A common kind of fainting appears to run in families, a new study of twins shows.

About 1 in 4 people will faint at some point in their lives. Fainting is a sudden, brief loss of consciousness after blood pressure drops to the brain. Sometimes, that loss of blood pressure happens for internal reasons -- dehydration or heart problems, for example.

But puzzlingly, people sometimes black out in response to some kind of outside-the-body trigger, like the sight of blood or after some kind of emotional upset. This is called vasovagal syncope.

Fortunately, fainting isn't usually dangerous. Most people usually wake up a few seconds after they pass out, but they may fall in the process. Falling during a fainting spell, however, can lead to injuries.

The study, which is published in the journal Neurology, suggests that in some cases, fainting may be inherited. If researchers can identify the genes involved, they say it may point to a way to help the small percentage of people who pass out so regularly that it interferes with their lives.

"A small proportion of people with fainting actually have a disabling problem. They have a lot of fainting attacks, and it makes life a little challenging for them. And if we can develop better treatments for that, that would be great," says researcher Samuel F. Berkovic, MD, a neurologist and professor in the department of medicine at the University of Melbourne, in Australia.
The Genetic Link

Researchers in Australia interviewed 51 same-sex sets of twins for the study. Some of the twins were identical. The others were fraternal twins. At least one twin in each pair reported passing out at some point in their lives.

Researchers found that both members of a set of identical twins were more than twice as likely to be prone to fainting spells compared to fraternal twins. Their fainting was also more likely to be triggered by something external, like a stressful event or the sight of blood, rather than by a biological problem like dehydration.

"Simply put, there is now strong evidence that a simple faint, for example, one caused by sight of blood, fear, or unpleasant thoughts, can have a genetic component," says Ezriel Kornel, MD, a neurologist at Northern Westchester Hospital in Mount Kisco, N.Y.

Even though identical twins were more likely to report outside triggers for their attacks, it wasn't always the same trigger for both twins, Berkovic says.

"The evidence is that the genetic factors are more [responsible for] ... the fainting rather than the triggers that cause the fainting," he tells WebMD.

The frequency of fainting among non-twin relatives in the same family was much lower, suggesting a complex interaction between many genes that all have to be inherited together, rather than just one or two.

"It's not a 'clean' genetic disorder by any means. There's a huge environmental component," says Satish R. Raj, MD. Raj is an assistant professor of medicine and pharmacology who studies fainting at Vanderbilt University Medical Center in Nashville, Tenn. He was not involved in the research.

Raj says the concept that fainting might be inherited is intriguing, but he agrees with the researchers that more work needs to be done to understand how fainting might be hardwired into our genes.

"It would be useful to have some clinical data. Something to indicate what about these fainting twins is different," Raj says.

Knowing what makes fainters different might help scientists track down the gene or genes that are responsible for the reaction.

The study also had other limits. The number of twins who were surveyed for the study was relatively small. When a study is small, that makes it harder to apply the results of the study to the general population. The study also relied on people to remember when they fainted, how often, and what triggered their blackout. Raj says that's not a terrible fault, since most doctors diagnose fainting by asking people to remember their episodes.

Lastly, because no clinical tests were used in the study, doctors couldn't definitely rule out other possible causes of fainting, such as problems with the heart or nervous system.