Vasovagal syncope (vay-zoh-VAY-gul SING-kuh-pee) occurs when you faint because your body overreacts to certain triggers, such as the sight of blood or extreme emotional distress. It may also be 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 doctor may recommend tests to rule out more-serious causes of fainting, such as heart disorders.

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

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

During a vasovagal syncope episode, bystanders may notice:

• Jerky, abnormal 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 disorder. You may want to consult your doctor 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 malfunctions in response to a trigger, such as the sight of blood.

Your heart rate slows, and the blood vessels in your legs widen (dilate). This allows blood to pool in your legs, which lowers your blood pressure. Combined, the drop in blood pressure and slowed heart rate quickly reduce blood flow to your brain, and you faint.

Sometimes there is no classical vasovagal syncope trigger, but common triggers include:

• Standing for long periods of time
• Heat exposure
• Seeing blood
• Having blood drawn
• Fear of bodily injury
• Straining, such as to have a bowel movement

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 examination. During the physical exam, your doctor will listen to your heart and take your blood pressure. He or she may also massage the main arteries in your neck to see if that causes you to feel faint.

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

• Electrocardiogram. This test records the electrical signals your heart produces. It can detect irregular heart rhythms and other cardiac problems. 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 doctor may look for conditions, such as anemia, that can cause or contribute to fainting spells.
• Tilt table test. If no heart problems appear to cause your fainting, your doctor 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 doctor may help you identify your fainting triggers and discuss ways you might avoid them.

However, if you experience vasovagal syncope often enough to interfere with your quality of life, your doctor may suggest trying one or more of the following remedies:

• Medications. A drug called fludrocortisone acetate that's normally used to treat low blood pressure may be helpful in preventing vasovagal syncope. Selective serotonin reuptake inhibitors also may be used.
• Therapies. Your doctor may recommend ways to decrease 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 may help 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 doctor.

What you can do

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

What to expect from your doctor

Questions your doctor might ask you include:

• What were you doing just before you fainted?
• What signs and 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 medication?
• Have you ever had a head injury?
• Has anyone in your family died suddenly of heart problems?