Fainting, also called vasovagal syncope (by about five neurologists and no one else), is a brief loss of consciousness that occurs when blood flow to the brain drops.  Its causes are so numerous that it is basically unknown what causes it - everything from dehydration to medications to pregnancy, heart conditions and age - but these triggers all occur among people who have a strong genetic predisposition, according to new research.

For the study, 51 sets of twins of the same gender between the ages of nine and 69 were given a telephone questionnaire. At least one of the twins had a history of fainting. Researchers also gathered information about any family history of fainting. Of the 51 sets of twins, 57 percent reported having typical fainting triggers.

The research found that among twins where one fainted, those who were identical (from the same fertilized egg) were nearly twice as likely to both faint compared to fraternal twins (those from two different fertilized eggs). The risk of fainting not related to outside factors (such as dehydration) was also much higher in identical twins compared to fraternal twins. Identical twins were much more likely to both experience fainting associated with typical triggers than fraternal twins. The frequency of fainting in non-twin relatives was low, suggesting that the way fainting is inherited is usually not by a single gene.

"The question of whether fainting is caused by genetic factors, environmental factors or a mixture of both has been the subject of debate," said study author Samuel F. Berkovic, MD, FRS, with the University of Melbourne in Victoria, Australia.  "Our results suggest that while fainting appears to have a strong genetic component, there may be multiple genes and multiple environmental factors that influence the phenomenon."

 Published in Neurology®