Merck Manual

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Andrea D. Thompson

, MD, PhD, University of Michigan;

Michael J. Shea

, MD, Michigan Medicine at the University of Michigan

Reviewed/Revised Aug 2022 | Modified Sep 2022
Topic Resources

Light-headedness (near syncope) is a sense that one is about to faint.

Fainting (syncope) is a sudden, brief loss of consciousness during which the person falls to the ground or slumps in a chair followed by a return to consciousness. The person is motionless and limp and may have cool legs and arms, a weak pulse, and shallow breathing.

Some people feel light-headed or dizzy before they faint. Others may have nausea, sweating, blurred vision or tunnel vision, tingling of lips or fingertips, chest pain, or palpitations. Less often, people faint suddenly, without any warning symptoms.

Fainting can occur in people of any age, but dangerous causes of fainting are more common among older people.

Causes of Fainting

A person cannot lose consciousness unless brain function is generally disturbed. This disturbance usually occurs because overall blood flow to the brain is reduced. Sometimes, however, blood flow is adequate but the blood does not contain sufficient oxygen or glucose (blood sugar), which the brain needs to function.

Blood flow to the brain can be reduced in several ways. Most often, the cause is something that interferes with the normal return of blood to the heart (and thus reduces blood flow out from the heart). Less often, the cause is a disorder that interferes with blood pumping (typically a heart disorder). Although strokes reduce blood flow to the brain, they only reduce flow to part of the brain. Thus, strokes rarely cause fainting except for the few strokes that involve the part of the brain that maintains consciousness.

The most common causes of fainting are

  • Strong emotion (such as fear, pain, or sight of blood)

  • Coughing or straining to pass stool or urine

  • Prolonged standing

  • Standing up suddenly

  • Pregnancy

  • Use of certain drugs

  • Idiopathic (meaning that the cause cannot be determined)

These common causes nearly always cause fainting only when people are standing up. When they fall down, blood flow to the brain is increased, quickly restoring consciousness, although people may not feel completely normal for a few minutes to a few hours. Some people feel tired or exhausted for several hours. These causes tend not to be serious unless people are injured when they fall.

Most of these causes involve decreased return of blood to the heart. Strong emotion (particularly that triggered by the sight of blood) or pain can activate the vagus nerve. Vagus nerve activation widens blood vessels, reducing the return of blood to the heart, and slows the heart rate. Both of these factors cause light-headedness and sometimes fainting (called vasovagal syncope or neurocardiogenic syncope).

Straining during bowel movements or urination or coughing increases chest pressure. Increased chest pressure can activate the vagus nerve and also reduce the return of blood to the heart—two factors that may cause fainting.

Healthy people may faint when standing still for a long time (most common in soldiers, a phenomenon called parade ground syncope), because the leg muscles have to be active to help return blood to the heart.

Sitting or standing up too quickly can cause fainting, because the change in position causes blood to pool in the legs, resulting in a fall in blood pressure. Normally, the body quickly increases the heart rate and constricts blood vessels to maintain blood pressure The Body's Control of Blood Pressure High blood pressure (hypertension) is persistently high pressure in the arteries. Often no cause for high blood pressure can be identified, but sometimes it occurs as a result of an underlying... read more The Body's Control of Blood Pressure . If the body does not compensate in these ways, light-headedness is common and fainting may rarely occur. Certain brain and spinal cord disorders, prolonged bed rest, and certain drugs (particularly those used to treat high blood pressure) can interfere with this compensation and lead to fainting when standing up Dizziness or Light-Headedness When Standing Up In some people, particularly older people, blood pressure drops excessively when they sit or stand up (a condition called orthostatic or postural hypotension). Symptoms of faintness, light-headedness... read more .

Hormonal changes in early pregnancy sometimes lead to fainting.

Low blood sugar (hypoglycemia Hypoglycemia Hypoglycemia is abnormally low levels of sugar (glucose) in the blood. Hypoglycemia is most often caused by medications taken to control diabetes. Much less common causes of hypoglycemia include... read more ) initially causes confusion, light-headedness, shakiness, and other symptoms, but if hypoglycemia is severe or prolonged, people can lose consciousness. Because these other symptoms usually occur before fainting, people with hypoglycemia usually have some warning before they faint. Usually, the cause of hypoglycemia is use of drugs for diabetes, particularly insulin. Rarely, people have a tumor that secretes insulin.

Less common but more serious causes include

  • Heart valve disorders (most commonly, the aortic valve)

  • A heart rate that is too fast or too slow

  • Blockage of an artery to the lungs by a blood clot (pulmonary embolism)

  • Heart attack or other heart muscle disorders

Heart valve disorders can block blood from leaving the heart. A very fast heart rate may not allow the heart enough time to refill with blood, so less blood is pumped. A very slow heart rate may not pump enough blood. Blood clots in the lungs can keep the heart from pumping enough blood. People with a heart attack rarely have fainting when the heart attack occurs (more common in older people). Other uncommon heart muscle disorders called cardiomyopathies can cause fainting, particularly during exercise, typically because of an abnormal heart rhythm.

Although most strokes do not cause fainting, a stroke or transient ischemic attack (TIA) that involves certain blood vessels at the base of the brain (posterior circulation stroke) can cause fainting. Similarly, a migraine that involves these blood vessels sometimes causes fainting.

Evaluation of Fainting

If possible, someone who witnessed the faint should provide the doctor with a description of the event because the person who fainted may not remember.

Warning signs

In people who have fainted, certain symptoms and characteristics are cause for concern. They include

  • Fainting during exercise

  • Several episodes within a short time

  • Sudden fainting without any warning symptoms or any apparent trigger

  • Fainting preceded or followed by possible heart symptoms such as chest pain, palpitations, or shortness of breath

  • Older age

  • Significant injury as a result of fainting

  • Family history of sudden unexpected death, fainting during exercise, or repeated episodes of fainting or seizures for which a cause has not been found

When to see a doctor

Although most causes of fainting are not serious, a doctor's evaluation is needed to distinguish serious causes from relatively harmless ones. People who fainted should see a doctor right away, especially if they have any warning signs.

What the doctor does

Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What doctors find during the history and physical examination often suggests a cause of the fainting and the tests that may need to be done.

Doctors ask about the events leading up to the fainting episode. They ask whether the person was exercising, arguing, or in a potentially emotional situation. They ask whether the person was lying or standing, and, if standing, for how long. They also ask about symptoms that occurred immediately before or after the event, including whether the person felt light-headed or dizzy or experienced nausea, sweating, blurred or tunnel vision, tingling of lips or fingertips, chest pain, or palpitations.

Doctors also ask any witnesses to describe the episode. A sudden, abrupt faint without any warning symptoms or apparent trigger suggests a heart disorder. If fainting is preceded by a brief period of symptoms such as light-headedness, nausea, yawning, blurred vision, or sweating and occurs during a painful or unpleasant situation, it is probably vasovagal syncope, which is not dangerous.

Doctors ask about previous episodes of dizziness or fainting and about other disorders, drugs, or symptoms that may be related to fainting. Doctors also check the person for injuries resulting from the fainting episode.

Then doctors measure the person's vital signs. Heart rate and blood pressure are measured with the person lying down and after 1 to 3 minutes of standing. Doctors listen to the heart for signs of an abnormal heart valve or abnormal blood flow. They do a neurologic examination for signs of a stroke.



Testing typically is done unless people have an obvious, harmless emotional trigger and otherwise feel well. Doctors choose tests based on the suspected cause.

  • Electrocardiography (ECG)

  • Continuous ambulatory ECG (Holter monitor or event monitor)

  • Measurement of oxygen in the blood (pulse oximetry)

  • Fingerstick blood sugar measurement

  • Sometimes ultrasonography of the heart (echocardiography)

  • Sometimes tilt table testing

  • Sometimes blood tests

  • Rarely imaging of the central nervous system (brain and spinal cord)

In general, if fainting results in an injury or has happened several times (particularly within a brief period), more intensive evaluation is warranted. Imaging of the heart and brain are not routinely done unless the doctor suspects a heart or brain problem.

People with suspected heart disorders, including heart attack, abnormal heart rhythm, or heart valve abnormality are usually admitted to the hospital for evaluation.

ECG Electrocardiography Electrocardiography (ECG) is a quick, simple, painless medical test that measures the heart’s electrical impulses. During an ECG, the heart's electrical impulses are measured, amplified, and... read more Electrocardiography is done. ECG may show a heart rhythm disturbance or other heart problem but sometimes is normal if the abnormal heart rhythm has resolved. Sometimes doctors admit the person to a hospital to monitor the heart's activity for 24 hours. Less often, doctors may have the person wear a small heart monitor while at home (called continuous ambulatory ECG Continuous Ambulatory Electrocardiography A continuous ambulatory electrocardiography (ECG) is a painless medical test that records the heart's electrical activity continuously for 24 to 48 hours while the person engages in normal daily... read more ). Or, rarely, doctors may implant a recording device under the skin.

Oxygen in the blood is measured. Pulse oximetry is done during or immediately after an episode to identify low levels of oxygen in the blood (which may indicate a blood clot). If the level of oxygen in the blood is low, doctors do computed tomography (CT) angiography (using intravenous contrast) or a lung scan to check for a blood clot.

Laboratory tests are done if physical examination findings suggest they are needed. However, all women of childbearing age should have a pregnancy test.

Tilt table testing Tilt Table Testing Tilt table testing is a medical test that measures how being in different positions affects heart rate, heart rhythm, and blood pressure. People with unexplained lightheadedness, dizziness,... read more Tilt Table Testing is sometimes done if people have fainting when they stand up. It is also used to evaluate fainting that is caused by exercise if echocardiography or exercise stress testing does not reveal a cause.

Electrophysiologic (EP) testing Electrophysiologic Testing Electrophysiologic testing measures the heart's electrical activity through wire electrodes inserted into all 4 heart chambers. Electrophysiologic testing is used to evaluate serious abnormalities... read more includes tests that record the electrical activity and electrical pathways of the heart by means of wires passed through blood vessels into the heart. EP testing is sometimes done if other testing does not identify a heart rhythm disturbance in a person with unexplained recurrent episodes of fainting, in people with unexplained warning signs and in people with a history of heart failure Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more Heart Failure (HF) due to heart attack.

Treatment of Fainting

Specific treatment depends on the cause. For example, people who have fainting caused by an abnormal heart rhythm may need to have a pacemaker and/or defibrillator implanted.

If people see someone faint, they should check whether the person is breathing. If the person is not breathing, bystanders should call for emergency medical assistance and begin cardiopulmonary resuscitation Cardiac Arrest and CPR Cardiac arrest is when the heart stops pumping blood and oxygen to the brain and other organs and tissues. Sometimes a person can be revived after cardiac arrest, particularly if treatment is... read more Cardiac Arrest and CPR (CPR), including applying an automated external defibrillator Automated External Defibrillator: Jump-Starting the Heart Automated External Defibrillator: Jump-Starting the Heart (AED) if one is available. Once the person reaches the hospital, doctors will treat the cause of the fainting with drugs or appropriate measures, such as direct-current cardiac defibrillation to restart the heart or drugs or surgery to open blocked arteries.

A person who is breathing should remain lying down. If the person sits upright too rapidly, fainting may recur.

Doctors may ask a person who has fainted to restrict driving and operating machinery until the cause of fainting is determined and treated because if the cause is an unrecognized heart disorder, the next manifestation may be fatal.

Essentials for Older People

Older people are particularly susceptible to fainting because blood flow to the brain decreases as people age. The most common cause of fainting in older people is inability of blood pressure to rapidly adjust when the person stands. Blood flow decreases because arteries become more rigid and less able to adjust rapidly, physical inactivity reduces the muscle activity that pushes blood through the veins and back to the heart, and heart disease decreases the effectiveness of blood pumping.

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

Key Points

  • Fainting usually results from insufficient blood flow to the brain.

  • Most causes of fainting are not serious.

  • Some less common causes are serious or potentially fatal.

  • If fainting has an apparent trigger (such as strong emotion), is preceded by symptoms (such as light-headedness, nausea, or sweating), and requires a few minutes to recover, it is probably vasovagal syncope and is not serious.

  • Fainting due to heart rhythm disturbances typically occurs abruptly and with a quick recovery.

  • Doctors may ask a person who has fainted to restrict driving and operating machinery until the cause of fainting is determined and treated because if the cause is an unrecognized heart disorder, the next manifestation may be fatal.

  • If fainting is caused by a slow heart rate, a pacemaker may be needed.

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