Syncope: Etiology and diagnostic approach
ABSTRACTThere are three major types of syncope: neurally mediated (the most common), orthostatic hypotensive, and cardiac (the most worrisome). Several studies have shown a normal long-term survival rate in patients with syncope who have no structural heart disease, which is the most important predictor of death and ventricular arrhythmia. The workup of unexplained syncope depends on the presence or absence of heart disease: electrophysiologic study if the patient has heart disease, tilt-table testing in those without heart disease, and prolonged rhythm monitoring in both cases if syncope remains unexplained.
KEY POINTS
- Neurally mediated forms of syncope, such as vasovagal, result from autonomic reflexes that respond inappropriately, leading to vasodilation and relative bradycardia.
- Orthostatic hypotension is the most common cause of syncope in the elderly and may be due to autonomic dysfunction, volume depletion, or drugs that block autonomic effects or cause hypovolemia, such as vasodilators, beta-blockers, diuretics, neuropsychiatric medications, and alcohol.
- The likelihood of cardiac syncope is low in patients with normal electrocardiographic and echocardiographic findings.
- Hospitalization is indicated in patients with syncope who have or are suspected of having structural heart disease.
If the electrophysiologic study is negative
If the electrophysiologic study is negative, the differential diagnosis still includes arrhythmia, as the yield of electrophysiologic study is low for bradyarrhythmias and some ventricular tachycardias, and the differential diagnosis also includes, at this point, neurally mediated syncope.
The next step may be either prolonged rhythm monitoring or tilt-table testing. An event monitor or an implantable loop recorder can be placed for prolonged monitoring. The yield of the 30-day event monitor is highest in patients with frequently recurring syncope, in whom it reaches a yield of up to 40% (10% to 20% will have a positive diagnosis of arrhythmia, while 15% to 20% will have symptoms with a normal rhythm).31,34 The implantable recorder has a high overall diagnostic yield and is used in patients with infrequent syncopal episodes (yield up to 50%).1,35,36
In brief, there are two diagnostic approaches to unexplained syncope: the monitoring approach (loop recorder) and the testing approach (tilt-table testing). A combination of both strategies is frequently required in patients with unexplained syncope, and, according to some investigators, a loop recorder may be implanted early on.21
Heart disease with left ventricular dysfunction and low ejection fraction
In patients with heart disease with left ventricular dysfunction and an ejection fraction of 35% or less, an implantable cardioverter-defibrillator can be placed without the need for an electrophysiologic study. These patients need these devices anyway to prevent sudden death, even if the cause of syncope is not an arrhythmia. Patients with a low ejection fraction and a history of syncope are at a high risk of sudden cardiac death.32 Yet in some patients with newly diagnosed cardiomyopathy, left ventricular function may improve with medical therapy. Because the arrhythmic risk is essentially high during the period of ventricular dysfunction, a wearable external defibrillator may be placed while the decision about an implantable defibrillator is finalized within the ensuing months.
In patients with hypertrophic cardiomyopathy, place an implantable cardioverter-defibrillator after any unexplained syncopal episode.
Valvular heart disease needs surgical correction.
If ischemic heart disease is suspected, coronary angiography is indicated, with revascularization if appropriate. An implantable cardioverter-defibrillator should be placed if the ejection fraction is lower than 35%. Except in a large acute myocardial infarction, the substrate for ventricular tachycardia is not ameliorated with revascularization.32,37 Consider an electrophysiologic study when syncope occurs with coronary artery disease and a higher ejection fraction.
A note on left or right bundle branch block
Patients with left or right bundle branch block and unexplained syncope (not clearly vasovagal or orthostatic) likely have syncope related to intermittent high-grade atrioventricular block.38
One study monitored these patients with an implanted loop recorder and showed that about 40% had a recurrence of syncope within 48 days, often concomitantly with complete atrioventricular block. About 55% of these patients had a major event (syncope or high-grade atrioventricular block).39 Many of the patients had had a positive tilt test; thus, tilt testing is not specific for vasovagal syncope in these patients and should not be used to exclude a bradyarrhythmic syncope. Also, patients selected for this study had undergone carotid sinus massage and an electrophysiology study with a negative result.
In another analysis, an electrophysiologic study detected a proportion of the bradyarrhythmias but, more importantly, it induced ventricular tachycardia in 14% of patients with right or left bundle branch block. Although it is not sensitive enough for bradyarrhythmia, electrophysiologic study was highly specific and fairly sensitive for the occurrence of ventricular tachycardia on follow-up.38 Thus, unexplained syncope in a patient with right or left bundle branch block may warrant carotid sinus massage, then an electrophysiologic study to rule out ventricular tachycardia, followed by placement of a dual-chamber pacemaker if the study is negative for ventricular tachycardia, or at least placement of a loop recorder.
INDICATIONS FOR HOSPITALIZATION
Patients should be hospitalized if they have severe hypovolemia or bleeding, or if there is any suspicion of heart disease by history, examination, or electrocardiography, including:
- History of heart failure, low ejection fraction, or coronary artery disease
- An electrocardiogram suggestive of arrhythmia (Table 3)
- Family history of sudden death
- Lack of prodromes; occurrence of physical injury, exertional syncope, syncope in a supine position, or syncope associated with dyspnea or chest pain.2,40
In these situations, there is concern about arrhythmia, structural heart disease, or acute myocardial ischemia. The patient is admitted for immediate telemetric monitoring. Echocardiography and sometimes stress testing are performed. The patient is discharged if this initial workup does not suggest underlying heart disease. Alternatively, an electrophysiologic study is performed or a device is placed in patients found to have structural heart disease. Prolonged rhythm monitoring or tilt-table testing may be performed when syncope with underlying heart disease or worrisome features remains unexplained.
Several Web-based interactive algorithms have been used to determine the indication for hospitalization. They incorporate the above clinical, electrocardiographic, and sometimes echocardiographic features.2,24,25,40–42 A cardiology consultation is usually necessary in patients with the above features, as they frequently require specialized cardiac testing.
Among high-risk patients, the risk of sudden death, a major cardiovascular event, or significant arrhythmia is high in the first few days after the index syncopal episode, justifying the hospitalization and inpatient rhythm monitoring and workup in the presence of the above criteria.24,40,42
SYNCOPE AND DRIVING
A study has shown that the most common cause of syncope while driving is vasovagal syncope.6 In all patients, the risk of another episode of syncope was relatively higher during the first 6 months after the event, with a 12% recurrence rate during this period. However, recurrences were often also seen more than 6 months later (12% recurrence between 6 months and the following few years).6 Fortunately, those episodes rarely occurred while the patient was driving. In a study in survivors of ventricular arrhythmia, the risk of recurrence of arrhythmic events was highest during the first 6 to 12 months after the event.43
Thus, in general, patients with syncope should be prohibited from driving for at least the period of time (eg, 6 months) during which the risk of a recurrent episode of syncope is highest and during which serious cardiac disease or arrhythmia, if present, would emerge. Recurrence of syncope is more likely and more dangerous for commercial drivers who spend a significant proportion of their time driving; individualized decisions are made in these cases.
