Testing tears may be a cheap, non-invasive as well as reliable technique for early diagnosis of Parkinson's disease, say scientists.

It is because tears contain various proteins produced by the secretory cells of the tear gland, which is stimulated by nerves to secrete these proteins into tears. The differences in protein can accurately predict the neurological disorder, the findings showed. "We believe our research is the first to show that tears may be a reliable, inexpensive and non-invasive biological marker of Parkinson's disease," said Mark Lew, from the University of Southern California in Los Angeles.

As Parkinson's can affect nerve function outside of the brain, the research team hypothesised that any change in nerve function may be seen in the protein levels in tears and thus help differentiate between people with and with Parkinson's. "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," Lew noted.

For the preliminary study, to be presented at the 2018 AAN Annual Meeting in Los Angeles in April, tear samples from a small group with Parkinson's were compared to tear samples with those who did not have the disease. The results showed that total levels of protein alpha-synuclein were decreased in people with Parkinson's, while the levels of protein oligomeric alpha-synuclein were increased in people with Parkinson's, compared to people without Parkinson's.

It is possible that the tear gland secretory cells themselves produce these different forms of alpha-synuclein that can be directly secreted into tears. However, 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.