Comparison of magnification corrected optic disc size by slit-lamp biomicroscopy, fundus photography, and optical coherence tomography.

Purpose: Optic disc size measurement is essential for determining landmarks, lesions, and distances of the retina. Due to the size variability in nature and when measured with different instruments, newer methods to compare sizes between different instruments are needed. We aimed to determine the comparability of slit lamp, fundus photography, and optical coherence tomography (OCT) measurements of optic disc size after correcting for both ocular and instrument magnification.

Materials and methods: In this prospective, observational study, optic disc size was measured by slit-lamp biomicroscopy, fundus photography, and OCT. Instrument and ocular magnification was calculated, and the measured size was corrected for both using the Littman formula. The corrected values were analyzed using Bland-Altman plots and intra-class correlation (ICC) for agreeability and correlation, respectively. Linear regression analysis was conducted to estimate the systematic errors and interconvertibility in cases of high correlation.

Results: Fifty-six eyes were included for the analysis. Uncorrected and corrected values for each method had excellent correlation and agreeability (ICC absolute agreement >0.75, P < 0.01). Among magnification corrected values, slit lamp and OCT had excellent correlation and agreement (ICC consistency = 0.846, confidence interval 0.75-0.91, P < 0.01). This pair had R ² = 0.73 on linear regression (P < 0.01). Linear regression formulas for interconverting uncorrected and corrected values for the three methods had variable accuracy (R ² from 0.47 to 0.66).

Conclusion: Uncorrected values of different methods for disc size measurement cannot be compared directly; they need ocular and instrument magnification correction. Among the three methods compared, the best interconvertible pairs of measurement were magnification corrected slit lamp and OCT values.