac disease can and should be evaluated for an implantable cardioverter-defibrillator.[17][10](A1)

All patients presenting with cardiac syncope who have not had a definitive intervention to prevent future events should be advised not to drive. Patients who have syncopized are 2 to 4 times more likely to be involved in motor vehicle collisions than the general population.[10](A1)

Differential Diagnosis

All patients presenting with a transient, apparent loss of consciousness should be assessed for the possibility of cardiac syncope. However, they could also have any of the many other causes of loss of consciousness. In cases of head trauma preceding loss of consciousness, a patient is considered to have had a concussion. Nontraumatic transient loss of consciousness can occur from other sources of syncope or non-syncopal events such as seizures; metabolic disturbances like hypoglycemia, hyponatremia, hypoxia, or hypocapnia); vertebrobasilar cerebral transient ischemic events; alcohol intoxication or other central nervous system depressant substance use; pseudoseizures; or psychogenic pseudosyncope. In these instances, loss of consciousness or presumed loss of consciousness is not due to global cerebral hypoperfusion, as is the case in syncope. Among transient loss of consciousness events deemed syncopal, any event that reduces either cardiac output or peripheral vascular resistance can result in this common outcome. 

The following are several noncardiac etiologies:

Reflex (Neurogenic)

• Vasovagal - typically preceded by an inciting stressful event such as fear, seeing blood, hearing bad news, emotional stress, or pain
• Situational - preceded by a specific action such as sneezing, laughing, coughing, urinating, defecating, eating, or exercise
• Carotid sinus stimulation

Orthostatic Hypotension

• Autonomic dysfunction - usually a symptom of another degenerative disease such as Parkinson disease, multiple system atrophy, Lewy body dementia or can be a primary disease on its own; can also occur as a direct result of diabetes, amyloidosis, and spinal cord trauma
• Drug-induced - in the setting of diuretics, vasodilating agents, alcohol, antidepressants, or any other medications that reduce cardiac output or vascular resistance.
• Volume depletion - traumatic hemorrhage, atraumatic blood loss, diarrhea, vomiting, sweating, decreased oral hydration[17]

Prognosis

Patients with easily treatable etiologies of cardiac syncope will generally have a good prognosis with adequate treatment. Patients with advanced heart failure and syncope, however, have a 45% one-year mortality.[6] Patients with cardiac syncope are also more likely to have recurrent syncopal events with an estimated recurrence rate of 33% within 3 years. Recurrent syncope can worsen a patient's quality of life, especially if syncopal events result in broken bones, intracranial bleeds, or other traumatic injuries.[17]

Complications

Although the treatment and diagnostic modalities employed in the management of cardiac syncope are largely targeted at preventing a future sudden cardiac death, syncopal events can have immediate complications. These complications include costs of hospitalizations, treatment, and lost work time, as well as physical injuries related to falling during the syncopal event.