Vasovagal syncope (vay-zoh-VAY-gul SING-kuh-pee) occurs when you faint because your body overreacts to certain triggers. These may include the sight of blood or extreme emotional distress. Vasovagal syncope also is called neurocardiogenic syncope.

The vasovagal syncope trigger causes your heart rate and blood pressure to drop suddenly. That leads to reduced blood flow to your brain, causing you to briefly lose consciousness.

Vasovagal syncope is usually harmless and requires no treatment. But it's possible that you may injure yourself during a vasovagal syncope episode. Your healthcare professional may recommend tests to rule out more-serious causes of fainting, such as heart conditions.

Before you faint due to vasovagal syncope, you may experience some of the following symptoms:

• Change in skin color.
• Lightheadedness.
• Tunnel vision — your field of vision narrows so that you see only what's in front of you.
• Upset stomach.
• Feeling warm.
• A cold, clammy sweat.
• Blurred vision.

During a vasovagal syncope episode, people around you may notice:

• Jerky movements.
• A slow, weak pulse.
• Dilated pupils.

Recovery after a vasovagal episode generally begins in less than a minute. However, if you stand up too soon after fainting — within about 15 to 30 minutes — you're at risk of fainting again.

When to see a doctor

Fainting can be a sign of a more serious condition, such as a heart or brain condition. You may want to consult your healthcare professional after a fainting spell, especially if you've never had one before.

Vasovagal syncope occurs when the part of your nervous system that regulates heart rate and blood pressure overreacts to a trigger. Common triggers include:

• Standing for long periods of time.
• Heat exposure.
• Seeing blood.
• Having blood drawn.
• Fear of bodily injury.
• Straining, such as to pass stool.

Vasovagal syncope can happen without a trigger.

During a vasovagal syncope episode, your heart rate slows, and the blood vessels in your legs widen. This allows blood to pool in your legs, which lowers your blood pressure. The drop in blood pressure and slowed heart rate quickly reduce blood flow to your brain, and you faint.

The risk of vasovagal syncope may be higher in:

• Children and young adults, although vasovagal syncope can happen at any age.
• People who stand for long periods of time.

You may not always be able to avoid a vasovagal syncope episode. If you feel like you might faint, lie down and lift your legs. This allows gravity to keep blood flowing to your brain. If you can't lie down, sit down and put your head between your knees until you feel better.

Diagnosing vasovagal syncope often begins with a physical exam. During the physical exam, your healthcare professional listens to your heart and takes your blood pressure. Your healthcare professional may massage the main arteries in your neck to see if that causes you to feel faint.

Your healthcare professional also may recommend several tests to rule out other possible causes of your fainting — particularly heart-related conditions. These tests may include:

• Electrocardiogram. This test records the electrical signals your heart produces. It can detect irregular heart rhythms and other cardiac conditions. You may need to wear a portable monitor for at least a day or as long as a month.
• Echocardiogram. This test uses ultrasound imaging to view the heart and look for conditions such as valve problems that can cause fainting.
• Exercise stress test. This test studies heart rhythms during exercise. It's usually conducted while you walk or jog on a treadmill.
• Blood tests. Your healthcare professional may look for conditions such as anemia that can cause or contribute to fainting spells.
• Tilt table test. If no heart conditions appear to cause your fainting, your healthcare professional may suggest that you undergo a tilt table test. During the test, you lie flat on your back on a table that changes positions, tilting you upward at various angles. A technician monitors your heart rhythms and blood pressure during the test to see if changing your posture affects them.

In most cases of vasovagal syncope, treatment is unnecessary. Your healthcare professional may help you understand your fainting triggers and discuss ways you might avoid them.

If vasovagal syncope interferes with your quality of life, your healthcare professional may suggest trying one or more of the following remedies:

• Medicines. A drug called fludrocortisone acetate that's usually used to treat low blood pressure may be helpful in preventing vasovagal syncope. Selective serotonin reuptake inhibitors also may be used.
• Therapies. Your healthcare professional may recommend ways to lessen the pooling of blood in your legs. These may include foot exercises, wearing compression stockings or tensing your leg muscles when standing. You may need to increase salt in your diet if you don't usually have high blood pressure. Avoid prolonged standing — especially in hot, crowded places — and drink plenty of fluids.
• Surgery. Very rarely, inserting an electrical pacemaker to regulate the heartbeat helps some people with vasovagal syncope who haven't been helped by other treatments.

It's a good idea to prepare for your appointment to make the most of your time with your healthcare professional.

What you can do

• Write down details of your symptoms, including any triggers that may have caused you to faint.
• Make a list of all medicines, vitamins and supplements you're taking.
• Write down questions you want to ask, including questions about potential tests and treatments.

What to expect from your doctor

Questions your healthcare professional might ask include:

• What were you doing just before you fainted?
• What symptoms, if any, did you experience before you fainted?
• Have you ever fainted before? If yes, what were you doing before you fainted then?
• Have you recently started taking a new medicine?
• Have you ever had a head injury?
• Has anyone in your family died suddenly of a heart condition?