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Editors Selection IGR 15-1

Anatomical Structures: Peripapillary atrophy

Comment by Claude Burgoyne on:

52740 Microstructure of parapapillary atrophy: beta zone and gamma zone, Dai Y; Jonas JB; Huang H et al., Investigative Ophthalmology and Visual Science, 2013; 54: 2013-2018

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In this report, Dai and co-authors co-localize clinical optic disc photographs to the CSLO infra-red (IR) image associated with Spectralis SDOCT horizontal B-scan data sets, so as to test hypotheses regarding the size of 'gamma' and 'beta' peripapillary 'atrophy' in glaucoma and normal human eyes. Eighty normal eyes of 46 subjects and 80 eyes of 46 patients were studied. Gamma zone 'atrophy' was defined to be regions outside of the clinically defined disc margin (in clinical photos) that do not have Bruch's Membrane (BM) by SDOCT. Betazone atrophy was defined to be regions outside of the gamma zone in clinical photos in which BM does not have RPE (by SDOCT). Using multivariate strategies, they found gamma zone 'atrophy' to be significantly associated with longer axial length, longer vertical disc diameter, older age and the absence of glaucoma. Beta zone atrophy was associated with longer axial length and the presence of glaucoma.

While this is a serious study, there are several methodological and conceptual issues which undermine the importance of its findings. First, radial B-scans were not employed to characterize these anatomic relationships. Radial B-scans centered on Bruch's membrane opening (BMO) would provide the most geometrically appropriate (i.e., perpendicular to the tissues being measured) and statistically robust (i.e., equally distributed) A-scan pattern. Second, by using a description of disc size that is based on the clinical disc margin, the reported disc size and shape measurements, instead of being based on SDOCT anatomy, remain clinical estimates that have no anatomic meaning. This is an issue that all investigators now face. What do we anatomically mean by disc size and shape? Should it be BMO which can be clinically invisible within regions of most eyes? Or should it be the anterior scleral canal opening (ASCO), which is clinically invisible in most human eyes. By failing to address this issue with SDOCT anatomy, this study confounds rather than clarifies this issue. Third, and most seriously, the authors fail to recognize that what they describe as gamma zone 'atrophy' is either externally oblique border tissues of Elschnig (BTE ‐ Figure 2 of the article) alone or externally oblique scleral canal and BTE in combination (Figure 1 of the article). Neither of these tissues ever had BM or RPE present and are only rarely pigmented (in the setting of a pigment crescent as previously described by Shields). Calling these tissues 'atrophic' is therefore inappropriate. The fact that there is no relationship between the presence of these tissues and glaucoma emphasizes this point. These tissues, when present and clinically visible, should be referred to as externally oblique border tissues and sclera. As the authors suggest, a series of papers by Strouthidis on monkeys and Reis on humans have described their presence. The nasally oblique passage of the scleral canal required for the retinal ganglion cell axons to achieve the mid-line chiasm is a natural explanation for their presence on the temporal side of the disc (in most eyes). Finally, as the authors suggest and as recently described by Kim and colleagues, progressive, temporal displacement of BMO relative to the underlying ASCO, with increasing axial length, likely explains their association with axial length. This field of study needs to bear these anatomic relationships in mind as it moves forward using SDOCT.

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