Antwerp researchers find that acting out dreams is the most reliable clue for spotting body first Parkinson's

For several years, scientists have been debating a fascinating idea about where Parkinson's actually begins. Known as the brain first versus body first theory, it suggests the condition can take two very different starting routes. In the brain first version, changes begin deep inside the brain, often in the smell centre, before spreading outward. In the body first version, the story starts in the gut or peripheral nerves, travelling upward to the heart and the brainstem. While this neat divide looks convincing when researchers average out data from large groups of people, a team at Antwerp University Hospital in Belgium wanted to know if it actually holds up in the real world for one person sitting in a clinic. Between 2024 and 2025, Kaat Colman, David Crosiers, Femke Dijkstra, and their colleagues studied 20 people who had been diagnosed with Parkinson's within the previous two years. To establish a baseline, the team used a specialised heart nerve scan called an MIBG scan. Because the body first route affects the nerves around the heart early on, an abnormal heart scan placed nine participants in the body first group, while a normal scan placed eleven participants in the brain first group. The researchers then put a wide range of everyday clinical checks and brain scans to the test to see if any of them could accurately predict which group each person belonged to. One clue stood head and shoulders above the rest. Acting out vivid dreams during sleep, known as probable REM sleep behaviour disorder, correctly sorted 85% of the participants. Every single person in the body first group reported these lively nights, which the team assessed using a questionnaire, an expert interview, and feedback from bed partners. Most people in the brain first group slept more quietly, though three of them did show signs of acting out dreams. Everything else, however, turned out to be surprisingly blurry. Symptoms often linked to the body first theory, such as constipation, blood pressure drops upon standing, and a complete loss of smell, showed up in both groups without reliably telling them apart. The same was true for mood changes, memory scores, and whether someone mainly had a tremor or balance difficulties. Even dopamine brain scans, known as DAT scans, failed to draw a clear line between the two groups. Rather than two separate boxes, the researchers suggest that Parkinson's may sit along a sliding scale where different paths overlap and shift over time. Someone might start with changes in the brain that eventually reach the brainstem and trigger dream enactment, or changes might begin in several places at once. The authors are also candid about the limits of their work. With only 20 participants, the study is small. Sleep behaviour was measured through interviews rather than an overnight sleep lab, not everyone had a dopamine brain scan, and genetic testing was not included. Even with those caveats, the findings offer a practical takeaway for future research. If larger studies confirm these results, a simple conversation about sleep habits could become a low cost way to identify people who are most likely to benefit from targeted clinical trials, such as gut focused therapies like faecal microbiota transplantation. It is a grounded reminder that while every person's experience of Parkinson's is unique, paying close attention to something as everyday as sleep could help scientists build more personalised treatments in the years ahead.