Ask the doctors: No saliva test for Parkinson’s disease

Dear Doctor: What can you tell me about the new saliva test for Parkinson’s disease? My older sister just learned she might have Parkinson’s, but her doctors aren’t sure. Why is it so hard to diagnose?

Dear Reader: Parkinson’s disease is a progressive disorder of the nervous system that leads to a deficit of dopamine, which is a neurotransmitter. Neurotransmitters help nerve cells communicate with each other, and dopamine plays a crucial role in our ability to produce smooth, deliberate and sustained movement. It’s the loss of dopamine that gives rise to some of the symptoms of Parkinson’s, which include difficulties with walking and balance, tremors, slurred speech and rigidity. The disorder also affects sleep, smell and blood pressure.

Diagnosing Parkinson’s disease can be difficult because the symptoms and their onset vary from person to person and are similar to other neurodegenerative disorders. There is no specific test for Parkinson’s disease, so emerging research into chemical changes in the saliva of Parkinson’s patients has drawn a lot of interest. While it’s true that some of this research shows promise in the area of diagnosis, at this time there is no saliva test for the disease.

Several studies have examined a range of biomarkers that are present in the saliva of people living with Parkinson’s. Many of them found significant differences when compared to the saliva of people without the disease. In a newer study, published earlier this year, researchers in India examined saliva samples taken from 76 patients living with Parkinson’s disease and samples from 37 healthy people, who served as controls. They found certain metabolic compounds in the Parkinson’s group saliva that suggest the disease process may be linked to an imbalance in the makeup and functioning of neurotransmitters, as well as to certain changes in the makeup of the gut microbiome.

This isn’t the first research to look into the potential role of the gut microbiome in neurodegenerative diseases such as Parkinson’s. The intestine is among the first organs affected by Parkinson’s disease, and health care professionals have long noted that symptoms such as constipation and gastrointestinal distress often precede the onset of neurological symptoms.

Recent research has also highlighted the gut-brain connection. One study tracked patients who had portions of their vagus nerve surgically removed. Five years after the surgery, the group had a 40% lower incidence of Parkinson’s disease than patients whose vagus nerve remained intact. This is significant because the vagus nerve runs from the brainstem to the colon, a direct link between the brain and the gut.

In another study, mice bred to be susceptible to Parkinson’s disease developed symptoms when they were implanted with fecal samples from Parkinson’s patients. The same type of mice implanted with fecal samples from healthy adults did not develop similar symptoms.

All of this has led to increasing interest in the idea that Parkinson’s disease originates in the gut and then spreads to the brain. Each new study sets the stage for further research into gut-based therapies, and even to a potential cure.