Diagnosing Parkinson's disease is still a challenge for doctors. Complex neurological exams can help identify the disease after symptoms appear but detecting the disease early is vital to better battle the neurological degeneration associated with the condition. A small, but promising, new study suggests there is a link between a thinning of the retina and the onset of Parkinson's, pointing toward a potential simple eye exam that may help diagnose the disease before major symptoms appear.

A flurry of research is currently underway trying to zero in on an effective and reliable biomarker for early diagnosis of Parkinson's disease. Blood tests, smell tests and tear analysis are all proving to be promising contenders for aiding early-stage diagnoses. However, a team of South Korean researchers is hoping the eye could offer a clear indication of the onset of the disease.

The new research initially found that a loss of brain cells that produce dopamine, which is a key characteristic of the disease, is linked with a gradual thinning of nerve cells in the inner layers of the retina. To explore this correlation the study recruited 49 subjects recently diagnosed with Parkinson's but yet to commence medication. These subjects were matched with a control group of healthy, age-similar subjects without the disease.

High-resolution eye scans were taken of all the participants, and those with Parkinson's were found to, on average, have thinner retinal layers compared to the healthy subjects. The degree of retinal thinning was also found to be directly associated with the severity of the disease.

The study also used PET imaging to measure the density of dopamine-producing cells in the brains of some of the Parkinson's patients. Again, retinal thinning positively correlated with a decline in these specific brain cells.

"Our study is the first to show a link between the thinning of the retina and a known sign of the progression of the disease – the loss of brain cells that produce dopamine," says Jee-Young Lee, one of the authors on the new study. "We also found the thinner the retina, the greater the severity of disease. These discoveries may mean that neurologists may eventually be able to use a simple eye scan to detect Parkinson's disease in its earliest stages, before problems with movement begin."

The study adds to a compelling body of evidence beginning to suggest that several neurodegenerative diseases may be detected through simple eye tests. Other recent work has found cell death in the eye could be traced to signs of early Parkinson's disease, and Alzheimer's researchers are beginning to target the eye as an indicator of early neurodegeneration.

It's still early days for this retinal thinning research, with more work certainly needing to be done to verify the results in a larger cohort and track subjects over a longer period of time. The hope is that this work will not only offer a potential diagnostic tool for clinicians but an accurate and easy way to objectively measure the efficacy of future treatments.

"If confirmed, retina scans may not only allow earlier treatment of Parkinson's disease but more precise monitoring of treatments that could slow progression of the disease as well," says Lee.

The new research was published in the journal Neurology.