March 25, 2019Ophthalmology
The retinal pigment epithelium (RPE) is a cell layer that lies next to and maintains the health of the retina's light-sensing photoreceptors. Because the cells contain pigment, and thus absorb incoming light, the thin layer of RPE tissue is difficult to image.
According to an article published in the journal JCI Insight (21 March 2019), the RPE forms unique patterns that can be used to track changes in the back of the eye. By using a combination of adaptive optics imaging and a fluorescent dye, the authors used the RPE patterns to track individual cells in healthy volunteers and people with retinal disease. This new finding could provide a way to study the progression and treatment of blinding diseases that affect the RPE.
However, even using adaptive optics, a specialized imaging technology that can distinguish individual cells in the eye, visualizing the RPE layer can be very challenging. As a result, the authors used an FDA-approved fluorescent dye called indocyanine green (ICG) that is used to visualize the blood vessels in the back of the eye. While the dye fades from the blood vessels quickly, within about thirty minutes, the dye persists in the RPE for several hours, revealing a fluorescent mosaic pattern, with some cells appearing more brightly and others more dimly.
The authors designed software that recognizes RPE patterns and then computes changes that occur from one imaging session to the next. For healthy volunteers, there was very little change in the RPE over several months, with the vast majority of the cells retaining a stable amount of ICG staining. However, to find out whether this technique could detect the early stages of damage to the RPE, the authors also imaged the eyes of people with conditions that can affect that part of the eye. First, the authors imaged the retinas of a patient with late-onset retinal degeneration (L-ORD), a condition that is thought to affect the RPE in later stages of the disease. Results showed that the mosaic pattern of the RPE in a patient in the earlier stages of L-ORD was only slightly less stable than in healthy eyes, showing relatively minor changes in a few areas of the retina. Next, the researchers imaged the eyes of a patient with Bietti crystalline dystrophy (BCD), a disease that causes progressive loss of RPE cells. Adaptive optics with ICG dye revealed not only that RPE cells in the patient with BCD were larger and less well organized than healthy cells at all time points, but also that there were drastic changes in the RPE mosaic pattern over time. While this study uses adaptive optics imaging, Tam believes that, with additional efforts, it will be possible to image this RPE mosaic pattern with conventional imaging methods. Being able to visualize this pattern over time will help researchers better understand how the RPE layer changes over time, and eventually help guide the development of new treatments to prevent damage to or repair the RPE.