Reduced photoreceptor layer thickness in early type 2 diabetes mellitus is associated with decreased contrast sensitivity: a prospective cross-sectional study

Purpose To investigate changes in contrast sensitivity (CS) in patients with Type 2 Diabetes Mellitus (T2DM) with no diabetic retinopathy (DR) or nonproliferative diabetic retinopathy (NPDR) and to assess the relationships between these alterations and early retinal microstructure. Methods This prospective cross-sectional study included subjects aged 40–60 years, including 30 healthy controls, 30 individuals with T2DM without DR, and 30 individuals with T2DM and NPDR. General data from all groups were recorded. Contrast sensitivity was assessed using the CSV-1000E chart, and AULCSF was calculated as a summary measure of overall contrast sensitivity. Optical coherence tomography (OCT) and OCT angiography (OCTA) were used to analyze the thickness as well as blood flow of retinal sublayers. Differences in inflammatory factors, including PLR (PLT-to-lymphocyte ratio), SII (Systemic Immune-Inflammation Index), and NLR (Neutrophil-to-lymphocyte ratio), were compared among the three groups. Pearson correlation analyses were performed to assess the associations between contrast sensitivity, retinal structural parameters, and inflammatory markers. Exploratory ROC curve analysis was performed to evaluate the ability of photoreceptor layer thickness to identify control-defined contrast sensitivity abnormalities in participants with T2DM. Results Significant between-group differences were observed in AULCSF, middle-spatial-frequency CS, photoreceptor layer thickness, and PLR. GCIPL thickness was reduced in patients with NPDR. No significant differences were observed in total, inner, or outer retinal thickness, FAZ, VD, AFI, SII, or NLR. Middle CS was positively correlated with photoreceptor thickness and PLR. Additionally, photoreceptor thickness differed significantly between DM patients with normal and abnormal CS, and Pearson correlation analysis confirmed a positive association between photoreceptor thickness and middle CS. The AUC for identifying abnormal CS among participants with T2DM based on photoreceptor layer thickness was 0.920 (95% CI: 0.852–0.989), indicating good discriminatory performance within the present study cohort. Conclusion This study identified an association between retinal microstructure and visual function in individuals with T2DM. These findings suggest that CS abnormalities and reduced photoreceptor layer thickness are associated with early structural and functional retinal alterations before clinically detectable DR. Longitudinal studies are required to determine whether these parameters predict future DR development.