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Editors Selection IGR 19-2

Anatomical Structures: Peripapillary Choroid

Comment by Jost Jonas on:

75572 Thin minimal rim width at Bruch's membrane opening is associated with glaucomatous paracentral visual field loss, Taniguchi EV; Paschalis EI; Li D et al., Clinical Ophthalmology, 2017; 11: 2157-2167

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In their study, Taniguchi and colleagues report on the association between a low minimal neuroretinal rim width at Bruch's membrane opening and glaucomatous versus peripheral paracentral visual field loss. In discussing the results, one may consider that eyes with a paracentral visual field defect have a retinal nerve fiber layer (RNFL) defect relatively close to the temporal region of the optic nerve head, since the retinal ganglion cell axons associated with a paracentral visual field loss as compared to those associated with a peripheral visual field loss enter the optic disc closer to the disc-fovea line. Since in normal eyes the RNFL is usually the thinnest and the BMO-MRW is usually the smallest in the temporal optic nerve head region as compared to any other optic nerve head region (as it was the case in 90% of the control eyes also in the present study), any additional loss in retinal nerve fiber tissue will further reduce the anyway thin BMO-MRW in that area. The question arises therefore whether it might not have been anticipated that an eye with a glaucomatous paracentral visual field defect as compared to an eye with a peripheral glaucomatous visual field defect was expected to have a thinner minimal BMO-MRW. In an eye with a peripheral glaucomatous visual field defect, the loss in RNFL occurs at a location which before onset of the optic nerve damage is relatively thick so that a local RNFL loss may reduce the BMO-MRW at that location with a primarily relatively thick RNFL while the thinnest (or minimal) BMO-MRW is still be located, and would be unchanged, in the temporal optic nerve head region. If these thoughts are valid, the association between a paracentral glaucomatous visual field defect and a low minimal BMO-MRW could be explained by the spatial structural/functional relationship and it would not necessarily be that a very low minimal BMO-MRW value is a special biomarker for a paracentral versus peripheral glaucomatous visual field defect.

Another potentially confounding factor could have been a difference between the glaucoma eyes with paracentral visual field defects and glaucoma eyes with peripheral visual field defects in the amount of the optic disc rotation around its vertical axis. The vertical optic disc rotation increases with axial myopia and is associated with a thinning of the BMO-MRW in the temporal optic nerve head region. The refractive error or axial length and potential differences between the glaucoma groups in these parameters have not been mentioned in the article.

Also in view of the relatively small study samples, in view of the relatively high P-value of 0.03 for the difference between the eyes with early paracentral visual field loss versus eyes with peripheral visual field loss, and in view of potential selection artefacts due to the hospital-based recruitment of the study participants, one may infer that additional studies may be warranted to further strengthen the conclusions drawn by the authors.

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