Human eyes are the only natural window we have into a person's central nervous system.
By looking through them, scientists have found very early signs of Parkinson's disease, up to seven years before symptoms emerge.
The findings are based on three-dimensional eye scans, which are commonly used by eye specialists to examine the health of someone's retina – the layer of nerve cells at the back of the eye.
For the past decade or so, researchers have been trying to use that data to diagnose neurological diseases and disorders like Alzheimer's disease, multiple sclerosis, and schizophrenia – in an emerging field known as 'oculomics'.
Parkinson's, which is a disorder of the nervous system with unknown cause or cure, could very well be a part of that future.
"I continue to be amazed by what we can discover through eye scans," says ophthalmologist Siegfried Wagner from University College London and Moorfields Eye Hospital in the United Kingdom.
"While we are not yet ready to predict whether an individual will develop Parkinson's, we hope that this method could soon become a pre-screening tool for people at risk of disease."
Together with colleagues from several hospitals around the UK, Wagner has pulled off the largest study to date on retinal imaging for Parkinson's disease.
Previously, scientists had noticed the retinas of patients who died with Parkinson's showed abnormalities, but it was unclear if those changes could be detected in life.
The new findings suggest that they can. In fact, the eyes might be a window into how the disease arises and progresses.
Using the help of an artificial intelligence program, researchers compared the eye scans of 154,830 patients aged 40 years and over.
The 700 individuals in this group with prevalent Parkinson's disease showed a small but significant difference in the look of their retinas. In certain regions, they noticed a thinning in the layer of inner ganglion cells, which are a type of neuron that transmit visual information via dopamine.
Researchers then used this information in their examination of eye scans from 67,311 people in the UK Biobank database, 53 of whom were diagnosed with Parkinson's during the study.
They found that this thinning of the inner ganglion layer can appear in the early stages of Parkinson's, even before clinical symptoms show up.
The findings support some earlier reports of ganglion thinning in individuals with Parkinson's and add weight to the idea that the eyes show some of the first outward signs of the disease.
Currently, one of the trickiest parts of diagnosing and treating Parkinson's disease is that its symptoms lag far behind its pathology.
On average, by the time someone receives a diagnosis of Parkinson's, they have already lost between 60 and 80 percent of the dopamine-producing cells in a part of their brain known as the substantia nigra.
If the initial stages of the disorder can one day be glimpsed through the eye, it could revolutionize how doctors treat and researchers study the disease.
Parkinson's is a degenerative illness marked by physical tremors, slow movements, stiffness, or loss of balance that inevitably grows more severe over time. But current treatments can slow down clinical progression, and they work best when started early.
"Finding signs of a number of diseases before symptoms emerge means that, in the future, people could have the time to make lifestyle changes to prevent some conditions arising," says Wagner, "and clinicians could delay the onset and impact of life-changing neurodegenerative disorders."
The study was published in Neurology.