Syncope is defined as a sudden, transient loss of consciousness associated with the inability to maintain postural tone. In as many as 50 percent of cases, syncope defies diagnosis.[1-5] This article discusses a relatively new diagnostic tool, the head-upright tilt test, and its use in the evaluation of patients with syncope.
Scope and Impact of Syncope
Each year, 300,000 patients present to emergency departments because of syncope.[1-3,6] An equal number of patients are evaluated for syncope in physicians' offices. The estimated cost of evaluating patients with syncope is approximately $750 million annually.
In the Framingham study, 3.0 percent of 2,336 men and 3.5 percent of 2,873 women (combined mean age: 46 years) had an episode of syncope. Thirty percent of the men and 27 percent of the women with syncope had recurrent episodes.
The incidence of syncope has been found to increase with increasing age. In a study of 711 elderly patients 75 years of age or older living in a long-term care facility, the yearly incidence of syncope was 6 percent and the recurrence rate was 30 percent.[4,9]
Syncope is associated with high morbidity and mortality. The variety of injuries that may result from syncope or fainting range from small contusions and lacerations to major injuries such as cerebral concussions, subdural hematomas and hip fractures. In one study, 37 percent of 204 patients evaluated for syncope had suffered physical injury. In addition, the underlying cause of syncope can have prognostic value. A cardiac cause of syncope has been associated with a one-year mortality rate as high as 30 percent (Table 1).[1,2,10,11]
Evaluation of Syncopal Patients
Table 2 lists the basic pathophysiologic causes of syncope. They can be grouped into six categories: (1) abnormalities of neuroautonomic regulation, (2) cardiac causes, including mechanical or electrophysiologic abnormalities, (3) underlying neurologic disorders such as seizure disorders, (4) abnormalities of metabolic or endocrine function, (5) psychiatric disturbances and (6) syncope of undetermined origin. The history and physical examination are particularly helpful in the evaluation of patients with syncope or near-syncope. In one study, the cause of syncope was identified from the history and physical examination in 25 percent of the patients.
While a complete discussion of the approach to syncope is beyond the scope of this article, a brief algorithm for the evaluation of syncope is presented in Figure 1. Several recent articles provide an excellent review of this subject.[6,12,13]
Pathophysiology of Vasovagal Syncope
When a person moves to the upright position, 300 to 800 mL of blood is shifted from the thorax to the lower extremities and is lost from the central circulation. Subsequently, the decreased central venous return reduces left ventricular filling, with an immediate drop in cardiac output and in arterial blood pressure. The carotid sinus and aortic arch baroreceptors reduce their inhibitory drive to the vasomotor center of the medulla and permit an increased sympathetic tone, with a rise in epinephrine and vasopressin levels, an increased heart rate, increased cardiac contractility and an increase in arteriolar resistance][15-17] and venous tone. As a result, adequate blood pressure and cerebral perfusion are maintained (Figure 2).
In some individuals, the sudden onset of syncope results from an abnormal vasovagal reflex. The abnormal vasovagal response occurs because the compensatory mechanisms that occur in normal individuals on assuming an upright position are disrupted by a sudden drop in venous return to the heart. This reduction in venous return results in an increase in inotropy and, as the vigor of cardiac contractility increases, the left ventricular chamber size decreases while the wall stress increases. The mechanoreceptors, or nonmyelinated C-fibers, of the left ventricle paradoxically fire en masse resulting in an override of the carotid sinus and aortic arch baroreceptors.[15,17] This sudden surge in vagal afferent input to the brainstem somehow mimics the effects of hypertension, which in turn results in a paradoxic decrease in sympathetic tone and an overwhelming increase in vagal efferent tone. The clinical picture is that of hypotension secondary to vasodilatation from sympathetic withdrawal and an overriding vagal tone with bradycardia from direct vagal stimulation (Figure 3).
During the orthostatic stress produced by the tilt test, an abnormal vasovagal response may occur and result in syncope or near-syncope (Figure 4). Vasovagal episodes as long as 72 seconds have been reported (Figure 5).
Head-Upright Tilt Test
The head-upright tilt test has been used for a number of years in the laboratory to evaluate the human physiologic response to gravity.[20,21] Only recently has the tilt test become a clinical tool for the evaluation of patients with syncope of undetermined cause. The tilt test serves as a means of determining which patients are susceptible to vasovagal (neurally mediated) syncope.[23-25]
The physiologic response to the upright tilt was described several decades ago (Table 3). More recently, tilt testing has been used concurrently with cardiac electrophysiologic studies to evaluate the effect of cardiac arrhythmias on hemodynamic stability.
Although vasovagal syncope is classically associated with a distinct prodrome of nausea, diaphoresis, dizziness and other warning signals, it is not always preceded by these symptoms. Vasovagal syncope may occur suddenly without warning. In one study, 53 percent of patients sustaining syncope-related injury were shown by tilt testing to have vasovagal syncope.
Vasovagal syncope should not be confused with orthostatic hypotension, Which is manifested during tilt testing by a sudden drop in blood pressure as soon as the patient is tilted. Vasovagal syncope occurs in patients who initially have a normal response (a rise in blood pressure and heart rate) on tilt testing. Patients with vasovagal syncope exhibit an inability to maintain homeostatic blood pressure and cerebral perfusion due to abnormalities of autonomic regulation.
Method of Tilt-Table Testing
A standard protocol for the head-upright tilt test has not yet been defined. Methods differ in regard to the angle of tilt (60 degrees or greater is generally used), the duration of time for the tilt test and the use of adjuvant drugs such as isoproterenol (Isuprel) to provoke the vasovagal response.[28-30] The head-upright tilt test described in this article, using an 80-degree tilt angle for 30 minutes and adjuvant isoproterenol if needed, is used by the Medical College of Ohio at Toledo and is similar to the protocol used by others.
Usually, children and adults are referred for evaluation of unexplained syncope after a thorough evaluation, which would include a history and physical examination, magnetic resonance imaging (MRI) or computed tomographic (CT) scanning of the head (when indicated), electroencephalography, cerebral blood flow evaluations, echocardiography, cardiac catheterization and cardiac electrophysiology studies.
Head-upright tilt-table testing is performed following six hours of fasting. When possible, it is performed in the morning to obviate the effect of cyclic variation in parasympathetic tone. The patient is placed on a tilt table. An intravenous line is inserted, and continuous electrocardiographic monitoring is performed. In selected patients, transcranial Doppler cerebral blood flow is continuously monitored, and in some patients continuous electroencephalographic monitoring is used. The patient is secured to the tilt table with snug restraints to prevent the patient from falling if syncope occurs. Baseline hemodynamic parameters are measured, and the patient is tilted to an 80-degree head-upright position within 10 seconds (Figures 6 and 7).
Blood pressure is taken every three minutes, the heart rate is monitored, and symptoms and sign of the discomfort of syncope or near-syncope are constantly elicited. The baseline tilt lasts for 30 minutes or until syncope or near-syncope occurs. Vasovagal symptoms (nausea, lightheadedness, blurred vision and loss of sense of well-being) and signs (yawning, sweating, pallor, hypotension and bradycardia) rapidly resolve when the patient is returned to the supine position. If syncope does not occur after 30 minutes, the patient is returned to the supine position, and the tilt test is repeated using a continuous infusion of intravenous isoproterenol, 1 to 6 [Mu] g per minute, to increase the resting baseline heart rate by 20 percent.
ADJUVANT USE OF ISOPROTERENOL
Isoproterenol is added to the tilt-table testing regimen for two reasons.[28,30,35] An adrenergic agonist, isoproterenol can increase susceptibility to vasovagal syncope, since this reaction is triggered by a strong sympathetic stimulus. Early studies indicated that isoproterenol can also increase the sensitivity of the test by as much as 60 percent, without an appreciable change in specificity (Table 4). Contrary evidence was recently reported in another study in which the specificity of tilt testing using isoproterenol in young adults was only 33 percent. This study advised against adjuvant use of isoproterenol. To clarify this concern, further investigation is needed in this evolving area.
Convulsive Syncope vs. Epilepsy
Recognizing nonepileptic conditions that produce seizures is important. A misdiagnosis of epilepsy delays identification of the correct etiology and hinders therapy. Early in our experience, some patients displayed tonic-clonic seizure-like activity (convulsive syncope) during tilt test-induced syncope. It was postulated that some patients with the clinical presentation of epilepsy may actually have convulsive syncope.
In one study, 15 patients with idiopathic, recurrent, generalized seizures refractory to therapy underwent head-upright tilt testing while in a drug-free state. Syncope associated with tonic-clonic seizure-like activity occurred in six patients (40 percent) during the initial tilt and in four patients (27 percent) during isoproterenol infusion. Five of the patients with positive results initially on tilt testing underwent a second drug-free tilt test with continuous electroencephalographic monitoring. In each of these patients, the electroencephalogram demonstrated a diffuse slowing indicative of cerebral hypoxia rather than the hypersynchronous spike wave activity usually seen in epilepsy (Figure 8).
These results suggest that tilt-table testing may be useful in differentiating convulsive syncope from epilepsy in patients with recurrent idiopathic seizures. Other investigators reported similar findings with tilt testing among 16 patients who were found to have convulsive syncope during ocular compression testing.
Indications for Tilt Testing
The indications for tilt testing are currently evolving. Patients with unexplained syncope are diagnostically challenging. The integration of invasive and noninvasive studies before tilt testing must be considered. The decision to use tilt testing is based on the clinical situation. Certainly, the diagnosis of vasovagal-mediated syncope is attractive, since the prognosis is good and effective therapy exists. However, physicians should be careful not to overlook other serious coexisting problems, such as cardiac arrhythmias and seizure disorders.
Therapy for Vasovagal Syncope
Several principles guide therapy. [Beta.sub.1] adrenergic (cardioselective) blocking agents can prevent reflex hypotension and bradycardia, partly by their negative inotropic action.[29,34] Volume expansion with fludrocortisone acetate (Florinef Acetate), a salt-retaining mineralocorticoid, has also been employed.[35,38] Vagolytic agents (such as scopolamine and atropine) are believed to diminish the efferent hypervagotonia associated with this reflex and hence lessen the degree of induced bradycardia.
Disopyramide (Napamide, Norpace) exhibits both negative inotropic actions and potent anticholinergic effects and has recently been found to reduce episodes of vasovagal syncope.[34,36] Disopyramide possesses a peripheral vasoconstrictive effect that may diminish symptoms in patients who develop hypotension, as opposed to a prominent bradycardia, during vasovagal syncope.
Oral theophylline has proved effective in preventing recurrent episodes of syncope. Theophylline is thought to block the peripheral vasodilatory effects of adenosine.
Cardiac pacemakers have been advocated in patients with severe bradycardia and prolonged periods of asystole[19,24] (Table 5).
Initial reports seem to support good results with tilt test-directed therapy for vasovagal syncope. In one study of Medical therapy, prophylactic treatment prevented recurrent syncope over a 16-month period. In a 24-month study,pacemakers resulted in abolition of symptoms in 21 of 40 patients.
A 27-year-old man presented to our clinic with a four-year history of epilepsy. His seizures were marked by sudden loss of consciousness without warning, marked cyanosis and tonic-clonic movements of the extremities. Serial use of phenytoin, valproic acid and carbamazepine were not effective in preventing seizures. Because of continued epileptic episodes, he underwent a tilt test with continuous electroencephalographic monitoring. Eight minutes into the study, he developed asystole, loss of consciousness and suffered a grand mal seizure.
Anticonvulsants were subsequently discontinued. The patient was placed on a beta blocker, but developed severe fatigue. His medicine was changed to fludrocortisone, which was partially effective. He now has a longer prodrome, which allows him to sit or lie down and abort most spells.
The authors thank Sherri Sweet, Pam Brewster, Daniela Samoil, M.D., and Mary D. Wolfe for help in the preparation of this manuscript.
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DOUGLAS A. WOLFE, M.D. is a cardiology fellow at the Medical College of Ohio, Toledo. a graduate of the Medical University of South Carolina, Charleston, he completed a family practice residency at Spartanburg (S.C.) Regional Medical Center, and an internal medicine residency at the Medical University of South Carolina. Dr. Wolfe is currently doing clinical research in autonomic dysfunction and its relationship to syncope. BLAIR P. GRUBB, M.D. is assistant professor of medicine at the Medical College of Ohio, where he is also co-director of the electrophysiology laboratory. He completed an internal medicine residency at the Greater Baltimore Medical Center and cardiology and electrophysiology fellowships at Pennsylvania State University, Hershey. Dr. Grubb's research interests include the mechanism and management of syncope. SANFORD R. KIMMEL, M.D. is associate professor of clinical family medicine at the Medical College of Ohio. A graduate of the Ohio State University School of Medicine, Columbus, he completed a family practice residency at Saint Elizabeth Medical Center in Dayton, Ohio, and two years of pediatrics training at Columbus (Ohio) Children's Hospital.
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