Understanding How Low Luminance Visual Deficit Progresses With Advancing Diabetic Macular Ischemia and Diabetic Retinopathy.

Purpose: To assess whether low luminance deficit (LLD) correlates with diabetic macular ischemia on optical coherence tomography angiography (OCTA). Methods: Best-corrected visual acuity (BCVA) and low luminance visual acuity (LLVA; using 2.0-log unit neutral density filter) were assessed with Early Treatment Diabetic Retinopathy Study (ETDRS) charts and recorded as ETDRS letter scores. Low-luminance deficit (LLD) was calculated as BCVA - LLVA. From OCTA 3 × 3 mm scans, geometric perfusion deficits (GPD), vessel length density (VLD), and vessel density (VD) in deep (DCP) and superficial (SCP) capillary plexuses were quantified. Linear regression analyzed associations between LLD and OCTA metrics. Results: The study included 91 non-referable (no/mild DR), 94 referable (moderate/severe nonproliferative DR, treatment naïve PDR), and 76 quiescent laser-treated PDR eyes, all without macular edema. BCVA (86.6 ± 5.02, 83.5 ± 5.95, 76.8 ± 9.52; P < 0.001) and LLVA (78.5 ± 5.18, 73.8 ± 7.84, 69.5 ± 9.61; P < 0.001) declined with increasing DR severity. LLD was higher in referable DR (9.8 ± 4.4) versus non-referable DR (8.1 ± 3.12) and quiescent PDR eyes (7.3 ± 3.52; P < 0.001). LLD was negatively associated with DCP VLD in the entire cohort (referable and non-referable DR; β = -3.01; P = 0.030) and with DCP GPD in quiescent PDR eyes (β = -1.77; P = 0.032). Conclusions: Both BCVA and LLVA decline with DR progression; however, distinct LLD patterns have emerged across DR stages. In referable DR, LLD is larger as a result of LLVA decline, whereas in quiescent PDR smaller LLD reflects a greater decline in BCVA. These findings emphasize the nuanced relationship of LLD with photopic and mesopic visual functions, which decline in distinct patterns across DR stages. LLD may serve as a simple and practical tool for monitoring macular ischemia progression in diabetic eyes.