Abstract
Purpose :
Retinitis pigmentosa (RP) is the most common form of inherited irreversible visual loss worldwide. The biological mechanisms that cause cones to die–the most debilitating phase–remain unknown. To better describe the pattern of cone outer segment (OS) shortening (one of the earliest signs of cone degradation), we used adaptive optics optical coherence tomography (AO-OCT) to quantify OS length variability of individual cones.
Methods :
We used the Indiana AO-OCT and clinical OCT (Spectralis, Heidelberg) to image three subjects: one arRP with CDHR23-PDZD7 mutation, one RP with negative genetic results and one control. 450x450 μm2 AO-OCT volume videos were acquired along the temporal horizontal meridian at 2°, 4° and 8° retinal eccentricity, which sampled the inner edge of the transition zone from healthy to diseased retina of the RP subjects. For each cone, we identified reflections across the expected depth range of the OS and defined OS length as the distance between the inner segment (IS)/OS junction reflection and the deepest OS reflection identified. We quantified the variability of cone IS/OS depth, OS tip (COST) depth, and the separation of the two (OS length). We used Voronoi analysis to test for spatial clustering of cones based on their OS length.
Results :
Loss of outer retinal bands in the Spectralis B-scans of the RP subjects confirmed a transition zone between 5° and 9° from the fovea. Cone OS length was more varied in the RP subjects compared to the control at all retinal locations, even at 2° where the tissue was presumably healthy. OS length variability increased by 3.3X (2°), 3.9X (4°), and 2.7X (8°). Depth location of the cone IS/OS and COST reflections were also more varied in the RP subjects than the control at all retinal locations. IS/OS depth variability increased by 2.6X (2°), 2.2X (4°), and 2.3X (8°); COST depth variability increased by 2.3X (2°), 2X (4°), and 1.4X (8°). No evidence of cone clustering based on OS length was found in either RP subject.
Conclusions :
AO-OCT reveals increased local variation in the OS length and depth location of individual cones in RP, even in parts of the retina considered to be healthy. Undetected by clinical OCT, these variations may be fundamental to understanding the kinetics of OS degeneration.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.