Around half a million people in the United States are affected by Parkinson's disease, a neurodegenerative disorder. New research published in Neurology reveals that the gut may play a role in the disease and, furthermore, may be where Parkinson's disease begins.
Study author Bojing Liu, of the Karolinska Instituet in Stockholm, Sweden, and colleagues found that Parkinson's disease might start in the gut and reach the brain via the vagus nerve.
The vagus nerve is the tenth cranial nerve that extends to the abdomen from the brainstem. It supplies nerve fibers to the throat, voice box, windpipe, lungs, heart, esophagus, and intestinal tract, and it controls unconscious body processes such as heart rate and the digestion of food.
Previous studies have associated mechanisms between the gut and vagus nerve with Parkinson's disease. However, researchers say that empiric evidence so far has been scarce and inconsistent.
Investigators aimed to explore in the new study whether, when branches of the vagus nerve are cut - through a surgical procedure called a vagotomy - the risk of Parkinson's disease decreases. A vagotomy is typically performed to reduce acid secretion in the stomach for the treatment and prevention of peptic ulcers.
Data from the national registers of Sweden were studied to compare 9,430 people who had a vagotomy between the years 1970 and 2010, with 377,200 individuals from the general population. Follow-up occurred over the 40-year period from the date of the vagotomy until Parkinson's disease diagnosis, death, or emigration out of Sweden.
Reduced risk of Parkinson's disease with truncal vagotomy
Over the course of the study, 101 of the people (or 1.07 percent) who had a vagotomy developed Parkinson's disease, compared with 4,829 individuals in the control group (or 1.28 percent). The difference between the vagotomized group and the general population was "not significant."
However, when two types of vagotomy - truncal and selective - were compared in the analysis, the team observed a lower risk of Parkinson's disease among people with truncal vagotomy. Truncal vagotomy involves the full resection of the main trunk of the vagus nerve, whereas in selective vagotomy, only some of the nerve branches are resected.
Of the people who had truncal vagotomy and were followed up for at least 5 years, 19 of them (or 0.78 percent) developed Parkinson's disease, compared with individuals who had not had surgery. A total of 60 people (or 1.08 percent) who had selective vagotomy and were followed up for at least 5 years developed Parkinson's disease.
After adjusting for conditions such as chronic obstructive pulmonary disease, diabetes, vascular diseases, rheumatologic disease, and osteoarthritis, Liu and team found that people who had truncal vagotomy at least 5 years prior to follow-up "were 40 percent less likely to develop Parkinson's disease than those who had not had the surgery."
"These results provide preliminary evidence that Parkinson's disease may start in the gut. Other evidence for this hypothesis is that people with Parkinson's disease often have gastrointestinal problems, such as constipation, that can start decades before they develop the disease. In addition, other studies have shown that people who will later develop Parkinson's disease have a protein believed to play a key role in Parkinson's disease in their gut."
Scientists hypothesize that these proteins mistakenly fold the wrong way and that this error spreads from cell to cell. "Much more research is needed to test this theory and to help us understand the role this may play in the development of Parkinson's," says Liu. Moreover, as Parkinson's is classified as a syndrome, "there may be multiple causes and pathways" behind the condition.
Limitations of the study include the fact that while the study was large, the numbers of people in certain subgroups were small. Additionally, factors that affect the risk of Parkinson's disease, including genetics, smoking, or drinking coffee, could not be controlled for in this study.