Parkinson’s found to alter eye’s retina, impact vision in new study
Significant changes seen in retina structure, function in adults
Significant alterations in the structure and function of the eye’s retina — the layer at the very back of the eyeball that’s key in transmitting information to the brain — were revealed in adults with Parkinson’s disease in a new study.
Data demonstrated a significant impact of disease duration on the thickness of the retina, while retinal function correlated with the degree of disability.
“These results, therefore, reflect the presence of progressive degeneration in the retinal layers with disease progression and indicate that the neurodegenerative process runs in parallel in the brain and the retina in [Parkinson’s],” the researchers wrote.
Overall, according to the scientists, “[Parkinson’s disease] patients had significantly impaired retinal structure and function in comparison to healthy [people].”
The study, “Retinal functional and structural changes in patients with Parkinson’s disease,” was published in the journal BMC Neurology.
Comparing retina structure, function in patients vs. healthy people
Beyond the hallmark motor symptoms experienced by people with Parkinson’s disease, nonmotor symptoms, such as vision problems, have been reported. Such impairments can affect clarity of vision, color perception, eye muscle movement, and contrast sensitivity — the ability to distinguish between an object and the background behind it.
Now, scientists at the Beni-Suef University, in Egypt, sought to investigate disease-related vision changes in Parkinson’s. To that end, the team examined the structure and function of the retina of Parkinson’s patients.
The retina comprises several distinct layers of nerve and photoreceptor cells located at the back of the eye, which capture incoming light. The light is then converted to electrical signals that travel to the brain via optic nerve fibers.
The study enrolled 50 adults with Parkinson’s, 64% of them men, with a mean age of 60. Another 50 age- and sex-matched healthy individuals served as controls. Among the patients, the mean disease duration was about 3.6 years, and all were receiving treatment.
Noninvasive optical coherence tomography (OCT) was used to measure the thickness of layers within the retina.
The results showed that the retinal nerve fiber layer (RNFL), one of the innermost layers of the retina, was significantly thinner in both eyes of Parkinson’s patients compared with controls.
Likewise, the thickness of the layer immediately below the RNFL, known as the ganglion cell complex, or GCC, was significantly reduced in the patients’ right eye. The left eye GCC also was thinner than that of the controls, but the difference did not reach statistical significance.
A longer disease duration — the time from diagnosis to the eye assessment — significantly correlated with a thinner RNFL in various regions of the retina. This included the upper region (superior) of the left eye, the lower region (inferior) of the right eye, and the average left and right eye RNFL thickness.
No relationship was found between RNFL and GCC thickness and disease stage, as assessed by the Hoehn and Yahr (H&Y) staging scale, or the level of disability, measured with the Unified Parkinson’s Disease Rating Scale (UPDRS).
Noninvasive test use to measure retina’s response to light stimulus
To examine retinal function, the researchers applied electroretinography (ERG), a noninvasive test that measures the retina’s electrical activity in response to light stimulus.
Photoreceptor cells within the retina, located in the outer nuclear layer of the retina farthest from the brain, are either rods, responsible for vision in low light, or cones, for vision at higher light levels.
Rod activity is measured by a dim flash of light on an eye that has adjusted to darkness, while cone activity is assessed with a regular light flash on a light-adapted eye. The subsequent electrical signal is detected as an a-wave, produced by the photoreceptors, followed by a b-wave, created by a mixture of nerve cells, including photoreceptors.
The dark-adapted a- and b-wave responses of rods were significantly slower and weaker in Parkinson’s patients compared with controls, the researchers found. Similarly, the light-adapted cone a- and b-waves responses also were significantly slower and weaker.
In both light-adapted eyes, weaker a- and b-waves were significantly linked with worse UPDRS disability, as were slower b-wave signals in dark-adapted right eyes. No relationships were detected between these ERG parameters and disease duration, or H&Y stage, or total UPDRS scores and other ERG values.
“The retinal structure and function were significantly affected in patients with [Parkinson’s disease] in comparison to healthy controls,” the researchers concluded.
The team noted that “there was a significant impact of disease duration on retinal thickness, and there was a significant negative correlation between the degree of motor dysfunction in patients with [Parkinson’s disease] and retinal function.”