An optical scan of the retina may be able to detect changes marking the early stages of Parkinson’s disease (PD). Researchers in South Korea published a report in the journal Neurology finding that the thinning of the retina could be linked with the loss of dopamine on the left side of the substantia nigra in the midbrain in people with Parkinson’s.1 The retina is a layer of light-sensitive nerve cells at the back of the eye.2 Earlier studies have identified the thinning of the retina in people with Parkinson’s. Further research is needed to directly correlate it to the loss of dopamine and to consider retinal thinning as a PD biomarker.1,3
The Korean study1 had 49 participants with Parkinson’s and a group of control subjects matched for age and gender. Researchers sought to investigate the relationship between retinal thinning and dopaminergic deterioration—the breakdown or depletion of dopamine production in people newly diagnosed with PD and not yet taking medications.
The Korean study identified a relationship between retinal thinning and dopamine transporter loss. It applied retinal imaging to any pathologic changes that take place in the early stages of PD.
Opthalmologists (eye doctors) performed examinations on each participant. People with preexisting conditions of the eye such as macular degeneration or glaucoma were excluded from the study. OCT, optical coherence tomography, was used to measure the retinal layer thickness and volume of the macula. OCT is a high-resolution scanner capable of analyzing each layer of the retina.
The researchers were looking to examine any anatomical change of the retina resulting in functional implications. They correlated the thinning of the inner retinal layer of the eye with a special visual field test known as microperimetry.
The results demonstrated that people with Parkinson’s had a significant association between the thickness of their retinas and the dopamine transporter density in the left side of the substantia nigra region of the brain, but not in other areas.
There are limitations to the current research. The current technology requires expert usage and advanced calculations because the software and scanners used have limited specificity to easily detect changes in early Parkinson’s. The hope is that with advances in technology and larger longitudinal studies, scientists will be able to confirm the relationship between thinning of the retina and the loss of dopamine as PD progresses.
Using the eye for early detection
The findings were consistent with a 2014 study that found people with Parkinson’s had inner retinal layers that were thinner than control subjects. One hypothesis is that retinal thinning could be caused by proteins, like alpha-synuclein, in the retina or loss of retinal dopamine.1
We know that Parkinson’s tends to affect one side of the body before the other. There have been noted differences in the left and right brain in multiple neurodegenerative diseases, not just in Parkinson’s. This study evaluated both eyes and the thinner the retina in the left eye the more substantial the disease progression. Participants in this study experienced increased motor symptoms on the right side of their bodies and reduced dopamine levels in the left side of the brain.1 This is consistent with split-brain theories where one hemisphere of the brain controls the opposite side of the body, accordingly damage to one side of the brain affects the opposite side.
The retinal findings mirror this asymmetry, suggesting that retinal thinning in PD may also occur on one side before the other. This could make the retina a useful way to monitor the pathology of Parkinson’s as a tool for early diagnosis and measure of disease progression.3
There is not yet a biomarker for Parkinson’s, but it might be possible that one day there will be a simple eye scan to detect the disease.2,3
Lehmann, C. For Your Patients-Parkinson's Disease Changes: Retinal Imaging May Detect Pathologic Changes in Early Stages of Parkinson's Disease. Available at: https://journals.lww.com/neurotodayonline/Fulltext/2018/09200/For_Your_Patients_Parkinson_s_Disease_Changes_.7.aspx. Accessed 10.30.2018.
Kuhl, MM. Changes in the Eye May Be a Window to Parkinson's Disease. Available at: https://www.michaeljfox.org/foundation/news-detail.php?changes-in-the-eye-may-be-window-to-parkinson-disease. Accessed 10.30.2018.
George, J. Can Eyes Predict Parkinson’s Disease? Available at: https://www.medpagetoday.com/neurology/parkinsonsdisease/74575. Accessed 10.30.18.