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Michael J. Girard, Lyn Zimmo, Edward White, Jean Martial Mari, C Ross Ethier, Nicholas G. Strouthidis; 3D Deformation Mapping of the Human Optic Nerve Head In Vivo. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3182.
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© ARVO (1962-2015); The Authors (2016-present)
To map the 3D deformations of the human optic nerve head (ONH) in vivo following IOP-lowering by trabeculectomy (TE).
3 high-tension + 1 normal-tension glaucoma patients (pre-TE IOPs of 36, 24, 23, and 14 mmHg, respectively) were imaged using spectral domain optical coherence tomography (OCT; Heidelberg Spectralis) (< 5 days) before and (< 5 weeks) after TE. TE reduced IOP by 24 mmHg (67%), 13(54%), 6(26%), and 4(29%), respectively. At each imaging session an OCT scan of the ONH was acquired with 145 B-scans, each composed of 384 A-scans and averaged 9 times for speckle noise reduction. OCT image quality was improved by removing shadow artefacts and by increasing deep structure visibility using our compensation algorithm (IOVS 2011; 52:7738-48). Each pre-TE OCT volume was manually segmented to isolate: 1) retina and neural tissues; 2) choroid and Bruch’s membrane; 3) sclera; 4) lamina cribrosa (LC). Segmented ONHs were then split into smaller elements using c. 7,000 equally-spaced points covering all visible structures. An OCT-based 3D tracking algorithm, robust to speckle noise, was developed and validated (in submitted companion abstract). For each patient, this algorithm was applied to both pre- and post-TE OCT volumes to extract IOP-induced 3D displacements (referenced to the plane of Bruch’s membrane opening) at the defined points. Finally, strain (a measure of tissue stretch) was derived from the displacements and mapped in 3D for each ONH.
Following IOP lowering: 1) the mean LC displacement was 11.3, -39.4, 5.6 and 3.8 μm, respectively (a negative sign = anterior movement); 2) the mean LC compressive strains were -2.66%, 6.02%, -2.29% and -0.08% (a negative sign = thickening); 3) the scleral canal contracted by 1.23%, 2.55%, 0.65%, and 0.97%, respectively (Figure).
We present here novel engineering tools that can map 3D deformations of the ONH in vivo for the first time. We also demonstrate that ONH deformations are detectable in vivo and that TE can reduce scleral canal size and relieve compressive strains in the LC in almost all cases. Strain relief in the LC (to various degrees) could explain why glaucomatous progression may be slowed following TE. Our goal is to use these technologies to establish a relationship between ONH deformations and visual field loss.
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