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Michael Girard, Nicholas Strouthidis; Peripapillary Sclera (ppS) and Lamina Cribrosa (LC) 3D Strain Mapping in High- and Normal-tension Glaucoma Patients following IOP Lowering by Trabeculectomy (TE). Invest. Ophthalmol. Vis. Sci. 2013;54(15):3157.
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© ARVO (1962-2015); The Authors (2016-present)
To experimentally measure and compare ppS and LC strains (deformations) in the eyes of high- and normal-tension glaucoma patients following IOP-lowering by TE.
3 high-tension (pre-TE IOP: 27.7 ± 7.2 mmHg; post-TE IOP: 13.3 ± 3.2 mmHg) and 3 normal-tension (pre-TE IOP: 11.7 ± 2.5 mmHg; post-TE IOP: 7.0 ± 2.6 mmHg) glaucoma patients were imaged (1 treated eye per patient) using optical coherence tomography (OCT; Heidelberg Spectralis) (< 5 days) before and (< 5 weeks) after TE. At each imaging session an OCT scan of the ONH was acquired (384 x 496 x 145 voxels) and averaged 9 times to reduce noise. In each post-TE OCT volume, 4 groups of tissues were manually segmented: the pre-laminar tissue, the choroid, the ppS and the LC. Segmentations were conducted only when tissue was visible as detected from the OCT signal. Full-thickness segmentation of the LC and ppS could not be achieved. Segmented ONHs were meshed and each comprised approximately 50,000 tetrahedrons and 5,000 nodes. For each ONH, a 3D tracking algorithm was applied to both post- and pre-TE OCT volumes to extract IOP-induced 3D displacements (referenced to the plane of Bruch’s membrane opening) at the defined nodes. Displacements were filtered, then smoothed to remove noise. 3D strains (1st principal component) were then computed from the displacements and averaged in the ppS and LC for each ONH. Here, strains represent the amount of deformation relief following TE.
Following IOP lowering and for all ONHs, LC strains (4.2 ± 3.3%) were on average higher than those of the ppS (1.9 ± 0.7%). No statistical difference was observed when comparing ppS strains from high-tension glaucoma (2.1 ± 0.5%) to those from normal-tension glaucoma (1.9 ± 1.0%). Similarly, no statistical difference was observed when comparing LC strains from high-tension glaucoma (5.1 ± 4.9%) to those from normal-tension glaucoma (3.3 ± 1.3%).
Our data suggest that TE in normal-tension glaucoma patients can relieve ppS and LC strains of the same order as those relieved in high-tension glaucoma patients. Our data also suggest a wide patient-to-patient variability in ONH biomechanical properties. We believe that in vivo measurements of ppS and LC strains could become powerful predictors of glaucoma progression. More patients are needed to strengthen the outcome of this work.
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