Common triggers of vasovagal syncope and how to reduce your risk of fainting

If you've ever fainted at the sight of blood or from standing up too fast, you've experienced what's known as vasovagal syncope, the most common cause of fainting. Up to a third of people have experienced an episode of vasovagal syncope at some point in their lives.

Occasional episodes of vasovagal syncope are rarely a cause for concern. But if they happen often, it's a good idea to see a doctor to rule out more serious underlying causes.

What is vasovagal syncope?

Vasovagal syncope occurs when the vagus nerve, which carries signals from the brain to the rest of the body and controls functions including heart rate and blood pressure, becomes overstimulated in response to triggers such as stress, dehydration, or donating blood. The heart rate slows and blood vessels widen, causing a drop in blood pressure and reduced blood flow to the brain. This can lead to a loss of consciousness.

Lying or sitting down (and even falling) quickly restores blood flow to the brain, allowing blood pressure to return to normal. Most people regain consciousness within a few seconds after passing out.

While vasovagal syncope is not life-threatening in itself, it can lead to serious injuries as a result of falling.

Common triggers of vasovagal syncope

Common triggers of vasovagal syncope include:

• prolonged standing
• standing up too quickly from a sitting position
• having blood drawn or donating blood
• the sight of blood
• dehydration
• intense pain (such as from a back spasm)
• sudden emotional stress or physical trauma.

Less common triggers of vasovagal syncope include coughing, straining to have a bowel movement, or urinating while standing up (in men).

Symptoms of vasovagal syncope

Fainting due to vasovagal syncope is often preceded by feeling dizzy, lightheaded, or nauseous. Your skin may feel cold and clammy and you may black out or have blurry vision. If you're standing up, you will lose control of the muscles in your lower body and will slump or fall down.

How to manage syncope if you're prone to fainting

If you have fainted before and recognize the signs, here are some things you can do to restore blood flow to the brain before you actually pass out.

• Lie down and elevate your legs.
• Cross your legs and tense the muscles in the legs, abdomen, and buttocks.
• Make a fist, or grip a rubber ball or something else that you can easily wrap your hand around.
• Sit down and put your head between your legs.

If you know your triggers, take preventive measures to avoid fainting or to avoid falling if you do faint. For example, if having your blood drawn is a trigger, ask to lie down during the procedure. If you've fainted before due to dehydration, make sure you drink enough water or other liquids throughout the day.

When to see a doctor

See your doctor if you have recurrent fainting episodes or if you experience confusion or heart palpitations during an episode. Your provider may want to rule out other causes of fainting. If you also experience chest pain or shortness of breath, call 911 right away.

Not all fainting is related to the vasovagal nerve. Other common causes include heart problems, low blood sugar, panic disorder, seizure disorders, neurological disorders, substance use disorders, and some prescription medications.

Treatment options for recurrent syncope

In general, the best way to avoid fainting due to vasovagal syncope is to avoid your triggers, recognize warning signs, and adopt prevention strategies if you do feel warning signs coming on.

If the underlying cause of your fainting episodes is not related to overstimulation of the vasovagal nerve, your doctor will likely recommend further testing. In that case, treatment will address the underlying cause.