Syncope (Fainting)

Updated:Dec 21,2016

What is syncope?

Syncope is temporary loss of consciousness and posture, described as "fainting" or "passing out." It's usually related to temporary insufficient blood flow to the brain. It most often occurs when the blood pressure is too low (hypotension) and the heart doesn't pump a normal supply of oxygen to the brain.

What causes syncope?

It may be caused by emotional stress, pain, pooling of blood in the legs due to sudden changes in body position, overheating, dehydration, heavy sweating or exhaustion. Syncope may occur during violent coughing spells (especially in men) because of rapid changes in blood pressure. It also may result from several heart, neurologic, psychiatric, metabolic and lung disorders. And it may be a side effect of some medicines.

Some forms of syncope suggest a serious disorder:

  • those occurring with exercise 
  • those associated with palpitations or irregularities of the heart 
  • those associated with family history of recurrent syncope or sudden death

What is neurally mediated syncope?

Neurally mediated syncope (NMS) is called also neurocardiogenic, vasovagal, vasodepressor or reflex mediated syncope. It's a benign (and the most frequent) cause of fainting. However, life-threatening conditions may also manifest as syncope. NMS is more common in children and young adults, although it can occur at any age. NMS happens because blood pressure drops, reducing circulation to the brain and causing loss of consciousness. Typical NMS occurs while standing and is often preceded by a sensation of warmth, nausea, lightheadedness and visual "grayout." If the syncope is prolonged, it can trigger a seizure. Placing the person in a reclining position will restore blood flow and consciousness and end the seizure.

AHA Recommendation

The majority of children and young adults with syncope have no structural heart disease or significant arrhythmia (abnormal heart rhythm). So, extensive medical work-up is rarely needed. A careful physical examination by a physician, including blood pressure and heart rate measured lying and standing, is generally the only evaluation required.

In other cases an electrocardiogram (EKG or ECG) is used to test for abnormal heart rhythms such as long Q-T syndrome. This is a genetic heart condition that can cause sudden cardiac death. Other tests, such as exercise stress test, Holter monitor, echocardiogram, etc., may be needed to rule out other cardiac causes of syncope.

If EKG and cardiac tests are normal, the person will undergo a tilt test. The blood pressure and heart rate will be measured while lying down on a board and after the board is tilted up. Someone who has NMS will usually faint during the tilt, due to the rapid drop in blood pressure and heart rate. As soon as the person is placed on his or her back again, blood flow and consciousness are restored.

To help prevent syncope, people with NMS should be on a higher-salt diet (this is recommended only for persons diagnosed with NMS) and drink plenty of fluids to avoid dehydration and maintain blood volume. They should watch for the warning signs of fainting — dizziness, nausea and sweaty palms — and sit or lie down if they feel the warning signs. Some people also may need medication. 

This content was last reviewed September 2016.


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