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Editors Selection IGR 21-2

Progression: SAP, OCT, HRT Evaluation of RNFL progression

Comment by Vincent Michael Patella on:

45765 Evaluation of Retinal Nerve Fiber Layer Progression in Glaucoma. A Prospective Analysis with Neuroretinal Rim and Visual Field Progression, Leung CKS; Liu S; Weinreb RN et al., Ophthalmology, 2011;

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Leung et al. (750) found poor agreement in Retinal Nerve Fiber Layer (RNFL), optic nerve, and standardized automated perimetry (SAP) summary progression measures, both in terms of progression event detection and rate of change. The authors prospectively followed 108 eyes of 70 patients at four month intervals for at least 2.9 years, using SAP Visual Function Index (VFI), time domain Optical Coherence Tomography (OCT) mean RNFL thickness, and Heidelberg Retinal Tomography Neuro Retinal Rim area (NRR). They also reported large diversity among patients in progression rates for all three measurements.

As emphasized by the authors, detection and quantification of change depend critically upon measurement variability. We must wonder how well the three metrics might agree in a perfect world with perfect instrumentation. As the authors put it, 'The discordance in progression could be related to the fundamental differences in instrument variabilities and measurable dynamic ranges..' These findings serve to emphasize the conceptual difference between actual agreement ‐ which can only be known in a perfect world ‐ versus the measurements we can perform today using current, admittedly imperfect instrumentation.

Perhaps the most broadly important idea suggested in this paper has to do with correcting for RNFL and NRR floor effects in order to scale structural measurements according to available dynamic range. The authors used the minimum clinically observed finding as the measurement floor for each instrument, and the mean normal value as the nominal 100% level, thus making RNFL thickness, NRR area, and SAP findings at least conceptually more comparable. Further work to reduce measurement variability and to improve measurement comparability is needed and might lead to more efficient combination of diverse clinic clinical metrics into summary combination analyses as well.

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