Avoiding False-Positive Glaucoma Diagnosis in Myopic Eyes: Clinical Importance of OCT Scan Diameter

<b>Background/Objectives:</b> Diagnosing glaucoma in myopic eyes remains challenging because myopia-related structural changes can mimic glaucomatous damage on optical coherence tomography (OCT). This study aimed to identify the optimal circular scan diameter for differentiating physiological myopic thinning from glaucomatous loss by analyzing retinal nerve fibre layer thickness (RNFLT) and colour-code distribution across three scan diameters. <b>Methods:</b> In this prospective cross-sectional study, 204 eyes (41 controls, 44 mild myopia, 66 moderate myopia, and 53 high myopia) were examined using spectral-domain OCT (Spectralis, Heidelberg). Three concentric circumpapillary scans centred on the Bruch's membrane opening were obtained: C1 = 3.5 mm, C2 = 4.1 mm, and C3 = 4.7 mm. Global and sectoral RNFLT were evaluated in seven anatomical regions (TS, NS, N, NI, TI, T, and G). Statistical analyses included one-way ANOVA, repeated-measures ANOVA, and receiver operating characteristic (ROC) analysis. <b>Results:</b> RNFLT decreased significantly with increasing scan diameter (<i>p</i>< 0.001). Thinning was most pronounced in the nasal-superior (NS) and nasal-inferior (NI) quadrants. Across all diameters, C2 (4.1 mm) showed the highest and most consistent discriminatory performance between myopic and control eyes (NI: AUC = 0.674, <i>p</i> = 0.001; NS: AUC = 0.625, <i>p</i> = 0.014). A progressive shift in OCT colour-code distribution was observed from green to yellow/red with both increasing myopia and larger scan diameters, reflecting anatomical stretching in the nasal and inferior regions. This change was most prominent at the outer ring (C3), where the temporal-inferior (TI) quadrant showed a significant rise in yellow codes (<i>p</i> = 0.020). Repeated-measures ANOVA revealed significant between-group effects in NS and NI (<i>p</i> < 0.01) and notable group × diameter interactions in NS and TS (<i>p</i>< 0.05). <b>Conclusions:</b> RNFLT thinning in non-glaucomatous myopic eyes predominantly affects nasal quadrants, whereas temporal segments remain relatively preserved. The 4.1 mm (C2) scan provides the most balanced diagnostic performance and minimizes false-positive "red disease" results. Recognition of the ring-dependent colour-code shift and segment-specific thinning is crucial for accurate interpretation of OCT findings in myopic eyes.