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Patrick A. Sibony, Mark J. Kupersmith; Geometric Morphometrics Of The Peripapillary RPE Layer By OCT. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3004.
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© ARVO (1962-2015); The Authors (2016-present)
Geometric Morphometrics (GM) is an established statistical framework used in biology and paleontology to analyze shapes. Shape is defined as the geometric property that remains after filtering out position, scale and rotation. We applied this method in a novel way to analyze the peripapillary retinal-pigment-epithelium Bruch's-membrane layer (RPE/BM) imaged on SD-OCT raster in papilledema, anterior ischemic optic neuropathy (AION) and normals. We suggest that the RPE/BM layer mirrors the shape of the load bearing structures of the optic disc (lamina cribosa, sclera) and may provide insights into the biomechanics of disc edema and intracranial hypertension.
We compared three groups: (i) 30 normal optic discs (ii) 15 AION and (iii) 15 papilledema with Idiopathic Intracranial Hypertension. The methodology follows: 1.Image acquisition and processing; 2.grid placement of 20 landmarks along the RPE/BM spanning 2500 microns on each side of the neural canal opening (NCO); 3.Rohlf's tps-Software to perform a Generalized-Least-Squares Procrustes Superimposition on digitized landmarks and produce a thin plate spline and 4. Principle Component Analysis and 5.Permutation statistics.
The RPE/BM layer in normals and AION has a V-shape that gently steepens (away from vitreous) as it approaches the NCO. The RPE/BM in papilledema has an inverted-U shape, skewed nasally inward (toward vitreous). The difference was significant (pap vs nl p=.001 and pap vs AION p=.01). There was no difference between normals and AION. The shape correlates with RNFL thickness in papilledema but not in AION. The NCO was widened in both AION and papilledema compared to normals (using GM / length measurements). Sequential OCTs, pre and post treatment shows that the inverted - U resumes a normal V-shape with weight loss or shunting.
The differences in RPE/BM shape between patients with papilledema and normals or AION are statistically significant. The V-shape of the normal peripapillary RPE/BM defined on the OCT 5-line-raster in normals is statistically indistinguishable from AION. The inverted-U shape in papilledema cannot be explained by the presence of disc edema alone. We suggest that the difference is due to both translaminar pressure gradient (IOP - CSF) and the material properties of the peripapillary sclera and lamina cribosa. GM is potentially useful to quantitatively asses the shape of the globe.
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