One might think of the retina as an exposed part of the central nervous system, available for inspection, unlike the rest of it. One of the major challenges in the diagnosis, prevention, and treatment of neurodegenerative conditions is that it is difficult to establish what exactly is going on inside a living individual's brain. Even modern imaging systems have considerable limitations in what can be seen. Thus a number of research groups, such as the one noted here, are attempting to find ways to make use of retinal structure as a readout for the broader state of the aging brain.
In almost 3,000 participants of the Rhineland Study aged between 30 and 94 years, the retina was assessed using "spectral domain optical coherence tomography" (SD-OCT) - a technique that provides detailed images of the retina and its various layers. In addition, brain scans were performed by magnetic resonance imaging (MRI). The data were analyzed using sophisticated software algorithms. This allowed for automated identification and determination of thickness and volumes, of both the different retinal layers and the different structures of the brain. Next, researchers looked for associations between the volume of the retina and the volume of certain brain structures.
There was a close relation between layers of the inner retina and the white matter in the brain. The thinner these retinal layers, the smaller the volume of the brain's white matter. By contrast, sections of the outer retina were mainly associated with the gray matter of the cerebral cortex. In the brain's occipital lobe, where visual processing happens, these associations were particularly pronounced. And the researchers found further relationships. The thickness of different retinal layers correlated closely with the volume of the hippocampus. This is an area of the brain that plays a central role in memory and is often affected in dementia.
"Imaging of the retina using SD-OCT is relatively simple, non-invasive and inexpensive. The current results suggest that SD-OCT measurements of the retina could potentially serve as biomarkers for brain atrophy and to monitor progression of certain neurodegenerative diseases. Further population-based studies as well as studies in patient groups and over a longer period of time are now needed to verify these results in a clinical setting."