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Editors Selection IGR 11-3

Optical Coherence Tomography: Influence of eccentric scanning of OCT

Comment by Marco Vizzeri on:

24190 The influence of eccentric scanning of optical coherence tomography on retinal nerve fiber layer analysis in normal subjects, Yoo C; Suh IH; Kim YY, Ophthalmologica, 2009; 223: 326-332

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Imaging technology has rapidly become the status quo for an objective assessment of the retinal nerve fiber layer (RNFL). With Stratus OCT, for example, a circle scan of 3.4 mm diameter can be positioned on the optic disc to directly quantify average and sectoral RNFL thicknesses and to compare the results to those of an internal normative database. Unfortunately, because automatic scan registration is not available, the process of placing the circle scan evenly around the disc at each visit relies in the ability of the operator with the help of a landmark feature. Yoo et al. (1107) evaluated the changes occurring to average and sectoral RNFL thickness measurements in healthy eyes when the scan is purposely misaligned in four different directions, along the vertical axis (superior-inferior) and along the horizontal axis (nasal-temporal), so that the circle scan approaches the disc margin in one sector. Predictably, scan displacement prompted more sectors to fall outside normal limits compared to the results of the normative database. This is likely due to the fact that the greater the distance from the disc margin, the thinner the RNFL thickness. Interestingly, the authors were able to confirm the results of a previous study,¹ conducted in a similar fashion, that showed that average RNFL thickness significantly increased compared to centered scans only when the scan was displaced temporally along the horizontal axis. Scan displacement in other directions produced little or no change in the average RNFL thickness. These results should warrant a careful search for a possible explanation. If RNFL thickness decreases further away from the disc in one sector, while it increases closer to the disc margin in the opposite sector by a similar magnitude, average RNFL thickness should remain relatively unchanged.

Clinicians should be aware of the effect of scan misalignment when interpreting Stratus OCT RNFL thickness measurements

This proved true for all but the temporally displaced scans. Can RNFL thickness changes in other sectors account for some of these findings? Surprisingly, in this study quadrants or clock hours changes in RNFL thickness measurements due to scan misalignment were not quantified, neither the authors managed to provide a reasonable explanation as to why an increased average RNFL thickness occurred to temporally displaced scans only. However, it is well known that the superior and inferior arcuate axon bundles are thicker temporally than nasally resulting in added average thickness with temporal displacement. Further studies with Spectral Domain OCT devices will better reveal the pattern of the RNFL in an area adjacent to the optic disc that is not limited to a circle of a fixed diameter. Nevertheless, clinicians should be aware of the effect of scan misalignment when interpreting Stratus OCT RNFL thickness measurements for glaucoma diagnosis and for monitoring disease progression.

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