Syncope (pronounced SIN-ko-pee) is the modern medical term for fainting, i.e., the sudden loss of consciousness and posture due to lack of blood to the brain. This is most likely due to sudden emotional stress, postural hypotension, or loss of blood.

This serves the physiological purpose of shutting down non-essential functions and allowing the body to focus on redirecting blood supply to the brain. As it happens, falling down makes it easier for the heart to pump blood to the brain, as it is no longer fighting gravity. Moreover, the victim will usually start breathing faster and the heart rate will increase, although blood pressure may still be low in the extremities, as most of the increased blood flow will be directed to the brain.

There are specific terms for different causes of syncopes:

A neurocardiogenic syncope or vasovagal syncope is caused when the vagus nerve overreacts to an emotional trigger, and heart rate and blood pressure drop suddenly. Common triggers include the sight of blood, a sudden fright, or receiving bad news.

An occupational syncope follows from a purely physical cause, rather than emotional. This may sometimes be a case of a healthy person overexerting themselves, but usually results from an unhealthy person overexerting themselves through coughing, defecating, or sneezing.

A cardiovascular syncope results when damage to the heart prevents the heart from pumping enough blood to the brain. This would include a heart attack, among other conditions.

Orthostatic hypotension results from standing up too quickly, and the heart not increasing blood pressure quickly enough to maintain blood supply to the head. This is more common when a person is dehydrated, which may be secondary to a medical condition such as diabetes. Orthostatic hypotension is often blamed for fainting due to overconsumption of alcohol, although alcohol causes a wide range of effects that both make hypotension more likely and also mimics a vasovagal syncope. In most cases orthostatic hypotension results in dizziness (AKA a presyncope), but may cause fainting in some cases.

A cerebrovascular syncope occurs when there is a problem with the blood vessels bringing blood to the brain, most often as a result of cerebrovascular disease (e.g., stroke). This would also include any case where someone passes out due to compression of the carotid artery, as is used in some martial arts.

In addition, there are various random conditions that can cause syncope, and in many people, especially the elderly, multiple factors contribute to a syncope. There are other causes of losing consciousness that are not considered syncope, including seizure, concussion, low blood oxygen, low blood sugar, and drug intoxication. In any case, where the cause of loss of conscious is not immediately apparent and benign, you should see a doctor immediately.

We have receptors, called the carotid bulbs, in the carotid arteries in our neck. One on each side. Their job is to maintain good blood flow, that is, blood pressure, to the brain. If the pressure drops, their job is to fix the flow. By fixing the pressure. It goes black in front of your eyes first and then you faint. Presumably you will fall down, which will help with that pressure.

Except when you don't fall down. The first person I saw faint was my cousin, on a train. We were 17. She was sitting across from me between two people. I looked at her and her eyes were rolled back in her head. I elbowed my other cousin, her older sister. She looked up, said "Fainted." and reached across and shoved her sister's head down to her knees. My cousin (her sister) promptly woke up.

In residency a friend was visiting. We went to "Inner City Hot Tubs" in Portland. We hot tubbed for a good while. He was an athlete but had nearly no body fat. He and my then husband went in the sauna for more time. I don't love saunas. Too hot. They came out and we were showering. The friend said, "I feel strange...." I whirled and shoved my hands under his arms from behind as he went down. Got him. Which is good because he was headed for a head bang against tile corners. I laid him down and put his legs up. All of the other people sitting around in towels gave me loads of advice. He woke up and got water and salt.

In clinic I am stitching an older teen's forehead. Scalps bleed, messy. He is holding on to his mother with his left hand. I am on his right, gloved, hopefully following sterile technique as I stitch. My nurse steps out. I ask his mother something. She doesn't answer. I look up. She has fainted dead away, but her son has a death grip on her arm. "Um. Don't let go of your mother, ok?" My nurse comes in and lies mom down and puts her legs up on a chair. She revives.

In clinic I am describing a procedure that we are about to do to a young woman. Who faints. We wake her up. Her blood pressure is fine. So we make her talk during the procedure, asking questions about her beau, and she stays awake.

In clinic I am on the phone, trying to set up a colonoscopy. My patient is a bit fidgity, standing up. He makes a sound and then goes down right in front of me. "Uh." I say, "My patient fainted. I'll call you back." He stays down for a while. He has lost a lot of weight, over one hundred pounds. I had cut his blood pressure medicine in half, but that wasn't enough. We keep cutting it in half and now he's off. For every kilogram lost, the blood pressure drops about a point. Ok, if you lose over a hundred pounds, you don't drop 100 points, but you WILL need your blood pressure medicine cut back. That was his second visit with me. Some other clinic was not paying attention.

I have nearly fainted once. Medical school, pregnant, emergency room rotation, Friday night drunk tank with the car wreck drunks lined up on stretchers. I was with the orthopedic resident who was checking that all the tendons were working in an arm. "Wiggle this finger." All of the drunks were dripping blood and none of them knew they'd been in a wreck and all were talking. I felt all my blood drain south. "I need to use the ladies room." I walked out, to the bathroom, put my head way down. I did not faint.

In pregnancy the blood volume is increased by about 1/5. So if the woman doesn't drink enough fluid, she will complain of dizziness standing. Or she will faint.

Heart arrhythmias are not that common as a cause of fainting, but can be a sign of a heart attack, or a prelude to a stroke, or a medication complication. The history and exam and labs (and a urine tox screen and pregnancy test) all help, but history is most important.

We can faint from dehydration, from loss of blood, from a vasovagal response: that's the faint at the sight of blood or too hot or stress....

Syn"co*pe (?), n. [L. syncope, syncopa, Gr. a cutting up, a syncope; akin to to beat together, to cut up, cut short, weavy; with + to strike, cut.]

1. Gram.

An elision or retrenchment of one or more letters or syllables from the middle of a word; as, ne'er for never, ev'ry for every.

2. Mus.

Same as Syncopation.

3. Med.

A fainting, or swooning. See Fainting.


A pause or cessation; suspension.


Revely, and dance, and show, Suffer a syncope and solemn pause. Cowper.


© Webster 1913.

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