A meta-analysis published in the Journal of Neurology on July 15, 2026, pooled ultrasound data from 12 case-control studies covering 1,091 people, 541 with Parkinson's disease and 550 without, and found the vagus nerve's cross-sectional area was smaller, on average, in the patient group. The right vagus nerve measured 1.97 square millimeters in patients versus 2.21 in controls, a difference of 0.24 square millimeters; the left measured 1.70 versus 1.89, a difference of 0.19 square millimeters. Both differences were statistically significant, according to the analysis led by Laura Sijben and colleagues at Maastricht University Medical Center in the Netherlands.
The vagus nerve carries signals between the gut's nervous system and the brainstem, and for two decades a proposal known as the Braak hypothesis has held that misfolded alpha-synuclein protein, a hallmark of Parkinson's pathology, may travel along this route before reaching brain regions tied to movement. The new pooled analysis does not test that transmission theory directly; it reports only that, measured with ultrasound at a single point in time, the nerve itself is narrower on a group level in people already diagnosed with the disease.
The finding sits atop a decade of conflicting single-site studies. A 2018 ultrasound study from Rostock, Germany, for instance, reported vagus nerve atrophy in one Parkinson's cohort, while other individual studies over the years found no measurable difference at all. The new analysis's own authors flagged substantial heterogeneity among the 12 pooled studies, which used different ultrasound machines, operators, and patient populations, and rated study quality as only moderate to high. A separate 17-study pooled analysis published in Neurological Sciences in July 2025 reported a similar direction of effect, lending some independent support to the pattern.