Causes of Syncope

Table 10.1 lists the major causes of syncope, divided into five major categories. Diagnosing syncope associated with the first four categories depends on taking a careful history and performing a careful physical examination. The majority of patients with syncope, however, fall into the fifth category: syncope associated with cardiac arrhythmias. In most cases, therefore, the clinician is left with having to assess whether the patient has syncope directly caused by cardiac arrhythmias (bradyarrhythmias or tachyarrhythmias) or a variant of vasodepressor syncope, in which bradycardia is often a prominent feature.

Table 10.1 Major causes of syncope.

Bradyarrhythmias That Cause Syncope

Although bradyarrhythmias have been claimed as a common cause of syncope, they actually cause less than 5% of syncopal episodes. Nonetheless, bradyarrhythmias must be regarded as an important cause of syncope because they are always completely treatable. The evaluation of patients with bradyarrhythmias has been discussed in detail in Chapter 5. In this section, we review the causes and evaluation of bradyarrhythmias only briefly.

Sinus Nodal Dysfunction

Abnormalities of the sinus node are common in elderly patients and are most often caused by idiopathic fibrous degeneration of the sinus node. Sinus nodal dysfunction is frequently associated with a similar fibrous degeneration of the AV conduction system, producing AV block, or of the atrial tissue, producing atrial tachyarrhythmias. Although sinus nodal disease is most often benign, the potential for sudden death is real in patients whose sinus nodal dysfunction is severe enough to produce syncope.

In most patients with syncope due to sinus nodal dysfunction, abnormalities of the sinus node are overt, and are usually seen during simple cardiac monitoring. Occasionally, however, electrophysiologic testing is needed to diagnose sinus nodal dysfunction even in these patients. Therefore, electrophysiologic testing should be considered in patients with syncope when the etiology remains unknown after a full evaluation, especially in elderly patients.

AV Block

On one hand, AV nodal disease is a rare cause of syncope. On the other, block in the His–Purkinje tissue is the most common cause of syncope due to bradyarrhythmias. When syncope is due to AV block, the ECG and cardiac monitoring most often reveal clues as to the etiology of syncope. Obviously, complete heart block in a patient presenting with syncope is an indication for pacing. Second-degree AV block should also be regarded as a strong clue. Even more subtle findings that can usually be safely ignored, such as intraventricular conduction disturbances or first-degree AV block, should be regarded with a high degree of suspicion in patients presenting with syncope. In such patients, electrophysiologic testing should be strongly considered, especially if no other etiology for syncope presents itself.

Tachyarrhythmias That Cause Syncope

Supraventricular Tachycardias

Although supraventricular tachycardias are relatively frequent arrhythmias, they only rarely cause syncope. In most cases in which syncope is associated with supraventricular tachycardia, a second condition is responsible for it. Most commonly, this second condition is sinus nodal dysfunction. In a patient with sinus nodal dysfunction, supraventricular tachycardia (usually atrial fibrillation or flutter) causes exaggerated overdrive suppression of the diseased sinus node. When the arrhythmia terminates, there is a prolonged sinus pause, leading to loss of consciousness. Less commonly, syncope can accompany supraventricular tachycardias that occur without concomitant sinus nodal dysfunction. In these cases, recent evidence suggests that syncope is the result of a vasodepressor reflex, and that the tachycardia itself may simply be the triggering stimulus for the vasodepressor response that produces syncope.

When syncope is associated with supraventricular tachycardia, loss of consciousness is almost always preceded by a prominent and unambiguous sensation of palpitations. Such a history should lead the physician immediately to suspect tachycardia as an etiology. Electrophysiologic testing should be considered early in the evaluation of such patients.

Ventricular Tachyarrhythmias

Although ventricular arrhythmias were not generally recognized until the mid 1980s as a major cause of syncope, it is now apparent that these arrhythmias are frequently responsible for syncope, especially in patients with underlying cardiac disease. Ventricular tachycardia or fibrillation probably represents the cause of syncope in up to 40% of patients with heart disease who present with this symptom. Because syncope due to ventricular tachyarrhythmias is a sign of impending sudden death, the new onset of syncope in patients with significant underlying heart disease should be treated as a medical emergency.

Syncope caused by ventricular tachyarrhythmias usually occurs suddenly and without warning, although in some patients with sustained ventricular tachycardia the sensation of a rapid heart rate may precede loss of consciousness. The syncope can be quite fleeting, lasting only for moments, or it may present as dramatically as a self-terminating cardiac arrest. No other cause of syncope is likely to produce the type of pulseless, apneic, cyanotic patient produced by a ventricular arrhythmia. Many patients referred to electrophysiologists for syncope of unknown etiology are reclassified as having had an aborted cardiac arrest after careful interrogation of witnesses.

Because most ventricular tachyarrhythmias are reentrant in nature, and because most reentrant circuits require the substrate produced by myocardial fibrosis, ventricular arrhythmias are unlikely to be the cause of syncope unless a disorder of the ventricular myocardium is present. When such myocardial disease is present, however, ventricular arrhythmias must be considered as being the most likely cause of syncope until proven otherwise. When evaluating a patient with syncope of unknown etiology, one of the first questions the physician must answer is whether the patient has underlying cardiac disease. If so, the physician’s focus must immediately shift away from merely preventing syncope and toward preventing sudden death.

Accordingly, if a careful history and physical examination do not yield the cause of syncope, a noninvasive cardiac workup to assess the status of the ventricular myocardium must be considered an essential part of the evaluation of the patient. If ventricular function is normal and there is no ventricular hypertrophy, ventricular arrhythmias can usually be dismissed as a cause for syncope. (The clinician, however, should also be mindful of the relatively uncommon forms of ventricular arrhythmia that can produce syncope in the absence of structural heart disease, including the long QT syndromes, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia).

If the cardiac evaluation reveals segmental wall-motion abnormalities or a reduced left ventricular ejection fraction, potentially lethal ventricular arrhythmias must be strongly considered.

The signal-averaged ECG may also help to determine the likelihood that ventricular arrhythmias are the cause of a patient’s syncope. The sensitivity of the signal-averaged ECG has generally been reported as being 73–89%, and the specificity as 89–100%, in predicting whether a patient presenting with syncope will have a positive electrophysiologic study.

Ambulatory monitoring should play a very small role in diagnosing ventricular arrhythmias as a cause of syncope, for three reasons. First, ventricular arrhythmias producing syncope are sporadic and unpredictable. The odds of capturing a syncope-producing ventricular arrhythmia while monitoring for a few days or a few weeks are small, and in a patient with underlying cardiac disease, the absence of such arrhythmias on ambulatory monitoring is meaningless. Second, the presence or absence of asymptomatic ventricular ectopy in such patients has extremely small specificity (so finding ectopy on ambulatory monitoring does not bring the clinician any closer to making a diagnosis). Third, once a patient with significant underlying heart disease has syncope of unclear origin, that patient must be presumed to be in imminent danger of sudden death, and the time for leisurely outpatient monitoring has passed. The patient should be evaluated as if he or she had suffered not “just” syncope, but an aborted cardiac arrest.

Once it has been determined that ventricular arrhythmias are reasonably likely to be the cause of a patient’s syncope, that patient should immediately be admitted to a monitored bed until lethal ventricular tachyarrhythmias have been either definitively ruled out or adequately treated. In such patients, the electrophysiology study is often the most direct way of determining the cause of syncope and deciding on appropriate therapy.

Vasodepressor Syncope

Vasodepressor syncope is by far the most common cause of syncope. The fact that vasodepressor syncope is known by so many names (including vasovagal syncope, cardioneurogenic syncope, and reflex syncope) is a reflection of the fact that its mechanism is poorly understood. To make matters worse, clinical syndromes that are almost certainly subcategories of vasodepressor syncope (Table 10.2) have usually been discussed in the literature as if they were completely unique and unrelated entities. This practice has led clinicians to the widespread misconception that there must be scores of causes of syncope, and accordingly, to a widespread attitude of hopelessness when faced with a patient who has syncope. In fact, patients who are prone to vasodepressor syncope often have a history of multiple syncopal episodes, and their episodes frequently match several of the syndromes listed in Table 10.2. Recognition that these different syndromes are merely variants of the same basic mechanism leads the physician immediately to the diagnosis in the majority of cases, and is a major step toward prescribing effective treatment.

Table 10.2 Syndromes of vasodepressor syncope.

The common denominator in all varieties of vasodepressor syncope is most likely the stimulation of the medullary vasodepressor region of the brain stem. The pathways that stimulate the vasodepressor region (afferent pathways) can arise from numerous locations—the resultant clinical syndrome has most often been named by the event that results in afferent stimulation of the medullary vasodepressor region (see Table 10.2). Once the vasodepressor region has been stimulated, that region generates efferent signals that cause both increased vagal tone (via the vagus nerve) and vasodilation (by pathways that are not well understood). Diminished cardiac filling and bradycardia follow, leading to syncope.

It should be recognized that although bradycardia is therefore often a prominent feature of this type of syncope, it is only rarely as important as vasodilation in producing symptoms. This is why therapy with pacemakers is usually not of significant benefit to patients suffering from vasodepressor syncope. It is also why the author has chosen to use the term “vasodepressor syncope” from the available menagerie of names.

In many of the syndromes listed in Table 10.2, stimulation of the cardiac C fibers (mechanocardiac receptors in the left ventricle) appears to be the origin of afferent stimulation of the vasodepressor region. The C fibers are stimulated when a volume-depleted ventricle is contracting vigorously, a situation that most commonly occurs when the venous return is decreased and the sympathetic tone is high.

Vasodepressor syncope tends to have characteristic clinical features that should lead the physician directly to the correct diagnosis. Many individuals have a predisposition to vasodepressor syncope, so that episodes recur periodically during a patient’s lifetime. The initial episode of syncope often occurs during the patient’s teen years. Over time, such individuals have episodes that match several of the syndromes listed in Table 10.2. Vasodepressor syncope is most often preceded by at least a few seconds of prodromal symptoms (lightheadedness, ringing in the ears, visual disturbances, diaphoresis, and nausea are the most prominent) and almost always occurs when the patient is upright (sitting or standing). Syncope resolves almost immediately when the patient assumes the supine position (often by falling). The vasodilation tends to persist for several minutes, so that if the patient tries to get up immediately after such an episode, a second syncopal episode often occurs. A prolonged feeling of being “washed out” and unable to function is common after vasodepressor syncope and is probably related to a residual autonomic imbalance triggered by the episode; unfortunately, these postdromal symptoms are often mistaken by clinicians for a “postictal” state. Patients who are predisposed to vasodepressor syncope will often have episodes of syncope when they are in a warm environment, when they have a viral illness, when they are dehydrated, or when they are under significant stress. The syndromes related to cardiac C fiber stimulation are relatively uncommon in patients with significant cardiac dysfunction, possibly because the C fibers are affected by myocardial disease. Not uncommonly, patients prone to vasodepressor syncope will experience a flurry of syncopal events over a period of days or weeks. Usually the reason for these flurries is unclear. In some instances, however, a flurry may be a clue that a patient has developed an occult peptic ulcer or a urinary tract infection that predisposes to vasodepressor episodes.

Obviously, given these prominent clinical features of vasodepressor syncope, taking a careful history is vitally important in making the correct diagnosis. As can be seen by studying Table 10.2, a patient’s activity at the time of syncope yields strong clues as to the mechanism. Syncope that occurs while micturating, defecating, coughing, or swallowing is almost always vasodepressor in origin. The same holds for syncope associated with fright, pain, noxious stimulation, or severe emotional stress. These syncopal episodes rarely cause a diagnostic dilemma, but obtaining a history of such episodes in the past may yield clues as to the etiology of more recent and less clear-cut episodes of vasodepressor syncope. Syncope that occurs immediately after stopping prolonged or vigorous exercise is usually vasodepressor in origin (as opposed to syncope that occurs during vigorous exercise). In the author’s experience, syncope that occurs in church (especially during the winter holidays, when tightly packed worshippers remain bundled in layers of cold-weather raiment) is usually due to a vasodepressor response. In a related phenomenon, syncope among members of a choir is also vasodepressor in origin in most cases. No other form of syncope is as situational as vasodepressor syncope.

Tilt-Table Testing

In recent years, the upright tilt-table study has entered common usage for determining a patient’s propensity to develop vasodepressor syncope. When subjected to an upright, motionless tilt, patients who have vasodepressor syncope will often develop a frank syncopal episode.

The protocol used in performing tilt-table testing varies among laboratories, but most centers tilt patients for 15–45 minutes at 60–85̊. “Normal” individuals compensate for such a tilt by increasing both α- and β-adrenergic tone as a result of baroreceptor stimulation and thus compensating for the decrease in venous return. In susceptible patients, however, these compensatory mechanisms eventually collapse. In such individuals, venous return is apparently never completely compensated. Thus, sympathetic tone progressively increases until, eventually, vigorous squeezing of the relatively empty ventricles results in recruitment of the cardiac C fibers. This, in turn, causes stimulation of the medullary vasodepressor region. The result is a sudden withdrawal of sympathetic tone, a sudden increase in vagal tone, sudden vasodilation, and syncope. A positive tilt-table study therefore identifies a patient who is prone to vasodepressor syncope.

The tilt-table study is positive in 30–74% of patients with syncope of unknown origin. Occasionally, however, an isoproterenol infusion is necessary to bring out syncope during tilt-table testing, even in patients who, by history, clearly have vasodepressor syncope. Further, the reproducibility of tilt-table testing has not been demonstrated, so that serial tilt-table studies to measure the efficacy of pharmacologic treatment are of questionable value.

The tilt-table study has been reported as being positive in up to 7% of individuals who do not have a history of syncope. Whether this represents a “false-positive” response or a true propensity for vasodepressor syncope is unknown. It is likely, however, that aggressive upright tilting would eventually produce hemodynamic collapse in almost anybody—otherwise, crucifixion would never have become a popular and effective form of execution.

Treating Vasodepressor Syncope

The most effective form of therapy for vasodepressor syncope is educating the patient. The patient who has had one or more vasodepressor episode should be advised as to the types of situation that predispose to vasodepressor syncope. Aggressive hydration should be advised when the patient is exercising, suffering from a viral illness or other infectious condition, or working in a hot environment. The symptoms experienced by the patient before losing consciousness—the prodrome of vasodepressor syncope—should be stressed as a warning that syncope is imminent. The patient should be advised to immediately assume the supine position under such circumstances until the symptoms pass, in order to avoid losing consciousness. With such measures, especially in patients who have infrequent episodes, often no other therapy is necessary.

Pharmacologic therapy for vasodepressor syncope is occasionally effective. β-blocking agents have recently become popular for treating vasodepressor syncope. Such therapy may seem paradoxical in a condition in which bradycardia is often a prominent feature, but because hypersympathetic tone is necessary to engage the cardiac C fibers, there is some rationale for using β-blockers. Disopyramide has also been used. Disopyramide has a vagolytic effect, but it also has a direct negative inotropic effect on the heart, and thus presumably inhibits stimulation of the cardiac C fibers as well. β-blockers and disopyramide can be expected to be effective only when the patient experiences episodes of syncope that are mediated by the cardiac C fibers, however. In addition, theophylline preparations, transdermal scopolamine, midodrine, fludrocortisone, and fluoxetine and other serotonin uptake inhibitors have been used with mixed success. Often, combinations of medications must be tried. Pharmacologic therapy should be reserved for patients who have frequent episodes of syncope and in whom nonpharmacologic methods have not been helpful.

Because bradycardia alone is only rarely the proximate cause of syncope in the vasodepressor syndromes, pacemaker therapy prevents these episodes only rarely and is generally reserved for patients who have profound and prolonged bradycardia accompanying their episodes. If pacemaker therapy is to be used, a dual-chamber pacemaker should be implanted to maintain AV coordination during pacing.

Evaluation of the Patient With Syncope

Table 10.3 outlines a three-step approach for evaluating the patient with syncope. The most important step is to obtain a careful history and perform a physical examination. Although this step is important in the evaluation of any medical condition, it is particularly important in the patient with syncope. The history yields important clues relating to the presence of neurologic conditions, underlying cardiac conditions, and vasodepressor syncope. The physical examination is important in uncovering the presence of occult neurologic lesions and cardiac disorders. When you approach the patient with syncope, your attitude must be that you will not leave the bedside until a presumptive diagnosis is clear; for, once you decide that laboratory studies must be relied upon to make the diagnosis, you’ve got a very difficult task ahead.

Table 10.3 Evaluation of the patient with syncope.

The ECG is important in the patient with syncope. The presence of Q waves, an intraventricular conduction abnormality, or ventricular arrhythmias should alert the physician to the presence of underlying cardiac disease that may predispose to lethal ventricular arrhythmias. Heart block or the presence of sinus bradycardia or sinus pauses point to bradycardias as a potential etiology. A short PR interval or preexcitation indicate a bypass tract, which may be the cause of syncope. The ECG should also be examined for signs of left ventricular hypertrophy, Brugada syndrome, repolarization abnormalities that might indicate a propensity for torsades de pointes, or signs of underlying ischemic heart disease.

The routine performance of brain scanning or electroencephalography (EEG) has not been helpful, and these tests should not be ordered unless the history or physical examination suggests a neurologic lesion or seizures. In cases in which a seizure has occurred but the EEG is negative, tilt-table testing should be considered, because seizure-like activity can be reproduced by inducing vasodepressor syncope in some patients (thus sparing the patient the diagnosis of epilepsy).

Tilt-table testing is not necessary in patients with a classic history for vasodepressor syncope. In these patients, therapy for vasodepressor syncope is appropriate even if the tilt-table study is negative.

Exercise-related syncope is usually vasodepressor in origin in younger patients, but in older patients is more likely to be related to ventricular arrhythmias, especially if underlying cardiac disease is present. Even younger patients with exercise-related syncope should have treadmill testing, however, to look for exercise-induced arrhythmias, and echocardiograms to look for structural abnormalities that might produce syncope (especially hypertrophic cardiomyopathy).

The most important, and the most neglected, part of the evaluation of syncope is to rule out underlying cardiac disease. In middle-aged and elderly patients with syncope, especially if syncope is of recent onset, a noninvasive evaluation of ventricular function is essential. An echocardiogram is probably the most useful means of assessing cardiac function, because it also yields information relative to potential aortic outflow lesions. A signal-averaged ECG also may be performed as a screening study for ventricular arrhythmias in patients suspected of having underlying cardiac disease.

Although arrhythmias are an important cause of syncope, ambulatory monitoring has generally been disappointing as a means of diagnosing arrhythmic syncope. A cardiac loop recorder (event recorder) can be helpful when symptomatic sinus nodal dysfunction or supraventricular tachycardia is suspected. When ventricular arrhythmias are suspected as the cause of syncope, however, ambulatory monitoring is inappropriate.

Immediate electrophysiologic testing is indicated in patients with syncope of unknown origin who are found to have had a previous myocardial infarction, a depressed left ventricular ejection fraction, nonsustained ventricular tachycardia, or a positive signal-averaged ECG. These patients should be presumed to be at high risk for sudden death from ventricular arrhythmias until proven otherwise, and should be hospitalized and monitored until ventricular arrhythmias are either ruled out or controlled.

Electrophysiologic testing, usually in conjunction with tilt-table testing, should be strongly considered when a careful, noninvasive evaluation has failed to reveal a presumptive diagnosis for syncope. Such combined testing can yield a presumptive diagnosis in up to 74% of patients whose syncope was of unknown origin before testing.

The evaluation of patients with syncope has often been regarded as difficult and frustrating. Using the principles outlined in this chapter, however, a diagnosis can be made relatively quickly in the vast majority of patients presenting with syncope.

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