Fainting (syncope) is a sudden, brief loss of consciousness.
Fainting is a symptom of an inadequate supply of oxygen and other nutrients to the brain, usually caused by a temporary decrease in blood flow. Blood flow to the brain can decrease whenever the body cannot quickly compensate for a fall in blood pressure.
A person cannot lose consciousness unless there is a general disturbance of brain function. This disturbance usually involves a reduction in blood flow to the brain. Blood flow to the brain can be reduced by a heart disorder or, more commonly, by something that interferes with the normal return of blood to the heart, which necessarily reduces blood flow to the brain (and the rest of the body). Rarely, blood flow to the brain is reduced by a disorder of the blood vessels at the base of the brain. Although seizures—a brain disorder—can cause loss of consciousness, they are not considered fainting. People and their doctors may not be able to differentiate between fainting and seizures without careful testing.
Problems With the Heart's Pumping:
Fainting may occur if the heart cannot pump enough blood to maintain a normal blood pressure. For example, an abnormal heart rhythm or a heart valve disorder may impair the heart's pumping ability. People with such disorders may feel fine when resting. However, they feel faint or actually faint when exercising because the heart cannot pump enough blood to meet the body's increased demand for oxygen. This type of fainting is called exertional or effort syncope. People with these disorders may also faint after exercising. During exercise, the increase in heart rate may enable the heart to pump enough blood to maintain adequate blood pressure, although just barely. When exercise stops, the heart rate (and the amount of blood pumped) begins to decrease. However, the blood vessels in muscles, which dilate (widen) during exercise to move more blood to and from the muscles, remain dilated. (The arterioles in muscles remain dilated to help supply oxygen and nutrients to muscle tissue, and the veins remain dilated to remove metabolic waste products produced during exercise.) The decrease in the amount of blood pumped out combined with dilation of the arterioles and veins causes blood pressure to fall, and fainting results.
An abnormality of the heart called hypertrophic cardiomyopathy (see Cardiomyopathy: Hypertrophic Cardiomyopathy) can also cause fainting that usually occurs during exercise. Severe narrowing (stenosis) of the aortic valve can have the same effect. These disorders may occur in younger people as well as older people, particularly those who have high blood pressure. If untreated, they can lead to death.
Low Volume of Blood:
Fainting may occur if the blood volume is too low. An obvious cause of low blood volume is bleeding. Another cause is dehydration, which may be due to diarrhea, excessive sweating, inadequate intake of fluids, or excessive urination (which is a common symptom of untreated diabetes—see Diabetes Mellitus (DM): Diabetes Mellitus—or Addison's disease—see Adrenal Gland Disorders: Addison Disease). In older people, the use of diuretics is a common cause of dehydration, particularly during warm weather or during an illness when obtaining or drinking enough fluids may be difficult. (Diuretics help the kidneys eliminate salt and water by increasing urine formation and thus decrease fluid volume in the body.)
Vagus Nerve Stimulation:
Fainting may occur if the vagus nerve, which supplies the neck, chest, and intestine, is stimulated. When stimulated, the vagus nerve slows the heart. Such stimulation also causes nausea and cool, clammy skin. This type of fainting is called vasovagal (vasomotor) syncope. The vagus nerve is stimulated by pain, fear, other distress (such as that due to the sight of blood), vomiting, a large bowel movement, and urination. Fainting during or immediately after urination is called micturition syncope. Rarely, vigorous swallowing causes fainting due to stimulation of the vagus nerve.
Reduced Blood Flow:
Fainting may also occur if straining reduces the amount of blood flowing back to the heart. Fainting due to coughing (cough syncope) usually results from such straining. Fainting after urination (micturition syncope) or after a bowel movement is partly due to straining (in addition to stimulation of the vagus nerve). Older men who must strain to empty their bladder because of a large prostate gland are particularly susceptible. Fainting when lifting weights (weight lifter's syncope) results from the strain of trying to lift or push heavy weights without breathing adequately during the exercise.
Problems With Blood Pressure:
Fainting that occurs when a person sits or stands up too quickly is called orthostatic (postural) syncope. It is particularly common among older people. It is caused by orthostatic hypotension (see Low Blood Pressure: Orthostatic Hypotension). In orthostatic hypotension, the compensatory mechanisms, particularly the constriction of blood vessels and the increase in heart rate, do not adequately restore blood pressure when a person stands and gravity causes blood to pool in the leg veins. A related form of fainting, called parade ground syncope, occurs when people stand still for a long time on a hot day. If the leg muscles are not used, blood is not pumped back to the heart. As a result, blood pools in the leg veins, and blood pressure falls.
In older people, an excessive decrease in blood pressure after eating a meal (postprandial hypotension—see Low Blood Pressure: Postprandial Hypotension) may cause fainting.
Fainting may result from very rapid breathing (hyperventilation, or overbreathing), which may be due to anxiety. This type of fainting is called hyperventilation syncope. Hyperventilation removes large amounts of carbon dioxide from the body. The decreased level of carbon dioxide causes blood vessels in the brain to constrict, and the person may feel faint or actually faint.
Rarely, fainting results from a mild stroke in which blood flow to a certain part of the brain (at the base) suddenly decreases. Fainting due to a stroke is more common among older people. Many other disorders, such as a deficiency of red blood cells (anemia), lung disorders, a decreased blood sugar level (hypoglycemia), and diabetes can cause fainting, especially if the compensatory mechanisms are also impaired.
Certain drugs may cause fainting. They include many of those used to treat high blood pressure, angina, and heart failure. Doses of these drugs must be carefully adjusted to prevent blood pressure from decreasing too much.
Dizziness or light-headedness may precede fainting, especially if the person is standing. After the person falls, blood pressure increases, partly because the person is lying down (and blood can flow to the brain without having to fight gravity) and often because the cause of fainting has passed. However, getting up too quickly may make the person faint again.
When the cause is an abnormal heart rhythm (arrhythmia), fainting usually begins and ends suddenly. Sometimes the person feels palpitations (awareness of heartbeats) just before fainting.
Vasovagal syncope may occur when a person is sitting or standing. It is often preceded by nausea, weakness, yawning, blurring of vision, and sweating. The skin may become cool and clammy. The person becomes ghostly pale, the pulse becomes very slow, and the person faints.
Fainting that begins gradually with warning symptoms and also disappears gradually suggests changes in the blood, such as a decreased level of sugar (hypoglycemia) or carbon dioxide (hypocapnia). Hypocapnia is often preceded by a pins-and-needles sensation in the fingertips and around the lips.
Doctors try to determine the cause of fainting because some causes are more serious than others. Heart disease, such as an abnormal heart rhythm or narrowing (stenosis) of the aortic valve, can be fatal. Other causes are much less worrisome.
Sometimes the nature of the symptoms suggests a cause to the doctor. Descriptions from witnesses of the fainting episode may be helpful. Of concern is fainting that occurs without any warning symptoms (particularly during exertion), is accompanied by shortness of breath or chest pain, results in injury, or occurs in a person with an abnormal finding during an examination of the heart or nervous system. Doctors also need to know whether the person has any disorders and whether the person is taking any prescription or over-the-counter drugs.
If the fainting occurs during emotionally stressful situations or is preceded by symptoms of vasovagal syncope (such as nausea, sweating, cool and clammy skin, and paleness), fainting usually is not serious, and extensive diagnostic procedures and treatment are rarely necessary.
Doctors will often obtain an electrocardiogram (ECG), which records the electrical activity of the heart and can detect an underlying heart disorder. Continuous ECG may be required to determine the cause of fainting. For this procedure, the person wears a small battery-powered device (Holter monitor). It records the heart's electrical activity for 24 hours or more as the person engages in normal daily activities (see Holter Monitor: Continuous ECG Readings). If an irregular heart rhythm coincides with a fainting episode, it is probably—but not necessarily—the cause.
Other procedures, such as echocardiography, which uses ultrasound waves to produce an image of the heart (see Diagnosis of Heart and Blood Vessel Disorders: Echocardiography and Other Ultrasound Procedures), can detect whether the heart has a structural or functional abnormality. Blood tests may show that the person has hypoglycemia or anemia.
Loss of consciousness due to a seizure (see Seizure Disorders) is distinguished from fainting because the causes and treatment are different. To distinguish between the two, doctors may use electroencephalography (EEG), which records the brain's electrical activity (see Symptoms and Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Electroencephalography). Also, after a seizure, recovery from unconsciousness is much slower, causing drowsiness that usually lasts for at least 10 minutes.
To confirm a suspected cause, doctors may attempt to re-create a fainting episode under safe conditions. For example, the person may be asked to breathe quickly and deeply. Or, while monitoring the heartbeat with ECG, a doctor may press gently over the carotid sinus (a part of the internal carotid artery containing sensors that monitor blood pressure). This pressure temporarily increases blood pressure inside the carotid sinus, tricking the body into thinking that blood pressure has increased throughout the body. The sinus then sends signals to the brain to reduce blood pressure, and faintness or fainting may result.
Tilt table testing (see Diagnosis of Heart and Blood Vessel Disorders: Tilt Table Testing) is commonly done to determine the cause of fainting. The person is strapped to a motorized table. Then the table tilts until the person is almost standing. This position is held for up to 45 minutes. Blood pressure and heart rate are continuously monitored during the test. If blood pressure does not decrease, the person is given isoproterenol (a drug that stimulates the heart), and the test is repeated. Use of this drug makes the test more sensitive.
Usually, lying flat restores consciousness. Raising the legs can speed recovery by increasing blood flow to the heart and brain. If the person sits up too rapidly or is propped up or carried in an upright position, another fainting episode may occur. Therefore, the person should remain lying down until fully recovered.
A heart rate that is too slow can be corrected by surgically implanting a pacemaker, an electronic device that stimulates heartbeats (see Keeping the Beat: Artificial Pacemakers). A heart rate that is too rapid can be slowed by using drugs, particularly a beta-blocker (such as atenolol or metoprolol). A defibrillator can be implanted to restore normal rhythm if the heart beats irregularly (see Abnormal Heart Rhythms: Restoring Normal Rhythm). Other causes of fainting—such as hypoglycemia and anemia—can be treated. If blood volume is very low, fluids may be given intravenously. Surgery may be considered for heart valve disorders.
Last full review/revision May 2007 by George L. Bakris, MD