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Editors Selection IGR 18-4

Clinical Examination Methods: Neuroretinal rim imaging

Comment by Ki Ho Park on:

51993 Enhanced Detection of Open-angle Glaucoma with an Anatomically Accurate Optical Coherence Tomography-Derived Neuroretinal Rim Parameter, Chauhan BC; O'Leary N; Almobarak FA et al., Ophthalmology, 2013; 120: 535-543

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Accurate quantification of neuroretinal rim tissue is challenging, due to the problematic nature of neuroretinal rim-cup and disc-margin determinations. Rim-width measurement utilizing a reference plane might, if the rim is located in the sloped sector of tilted discs, result in overestimation of the amount of axons. In some regions of the optic nerve heads furthermore, Bruch's membrane opening (BMO) extends toward the center of the disc, beyond the clinically identified disc margin, in which cases, the true rim is narrower than clinically evaluated.

Chauhan et al. has introduced a novel concept of defining the neuroretinal rim tissue: the BMO-minimum rim width (MRW), which is the minimum distance between BMO and the internal limiting membrane (ILM). This measurement modality is a more precise means of determining the amount of axons. Globally, BMO-MRW yielded better glaucoma-diagnostic performance than the other parameters, which included circumpapillary retinal nerve fiber layer thickness (RNFLT), BMO-horizontal rim width (BMO-HRW), the distance between BMO and the ILM on the BMO reference plane, and Moorfields Regression Analysis (MRA) parameters.

With the new structural parameter BMO-MRW, the total amount of axons can be represented more accurately than with circumpapillary RNFLT, because (1) the neural tissue is measured at BMO, which is closer to the damaged site than at the diameter of 3.46 mm circle; and (2) the width is measured perpendicular to the axons, which may overcome the measurement error due to an individual variation in the orientation of the neural tissue around the disc margin. This study had limited demographic data (age and MD only), as well as a smaller control sample size (n = 48) relative to the number of glaucoma patients (n = 107). A future study with a larger control sample size along with both patient- and control-group demographic data on the degree of disc tilt, the refractive error, the axial length, and the extent of peripapillary atrophy, will strengthen the present conclusions. Further, we expect, on the basis of the 48 radial scans performed in this study, that 360-degree scanning of BMO-MRW by automatic detection of BMO and ILM will offer, relative to manual detection, an enhanced glaucoma detection efficacy.

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