Watching the Nerve That Links the Gut and the Brain

A new Japanese study has found that ultrasound scans of the vagus nerve – the long nerve connecting the brain to the gut – may help track early Parkinson’s changes over time. Dr Tomoyuki Miyamoto and colleagues from Dokkyo Medical University followed 57 people with isolated REM sleep behaviour disorder (IRBD), 41 with Parkinson’s disease (PD), and 27 healthy volunteers. IRBD is often seen as an early warning sign of Parkinson’s and related conditions because people act out their dreams during sleep – a sign of changes in brain circuits that later become involved in PD. The researchers used ultrasound to measure the cross-sectional area (CSA) of the vagus nerve on both sides of the neck. This is a simple, non-invasive scan similar to those used in pregnancy, and it allows doctors to see if the nerve appears smaller or thinner over time – a possible sign of nerve degeneration. At the start of the study, there were no major differences in vagus nerve size between healthy participants, those with IRBD, or those already diagnosed with Parkinson’s. But when the researchers looked again after about two years, they found something interesting: the left vagus nerve in people with IRBD had become smaller, suggesting it was gradually shrinking as the disease process advanced. In people with Parkinson’s, the smaller the vagus nerve, the worse their motor symptoms tended to be. This supports the idea that Parkinson’s may begin in the gut and travel to the brain via the vagus nerve, a theory known as the “gut–brain axis” or “dual-hit hypothesis.” The findings suggest that keeping an eye on this nerve’s size could one day help doctors monitor disease progression or even spot when someone with IRBD is moving closer to developing Parkinson’s. However, the authors stressed that the vagus nerve measurement is not yet reliable enough to diagnose Parkinson’s or distinguish it from healthy ageing. Changes were modest, and ultrasound measurements can vary depending on technique. They called for larger, longer-term studies to confirm whether this approach can become a practical clinical tool.