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Pranav H. Patel, Robert A. Honkanen, Crystal Kania, Patrick A. Sibony, F. J. Rohlf; Geometric Morphometrics Analysis of Peripapillary Retinal Pigment Epithelium/Basement Membrane and Optic Disc Surface in Glaucoma Patients with Acute IOP Changes. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2834.
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Geometric morphometrics (GM) is a framework used to analyze shape variation (SV). We hypothesize that ppRPE/BM layer mirrors the shape of the load bearing structures of/around the optic disc and may provide insight into the biomechanics of optic neuropathy due to glaucoma (Glc). Here the effect of acute changes in IOP on SV of the ppRPE/BM and optic disc surface (ODS) in eyes with Glc is studied.
Fifteen eyes of 14 Glc patients undergoing filtering surgery were examined. Methodology follows: 1. Image acquisition and processing; 2. Grid placement of 14 landmarks along the ppRPE/BM spanning 1667 microns on each side of the neural canal opening (NCO); and placement of 10 evenly spaced landmarks along the ODS spanning the NCO; 3. Rohlf's tps-Software to perform a Generalized-Least-Squares Procrustes Superimposition on landmarks and produce a thin plate spline and 4. Principle Component (PC) Analysis and 5. Permutation statistics. OCT images taken for each eye during higher/lower periods of IOP were compared in a paired manner.
Mean IOP was 24.1/10.7 mmHg in the high/low groups respectively (p<0.0001). Permutation tests (10,000) showed a statistically significant SV difference in ppRPE/BM during times of lower IOP compared to higher IOP (p=0.01). 5 patterns of SV in the ppRPE/BM were seen: Group 1: 5 eyes with steepening of the V shape of ppRPE/BM (VSRPEBM) and narrowing of the NCO; Group 2: 5 eyes with VSRPEBM flattening and slight narrowing of the NCO; Group 3: 2 eyes with VSRPEBM steepening without change in NCO width; Group 4: 2 eyes with minimal VSRPEBM steepening and slight enlargement of the NCO; Group 5: 1 eye with VSRPEBM flattening with slight enlargement of the NCO. These patterns of SV were not related to magnitude of IOP change. The magnitude of SV change, classified as small, medium or large, also was not dependant on amount of IOP reduction. SV of the ODS in the high/low pressure groups did not show any significant change.
1. ppRPE/BM shape is statistically different, while ODS shape is not when IOP is acutely lowered in Glc patients.2. Five patterns of ppRPE/BM SV not dependant on the degree of IOP lowering were seen. 3. The optic disc and surrounding sclera/lamina cribosa compose a complex structure with varying responses to IOP lowering underscoring the importance for a better understanding of the biomechanical properties of these tissues.
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