Like many other medical conditions, fainting runs in families, a new study finds. The study included 51 sets of same-sex twins. At least 1 twin in each pair had fainted in the past. Fainting in both twins was twice as common among identical twins as among fraternal twins. Identical twins come from the same fertilized egg, which divides. All of their genes are the same. Fraternal twins come from two fertilized eggs. They have fewer genes in common. Other relatives of the twins in the study did not have an unusually high risk of fainting. Researchers said this suggests that fainting is not inherited through just one gene. The journal Neurology published the study online. HealthDay News wrote about it August 8.

By Howard LeWine, M.D.
Harvard Medical School

What Is the Doctor's Reaction?

Fainting or blacking out is the temporary loss of consciousness. Doctors call it syncope. It can be embarrassing and scary. And it's common. An estimated one person out of every three faints at least once in a lifetime.

People pass out when there is a substantial but brief drop in the amount of blood flowing to the brain. It's surprising that fainting is not even more common. Every time you stand up, gravity pulls nearly one- quarter of your body's blood supply into your legs and lower body. Your heart and circulation must respond within seconds to keep enough blood flowing up to your brain.

To meet the challenge, your heart will speed up by 10 to 15 beats a minute. Your nervous system will tell your arteries to narrow so your blood pressure rises. But if everything is not just right, your blood pressure may drop -- and you may, too.

Fainting almost always occurs shortly after you get up or after you've been standing for a while. It's much less frequent when people are sitting or lying down.

Most faints are caused by a temporary abnormal response to standing. Instead of your heart speeding up, it slows down. And your arteries should narrow, but instead they relax and widen.

The result is a fall in blood pressure that deprives the brain of its full supply of blood. But from the brain's point of view, the lapse of consciousness serves a protective function. By falling down, you restore blood flow to your brain long before any damage can develop.

Doctors call this vaso-vagal syncope.

As with many other medical conditions, your risk of fainting is higher if you have family members who faint. Because vaso-vagal syncope is so common, making the genetic connection took a study of identical twins. Researchers reported their findings in the August 7 issue of the journal Neurology.

What Changes Can I Make Now?

Vaso-vagal syncope is not life-threatening. But you can suffer a fracture when you fall to the ground.

Most people who lose consciousness from vaso-vagal syncope will first feel lightheaded, nauseous and clammy. If you are standing and have these symptoms, sit down right away, even if it means sitting on the floor. It is even better if you can lie down and get your feet up.

If you feel lightheaded and can't sit right away, cross your legs and tense the muscles in your lower body. This will help boost blood pressure. But it won't work for long. Sit as soon as you can.

If you are a fainter, recognize the situations that increase your risk of passing out:

• Strong emotion
• Intense pain
• An urgent need to use the bathroom but not being able to get there
• Dehydration -- When the tank isn't full, it's harder to keep the pressure up.
• Standing in one spot for a long time -- Walking squeezes blood from the legs up to the heart, but standing still has the opposite effect.
• Medicines, especially ones that lower blood pressure or slow the heart rate
• Alcohol, especially if you don't usually drink

Always notify your doctor after your first faint. You want to be sure that some more serious reason, such as a heart problem, did not cause you to pass out.

What Can I Expect Looking to the Future?

Researchers will find one or more specific genes that cause a person to be more likely to faint. But if you have ever passed out even once, you have a good chance of fainting again. Recognize the situations that increase your fainting risk and always drink plenty of liquids. These steps will greatly lower your risk of fainting in the future.