Gut microbiota analysis and its significance in vasovagal syncope in children

Vasovagal syncope (VVS)—the sudden, temporary loss of consciousness triggered by standing too long, stress, or fear—is the most common type of fainting in kids. It affects over half of children who visit the emergency room for syncope, and while it’s usually benign, recurrent episodes can disrupt daily life and mental health. Yet despite its prevalence, the exact cause of VVS remains unclear. Now, a study from Peking University First Hospital and collaborators hints that the trillions of bacteria living in our guts—our gut microbiota—might be a missing piece of the puzzle.

Led by Wei Bai from the Department of Pediatrics at Peking University First Hospital and Selena Chen from the University of California, San Diego, the research compared gut bacteria in 20 children with VVS and 20 healthy, age- and sex-matched controls. All participants had normal stool tests, no recent antibiotic or probiotic use, and no underlying heart, brain, or metabolic issues. The team also collected clinical data from the VVS group, including how often they fainted and results from the head-up tilt test (HUTT)—a standard test where patients lie on a table that tilts upright to mimic standing, measuring blood pressure and heart rate changes.

What the Study Found

The researchers used 16S rRNA gene sequencing—a tool to identify and count gut bacteria—to analyze fecal samples. While the overall diversity of gut microbiota (the number and variety of bacterial species) was similar between kids with VVS and healthy controls, one family of bacteria stood out: Ruminococcaceae.

Children with VVS had significantly more Ruminococcaceae than healthy kids—about 22% of their gut microbiota, compared to 14% in controls. A statistical tool called LEfSe confirmed that Ruminococcaceae was a key “biomarker” distinguishing the two groups (meaning it reliably separated kids with VVS from healthy ones).

But the real insight came from linking Ruminococcaceae to clinical symptoms:

Fainting frequency: The more Ruminococcaceae a child had, the more often they fainted (a strong positive correlation, r=0.616, P<0.01).
Blood pressure changes: Higher Ruminococcaceae levels correlated with bigger drops in diastolic (lower number) and mean blood pressure during the HUTT, as well as faster declines in those pressures when fainting occurred. For example, kids with more Ruminococcaceae had a 58% stronger link to faster diastolic pressure drops (r=0.589, P<0.01).

Why Ruminococcaceae Matters

Ruminococcaceae is a “commensal” bacteria—meaning it’s part of a healthy gut. Its main job is to break down fiber into short-chain fatty acids (SCFAs) like butyrate and acetate. SCFAs are critical for gut health: they feed intestinal cells, reduce inflammation, and even affect other body systems, including blood pressure and the brain.

But too much of a “good” thing might be problematic for kids with VVS. Here’s how Ruminococcaceae could tie to fainting:

Blood pressure regulation: SCFAs like butyrate can lower blood pressure by interacting with the renin-angiotensin system (RAS)—a pathway that controls blood pressure. In VVS, a sudden drop in blood pressure is the main cause of fainting. More Ruminococcaceae could mean more SCFAs, leading to bigger, faster blood pressure drops during triggers like standing.
Gut-brain axis: The gut and brain communicate via the vagus nerve, a major pathway in the autonomic nervous system (which regulates automatic functions like heart rate and blood pressure). Imbalances in the autonomic nervous system are a known factor in VVS. SCFAs act as “signals” in this axis—too many might disrupt the vagus nerve’s ability to keep blood pressure stable.
Anxiety link: Studies in mice show Ruminococcaceae increases with anxiety, a common trigger for VVS. This creates a cycle: more Ruminococcaceae could worsen anxiety, leading to more fainting, which in turn might increase Ruminococcaceae further.

What This Means for Kids with VVS

This study doesn’t prove that Ruminococcaceae causes VVS—but it suggests gut microbiota imbalance could be a contributing factor. For parents and doctors, this is a new way to think about recurrent fainting: it’s not just about the heart or brain—it might also involve the gut.

Future research could explore whether modifying gut microbiota (e.g., with probiotics, diet changes) helps reduce syncope frequency. For example, if Ruminococcaceae is too high, a diet lower in fermentable fiber (which feeds this bacteria) might help. Or, if SCFAs are the culprit, targeted therapies to regulate their levels could be tested.

Conclusion

Vasovagal syncope is a frustrating condition for kids and families, but this study opens a new door to understanding it. By linking Ruminococcaceae to fainting frequency and blood pressure changes, the research adds gut microbiota to the list of factors that might drive VVS. While more work is needed to confirm these findings and explore treatments, the message is clear: our guts are more connected to our bodies—and our health—than we ever thought.

This study was published in the Chinese Medical Journal in 2019. The full paper is available at doi.org/10.1097/CM9.0000000000000086.

Led by Wei Bai (Peking University First Hospital) and Selena Chen (UC San Diego), the research included collaborators from Peking University Health Sciences Centre and Peking University Third Hospital. All procedures were approved by Peking University First Hospital, and informed consent was obtained from all participants.

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