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Editors Selection IGR 19-4
Anatomical Structures: Neuroretinal Rim Assessment after Trabeculectomy
Comment by Massimo Fazio on:
78205 Structural Reversal of Disc Cupping After Trabeculectomy Alters Bruch Membrane Opening-Based Parameters to Assess Neuroretinal Rim, Gietzelt C; Lemke J; Schaub F et al., American Journal of Ophthalmology, 2018; 194: 143-152
This retrospective study quantified longitudinal changes in Bruch membrane opening minimum rim width and area (MRW and MRA, as defined by Gardiner et al.1) as measured by a Spectralis SD-OCT following trabeculectomy with mitomycin C at three, six, and 12 months.
The primary finding of this study was that MRW and MRA significantly increased post trab at three months (p = 0.012), at six months (p = 0.007), and at 12 months (p=0.010). Importantly, RNFL thickness remained stable between baseline and follow-up at three, six, and 12 months and showed a moderate loss after 18 months (p = 0.021) of follow-up. No statistically significant changes in global visual field after trab were observed.
By now, it has been investigated by many whether structural changes of the optic nerve head (ONH) following IOP reduction by trabeculectomy are associated with betterments in the visual field function. This study confirms that, once again,2,3 structural changes of the ONH following trabeculectomy are correlated with the magnitude of change in IOP but not with visual function.
Of particular interest in this study is that RNFL thickness was not affected by IOP reduction, while MRA was. The authors do not stretch their discussion to try to provide a possible explanation of this observation.
MRA is an interesting novel parameter that attempts to provide a surrogate measure for the number of axons entering the optic nerve head. It has been shown to be better correlated to RNFL thickness than rim area measured by confocal scanning laser ophthalmoscopy (CSLO).1 The area depicted by MRA is inclusive of both neural tissue (axons) and non-neural tissue (primarily vasculature). The fraction of volume described as vasculature is mainly constituted by blood. Blood, being a fluid, provides no resistance to the mechanical deformations induced by IOP. Consequentially, under varying IOP, vascular tissue in the ONH deforms more than the surrounding tissue, as recently quantified by Fazio et al.4 While reduction in mechanical stretching following trabeculectomy of the rim tissues5 would certainly contribute to the sensitivity of rim morphology to IOP, because of the weak resistance to compression of vascular tissue, one would wonder if findings like in this study of changes in MRA or MRW following IOP reduction are a consequence of deformations in the vasculature tissue first of anything else. Deformations of the ONH vascular tissue under varying IOP has been scarcely investigated. Novel mechanistic models6 postulate that vasculature compression under increasing IOP may be a primary factor determining blood-flow autoregulation and provide a plausible explanation to otherwise controversial findings on changes in hemoregulation following trab.7,8
The contribution of vasculature anatomy to ONH morphologic changes with IOP and glaucoma progression is currently poorly understood. The rise of OCT-angiography, with its ability to separate structural from vascular tissue, will certainly facilitate an explanation to findings like those proposed by this study.
References
- Gardiner SK, Ren R, Yang H, et al. A method to estimate the amount of neuroretinal rim tissue in glaucoma: comparison with current methods for measuring rim area. Am J Ophthalmol. 2014;157(3):540-9 e1-2.
- Waisbourd M, Ahmed OM, Molineaux J, et al. Reversible structural and functional changes after intraocular pressure reduction in patients with glaucoma. Graefes Arch Clin Exp Ophthalmol 2016;254(6):1159-1166.
- Kiessling D, Christ H, Gietzelt C, et al. Impact of ab-interno trabeculectomy on Bruch's membrane opening-based morphometry of the optic nerve head for glaucoma progression analysis. Graefe's Arch Clin Exp Ophthalmol. 2018. doi: 10.1007/s00417-018-4187-2.
- Fazio MA, Clark ME, Bruno L, Girkin CA. In vivo optic nerve head mechanical response to intraocular and cerebrospinal fluid pressure: imaging protocol and quantification method. Scientific Reports. 2018;8(1):12639.
- Girard MJA, Beotra MR, Chin KS, et al. In Vivo 3-Dimensional Strain Mapping of the Optic Nerve Head Following Intraocular Pressure Lowering by Trabeculectomy. Ophthalmology. 2016;123(6):1190-1200.
- Guidoboni G, Harris A, Cassani S, et al. Intraocular Pressure, Blood Pressure, and Retinal Blood Flow Autoregulation: A Mathematical Model to Clarify Their Relationship and Clinical Relevance. Investigative Ophthalmology & Visual Science. 2014;55(7):4105-4118.
- Trible JR, Sergott RC, Spaeth GL Trabeculectomy is associated with retrobulbar hemodynamic changes. A color Doppler analysis. Ophthalmology. 1994;101:340-351.
- Cantor LB. The effect of trabeculectomy on ocular hemodynamics. Trans Am Ophthalmol Soc. 2001;99:241-252.
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