Merck Manual

Please confirm that you are a health care professional

honeypot link



Andrea D. Thompson

, MD, PhD, University of Michigan;

Michael J. Shea

, MD, Michigan Medicine at the University of Michigan

Reviewed/Revised Jul 2022 | Modified Sep 2022
Topic Resources

Syncope is a sudden, brief loss of consciousness with loss of postural tone followed by spontaneous revival. The patient is motionless and limp and usually has cool extremities, a weak pulse, and shallow breathing. Sometimes brief involuntary muscle jerks occur, resembling a seizure.

Near-syncope is light-headedness and a sense of an impending faint without loss of consciousness. It is usually classified and discussed with syncope because the causes are the same.

Seizures Seizure Disorders A seizure is an abnormal, unregulated electrical discharge that occurs within the brain’s cortical gray matter and transiently interrupts normal brain function. A seizure typically causes altered... read more can cause sudden loss of consciousness but are not considered syncope. However, seizures must be considered in patients presenting for apparent syncope because history may be unclear or unavailable, and some seizures do not cause tonic-clonic convulsions. Furthermore, a brief (< 5 second) seizure sometimes occurs with true syncope.

Diagnosis depends on a careful history, eyewitness accounts, or fortuitous examination during the event.

Pathophysiology of Syncope

Most syncope results from insufficient cerebral blood flow. Some cases involve adequate flow but with insufficient cerebral substrate (oxygen, glucose, or both).

Insufficient cerebral blood flow

Most deficiencies in cerebral blood flow result from decreased cardiac output (CO).

Decreased CO can be caused by

  • Cardiac disorders that obstruct outflow

  • Cardiac disorders of systolic dysfunction

  • Cardiac disorders of diastolic dysfunction

  • Arrhythmias (too fast or too slow)

  • Conditions that decrease venous return

Outflow obstruction can be exacerbated by exercise, vasodilation, and hypovolemia (particularly in aortic stenosis and hypertrophic cardiomyopathy), which may precipitate syncope.

Arrhythmias Overview of Arrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more Overview of Arrhythmias cause syncope when the heart rate is too fast to allow adequate ventricular filling (eg > 150 to 180 beats/minute) or too slow to provide adequate output (eg, < 30 to 35 beats/minute).

Venous return can be decreased by hemorrhage, increased intrathoracic pressure, increased vagal tone (which can also decrease heart rate), and loss of sympathetic tone (eg, from drugs, carotid sinus pressure, autonomic dysfunction). Syncope involving these mechanisms (except for hemorrhage) is often termed vasovagal or neurocardiogenic and is common and benign.

Orthostatic hypotension Orthostatic Hypotension Orthostatic (postural) hypotension is an excessive fall in blood pressure (BP) when an upright position is assumed. The consensus definition is a drop of > 20 mm Hg systolic, > 10 mm Hg... read more , a common benign cause of syncope, results from failure of normal mechanisms (eg, sinus tachycardia, vasoconstriction, or both) to compensate for the temporary decrease in venous return that occurs with standing.

Cerebrovascular disorders (eg, strokes Overview of Stroke Strokes are a heterogeneous group of disorders involving sudden, focal interruption of cerebral blood flow that causes neurologic deficit. Strokes can be Ischemic (80%), typically resulting... read more Overview of Stroke , transient ischemic attacks Transient Ischemic Attack (TIA) A transient ischemic attack (TIA) is focal brain ischemia that causes sudden, transient neurologic deficits and is not accompanied by permanent brain infarction (eg, negative results on diffusion-weighted... read more ) rarely cause syncope because most of them do not involve the centrencephalic structures that must be affected to cause loss of consciousness. However, basilar artery ischemia, due to transient ischemic attack, stroke, or migraine, may cause syncope. Rarely, patients with severe cervical arthritis or spondylosis develop vertebrobasilar insufficiency with syncope when the head is moved in certain positions.

Insufficient cerebral substrate

The central nervous system (CNS) requires oxygen and glucose to function. Even with normal cerebral blood flow, a significant deficit of either will cause loss of consciousness . In practice, hypoglycemia is the primary cause because hypoxia rarely develops in a manner causing abrupt loss of consciousness (other than in flying or diving incidents). Loss of consciousness due to hypoglycemia is seldom as abrupt as in syncope or seizures because warning symptoms occur (except in patients taking beta-blockers); however, the onset may be unclear to the examiner unless the event was witnessed.

Etiology of Syncope

Causes are usually classified by the mechanism (see table Some Causes of Syncope Some Causes of Syncope Some Causes of Syncope ).

The most common causes are

  • Vasovagal (neurocardiogenic)

  • Idiopathic

Many cases of syncope never have a firm diagnosis but lead to no apparent harm. A smaller number of cases have a serious cause, usually cardiac.


Evaluation of Syncope

Evaluation should be done as soon as possible after the event. The more remote the syncopal event, the more difficult the diagnosis. Information from witnesses is quite helpful and best obtained as soon as possible.


History of present illness should ascertain events leading up to the syncope, including the patient’s activity (eg, exercising, arguing, in a potentially emotional situation), position (eg, lying or standing), and, if standing, for how long. Important associated symptoms immediately before or after the event include whether there was a sense of impending loss of consciousness, nausea, sweating, blurred or tunnel vision, tingling of lips or fingertips, chest pain, or palpitations. Length of time recovering should also be ascertained. Witnesses, if any, should be sought and asked to describe events, particularly the presence and duration of any seizure activity.

Review of systems should ask about any areas of pain or injury, episodes of dizziness or near-syncope upon arising, and episodes of palpitations or chest pain with exertion. Patients should be asked about symptoms suggesting possible causes, including bloody or tarry stools, heavy menses (anemia Etiology of Anemia Anemia is a decrease in the number of red blood cells (RBCs), which leads to a decrease in hematocrit and hemoglobin content. (See also Red Blood Cell Production.) The RBC mass represents the... read more ); vomiting, diarrhea, or excess urination (dehydration or electrolyte abnormalities); and risk factors for pulmonary embolism (recent surgery or immobilization, known cancer, previous clots or hypercoagulable state).

Past medical history should ask about previous syncopal events, known cardiovascular disease, and known seizure disorders. Drugs used should be identified (particularly antihypertensives, diuretics, vasodilators, and antiarrhythmics—see table Some Drug Causes of Syncope Some Drug Causes of Syncope Some Drug Causes of Syncope ). Family history should note presence at a young age of heart disease or sudden death in any family member.

Physical examination

Vital signs are essential. Heart rate and blood pressure are measured with the patient supine and after 3 minutes of standing. Pulse is palpated for irregularity.

General examination notes patient’s mental status, including any confusion or hesitancy suggesting a postictal state and any signs of injury (eg, bruising, swelling, tenderness, tongue bite).

The heart is auscultated for murmurs; if present, any change in the murmur with a Valsalva maneuver, standing, or squatting is noted.

Careful evaluation of the jugular venous waves (see figure ) while palpating the carotid or auscultating the heart may allow diagnosis of an arrhythmia if an ECG is not available. For example, cannon "a" waves occur when the atria contraction takes place against a closed tricuspid valve and indicate atrial-ventricular dissociation.

Abdomen is palpated for tenderness, and a rectal examination is done to check for gross or occult blood.

A neurologic examination is done to identify any focal abnormalities, which suggest a central nervous system cause (eg, seizure disorder).

Red flags

Certain findings suggest a more serious etiology:

  • Syncope during exertion

  • Multiple recurrences within a short time

  • Heart murmur or other findings suggesting structural heart disease (eg, chest pain)

  • Older age

  • Significant injury during syncope

  • Family history of sudden unexpected death, exertional syncope, or unexplained recurrent syncope or seizures

Interpretation of findings

Although the cause is often benign, it is important to identify the occasional life-threatening cause (eg, tachyarrhythmia, heart block) because sudden death is a risk. Clinical findings (see table Some Causes of Syncope Some Causes of Syncope Some Causes of Syncope ) help suggest a cause in 40 to 50% of cases. A few generalizations are useful.

Benign causes often lead to syncope.

  • Syncope precipitated by unpleasant physical or emotional stimuli (eg, pain, fright), usually occurring in the upright position and often preceded by vagally mediated warning symptoms (eg, nausea, weakness, yawning, apprehension, blurred vision, diaphoresis), suggests vasovagal syncope.

  • Syncope that occurs most often when assuming an upright position (particularly in elderly patients after prolonged bed rest or in patients taking drugs in certain classes) suggests orthostatic syncope.

  • Syncope that occurs after standing for long periods without moving is usually due to venous pooling.

Dangerous causes are suggested by red flag findings.

Loss of consciousness during a seizure or postictal confusion can sometimes be confused with syncope, but muscular jerking or convulsions that last more than a few seconds, incontinence, drooling, or tongue biting, if present, usually point to a seizure.


Testing typically is done.

  • ECG

  • Pulse oximetry

  • Sometimes echocardiography

  • Sometimes tilt table testing

  • Blood tests only if clinically indicated

  • Central nervous system imaging rarely indicated

In general, if syncope results in an injury or is recurrent (particularly within a brief period), more intensive evaluation is warranted. Cardiac and brain imaging are not done unless indicated by clinical findings (suspected cardiac etiology or neurologic deficits).

Patients with suspected arrhythmia, myocarditis, or ischemia should be evaluated as inpatients. Others may be evaluated as outpatients.

ECG is done for all patients. The ECG may reveal arrhythmia, a conduction abnormality, ventricular hypertrophy, pre-excitation, QT prolongation, pacemaker malfunction, myocardial ischemia, or myocardial infarction. If the diagnosis is questionable after this basic evaluation, measuring cardiac markers and obtaining serial ECGs to rule out MI in older patients plus ECG monitoring for at least 24 hours are prudent. Any detected arrhythmia must be associated with altered consciousness in order to be implicated as the cause, but most patients do not experience syncope during monitoring. On the other hand, the presence of symptoms in the absence of rhythm disturbance helps rule out a cardiac cause. An event recorder (which can record cardiac rhythm for longer periods) may be useful if warning symptoms precede syncope. A signal-averaged ECG may identify predisposition to ventricular arrhythmias in patients with ischemic heart disease or in post-myocardial infarction patients. If syncopal episodes are infrequent (eg, < 1/month), an implantable loop recorder can be used for longer term recording. This device continuously records the rhythm and can be interrogated by an external machine that allows the cardiac rhythm to be printed.

Pulse oximetry should be done during or immediately after an episode to identify hypoxemia (which may indicate pulmonary embolism). If hypoxemia is present, CT angiography is indicated to rule out pulmonary embolism.

Laboratory tests are done based on clinical suspicion; reflexively obtained laboratory panels are of little use. However, all females of childbearing age should have a pregnancy test. Hematocrit is measured if anemia is suspected. Electrolytes are measured only if an abnormality is clinically suspected (eg, by symptoms or drug use). Serum troponin is measured if acute myocardial infarction is suspected.

Echocardiography Echocardiography This photo shows a patient having echocardiography. This image shows all 4 cardiac chambers and the tricuspid and mitral valves. Echocardiography uses ultrasound waves to produce an image of... read more Echocardiography is indicated for patients with clinically unexplained syncope, exercise-induced syncope, cardiac murmurs, or suspected intracardiac tumors (eg, those with positional syncope).

Tilt table testing Tilt Table Testing Tilt table testing is used to evaluate syncope in Younger, apparently healthy patients Elderly patients when cardiac and other tests have not provided a diagnosis Tilt table testing produces... read more Tilt Table Testing may be done if history and physical examination indicate vasodepressor or other reflex-induced syncope. It is also used to evaluate exercise-induced syncope if echocardiography or exercise stress testing is negative.

Stress testing Stress Testing In stress testing, the heart is monitored by electrocardiography (ECG) and often imaging studies during an induced episode of increased cardiac demand so that ischemic areas potentially at risk... read more Stress Testing (exercise or pharmacologic) is done when intermittent myocardial ischemia is suspected. It is often done for patients with exercise-induced symptoms. Exercise testing is less valuable unless physical activity precipitated syncope.

  • Unexplained recurrent syncope

  • Unexplained red flag findings

  • Ischemic cardiomyopathy, non-ischemic cardiomyopathy, and adult congenital disease, or unexplained syncope that does not otherwise meet criteria for an ICD used for primary prevention

A negative response defines a low-risk subgroup with a high rate of remission of syncope. The use of electrophysiologic testing is controversial in other patients.

EEG is warranted if a seizure disorder is suspected.

CT and MRI of the head and brain are indicated only if signs and symptoms suggest a focal CNS disorder.

Treatment of Syncope

In witnessed syncope, pulses are checked immediately. If the patient is pulseless, CPR is begun. If pulses are present, severe bradycardia is treated with atropine or external transthoracic pacing. Isoproterenol can be used to maintain adequate heart rate while a temporary pacemaker is placed.

Tachyarrhythmias are treated; a direct-current synchronized shock Direct Current (DC) Cardioversion-Defibrillation The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more is quicker and safer than drugs for unstable patients. Inadequate venous return is treated by keeping the patient supine, raising the legs, and giving IV normal saline. Tamponade is relieved by pericardiocentesis. Tension pneumothorax requires insertion of a pleural cannula and drainage. Anaphylaxis is treated with parenteral epinephrine.

Placing the patient in a horizontal position with legs elevated typically ends the syncopal episode if life-threatening disorders are ruled out. If the patient sits upright too rapidly, syncope may recur; propping the patient upright or transporting the patient in an upright position may prolong cerebral hypoperfusion and prevent recovery.

Specific treatment depends on the cause and its pathophysiology. Driving and use of machinery should be prohibited until the cause is determined and treated.

Geriatrics Essentials: Syncope

The most common cause of syncope in older adults is postural hypotension due to a combination of factors. Factors include rigid, noncompliant arteries, reduced skeletal muscle pumping of venous return due to physical inactivity, and degeneration of the sinoatrial node and conduction system due to progressive structural heart disease.

In older adults, syncope often has more than one cause. For example, the combination of taking several heart and blood pressure drugs and standing in a hot church during a long or emotional service may lead to syncope even though no single factor might cause syncope.

Key Points

  • Syncope results from global central nervous system dysfunction, usually resulting from insufficient cerebral blood flow.

  • Most syncope results from benign causes.

  • Some less common causes involve cardiac arrhythmia or outflow obstruction and are serious or potentially fatal.

  • Vasovagal syncope usually has an apparent trigger, warning symptoms, and a few minutes or longer of postrecovery symptoms.

  • Syncope due to cardiac arrhythmias typically occurs abruptly and with quick recovery.

  • If a benign etiology is not clear, driving and use of machinery should be prohibited until the cause is determined and treated—the next manifestation of an unrecognized cardiac cause may be fatal.

  • Treatment is directed at the underlying disorder.

More Information

Primary Writing Committee, Sandhu RK, Raj SR, et al: Canadian Cardiovascular Society Clinical Practice Update on the Assessment and Management of Syncope. Can J Cardiol 36(8):1167–1177, 2020. doi: 10.1016/j.cjca.2019.12.023

Drugs Mentioned In This Article

Drug Name Select Trade
Atreza, Atropine Care , Atropisol , Isopto Atropine, Ocu-Tropine, Sal-Tropine
Adrenaclick, Adrenalin, Auvi-Q, Epifrin, EpiPen, Epipen Jr , Primatene Mist, SYMJEPI, Twinject
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!