WASHINGTON, Feb. 24 (Xinhua) -- American scientists found that tears may hold clues to whether someone has Parkinson's disease, according to a preliminary study that will be presented at the American Academy of Neurology's 70th Annual Meeting in Los Angeles, April 21 to 27, 2018.

"We believe our research is the first to show that tears may be a reliable, inexpensive and noninvasive biological marker of Parkinson's disease," said the paper's author Mark Lew from the Keck School of Medicine of the University of Southern California in Los Angeles.

Lew said the research team investigated tears because they contain various proteins produced by the secretory cells of the tear gland, which is stimulated by nerves to secrete these proteins into tears.

Given that Parkinson's can affect nerve function of the brain, the research team hypothesized that any change in nerve function may be seen in the protein levels in tears, Lew said.

They got tear samples from 55 people with Parkinson's and compared with tear samples from 27 people who did not have Parkinson's but who were the same age and gender. Tears were analyzed for the levels of four proteins.

Researchers found differences in the levels of a particular protein, alpha-synuclein, in the tears of people with Parkinson's compared to controls.

"Knowing that something as simple as tears could help neurologists differentiate between people who have Parkinson's disease and those who don't in a noninvasive manner is exciting," said Lew.

"Because the Parkinson's disease process can begin years or decades before symptoms appear, a biological marker like this could be useful in diagnosing, or even treating, the disease earlier," said Lew.

More research now needs to be done in larger groups of people to investigate whether these protein changes can be detected in tears in the earliest stages of the disease, before symptoms start.