To evaluate whether the vagus nerve (VN) cross-sectional area (CSA), measured by high-resolution ultrasound, differs between Parkinson’s disease (PD) patients and healthy controls.

The VN is central to the gut-brain axis and autonomic regulation, both implicated in PD pathophysiology. High-resolution ultrasound offers a precise, non-invasive method to assess VN CSA, but case-control studies report inconsistent findings.

We systematically reviewed and meta-analyzed PubMed-indexed case-control studies comparing VN CSA in PD versus controls. Standardized mean differences (SMD) were pooled using a random-effects model (DerSimonian-Laird). Subgroup analyses were performed by side (right/left) and anatomical level (thyroid and carotid-bulb). Publication bias was assessed using Egger’s and Begg’s tests and adjusted via trim-and-fill.

Ten studies including 809 participants (411 PD and 398 controls) were analyzed. The pooled random-effects estimate showed significantly smaller VN CSA in PD compared to controls (SMD = -0.93; 95% CI: -1.31 to -0.56; p < 0.001). The prediction interval ranged from -2.67 to 0.81, indicating variability across studies. Subgroup analysis revealed the largest reduction at the right thyroid level (SMD = -1.71; 95% CI: -2.57 to -0.86), followed by the left thyroid level (SMD = -1.19; 95% CI: -1.72 to -0.65). At the carotid-bulb, reductions were smaller and less consistent, with pooled estimates ranging from -0.42 to -0.76. Heterogeneity was significant (Q = 221.8, p < 0.001; τ² = 0.65), though I² was low (0.9%). Egger’s test indicated publication bias (p = 0.017), and trim-and-fill adjustment reduced the effect size to -0.66 (30.6% change). Fail-safe N using Rosenthal’s method was 442, suggesting robustness of the findings.

High-resolution ultrasound demonstrates significant VN CSA reduction in PD, more pronounced on the right side and at the thyroid level. Despite evidence of publication bias, findings remain robust, supporting ultrasound as a promising biomarker for autonomic involvement in PD.