Fixed-diameter scan protocol preferable for retinal nerve fibre layer measurement by optical coherence tomography in all sizes of optic discs

Background: Retinal nerve fibre layer (RNFL) measurements using a fixed-diameter versus a user-defined scan-protocol of optical coherence tomography (Stratus OCT) were compared in 32 normal, 62 glaucoma suspects and 36 glaucomatous eyes. Methods: Peripapillary RNFL thickness was measured using the standard "fast'' RNFL scan-protocol and proportional 2.27 x disc scan protocol. Disc size was measured using the "fast'' optic disc protocol. The correlation between RNFL thickness for each scan-protocol and disc size was analysed. Results: In normal eyes, RNFL thickness was independent of the optic-disc area using the fixed-diameter protocol (p = 0.92) but was inversely proportional to disc size using the proportional protocol (p<0.001). In glaucoma suspects, the optic-disc area correlated with RNFL thickness using the fixed-diameter protocol (p<0.001). In the multivariate analysis in glaucomatous eyes, the RNFL thickness using the fixed-diameter protocol was significantly affected by the mean deviation on visual fields but not by disc area (p<0.001 and p = 0.64 respectively) Conclusion: In normal subjects, disc size does not appear to affect RNFL measurements by OCT using the fixed-diameter protocol, thus indicating that RNFL thickness may be related to distance from the centre of the optic disc rather than the margin. The thicker RNFL observed in large glaucomatous discs in this study may be related to the earlier stage of glaucoma, though it may not apply to the general population.