Episodic Impairment of Consciousness

Temporary loss of consciousness may be caused by transient impaired cerebral perfusion (the presumed mechanism for syncope), cerebral ischemia, migraine, epileptic seizures, metabolic disturbances, sudden increases in intracranial pressure (ICP), or sleep disorders. These conditions may be difficult to distinguish from anxiety attacks, psychogenic nonepileptic spells (PNESs), panic disorder, and malingering, which should always be considered.

Syncope is defined as an abrupt, transient, complete loss of consciousness, associated with inability to maintain postural tone, with rapid and spontaneous recovery ( ). Syncope may result from both cardiac and noncardiac causes. Specific causes of a transient impairment in cerebral perfusion include vasovagal episodes (typically a surge in parasympathetic autonomic tone), decreased cardiac output secondary to cardiac arrhythmias, outflow obstruction, hypovolemia, orthostatic hypotension, and decreased venous return. Cerebrovascular disturbances from transient ischemic attacks of the posterior cerebral circulation perfusing the brainstem, or cerebral vasospasm from migraine, subarachnoid hemorrhage, or hypertensive encephalopathy may result in temporary loss of consciousness. Situational syncope may occur in association with cough, micturition, defecation, swallowing, Valsalva maneuver, or diving. These spells are often mediated via a decrease in venous return to the thorax and/or an increase in sympathetic tone. Metabolic disturbances due to hypoxia, drugs, anemia, and hypoglycemia may result in frank syncope or, more frequently, the sensation of an impending faint (presyncope).

Absence seizures, generalized tonic-clonic seizures, and complex partial seizures are associated with alterations of consciousness and are usually easily distinguished from syncope by careful questioning. Seizures may be difficult to distinguish from PNESs, panic attacks, and malingering. In children, breath-holding spells, a form of syncope (discussed later under “Miscellaneous Causes of Altered Consciousness”), can cause a transitory alteration of consciousness that may mimic epileptic seizures in this population. Although rapid increases in ICP (which may result from intermittent hydrocephalus, severe head trauma, brain tumors, intracerebral hemorrhage, certain severe metabolic derangements or Reye syndrome) may produce sudden loss of consciousness, affected patients frequently have other neurological manifestations that lead to this diagnosis.

In patients with episodic impairment of consciousness, diagnosis relies heavily on the clinical history described by the patient, and obtaining a detailed history from unaffected observers is often essential to clarifying the diagnosis. Laboratory investigations may also provide useful information. In a minority of patients, a cause for the loss of consciousness may not be established, and these patients may require longer periods of observation. Table 2.1 compares the clinical features of syncope and seizures.

Table 2.1

Comparison of Clinical Features of Syncope and Seizures

Features	Syncope	Seizure
Relation to posture	Common	No
Time of day	Diurnal	Diurnal or nocturnal
Precipitating factors	Emotion, injury, pain, crowds, heat, exercise, fear, dehydration, coughing, micturition, venipuncture, prolonged standing	Sleep deprivation, drug/alcohol withdrawal, illness, medication nonadherence
Skin color	Pallor	Cyanosis or normal
Diaphoresis	Common	Rare
Aura or premonitory symptoms	Often minutes or longer, but can be very brief	Brief
Convulsion	Rare	Common
Other abnormal movements	Minor irregular twitching	Rhythmic jerks
Injury	Rare	Common (with convulsive seizures)
Urinary incontinence	Rare	Common
Tongue biting	No	Common with convulsive seizures
Postictal confusion	Rare	Common
Postictal headache	No	Common
Focal neurological signs	No	Occasional
Cardiovascular signs	Common to have low blood pressure and heart rate during event; cardiovascular exam may be completely normal after event unless there is an underlying cardiac disorder	Rare
Abnormal findings on EEG	Rare (generalized slowing may occur during the event)	Common

EEG , Electroencephalogram.

Syncope

The pathophysiological basis of syncope is the temporary failure of cerebral perfusion, with a reduction in cerebral oxygen availability. Syncope refers to a symptom complex characterized by lightheadedness, generalized muscle weakness, giddiness, visual blurring, tinnitus, and gastrointestinal (GI) symptoms. The patient may appear pale and feel cold and “sweaty.” The onset of loss of consciousness generally is gradual but may be rapid if related to certain conditions such as a cardiac arrhythmia or in the elderly. The gradual onset may allow patients to protect themselves from falling and injury. Factors precipitating a vasovagal syncopal episode (also known sometimes as a simple faint) include emotional stress, unpleasant visual stimuli, prolonged standing, venipuncture, and pain. Although the duration of unconsciousness is brief, it may range from seconds to minutes. During the faint, the patient may be motionless or display myoclonic jerks but never tonic-clonic movements. Urinary incontinence is uncommon. The pulse is weak and often slow because patients may be briefly bradycardic (from parasympathetic tone) and vasodilated. Breathing may be shallow and the blood pressure barely obtainable. As the fainting episode corrects itself by the patient becoming horizontal, normal color returns, breathing becomes more regular, and the pulse and blood pressure return to normal. After the faint, the patient experiences some residual weakness, but unlike the postictal state, confusion, headaches, and drowsiness are uncommon. Nausea may be noted before the episode and may still be present when the patient regains consciousness. The causes of syncope, which may often overlap, are classified by their pathophysiological mechanism ( Box 2.1 ), but cerebral hypoperfusion is always the common final pathway. Rarely, vasovagal syncope may have a genetic component suggestive of autosomal dominant inheritance ( ). reviewed the clinical features of the successive phases of syncope, as discussed earlier.

BOX 2.1

Classification and Etiology of Syncope

- Arrhythmias:
- Bradyarrhythmias
- Tachyarrhythmias
- Reflex arrhythmias (temporary sinus pause or bradycardia)
- Decreased cardiac output:
- Outflow obstruction
- Inflow obstruction
- Cardiomyopathy
Hypovolemic
Hypotensive:
- Vasovagal attack
- Drugs
- Dysautonomia
Cerebrovascular:
- Carotid disease
- Vertebrobasilar disease
- Vasospasm
- Takayasu disease
Metabolic:
- Hypoglycemia
- Anemia
- Anoxia
Hyperventilation
Multifactorial:
- Vasovagal (vasodepressor) attack
- Cardiac syncope
- Situational: cough, micturition, defecation, swallowing, diving, Valsalva maneuver

History and Physical Examination

The history and physical examination are the most important components of the initial evaluation of syncope. Significant age and sex differences exist in the frequency of the various types of syncope. Syncope occurring in children and young adults is most frequently due to hyperventilation or vasovagal (vasodepressor) attacks and less frequently due to congenital heart disease ( ). Fainting associated with benign tachycardias without underlying organic heart disease also may occur in children. Syncope due to basilar migraine is more common in young females. Although vasovagal syncope can occur in older patients ( ), when repeated syncope begins in later life, organic disease of the cerebral circulation or cardiovascular system usually is responsible and requires exhaustive investigation.

A thorough history is the most important step in establishing the cause of syncope. The patient’s description usually establishes the diagnosis. The neurologist should always obtain as full a description as possible of the first faint. The clinical features should be established, with emphasis on precipitating factors, posture, type of onset of the faint (including whether it was abrupt or gradual), position of head and neck, the presence and duration of preceding and associated symptoms, duration of loss of consciousness, rate of recovery, and sequelae. If possible, question an observer about clonic movements, color changes, diaphoresis, pulse, respiration, urinary incontinence, and the nature of recovery. Be certain to ask about any prior events, and gather these same details for each event that the patient recalls.

Cardiac syncope is defined as syncope caused by bradycardia, tachycardia, or hypotension due to low cardiac index, blood flow obstruction, vasodilation, or acute vascular dissection ( ). Cardiac syncope should be suspected in patients with known cardiac disease. Clues in the history that suggest cardiac syncope include a history of palpitations or a fluttering sensation in the chest before loss of consciousness. These symptoms are common in arrhythmias but do not definitively establish that diagnosis as the cause for the syncope. In vasodepressor syncope and orthostatic hypotension, preceding symptoms of lightheadedness are common. Episodes of cardiac syncope generally are briefer than vasodepressor syncope, and the onset usually is rapid. Episodes due to cardiac arrhythmias occur independently of position, whereas in vasodepressor syncope and syncope due to orthostatic hypotension the patient usually is standing.

Attacks of syncope precipitated by exertion suggest a cardiac etiology. Exercise may induce arrhythmic syncope or syncope due to decreased cardiac output secondary to blood flow obstruction, such as may occur with hypertrophic cardiomyopathy with dynamic outflow obstruction, or with aortic or subaortic stenosis. Exercise syncope also may be due to cerebrovascular disease, aortic arch disease, congenital heart disease, severe stenosis of any of the cardiac valves, pulseless disease (Takayasu disease, a type of vasculitis), pulmonary hypertension, anemia, hypoxia, and hypoglycemia. A family history of sudden cardiac death, especially in females, suggests the long QT syndrome. Postexercise syncope may be secondary to orthostasis in the setting of dilated vascular beds in the large muscles (cardiac output may normalize faster than systemic vascular resistance), vasovagal syncope brought on by relative hypovolemia (in a setting of dilated vasculature), or autonomic dysfunction. A careful and complete medical and medication history is mandatory to determine whether prescribed drugs have induced either orthostatic hypotension or cardiac arrhythmias. To avoid missing a significant cardiac disorder, one should always consider a comprehensive cardiac evaluation in patients with exercise-related syncope. Particularly in the elderly, cardiac syncope must be distinguished from more benign causes because of increased risk of sudden cardiac death ( ).

The neurologist should inquire about the frequency of attacks of loss of consciousness and the presence of cerebrovascular or cardiovascular symptoms between episodes. Question the patient whether all episodes are similar, because some patients experience more than one type of attack. In the elderly, syncope may cause unexplained falls lacking prodromal symptoms. With an accurate description of the attacks and familiarity with clinical features of various types of syncope, the physician will correctly diagnose most patients ( ), but confirmatory testing to rule in, or to exclude, some high-risk diagnoses may be required. Features that distinguish syncope from seizures and other alterations of consciousness are discussed later in the chapter.

After a complete history, the physical examination is of next importance. Examination during the episode is very informative but frequently impossible unless syncope is reproducible by a Valsalva maneuver or by recreating the circumstances of the attack, such as by position change. In the patient with suspected cardiac syncope, pay particular attention to the vital signs and determination of supine and erect blood pressure. Normally, with standing, the systolic blood pressure is stable or rises and the pulse rate may increase . An orthostatic drop in blood pressure greater than 15 mm Hg may suggest autonomic dysfunction. Assess blood pressure in both arms when suspecting cerebrovascular disease, subclavian steal, or Takayasu arteritis.

During syncope due to a cardiac arrhythmia, a heart rate faster than 140 beats/ min often indicates that the rhythm is not sinus tachycardia (may be a supraventricular tachycardia, an ectopic atrial or ventricular tachycardia, or atrial fibrillation or flutter), whereas a bradycardia with heart rate of less than 40 beats/min suggests complete atrioventricular (AV) block or a prolonged sinus pause. An irregular pulse indicates possible atrial fibrillation but may also be seen with frequent premature atrial or ventricular contractions, and with intermittent AV block. Vagal maneuvers, which include Valsalva and cold water to the face, sometimes terminate a supraventricular tachycardia. Carotid sinus massage may also be effective, but this maneuver is not advisable in the acute setting because of the risk of cerebral embolism from potential atheroma in the carotid artery wall. In contrast, an ectopic atrial or ventricular tachycardia will usually not be terminated by vagal maneuvers. It is recommended that all patients with syncope undergo a resting electrocardiogram as part of their initial evaluation ( ).

All patients with syncope should also undergo cardiac auscultation for the presence of cardiac murmurs and abnormalities of the heart sounds. Possible murmurs of concern include aortic stenosis, hypertrophic cardiomyopathy with outflow tract obstruction, and mitral valve stenosis. An intermittent posture-related murmur may be associated with an atrial myxoma. A systolic click and late systolic murmur of mitral regurgitation in a young person suggests mitral valve prolapse. A pericardial rub suggests pericarditis. The finding of a murmur, rub, or abnormal click in a patient with syncope should prompt the physician to order an echocardiogram.

All patients should undergo observation of the carotid and jugular venous pulses and auscultation of the neck. The degree of aortic stenosis may be reflected at times in a delayed or weakened carotid upstroke. Carotid, ophthalmic, and supraclavicular bruits suggest underlying cerebrovascular disease. Jugular venous distention suggests congestive heart failure or other abnormal filling of the right heart, whereas a very low jugular venous pressure suggests hypovolemia. Carotid sinus massage should be avoided in patients with carotid bruits but may be useful in patients suspected of having carotid sinus syncope. It is important to keep in mind that up to 25% of asymptomatic persons may have some degree of carotid sinus hypersensitivity. Carotid massage should be avoided in patients with suspected cerebrovascular disease, even if they have no carotid bruit, and when performed should be under properly controlled conditions with electrocardiographic (ECG) and blood pressure monitoring. The response to carotid massage may be vasodepressor, cardioinhibitory, mixed, or minimal.

Causes of Syncope

Cardiac Arrhythmias

Both bradyarrhythmias and tachyarrhythmias may result in syncope, and abnormalities of cardiac rhythm due to dysfunction from the sinoatrial (SA) node to the Purkinje network may be involved. Always consider arrhythmias in all cases in which an obvious alternative mechanism is not established. Syncope due to cardiac arrhythmias generally occurs more quickly than syncope from other causes. Cardiac syncope may occur in any position, is occasionally exercise induced, and may occur in both congenital and acquired forms of heart disease.

Although palpitations sometimes occur during arrhythmias, others are unaware of any cardiac symptoms. Syncopal episodes secondary to cardiac arrhythmias may be more prolonged than benign syncope and often occur with less warning. Patients may injure themselves significantly during their fall. The most common arrhythmias causing syncope are AV block, SA block, and paroxysmal supraventricular and ventricular tachyarrhythmias. AV block describes disturbances of conduction occurring in the AV conducting system, which include the AV node to the bundle of His and the Purkinje network. SA block describes a failure of consistent pacemaker function of the SA node. Paroxysmal tachycardia refers to a rapid heart rate that comes on intermittently. It may be secondary to an ectopic focus or reentrant loop outside the SA node but above the ventricle (supraventricular), or it may be from a source below the AV node (ventricular). In patients with implanted pacemakers, syncope can occur because of pacemaker malfunction.

Atrioventricular Block

AV block is probably the most common cause of arrhythmic cardiac syncope. The term Stokes-Adams attack describes disturbances of consciousness occurring in association with a complete AV block. Complete AV block occurs primarily in elderly patients and is often also seen in patients with a history of aortic valve disease. The onset of a Stokes-Adams attack generally is sudden, although a number of visual, sensory, and perceptual premonitory symptoms may be experienced. During the syncopal attack, the pulse disappears and no heart sounds are audible. The patient is pale and, if standing, falls down, often with resultant injury. If the attack is sufficiently prolonged, respiration may become labored, and urinary incontinence and clonic muscle jerks may occur. Prolonged confusion and neurological signs of cerebral ischemia may be present. Regaining of consciousness generally is rapid.

The clinical features of complete AV block include a slow pulse and elevation of the jugular venous pressure, sometimes with cannon waves. The first heart sound is of variable intensity, and heart sounds related to atrial contractions may be audible. An ECG confirming the diagnosis demonstrates independence of atrial P waves and ventricular QRS complexes. During Stokes-Adams attacks, the ECG generally shows ventricular standstill or a very slow ventricular escape rhythm, but ventricular fibrillation or tachycardia also may occur.

Sinoatrial Block

SA block may result in dizziness, lightheadedness, and syncope. It is most frequent in the elderly. Palpitations are common, and the patient appears pale. Patients with SA node dysfunction frequently have other conduction disturbances, and certain drugs (e.g., verapamil, digoxin, beta-blockers) may further impair SA node function. On examination, the patient’s pulse may be regular between attacks. During an attack, the pulse may be slow or irregular, and any of a number of rhythm disturbances may be present.

Paroxysmal Tachycardia

Supraventricular tachycardias include atrial fibrillation with a rapid ventricular response, atrial flutter, AV nodal reentry, and the Wolff-Parkinson-White syndrome (AV reentry involving an accessory pathway). These arrhythmias may suddenly reduce cardiac output enough to cause syncope. Ventricular tachycardia may result in syncope if the heart rate is sufficiently fast, and ventricular fibrillation will almost always result in nearly immediate syncope. Ventricular arrhythmias are more likely in the elderly and in patients with cardiac disease. Ventricular fibrillation may be part of the long QT syndrome, which has a cardiac-only phenotype or may be associated with congenital sensorineural deafness in children. In most patients with this syndrome, episodes begin in the first decade of life, but onset may be much later. Exercise may precipitate an episode of cardiac syncope. Long QT syndrome may be congenital or acquired and sometimes is misdiagnosed in adults as epilepsy. Acquired causes include cardiac ischemia, mitral valve prolapse, myocarditis, and electrolyte disturbances; there are also many drugs that can prolong the QT. In the short QT syndrome, signs and symptoms are highly variable, ranging from complete absence of clinical manifestations to recurrent syncope to sudden death. The age at onset often is young, and affected persons frequently are otherwise healthy. A family history of sudden death in a patient with a short QT may indicate a familial short QT syndrome inherited as an autosomal dominant mutation. The ECG demonstrates a short QT interval and a tall and peaked T wave, and electrophysiological studies may induce ventricular fibrillation. Brugada syndrome may produce syncope as a result of ventricular tachycardia or ventricular fibrillation ( ). The ECG in Brugada syndrome may or may not show a typical Brugada pattern at rest (i.e., an incomplete right bundle-branch block in leads V ₁ and V ₂ and significant downsloping ST elevation leading to inverted T waves in those two leads).

Reflex Cardiac Arrhythmias

A hypersensitive carotid sinus may be a cause of syncope in the elderly, most frequently men. Syncope may result from a reflex sinus bradycardia, sinus arrest, or AV block; peripheral vasodilatation with a fall in arterial pressure; or a combination of both. Although 10% of the population older than 60 years of age may have a hypersensitive carotid sinus, not all such patients experience syncope. Accordingly, consider this diagnosis only when the clinical history is compatible. Carotid sinus syncope may be initiated by wearing a tight collar, by rapidly turning the head (including when patients do so on their own), or by carotid sinus massage on clinical examination. When syncope occurs, the patient usually is upright, and the duration of the loss of consciousness generally is a few minutes. On regaining consciousness, the patient is mentally clear. Unfortunately, no accepted diagnostic criteria exist for carotid sinus syncope, and the condition is likely overdiagnosed.

Syncope in certain patients can be induced by unilateral carotid massage or compression; however, in those with atherosclerotic carotid disease, this can sometimes cause partial or complete occlusion of the ipsilateral carotid artery or release of atheromatous emboli and subsequent stroke. Because of these risks, carotid artery massage is contraindicated in those with known or suspected carotid atherosclerotic disease.

The rare syndrome of glossopharyngeal neuralgia is characterized by intense paroxysmal pain in the throat and neck accompanied by bradycardia or asystole, severe hypotension, and, if prolonged, seizures. Episodes of pain may be initiated by swallowing but also by chewing, speaking, laughing, coughing, shouting, sneezing, yawning, or talking. The episodes of pain always precede the loss of consciousness (see Chapter 20 ). Rarely, cardiac syncope may be due to bradyarrhythmias consequent to vagus nerve irritation caused by esophageal diverticula, tumors, or aneurysms in the region of the carotid sinus or by mediastinal masses or gallbladder disease.

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