Genetics and stem cells

'Fainting runs in families'

“Feeling faint? Blame your parents” advises the Daily Mail. The Mail went on to say that scientists have discovered that people who faint “could be genetically pre-disposed to swooning”.

The research looked at whether genetic factors are involved in fainting. The researchers examined identical and non-identical twins where at least one twin had a history of fainting. Identical twins share the same DNA, whereas non-identical twins are no more alike than any two non-twin siblings. Therefore, if the researchers found that fainting affected both identical twins more frequently than it affected both non-identical twins, this would suggest that genetic factors may have a role in fainting.

The study found that fainting affected both twins of an identical pair more often than it affected both twins of a non-identical pair. Identical twins were also more likely than non-identical twins to have both fainted at least twice, unrelated to any common fainting triggers (such as heat) and to have both fainted in relation to specific common triggers.

Twin studies are often used to investigate whether genetic factors are involved in certain characteristics in the population, yet the results can be oversimplified. This study was also problematic because it was very small and relied on telephone interviews with the twins, rather than diagnostic tests. This means that we cannot be certain that all cases were simple faints, and not caused by other factors, such as a heart condition.

Overall, the study suggests that fainting may involve both genetic and environmental factors. There is little that can be done to stop fainting, and of itself, fainting is not necessarily harmful. These facts, coupled with the inconclusive findings, do not really warrant national newspaper coverage.

Where did the story come from?

The study was carried out by researchers from the University of Melbourne, Australia, and the University Hospitals Giessen and Marburg, and Philipps-University Marburg, Germany. It was funded by the National Health and Medical Research Council Australia. The study was published in the peer-reviewed journal Neurology.

The study was over-simplified in some papers, with both The Daily Telegraph and the Mail reporting that fainting seems to run in families. This study found that fainting more commonly affects both identical twins than both non-identical twins. Its findings suggest that genetic factors are involved, but that does not mean that there is no role for environmental factors.

It should also be noted that there is a big difference between fainting and the suggestion of ‘feeling faint’ as in the Mail’s headline.

What kind of research was this?

This was a twin study investigating whether genetic factors have a role in ‘simple faint’ – medically termed ‘vasovagal syncope’. This affects about 25% of people at least once during their life. In vasovagal syncope, the oxygen supply to the brain is temporarily interrupted, usually resulting in a brief loss of consciousness. This is often the result of a drop in heart rate and blood pressure in response to external triggers, such as:

  • standing for too long
  • heat
  • dehydration
  • exposure to an unpleasant experience, such as the sight of blood

Twin studies are often used to estimate to what extent genetics contribute to variation in a particular characteristic in a population. To do this they compare identical and non-identical twins. Identical twins develop from the same fertilised egg and share the same DNA, whereas non-identical twins develop from two separate fertilised eggs, and are no more alike genetically than any two non-twin siblings, sharing only on average about half of their DNA. Therefore, if identical twins are more similar than non-identical twins for a particular characteristic, this suggests that genetic factors are contributing to variation in the characteristic.

The authors say that the role of genetic factors in this type of fainting is currently under debate, with conflicting results from research. Their twin study and assessment of family history aimed to clarify whether genetic factors play a role in this type of fainting. However, this study does not provide the full answer and does not mean that there is no role for environmental factors in susceptibility to fainting.

What did the research involve?

The researchers recruited 57 same-sex sets of twins through a national twin registry (36 pairs of identical and 21 pairs of non-identical twins). All of the twins were interviewed individually by telephone using a standardised questionnaire about this type of fainting. The questions covered:

  • potential triggers
  • early symptoms
  • recovery
  • family history of fainting, cardiac events and sudden death

They confirmed vasovagal syncope in at least one twin in 50 of the pairs, 47 of them adults and three pairs aged under 18 (the average age was about 35). They recruited an additional twin pair through their family studies to give a total of 33 identical twin pairs and 18 non-identical.

Researchers developed a score to describe the severity of fainting and typical triggers.

They obtained information on the experience of fainting among first and second-degree relatives from 19 pairs of identical twins, where both twins had fainted. This information was confirmed by a parent in most cases.

They assessed whether twins were identical or non-identical using a validated questionnaire, and further validated the results of this by looking at DNA markers extracted from blood or saliva.

What were the basic results?

When examining all twins, identical twins tended to be more likely to have both fainted, compared to non-identical twins, though the difference between the two groups did not reach statistical significance.

  • When they only analysed those twins who had suffered from more severe faints (syncope score four or five), identical twins were significantly more likely to have both had severe faints compared to non-identical twins.
  • Identical twins were significantly more likely than non-identical twins to have both fainted in response to common triggers. 
  • Identical twins were also significantly more likely than non-identical twins, to have both fainted at least twice, unrelated to any external trigger.
  • In 12 out of 19 of the identical sets of the twins who both fainted, relatives were either not affected at all by fainting, or only “sparsely” affected. In the remaining seven pairs, several close relatives had experienced fainting. The fact that several family members were affected in some families suggests that genetics could be playing a greater role in these families than others.

How did the researchers interpret the results?

The researchers say the findings strongly support the relevance of genetic factors in fainting, whether it is triggered by external events, or if it occurs frequently and independently of triggers. Genetic factors, they say, appear to be less relevant in those who faint infrequently and in association with strong triggers.

The researchers say the results suggest that syncope comprises a spectrum “between predominantly genetic factors on one side … and predominantly environmental factors on the other”. The results of their family history suggest that “multiple genes and environmental factors” play a role.


This research looked at whether genetic factors could have a role in susceptibility to fainting in twins. Researchers found that fainting affected both identical twins more frequently than it affected both non-identical twins. This, they stated, would suggest that genetic factors have a role in fainting.

The biggest limitation of this study is the small number of twins involved. For example, 33 of the 36 identical twin pairs had both experienced fainting, compared to 18 of the 21 non-identical pairs, while 22 of the identical pairs both experienced at least two faints compared with 13 of non-identical pairs. Comparisons involving such small numbers may affect the reliability of the study’s findings.

The study relied on telephone interviews to assess whether participants had experienced a simple faint. Though the questions covered typical features of vasovagal syncope, it is possible that some faints could have an undiagnosed disease cause, such as heart problems. It is also possible that information about fainting among other family members could have contained inaccuracies.

In conclusion, the results are interesting in that they suggest that genetic factors may have a role in susceptibility to fainting, but they do not provide the whole answer. A larger twin study, which also involves formal medical diagnosis of vasovagal syncope, would be helpful to further assess the role that genetic factors may have in susceptibility to fainting.  

NHS Attribution