EDITORIAL
NEJM
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Syncope is the sudden and transient loss of consciousness accompanied by a loss of postural tone. The term derives from the Greek word synkoptein, meaning "to cut short," and purportedly, Hippocrates himself provided the first description of a patient with the disorder.1 Syncope accounts for 3 percent of emergency room visits and 1 to 6 percent of all hospital admissions, and it costs $750 million per year to diagnose and treat.2,3 The causes of syncope range from the benign to the lethal. Rational patient care is facilitated by an understanding of the pathophysiology and epidemiology of syncope, as well as of the indications for and limitations of the available diagnostic tests.
Although a number of disease processes can cause syncope, hypoperfusion of the cerebral cortexes and reticular activating system is the common final pathway for the majority of episodes.4 Interruption of cerebral blood flow for 8 to 10 seconds usually produces loss of consciousness. In contrast, seizures, which are difficult to distinguish from syncope, impair consciousness by disturbing cerebral electrical activity without necessarily affecting perfusion.
In this issue of the Journal, Soteriades et al. provide valuable information about the incidence, causes, and prognosis of syncope in a suburban population.5 Among 7814 male and female participants in the Framingham Heart Study, 822 had syncope during an average follow-up of 17 years. Vasovagal syncope, also known as neurally mediated or neurocardiogenic syncope, occurred most frequently (in one fifth of patients) but, reassuringly, had a benign prognosis with no increase in the risk of death. In contrast, cardiac causes of syncope, which were identified in close to 10 percent of cases, were associated with an increase in the risk of death by a factor of 2. Patients with neurologic causes of syncope, including stroke, transient ischemic attack, and seizures, as well as those with syncope of unknown cause, were at intermediate risk. Although the type of syncope was classified on the basis of physicians' review of medical records, an approach that is subject to error, the findings are consistent with those of a number of prior studies.4
It is important to recognize that cardiac syncope can be a harbinger of sudden death. Because patients with this condition have a six-month mortality rate that exceeds 10 percent,5 timely and thorough evaluation is warranted. A history should be obtained and a physical examination and electrocardiography should be performed in all patients, with the initial goal of excluding an acute illness, such as myocardial infarction or pulmonary embolism, as the cause of syncope. The precise cause of syncope is identified during the initial evaluation in fewer than half the patients, but the presence or absence of heart disease offers important information about the need for further evaluation.6 Young patients without structural heart disease who have a history consistent with the presence of vasovagal, orthostatic, or medication-induced syncope have a good prognosis and can be followed expectantly. Elderly patients have a higher incidence of syncope as a result of impaired regulation of cerebral blood flow and polypharmacy.
Transient ventricular tachycardia and bradyarrhythmias, the most common cardiac causes of syncope, usually occur in patients with structural heart disease.7 Echocardiography to assess left ventricular function and tests for coronary artery disease are often warranted. Patients with known or suspected cardiac disease should generally be evaluated in the hospital, during continuous electrocardiographic monitoring, because of the possibility of a fatal arrhythmia and because of their high rate of early death.7
Further diagnostic evaluation and treatment should be guided by the findings on the initial assessment. For patients with structural heart disease, the risk of ventricular arrhythmias and sudden death increases with the severity of ventricular dysfunction. Patients with unexplained syncope, prior myocardial infarction, and severely reduced left ventricular function (as evidenced by an ejection fraction of less than 0.30) are at particularly high risk and should receive an implantable cardioverter�defibrillator.8 Patients with a prior myocardial infarction but less severe left ventricular dysfunction should undergo electrophysiologic testing to determine whether they have ventricular arrhythmias, supraventricular arrhythmias, or bradyarrhythmias. Because patients with syncope and inducible ventricular tachycardia have a prognosis similar to that of patients presenting with spontaneous, sustained ventricular arrhythmias, they, too, should be considered strong candidates for an implantable cardioverter�defibrillator.9 Electrophysiologic testing is rarely warranted in patients with normal hearts and normal electrocardiograms unless syncopal episodes are severe, recurrent, or accompanied by palpitations.7 In patients with normal left ventricular function, the absence of inducible ventricular tachycardia is associated with a benign prognosis.
Even after a thorough evaluation, the cause of syncope remains unknown in many patients. Indeed, Soteriades et al. report that syncope of unknown cause was the single largest category among patients with syncope in their cohort. This heterogeneous group includes patients with a benign cause of syncope (and thus a benign prognosis) as well as those with an undiagnosed cardiac cause. The challenge is to identify those who are at high risk for death. Certain patients remain at high risk for sudden death even in the absence of evident arrhythmias. Electrophysiologic studies are insensitive for the detection of arrhythmia in patients with nonischemic cardiomyopathy, and over half of these patients who have syncope will have a spontaneous episode of ventricular tachycardia within two years after the diagnosis of syncope.9 Empirical implantation of a cardioverter�defibrillator should be strongly considered in these patients unless a benign cause of syncope, such as hypotension from excessive diuresis or vasodilator therapy, can be identified.
Electrophysiologic studies are also insensitive for the detection of bradyarrhythmias.9 Patients with syncope and bifascicular or trifascicular block are at increased risk for sudden death and should undergo careful evaluation of the need for empirical implantation of a permanent pacemaker.10 A family history of unexplained sudden death or electrocardiographic abnormalities consistent with the presence of rare, but increasingly recognized, genetic causes of sudden death � including the long-QT syndrome, the Brugada syndrome (associated with ST-segment elevation in leads V1, V2, and V3), arrhythmogenic right ventricular dysplasia (often characterized by T-wave inversions in leads V1, V2, and V3), and hypertrophic cardiomyopathy � should prompt referral of the patient to an arrhythmia specialist for additional evaluation.
Recurrent, unexplained syncope can be a difficult diagnostic problem. Tilt-table testing can help establish a diagnosis of vasovagal syncope, although false negative and false positive results are common.7 The test is also useful for identifying patients with a prominent bradycardic response who may benefit from the implantation of a permanent pacemaker.11 In patients with rare episodes of syncope, the long intervals between episodes limit the value of Holter monitoring. Implantable cardiac monitors should be considered in these patients. Placed subcutaneously in the pectoral region with the patient under local anesthesia, these devices function as permanent loop recorders, recording rhythm abnormalities automatically or when activated by the patient. A diagnosis can be made in up to 90 percent of patients who have these monitors; the most common finding is either sinus rhythm during syncope, thereby excluding an arrhythmic cause, or an infrequent bradyarrhythmia.12 An evaluation for neurologic and psychiatric causes of syncope is warranted in some patients.
Syncope is a common and challenging problem. It occurs in the old and the young; it can be infrequent or recurrent; and it may have a benign or fatal prognosis. Patients with vasovagal syncope can be comforted by the knowledge that their prognosis is excellent and their risk of death is not increased. Similarly, patients who have cardiac syncope as a result of arrhythmia, although at increased risk for sudden death in the absence of therapy, may be protected by a pacemaker or an implantable cardioverter�defibrillator. The approach to patients with syncope of unknown cause presents the greatest challenge. A strategy that focuses on getting to the heart of the matter will identify patients at greatest risk.
William H. Maisel, M.D., M.P.H.
William G. Stevenson, M.D.
Brigham and Women's Hospital
Boston, MA 02115
References
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- Linzer M, Yang EH, Estes NAM III, Wang P, Vorperian VR, Kapoor WN. Diagnosing syncope. I. Value of history, physical examination, and electrocardiography. Ann Intern Med 1997;126:989-996. [ISI][Medline]
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- Stevenson WG, Epstein LM, Maisel WH, Sweeney MO, Stevenson LW. Syncope. In: Camm AJ, ed. Arrhythmias in heart failure. Armonk, N.Y.: Futura Publishing, 2002:56-60.
- Gregoratos G, Cheitlin MD, Conill A, et al. ACC/AHA guidelines for implantation of cardiac pacemakers and antiarrhythmia devices: executive summary -- a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Pacemaker Implantation). Circulation 1998;97:1325-1335. [Full Text]
- Sutton R, Brignole M, Menozzi C, et al. Dual-chamber pacing in the treatment of neurally mediated tilt-positive cardioinhibitory syncope: pacemaker versus no therapy: a multicenter randomized study. Circulation 2000;102:294-299. [Abstract/Full Text]
- Krahn AD, Klein GJ, Yee R, Takle-Newhouse T, Norris C. Use of an extended monitoring strategy in patients with problematic syncope. Circulation 1999;99:406-410. [Abstract/Full Text]