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Editors Selection IGR 24-3
Optical Coherence Tomography: Optic disc margin anatomy
Comment by Kouros Nouri-Mahdavi on:
48867 Optic disc margin anatomy in patients with glaucoma and normal controls with spectral domain optical coherence tomography, Reis AS; Sharpe GP; Yang H et al., Ophthalmology, 2012; 119: 738-747
Reis and colleagues presented SD-OCT findings in 30 glaucomatous eyes (with generalized rim loss, focal damage, and senile sclerotic damage) and ten control eyes regarding the anatomical structures corresponding the optic nerve head (ONH) border as observed clinically on ONH photographs. Contrary to our belief that the disc border is a uniform anatomical landmark, they reported that the SD-OCT (in-vivo histological) structure observed as the edge of the ONH varied along the circumference of the disc border and consisted of either the edge of the Bruch's membrane (BM), some part of the Border Tissue of Elschnig (BTE) or a combination thereof. Various configurations of the BTE were observed in individual eyes and across the study eyes and a trend was seen for the inferior and temporal regions of the ONH border to demonstrate an externally oblique BTE configuration as compared to the superior and nasal regions. Interestingly, an externally oblique BTE was more frequent in eyes with focal glaucomatous disc damage as opposed to eyes with generalized rim loss or senile sclerosis. In contrast to findings in monkeys, the edge of BM was not visible in humans despite the fact that an overhang of the BM beyond the clinically visible disc border was a frequent finding in the study eyes.
The findings put forward by Reis et al., and their potential implications have revolutionized our concepts regarding definition of the ONH border. Our simplistic belief that the scleral ring is a homogeneous structure around the disc is simply negated by the above findings. What are the other implications of Reis et al.'s findings? Externally oblique BTE was seen mostly in the inferotemporal region where glaucomatous damage most commonly occurs. This begs the question whether the configuration of the BTE or the relationship of the BM edge to the BTE and to the overlying retinal nerve fiber layer are related to the magnitude of glaucomatous damage. Given the limited sample size, the authors did not address this question. We need to further explore the relationship between the BM edge and BTE among various ethnicities or whether it varies according to disc size or refractive error/axial length of individual eyes? The ONH junction to the posterior eye wall is frequently oblique, causing various degrees of vertical tilt. It has been recently shown that the BM stretches away from the edge of the BTE (or the clinical border of the ONH), that an externally oblique configuration to BTE is commonly present and that some part of BTE is frequently identified as the clinical ONH border in highly myopic individuals. It does not seem that any eyes with significant vertical tilt were included in the study, but the role of BM/BTE topography and configuration with regard to glaucomatous optic neuropathy is of significant interest and warrants further research.
This new definition of ONH border can significantly affect neuroretinal rim measurements
As the authors have rightly mentioned, this new definition of ONH border can significantly affect neuroretinal rim measurements. The same investigators have recently presented data (as yet unpublished) that revised rim area measurements (minimum rim width as measured from the edge of the BM), actually improve detection of glaucoma by SD-OCT.
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