One day, it might be possible to diagnose Parkinson's disease noninvasively by checking for biological markers in tears, new research suggests.
The study — led by the University of Southern California (USC) in Los Angeles — found that tear samples from individuals with Parkinson's disease had different levels of a protein linked to the disease than those who did not have it.
"Knowing that something as simple as tears," explains study author Mark F. Lew, who is a professor of clinical neurology in USC's Keck School of Medicine, "could help neurologists differentiate between people who have Parkinson's disease and those who don't in a noninvasive manner is exciting."
He explains that such a marker could be very useful in helping to diagnose — and perhaps even treat — Parkinson's because the disease can begin many years before its symptoms appear.
The findings are to feature at the 70th annual meeting of the American Academy of Neurology, which is due to be held in Los Angeles, CA, in April.
Death of cells that make brain messengers
Parkinson's disease is a progressive disorder that kills brain cells that produce dopamine, which is a chemical messenger important for the control of movement.
The main symptoms of Parkinson's include slowness of movement, tremors, rigidity, and difficulty maintaining balance and coordination. These are sometimes accompanied by sleep disruption, depression, emotional disturbances, constipation, and difficulties with speech and swallowing.
There is also evidence to suggest that Parkinson's destroys cells that create another chemical messenger called norepinephrine, which helps to regulate many automatic functions in the body. This might explain why some of the symptoms are not related to movement.
Eventually, Parkinson's disease can progress to a stage when managing everyday tasks and living independently become too difficult.
More than 10 million of the world's population has Parkinson's disease. And in the United States — where around 60,000 cases are diagnosed every year — there are approximately 1 million people with the disease.
As yet, there is no cure for Parkinson's disease, although there are drugs that can help to ease symptoms in many cases.
While it is not yet clear how Parkinson's disease kills brain cells, researchers have discovered that toxic protein deposits known as Lewy bodies are often present in many brain cells of people with the disease. These deposits contain clusters of proteins that have not folded correctly.
A major component of Lewy bodies is an oligomeric form of the protein alpha-synuclein. The oligomeric form of a protein comprises several repeats of the protein's essential amino acids, but not as many as the polymeric form.
An author of a recently published study of alpha-synuclein in Parkinson's has suggested that the oligomeric protein's ability to "disrupt the integrity of the membrane" might be a key step in the process that ultimately kills the cell.
Since Parkinson's influences nerve function that lies outside the brain, perhaps evidence of marker proteins is present in tears; their secretion by tear glands is also triggered by nerves. This idea is what sent Prof. Lew and colleagues in search of the possible evidence.
To proceed, they compared tear samples taken from 55 individuals diagnosed with Parkinson's with those of 27 counterparts — matched for age and sex — who did not have the disease.
Oligomeric alpha-synuclein more abundant
The findings revealed that total alpha-synuclein was lower in the tears taken from the Parkinson's disease group than those of the healthy group; their average levels were 423 and 704 picograms per milligram, respectively.
However, the oligomeric form of alpha-synuclein was higher in the Parkinson's disease group than in the healthy group; their averages were 1.45 and 0.27 nanograms per milligram of tear protein, respectively.
There is still work to be done to confirm whether measuring this biological marker in tears is a viable diagnostic for Parkinson's disease.
In particular, studies are needed that involve larger groups and that will investigate whether the differences in alpha-synuclein are present in tears in the early stages of Parkinson's — before disease symptoms emerge.
"We believe our research is the first to show that tears may be a reliable, inexpensive, and noninvasive biological marker of Parkinson's disease."
Prof. Mark F. Lew