Peripheral Circulatory Collapse

Sudden extreme fall in blood pressure with resultant decrease in cerebral perfusion can lead to syncope. The most common cause is Vasodepressor or vasovagal syncope. Micturition syncope, defecation syncope, cough syncope, orthostatic hypotension, and the Vasodepressor variety of carotid sinus syncope also fall under this category.

Vasodepressor syncope occurs in response to sudden emotional stress. There is a sudden marked decline in total peripheral vascular resistance, primarily resulting from a decrease in vascular resistance in skeletal muscles, although resistance is reduced in other vascular areas, such as the renal, mesentary, and cerebral beds. This fall in vascular resistance is not compensated by increased cardiac output.

Micturition syncope and defecation syncope probably have similar mechanisms. At least four important physiologic changes have been described during sleep or micturition that may affect the cardiovascular system, leading to micturition syncope. First, sudden decompression of the bladder can lead to hypotension and circulatory collapse. Second, blood pressure and heart rate decrease during sleep. Third, micturition may be associated with a strong vagal stimulus, leading to bradycardia or hypotension. Finally, the Valsalva maneuver may be performed and lead to a decline in blood pressure. The mechanism of cough syncope is poorly understood, but increased intrathoracic pressure caused by a prolonged Valsalva maneuver has been considered. Others have noted a rapid rise in cerebrospinal fluid pressure, leading to a decline in cerebral blood flow.

Many drugs and disease entities are associated with orthostatic hypotension (Table 12.4). In patients with orthostatic hvpotension there may be a marked decline in blood pressure and cerebral perfusion upon standing.

Table 12.4

Causes of Orthostatic Hypotension.

The carotid sinus is important in the control of heart rate and blood pressure through a reflex arch. The afferent limb of the carotid sinus reflex begins with specialized nervous receptors situated in the wall of the carotid sinus, a fusiform dilation of the internal carotid artery immediately above the carotid bifurcation. From the carotid sinus, impulses are transmitted via the carotid sinus nerve or the nerve of Hering and join the glossopharyngeal nerve. These communications ascend to the cardioinhibitory or vasoniotor centers in the reticular formation of the brain. Impulses from hyperactive baroreceplors inhibit the vasomotor center and stimulate the vagal center. The former action leads to vasodilation; the latter slows the heart by suppressing the sinus node. Three types of responses of carotid sinus stimulation have been described.

1. Cardioinhibitory, manifested by bradycardia or asystole with or without hypotension. This is the commonest response, amounting to 50 to 80% of all carotid sinus hypersensitivity. It can be inhibited by atropine.
2. A vasodepressor reflex that produces hypotension without bradycardia or asystole.
3. A third response, manifested by cerebral symptoms without bradycardia or hypotension. Often, the response is a combination of cardioinhibitory and vasodepressor types.

Inadequate Cardiac Output

Syncope may result from inadequate cardiac output leading to decreased cerebral blood flow. This type of syncope can be divided into two categories, that caused by obstruction to outflow or pump failure, and syncope due to arrhythmias.

Syncope may result from obstruction to left ventricle outflow (see Table 12.1). Patients with aortic stenosis may present with exertional syncope. The mechanism of exertional syncope has been thought to be a fall in peripheral vascular resistance with exercise in combination with the inability to increase cardiac output appropriately. Pulmonary hypertension may also present with syncope related to effort. In this circumstance, the limitation to right ventricular outflow markedly inhibits the cardiac output response during increased peripheral demand. A similar mechanism is probably responsible for syncope in patients with pulmonary stenosis. Syncope associated with effort or strenuous exercise is also observed in patients with hypertrophic cardiomyopathy. Atrial myxomas may obstruct ventricular filling leading to low cardiac output and syncope.

Paroxysmal or sustained bradyarrhythmias or tachyarrhythmias can also lead to inadequate cardiac output with hypertension and syncope

Obstruction to Cerebral Blood Flow

Patients with severe diseases of the cerebral vasculature may develop syncope if there is a sudden decrease in cerebral blood flow, especially to the brainstem. Such events may occur with emboli to the vertebrobasilar arterial system. The embolic source may be sclerotic plaque, endocarditis from prosthetic valves, myxomatous material of mitral valve prolapse, or even intracardiac thrombus or tumors. Syncope is a prominent feature only when the vertebrobasilar system is involved. It is thought that the reticular activating system controls consciousness and that ischemia of this region may lead to drop attacks or syncope. Other vascular diseases of cerebral arteries may also present with syncope when there is sudden ischemia. These diseases include cerebral vasculitis such as giant cell arteritis and Takayasu's disease, basilar artery migraine, and subclavian steal syndrome.

Other Causes of Syncope

Other diseases that may present as syncope include akinetic seizures and hysterical reactions. In addition, in a large number of patients, a cause of syncope is not determined by currently available techniques.

Noncardiac Causes

Vasodepressor syncope occurs in all age groups, although it is more common in younger patients and rare in the elderly. Vasodepressor syncope occurs in response to sudden emotional stress or in a setting of real, threatened, or imagined injury. Some of the situations commonly leading to vasodepressor syncope include pain, sight of blood, instrumentation, and venipuncture. Vasodepressor syncope occurs primarily in the standing position and less frequently in the sitting position. Patients usually experience several minutes of prodromal symptoms including weakness, pallor, sweating, nausea, increased peristalsis, yawning, belching, and dimming of vision followed by a loss of consciousness associated with hypotension and sinus bradycardia. The patient generally appears pale and diaphoretic, but the patient's blood pressure, pulse, and mental status return to normal within minutes. Syncope may recur when the patient stands up.

Micturition syncope occurs in healthy, young-to-middle-aged men and also elderly men and women. Syncope usually occurs in the middle of the night during or immediately following voiding, often without premonitory symptoms. In the young, the predisposing factors include excessive alcohol consumption, a recent viral infection, fatigue, or recent reduced food intake. In the elderly, the predisposing factors are diuretics and chronic orthostatic hypotension. Syncope is usually not recurrent; however, recurrent micturition syncope has been reported with bladder neck obstruction, severe chronic orthostatic hypotension, and paroxysmal complete atrioventricular block.

Defecation syncope occurs in elderly patients, usually in the early morning hours. There are no known predisposing factors. The evaluation of defecation syncope is similar to other patients with syncope in whom a cause is not known.

Syncope in association with swallowing is rare and generally occurs in patients with structural diseases of the esophagus or the heart. Esophageal diseases that can cause swallow syncope include esophageal spasm, a diverticulum, or other lesions. Transient atrioventricular block or bradycardias associated with swallowing are also reported to cause swallow syncope.

Syncope can occur in association with a paroxysm of severe coughing. Patients with cough syncope are usually middle-aged men who are mildly obese, and heavy smokers and alcohol users who often have associated pulmonary conditions such as chronic obstructive pulmonary disease, asthma, bronchiectasis, pneumoconiosis, sarcoidosis, or tuberculosis. Cough syncope has also been associated with hypertrophic cardiomyopathy and herniation of cerebellar tonsils.

Orthostatic hypotension is one of the most common causes of syncope. Orthostatic hypotension occurs in a variety of clinical situations with volume depletion or decreased venous return. Various pharmacologic agents can predispose patients to orthostatic hypotension. Orthostatic hypotension is also a symptom of many central and peripheral nervous system disorders (Table 12.4).

In order to implicate orthostatic hypotension as causing syncope, orthostatic blood pressure chop should be severe (blood pressure fall to less than 90 mm Hg) or be concurrently associated with symptoms of dizziness or syncope.

In drug-induced syncope, rapidly acting medications (e.g., sublingual nitroglycerin) may cause syncope immediately after ingestion. More commonly, drugs may lead to effects on blood pressure or arrhythmias, leading to syncope. Some of the drug effects include the following:

1. Postural hypotension. In this category are drugs such as antihypertensives, diuretics, nitrates, other arterial vasodilators, l-dopa, phenothiazines, or other tranquilizers.
2. Anaphylactic reaction. Drugs may lead to an anaphylactic reaction with associated symptoms of anaphylaxis and hypotension leading to syncope.
3. Drug overdose.
4. Drug-induced ventricular tachycardia. This group includes drugs that lead to Q-T interval prolongation and torsades de pointes. The most commonly implicated drugs leading to torsades de pointes include quinidine, disopyramide, procainamide, psychotropic drugs, phenothiazines, and tricyclic antidepressants. In addition, drug-induced hypokalemia and hypomagnesemia may lead to prolonged Q-T interval and development of torsades de pointes.

When syncope is associated with hyperextension of the neck, head turning, tight collars, parallel parking, carrying shoulder loads, shaving, or other maneuvers that spontaneously stretch the carotid sinus, a diagnosis of carotid sinus syncope should be considered. Although elderly patients frequently have hyperactive carotid sinus reflexes, carotid sinus syncope is rare, occurring in only 5 to 20% of persons with a hyperactive reflex. Carotid sinus syncope occurs more often in men in the seventh and eight decade of life. Occasionally thyroid tumors, carotid body tumors, and inflammatory and malignant lymph nodes may precipitate carotid sinus syncope. Drugs such as digoxin, propranolol, and melhyldopa also may precipitate carotid sinus syncope.

In patients suspected of carotid sinus syncope, carotid massage is needed for diagnosis. Carotid massage should be performed in elderly patients only if other diagnostic studies have not been helpful, since rare transient and permanent neurologic deficits have been precipitated by this maneuver. The test should be done with electrocardiographic monitoring and frequent blood pressure determinations during the procedure. A sinus pause of 3 seconds or more or a decrease of systolic pressure of more than 50 mm Hg with no significant decrease in pulse rate represents a positive test diagnostic of a hypersensitive carotid sinus. The diagnosis of carotid sinus syncope is justified when an individual has a hyperactive carotid sinus reflex and syncope is clearly related to activities that press on or stretch the sinus.

In patients with symptoms compatible with a possible seizure as a cause of loss of consciousness, such as an aura, postictal state, and tonic-clonic movements, an EEC. is needed for further evaluation of a possible seizure. A head computerized tomography (CT) scan may also be needed as part of the workup of a seizure disorder.

Cerebrovascular disease is an uncommon cause of syncope, In patients with vertebrobasilar transient ischemic attacks or subclavian steal syndrome, syncope and drop attacks are almost always associated with other neurologic signs and symptoms of vertebrobasilar ischemia such as vertigo, visual disturbance, and sensory and motor findings.

In patients with a blood pressure difference greater than 20 mm Hg between the two arms or a decrease in pulse intensity in one arm, a diagnosis of subclavian steal syndrome should be considered. The diagnosis of subclavian steal syndrome requires cerebral angiography. Vertebrobasilar transient ischemic attack is a clinical diagnosis. If associated carotid artery disease is suspected, noninvasive flow studies, digital subtraction angiography. or carotid angiography will be needed for diagnosis and potential therapy.

Cardiac Causes

Structural abnormalities leading to decreased cardiac output include aortic stenosis, hypertrophic cardiomyopathy, acute myocardial infarction, aortic dissection, cardiac tamponade, and left atrial myxoma. Pulmonic stenosis, pulmonary hypertension, and tetralogy of Fallot that produce obstruction to the pulmonary flow also may cause syncope. Patients with aortic stenosis or pulmonary hypertension may present with exertional syncope.

Diseases resulting in syncope from structural abnormalities of the heart are suspected on the basis of the history and a physical examination that discloses a murmur and other related cardiovascular findings. Further evaluation for definitive diagnosis and determining the severity of these diseases requires echocardiography, cardiac catheterization, or both. If aortic dissection is suspected, an emergency aortogram is needed for diagnosis.

Bradycardias as well as tachycardias can lead to inadequate cardiac output and result in hypotension and syncope. The diagnosis of arrhythmias as a cause of syncope is based on an EGG or prolonged electrocardiographic monitoring. Prolonged electrocardiographic monitoring is indicated in all patients in whom a cause of syncope is not established by initial history, physical examination, and EGG. The following limitations should be recognized, however.

First, there is poor correlation between symptoms found during monitoring and concurrent EGG findings. Arrhythmias may be excluded as a cause if the patient has syncope during monitoring but no concurrent rhythm disturbance. Most patients do not have syncope during monitoring, however; presumptions about the cause of syncope are frequently necessary because of the prognostic importance and sporadic nature of the arrhythmias. Second, the optimal duration of ECG monitoring has not been determined. It has been suggested that patients should be continuously monitored until symptoms occur, but this is impractical. Third, the yield of prolonged ECG monitoring is variable, commonly showing transient arrhythmias as a potential cause of syncope in 10 to 65% of patients.

Despite these shortcomings, prolonged ECG monitoring is the single most useful diagnostic test in the evaluation of patients with syncope, the cause of which is still not known after history, physical examination, and ECG. In our study, prolonged ECG monitoring yielded a potential diagnosis in 12% of patients (Kapoor, Karpf, Wieand, et al. 1983).

Syncope of Unknown Cause

In a large number of patients, a cause of syncope may not be established despite a thorough evaluation. There are at least three possible reasons for the inability to define a cause of syncope in some patients. First, abnormalities causing syncope are frequently episodic or isolated events and may not be found at initial evaluation, but may become obvious in follow-up. Second, the diagnostic criteria may be too stringent. Last, most patients are evaluated after the syncopal event and may not recall the details surrounding their episode. If witnesses are not present, valuable observations are lost, and the cause of syncope may not be apparent.

All patients in whom a diagnosis cannot be established should be periodically reevaluated for new clues as to the etiology of their syncope. Patients having recurrent episodes of syncope of unknown cause may require repeated evaluation and more extensive diagnostic testing.