Light-headedness (near-syncope) is the feeling that one is about to faint. Fainting (syncope) is a sudden, brief loss of consciousness followed by spontaneous return of consciousness.
The causes of light-headedness and fainting tend to be the same. A person cannot lose consciousness unless brain function is generally disturbed. This disturbance usually occurs because blood flow to the brain is reduced. Brain blood flow can be reduced by a heart disorder or, more commonly, by something that interferes with the normal return of blood to the heart and thus reduces blood flow to the brain. Older people are particularly susceptible because blood flow to the brain decreases as people age. Brain disorders by themselves rarely cause fainting, unless they also affect the blood vessels. Seizures, a brain disorder, can cause loss of consciousness but are not considered fainting.
In heart disorders, blood flow to the brain may be reduced when the heart rate or rhythm is abnormal (too slow or too fast) or when the heart cannot pump blood adequately because blood flow is blocked. Blood flow can be blocked by a defective heart valve (most commonly, the aortic valve), by blood clots in the lungs or sometimes the heart, and, rarely, by certain heart tumors such as an atrial myxoma.
Many factors can interfere with the return of blood to the heart. Coughing or straining during bowel movements can increase chest pressure, reducing the return of blood to the heart. Healthy soldiers may feel faint or may faint when standing still for a long time (a phenomenon called parade ground syncope), because the leg muscles have to be active to help return blood to the heart. Strong emotion (particularly that triggered by viewing a bloody or gruesome scene) or pain can activate the vagus nerve. As a result, blood vessels widen (dilate), reducing the return of blood to the heart and sometimes causing fainting (called vasovagal syncope). Certain brain and spinal cord disorders and drugs (particularly those used to treat blood pressure) can also dilate blood vessels and cause fainting.
Sitting or standing up too quickly can cause a feeling of faintness or fainting, because the change in position causes blood to pool in the legs, resulting in a fall in blood pressure. Normally, the body quickly adjusts to maintain blood pressure. Inability to adjust quickly is called orthostatic hypotension. This disorder is particularly common among older people.
People are more likely to feel faint or to faint when they are standing up. When they lie or fall down, blood flow to the brain is increased, usually restoring consciousness.
Doctors must distinguish dangerous causes of fainting from relatively harmless ones. If fainting is preceded by brief warning symptoms such as light-headedness, nausea, yawning, blurred vision, or sweating and occurs during a painful or unpleasant situation, it is probably vasovagal, which is not dangerous. In such cases, doctors perform a physical examination. If results are normal, usually no further testing is needed. Fainting is worrisome in adolescents because it more often indicates a serious heart disorder.
Further testing is needed if fainting occurs without any warning symptoms (particularly during physical activity), is accompanied by shortness of breath or chest pain, or results in injury to the person or if the results of a heart or neurologic examination are abnormal. Electrocardiography (ECG) is often done. Other tests, such as ultrasonography of the heart (echocardiography), tilt table testing (see Diagnosis of Heart and Blood Vessel Disorders: Electrophysiologic Testing), and electrophysiologic testing (see see Diagnosis of Heart and Blood Vessel Disorders: Tilt Table Testing) are sometimes useful.
Last full review/revision April 2006 by Paul H. Tanser, MD