Evaluation of Retinal Pigment Epithelium Melanin Using Polarization-Sensitive OCT in Subthreshold Retinal Laser Therapy

Objectives: Retinal pigment epithelium (RPE) melanin absorbs laser energy during retinal laser therapy, potentially influencing treatment efficacy and safety. This study investigated the relationship between spot visibility threshold laser power and RPE melanin content, as measured by polarization-sensitive optical coherence tomography (PS-OCT), and examined differences in RPE melanin levels between the titration and treatment areas in subthreshold microsecond pulsing retinal laser therapy (SMPL). Methods: Twelve eyes from 11 patients who underwent SMPL for macular edema and serous retinal detachment were included. In SMPL therapy, the threshold laser power was determined through titration outside the macula, near the vascular arcade. RPE melanin levels were evaluated based on polarimetric entropy (PE) measured by PS-OCT. The relationships between the visibility threshold laser power observed in funduscopy, RPE-PE in the titration area, and RPE-PE in three distinct macular regions were analyzed. Results: A significant inverse correlation was observed between visibility threshold laser power (1125.0 ± 154.5 mW) and RPE-PE in the titration area (0.50 ± 0.09; p = 0.028; adjusted R² = 0.34; linear regression). RPE-PE values in the central macular region (0.57 ± 0.07) were correlated with those in the titration area (p = 0.025; adjusted R² = 0.35) but were significantly higher (p < 0.01; Wilcoxon signed-rank test). The ratio of macular to titration RPE-PE values was 1.15 ± 0.17, with considerable interindividual variability. Conclusion: The inverse correlation between individual visibility threshold laser power and RPE-PE, along with discrepancies in melanin density between the titration and macular regions, suggests that variations in RPE melanin density substantially influence the efficacy and safety of SMPL. These findings support the potential utility of PS-OCT–based RPE melanin assessment for optimizing SMPL parameters.