OCT scan diameter and false glaucoma diagnosis in myopia

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

Authors: Saadet Gültekin Irgat (1), Ramazan Demirel (1), Ecem Ulutürk (1), Alpaslan Koç (1), Fatih Özcura (1), Özlem Arık (2)

Date: Feb 2026

References: Gültekin Irgat S, Demirel R, Ulutürk E, Koç A, Özcura F, Arık Ö. Avoiding False-Positive Glaucoma Diagnosis in Myopic Eyes: Clinical Importance of OCT Scan Diameter. J Clin Med. 2026 Feb 23;15(4):1669.

[Link to open access paper]

Structural Optical Coherence Tomography (OCT) is central for glaucoma screening, but interpreting results in myopic eyes can be challenging where elongation and retinal nerve fibre layer (RNFL) thinning often create artefacts that mimic disease. Using the standard 3.5 mm scan circle may add to this difficulty, as the areas scanned may be more affected by parapapillary change, making the RNFL profile less reliable for distinguishing true glaucomatous damage from physiological myopic thinning. 

This prospective, cross-sectional study compared RNFL measurements across three concentric circles (3.5mm, 4.1mm and 4.7mm) in 204 myopic participants. The participants were classified according to myopia severity: mild myopia (−1.00 to −3.00D), moderate myopia (−3.00 to −6.00D), and high myopia (>−6.00D) and a control group with refraction of between +1.00 to −1.00D.

Primary outcomes included global and sectoral RNFL measurements across the three scan diameters, with particular focus on discriminating myopic eyes from controls.

Key study outcomes

• RNFL thinning in myopic eyes localized predominantly to nasal-superior and nasal-inferior sectors, contrasting with temporal-predominant glaucomatous patterns
• Mean global RNFL thickness decreased with increasing scan diameter: 98.7-101.6 micrometers at 3.5mm, 83.5-85.1 micrometers at 4.1mm, and 72.4-75.3 micrometers at 4.7mm
• In the nasal-inferior sector, the 4.1mm scan diameter provided a stronger ability to discriminate between myopic thinning and glaucomatous loss (area under curve (AUC) 0.674, performing slightly better than the standard 3.5mm scan (AUC 0.660) and the wider 4.7mm scan (AUC 0.637)
• Mean global RNFL thickness decreased with increasing scan diameter: 98.7-101.6 micrometers at 3.5mm, 83.5-85.1 micrometers at 4.1mm, and 72.4-75.3 micrometers at 4.7mm
• Automated color-coded classifications showed an increasing shift away from normative values with colors moving from green to yellow / red with higher myopia and circle diameters, especially nasally for high myopes and globally in mild myopes 

This study highlights the practical challenge of distinguishing myopia-related anatomical variation from suspected glaucomatous damage on OCT images. Recognising that myopic eyes tend to show nasal RNFL thinning, while early glaucoma more often affects temporal regions, may help reduce false-positive glaucoma diagnoses. 

The findings suggest that standard OCT protocols (typically 3.5 mm scan diameter) may not be optimal in myopic eyes. A slightly larger scan diameter (around 4.1 mm) appeared to reduce the influence of peripapillary distortion and showed modestly improved differentiation between myopic and normal eyes. However, although the 4.1mm scan showed a numerically higher performance in discriminating between physiological myopic and glaucomatous changes, it was a modest increase compared to the 3.5mm scan.

Eyecare practitioners should interpret OCT results cautiously in myopic patients, especially where there is nasal thinning (which could be considered typical in myopic eyes) and consider OCT findings overall alongside clinical and visual fields examination.

This study provides useful insights but has important limitations to consider in clinical practice. The cross-sectional design means it cannot determine whether nasal retinal nerve fibre layer thinning remains stable over time or predicts future glaucoma risk. Patients with glaucoma or suspected glaucoma were excluded, so the ability of the 4.1 mm scan to distinguish myopic change from true glaucomatous damage in clinically uncertain cases remains unproven.

The single-centre population and exclusion of more advanced myopic changes may limit how widely the findings apply. The color-coded classifications relied on manufacturer normative data developed for standard 3.5mm scans, meaning practitioners currently lack validated reference ranges to determine normal versus abnormal values at alternative diameters. The single-center population may also not represent the degree of myopic variation across different ethnic groups and age ranges. 

Although the 4.1 mm scan showed no clear relationship between axial length and RNFL thickness compared to the 4.7mm scan, suggesting a possible “neutral zone” where measurement effects are reduced, the 3.5mm scan also showed a similar correlation to the 4.1mm scan, meaning this may be a hypothetical difference at this time. A key limitation is the lack of normative databases for alternative scan diameters, as current systems are calibrated to standard protocols. This makes interpretation of results at larger diameters uncertain.

The performance of the varying scan diameters in eyes with significant disc tilt or parapapillary atrophy also remains unclear. Larger, longitudinal studies, including patients with suspected glaucoma, are needed before these findings can be applied confidently in routine practice.

 Background/Objectives: 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. 

Methods: 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. 

Results: RNFLT decreased significantly with increasing scan diameter (p< 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, p = 0.001; NS: AUC = 0.625, p = 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 (p = 0.020). Repeated-measures ANOVA revealed significant between-group effects in NS and NI (p < 0.01) and notable group × diameter interactions in NS and TS (p< 0.05). 

Conclusions: 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.

[Link to open access paper]