Fainting can run in the family, study says

LOS ANGELES -- If catching sight of blood or standing all day makes you woozy enough to black out, your genes are partly to blame. Fainting, like dimples and dyslexia, can run in the family, a new study shows.

Nearly one in four people experience fainting at least once in a lifetime. But researchers have long debated whether the behavior is written into our DNA or is more influenced by the environment around us.

Enter the identical twins. Studying such siblings, whose genetic material is almost exactly the same, is among the surest ways to learn how much genes influence a particular trait.

For this study, reported yesterday in the journal Neurology, researchers in Australia and Germany invited volunteers from the Australian Twin Registry, which helps connect twins with researchers. They recruited 36 pairs of identical twins and 21 pairs of same-sex fraternal twins (who are no more similar genetically than a regular brother or sister).

Researchers asked each twin about fainting history, whether they had particular triggers or warning symptoms, and if any other relatives also fainted.

In all, 57 percent of the study subjects said they reacted to typical triggers, such as the sight of blood, injury and pain, medical procedures, prolonged standing or scary thoughts. Factors such as illness or dehydration also brought on fainting.

Pairs of identical twins were much more likely to both experience fainting than were fraternal twins, the researchers reported. That was especially true for fainting associated with common triggers, and for frequent fainters (those who had experienced three or more fainting episodes).

Identical twins have essentially the same genes, and fraternal twins share only 50 percent of their genes on average, so this makes a strong case that fainting is partly genetic.

-- Los Angeles Times