“Not many people appreciate that the retina is an extension of the brain from the time of embryological development.  It is therefore not surprising that cells of the dopaminergic system are found in the retina. Moreover, that hallmarks of the disease such as alpha synuclein are also found in the eye. As the eye is accessible and imaged non-invasively and inexpensively, this research provides encouraging evidence that simple eye tests could be used to screen, monitor and assess therapies in patients with Parkinson’s Disease. Obviously, further validation will be required in longitudinal studies, but ocular biomarkers, including those such as DARC (detection of apoptosisng retinal cells) currently in clinical trials, may herald a new direction for PD.” – Professor Francesca Cordeiro “There are currently no diagnostic tests for Parkinson’s. Nor are there any readily available tools or methods of tracking the condition over time. What the community desperately needs are easy to administer techniques, which also relate to what is happening inside the body. This new research report from Ahn and colleagues presents just such an approach – a non-invasive eye test that measures the thickness of the retina, and appears to be closely related to the loss of dopamine neurons in the brain. If these results can be independently replicated in larger groups of people with Parkinson’s who are followed over time, this could be a very easy to establish diagnostic aid and method for monitoring Parkinson’s in the clinic.” – Dr Simon Stott

The retina is the inner layer of the eye which is responsible for transforming light into neural signals that travel up the optic nerve into the brain. Evidence from human and animal research now indicates that changes in the retina precede the onset of Parkinson’s. Recent studies have found alpha synuclein in tears, and in the retina of people with Parkinson’s. Retinal thinning has also been reported, using low resolution imaging with some inconsistencies. The current study took these findings further by assessing retinal thickness in people with Parkinson’s using a high resolution automated method, and asking how this relates to dopaminergic degeneration in the substantia nigra, a critical region for movement.

49 people who were newly diagnosed with Parkinson’s and 54 age matched controls underwent neurological and eye examination, and a DAT brain scan (which is an index of dopamine neuron loss in the substantia nigra and often used to help diagnose Parkinson’s). None of the people recruited into the study had an eye-related condition which might otherwise affect the results of the study. High resolution optical coherence tomography (OCT) and a standardized automated analysis method were used in this study. This is an important strength, because studies to date have been methodologically inconsistent. The use of a method which can be replicated consistently is an important advance toward evaluating retinal thickness as a potential biomarker for Parkinson’s. The researchers found that the innermost layer of the retina was thinner in the Parkinson’s group, and that greater retinal thinning was seen in people with more advanced symptoms. In addition, there was a close relationship between the retinal measurement and the DAT scan, indicating for the first time a direct link between the retina and dopaminergic loss in the brain. Since these newly diagnosed individuals had not yet started treatment at the time of recruitment into the study, these results are not obscured by the effects of dopaminergic medication.

As this was a cross-sectional study, which essentially takes a snapshot of people at one particular timepoint, future studies should aim to follow the same individuals over time as their condition progresses. This is an important step toward validating whether changes in retinal thickness track with changes in Parkinson’s.

See our summary on alpha-synuclein on tears: https://www.parkinsonsmovement.com/tears/