Gut Microbiota Analysis and Its Significance in Vasovagal Syncope in Children

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Gut Microbiota Analysis and Its Significance in Vasovagal Syncope in Children

Vasovagal syncope (VVS), characterized by transient loss of consciousness due to cerebral hypoperfusion, is the most common form of syncope in children, accounting for over 50% of cases presenting to emergency departments. Despite its prevalence, the mechanisms underlying VVS remain incompletely understood. Recent advancements in microbiome research have highlighted the role of gut microbiota in cardiovascular and neurological disorders, prompting investigations into its potential involvement in VVS pathogenesis. This study represents the first exploration of gut microbiota composition in pediatric VVS patients, identifying Ruminococcaceae as a key bacterial family associated with clinical symptoms and hemodynamic changes.

Introduction

VVS predominantly affects adolescents, with peak onset between 13–15 years. Episodes are often triggered by prolonged standing, emotional stress, or pain, accompanied by prodromal symptoms such as nausea, pallor, and diaphoresis. While generally benign, recurrent VVS significantly impacts quality of life and mental health. Current hypotheses for VVS pathogenesis include autonomic dysregulation, Bezold-Jarisch reflex activation, and imbalances in vasoactive substances like nitric oxide and angiotensin II. Notably, gastrointestinal symptoms frequently precede syncopal episodes, suggesting a potential gut-brain axis involvement.

The gut microbiota, a complex ecosystem of trillions of microorganisms, influences host physiology through metabolic, immune, and neural pathways. Dysbiosis has been implicated in hypertension, atherosclerosis, and neuropsychiatric disorders, often mediated by microbial metabolites like short-chain fatty acids (SCFAs). Given the overlap between these mechanisms and VVS pathophysiology, this study hypothesized that gut microbiota alterations might contribute to VVS development.

Methods

Study Design and Participants

The study enrolled 20 children aged 5–18 years diagnosed with VVS at Peking University First Hospital (July 2016–January 2017) and 20 age-/sex-matched healthy controls. VVS diagnosis followed established criteria, including a positive head-up tilt test (HUTT) and exclusion of structural cardiac, neurological, or metabolic abnormalities. Exclusion criteria included recent antibiotic/probiotic use, BMI >24 kg/m², or acute gastrointestinal illness.

Head-Up Tilt Test (HUTT)

Participants underwent HUTT after a 4-hour fast. Continuous blood pressure and ECG were monitored during 10 minutes of supine rest and 45 minutes of 60° tilt. Positive HUTT criteria included:

  • Systolic blood pressure (SBP) ≤80 mmHg or diastolic blood pressure (DBP) ≤50 mmHg
  • Mean arterial pressure (MAP) reduction ≥25%
  • Age-specific bradycardia (e.g., heart rate 8 years).

Gut Microbiota Analysis

Fecal samples were collected, stored at −80°C, and subjected to DNA extraction using the CTAB method. The V4 region of the 16S rRNA gene was amplified (515F/806R primers) and sequenced on an Illumina HiSeq 2500 platform. Operational taxonomic units (OTUs) were clustered at 97% similarity using UPARSE, with taxonomic annotation via the SILVA database.

Statistical Analysis

Alpha diversity (Shannon, Simpson, Chao1 indices) and beta diversity (weighted UniFrac) were assessed. Differential taxa were identified using LEfSe (linear discriminant analysis [LDA] score >4). Clinical correlations were analyzed via Pearson/Spearman tests.

Results

Participant Characteristics

No significant differences existed between VVS (n=20; 7 males, 13 females; mean age 11.1±2.0 years) and control groups (n=20; 8 males, 12 females; mean age 10.7±2.5 years) in age, sex, BMI, or anthropometrics (Table 1).

Gut Microbiota Diversity

Sequencing yielded 2,922,206 effective tags (73,055±11,305 per sample) and 7,012 OTUs. Alpha diversity indices (Shannon, Simpson, Chao1) showed no significant intergroup differences (Table 2). Beta diversity analysis (PCoA) revealed overlapping microbial communities (Anosim R=0.013, P >0.05) (Figure 1D).

Taxonomic Composition

At the phylum level, Firmicutes, Bacteroidetes, and Actinobacteria dominated both groups, with no abundance differences. At the family level, Ruminococcaceae was significantly enriched in VVS patients (median 22.10% [16.89–27.36%] vs. 13.92% [10.31–20.18%]; Z=−2.40, P <0.05) (Figure 2B). LEfSe confirmed Ruminococcaceae as a discriminative biomarker (LDA score >4, P <0.05) (Figure 3A–B).

Clinical Correlations

In VVS patients, Ruminococcaceae abundance correlated with:

  1. Syncope Frequency: Positive correlation (r=0.616, P <0.01) (Figure 4A).
  2. Hemodynamic Parameters:
    • Negative correlation with SBP and DBP during HUTT positivity (r=−0.489 and −0.448, P <0.05) (Figure 4B–C).
    • Positive correlation with DBP reduction (r=0.579) and decline rate (r=0.589, P <0.01) (Figure 4D–E).
    • Positive correlation with MAP reduction (r=0.489) and decline rate (r=0.467, P <0.05) (Figure 4F–G).

Discussion

This study provides novel evidence linking gut microbiota dysbiosis to pediatric VVS, with Ruminococcaceae emerging as a critical taxon. While overall microbial diversity remained unchanged, the selective enrichment of Ruminococcaceae—a SCFA-producing family within Firmicutes—suggests a potential mechanistic role in VVS pathophysiology.

Role of Ruminococcaceae and SCFAs

Ruminococcaceae colonizes the cecum and colon, fermenting dietary fiber into SCFAs like butyrate and acetate. SCFAs modulate blood pressure via:

  1. Renin-Angiotensin System (RAS) Regulation: Butyrate suppresses angiotensin II-induced hypertension and renal inflammation in animal models.
  2. Vasodilation: SCFAs activate G protein-coupled receptors (e.g., GPR41/43), promoting nitric oxide release and smooth muscle relaxation.
  3. Autonomic Nervous System (ANS) Modulation: SCFAs influence vagal tone and ANS balance, potentially exacerbating vasodilation and bradycardia during orthostatic stress.

The positive correlation between Ruminococcaceae abundance and syncope frequency may reflect cumulative SCFA effects on vascular tone and ANS stability. Conversely, the inverse relationship with blood pressure during HUTT suggests that excessive SCFA production could predispose to exaggerated hypotensive responses.

Gut-Brain Axis Implications

The gut-brain axis facilitates bidirectional communication between the enteric and central nervous systems. Ruminococcaceae-derived metabolites may influence syncopal triggers through:

  • Vagal Afferent Signaling: SCFAs activate intestinal enteroendocrine cells, transmitting signals to brainstem nuclei regulating cardiovascular function.
  • Neurotransmitter Modulation: Gut microbes produce γ-aminobutyric acid (GABA) and serotonin, which affect mood and autonomic outflow.
  • Immune Activation: Microbial dysbiosis can induce systemic inflammation, altering endothelial function and baroreceptor sensitivity.

Clinical and Therapeutic Perspectives

These findings align with observations that VVS patients often exhibit low BMI, as Ruminococcaceae abundance is inversely related to adiposity. Dietary interventions targeting fiber intake or probiotic supplementation could modulate SCFA production and mitigate syncopal episodes. Future studies should:

  • Validate these results in larger cohorts.
  • Measure fecal SCFA levels and plasma biomarkers (e.g., angiotensin II, nitric oxide).
  • Explore causal relationships via fecal microbiota transplantation in animal models.

Conclusion

This study identifies Ruminococcaceae as a hallmark gut microbial feature in pediatric VVS, correlating with syncope frequency and hemodynamic instability. While mechanistic details require further elucidation, these findings underscore the gut microbiota’s role in cardiovascular regulation and open avenues for microbiota-targeted therapies in VVS management.

doi.org/10.1097/CM9.0000000000000086

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