Purpose: To investigate the relationship between the foveal avascular zone (FAZ) and the inner nuclear layer (INL)-free zone in the center of the fovea in order to provide an improved approach to evaluation of the FAZ area with optical coherence tomography angiography (OCTA).
Methods: Thirty-six healthy individuals (36 eyes) and 15 age-matched patients (15 eyes) with nonproliferative diabetic retinopathy (nPDR) were included in this study. The FAZ, as well as INL-free zone, was measured on OCTA images. The FAZ area, INL-free zone area, and the ratio of the INL-free zone area to the FAZ area were compared between healthy subjects and nPDR patients.
Results: The mean FAZ area in healthy subjects and nPDR patients was 0.33 ± 0.1 mm2 and 0.41 ± 0.19 mm2 (p < 0.05), respectively. The mean INL-free zone area in healthy subjects and nPDR patients was 0.33 ± 0.07 mm2 and 0.25 ± 0.09 mm2 (p > 0.05), respectively. The INL-free zone area to the FAZ area ratio in healthy subjects and nPDR patients was 1.08 ± 0.25 and 0.56 ± 0.18 (p < 0.001), respectively. Receiver operating characteristic analysis showed that the INL-free zone area to the FAZ area ratio had a higher area under curve (0.94; 80.0% sensitivity and 97.2% specificity) compared to the FAZ area (0.61; 40.0% sensitivity and 94.4% specificity) for differentiating nPDR from healthy eyes.
Conclusion: This study showed that analysis of the FAZ based on the ratio of the INL-free zone area to the FAZ area has better accuracy compared to the conventional FAZ area measurement in differentiating eyes with mild to moderate nPDR without any structural abnormalities in the fovea from healthy eyes.
Keywords: Diabetic retinopathy; Foveal avascular zone; Inner nuclear layer; Optical coherence tomography angiography; Retinal ischemia.