Vasovagal Syncope Is Possibly the Strangest Thing Your Body Does. Here's Why It Happens

Some people might have a vasovagal reaction after long periods of exercise.
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There's no shortage of strange, impressive things our bodies can do. But vasovagal syncope might be the very strangest one of them all.


Even if you've never heard the term before, you're probably familiar with the concept. When someone passes out at the sight of blood or gets sweaty while having a bowel movement, that's vasovagal syncope at work.

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But uh, why exactly does this happen? And does it mean that something is seriously wrong with you?

Here are the answers to all your burning questions, plus how to cope if you deal with these common fainting spells.

What Is Vasovagal Syncope?

Vasovagal syncope, sometimes called a vasovagal response, is fainting that occurs when the brain experiences a sudden reduction in blood flow due to a drop in heart rate and blood pressure.

"It's known as a simple fainting spell. The fainting episodes are often accompanied by sweating, nausea or even vomiting," explains Minh Nghi, DO, an internist with Texas Health Harris Methodist Hospital Southwest Fort Worth and Texas Health Physicians Group.


The phenomenon happens when the vagus nerve — the longest nerve in the body, which runs from the brain to the gut — receives an exaggerated response from sensors in the heart, GI tract or neck.

"Because of crosstalk in the nerve, the heart can receive a signal that tells it to slow down," Dr. Nghi says. "If it slows down too much, blood pressure to the brain falls and one will lose consciousness."


While vasovagal episodes may sound weirdly terrifying, they're not particularly serious. Vasovagal syncope is benign, and a person will typically regain consciousness within a minute or so, once their heart rate and blood pressure pick back up, says Dr. Nghi.

However, a person can be at risk of potentially hitting their head when they faint or getting into an accident if the episode occurs while driving.


What Is the Vagus Nerve, Exactly?

Because the vagus nerve plays an essential role in vasovagal syncope, it makes sense to talk about it in a bit more detail. Running on an axis from the brain stem to the colon, the vagus or vagal nerves (there's actually a left and a right one) send information between the brain, heart and digestive system, according to the Cleveland Clinic.


The vagal nerves are the main nerves of the parasympathetic nervous system, the part of the nervous system that controls involuntary functions like digestion, heart rate and immunity. And it's this heart rate-control job that can trigger vasovagal syncope. When the nerve overreacts, it responds by causing a person's heart rate to become slower. This triggers the blood pressure drop that reduces blood flow to the brain, and ultimately, causes a person to faint or feel lightheaded.


What Causes Vasovagal Syncope?

There are all kinds of situations that can trigger a vasovagal reaction, and some of them are pretty strange.

"It's often brought on by a trigger such as strong emotions, certain odors, foods, coughing forcefully, vomiting or bowel movements," Dr. Nghi says. (In that way, you can think of vasovagal syncope as the instigator behind poop sweats or the reason you might feel dizzy when you're constipated.)


Other extremes can trigger vasovagal syncope too. When someone faints at the sight of blood (or even the thought of it), it's vasovagal syncope at work, according to the Mayo Clinic. The reaction can also happen from extreme heat, standing for too long, hyperventilating, intense pain, prolonged exercise, being dehydrated or even simply being afraid of sustaining a bodily injury (think: a fear of needles), according to Cedars-Sinai.

Less commonly, vasovagal syncope can be triggered by an underlying medical condition that affects the heart or nervous system. You might also be more likely to experience it if you have a condition that affects blood flow, which can cause your blood pressure to quickly drop when you stand up or change positions, the Cleveland Clinic notes.


Vasovagal Syncope Symptoms

With vasovagal syncope, a person doesn't typically go from feeling completely normal to just, well, blacking out. Usually their body will start to send signs that fainting could be around the corner. According to the Mayo Clinic, these can include:


  • Lightheadedness
  • Pale skin
  • Tunnel vision
  • Nausea
  • Feeling warm
  • Clammy sweating
  • Blurred vision

How Can You Treat or Prevent Vasovagal Syncope?

A person will typically regain consciousness from vasovagal syncope within a minute or so of fainting, so the focus is more on prevention.

"Knowing what might trigger a person's fainting will allow them to avoid the trigger and thus the spell," says Dr. Nghi.

If you've experienced vasovagal syncope before, be aware that you're prone to the condition and pay attention if you start showing symptoms.

"I recommend that patients be mindful of their situation and surroundings," Dr. Nghi says. "If they start to feel faint, sit or lie down. If driving, pull over immediately until they no longer feel faint."

Sometimes you might feel nauseated, clammy or sweaty first; in that case, get to a safe position before as soon as possible, before you start to feel faint.

Staying well hydrated and consuming more salt can also be helpful, particularly if a person is known to have low blood pressure, per Cedars-Sinai, as can stopping medications like diuretics that reduce blood pressure.

When to See a Doctor

It's worth mentioning a fainting episode to your doctor, even if it only happens once. While vasovagal syncope is often harmless, in some cases it could be a sign of an underlying condition like a heart or lung abnormality, notes the National Center for Biotechnology.

Together, you and your doctor can discuss your symptoms and health history to decide if you should undergo diagnostic testing.




Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.

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