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Nimesh Patel, Ronald Harwerth; Temporal Relationships in Structural Changes of the Optic Nerve Head and Macula in Experimental Glaucoma in Non-Human Primates. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3154.
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Assessment of structural changes in the retina and optic nerve head (ONH) with spectral domain optical coherence tomography (SD OCT) is important for the diagnosis and management of glaucoma. The present study was undertaken to investigate the temporal relationships of structural changes using a model of laser-induced glaucoma in rhesus monkeys, which parallels structural and functional defects in patients.
Unilateral Argon laser treatments were used to scar the trabecular meshwork and elevate the intraocular pressures in six monkeys. SD OCT transverse retinal scaling was calculated using a three surface schematic eye that incorporated individual ocular biometry measures. Radial and raster scans centered on the ONH and raster scans centered on the macula were used for morphological analysis of total retinal thickness and ONH cupping. Retinal nerve fiber layer (RNFL) area measures, both with and without removal of major retinal vascular, were determined from elliptical scan paths 550 µm from the ONH rim margin.
The earliest changes were in the ONH parameters. The maximum cup depth, increased linearly with increase in cumulative IOP (R2=0.09, p=0.03). In addition, an exponential decrease in neuroretinal rim (NRR) volume also occurred early (R2=0.78, p < 0.01). The decrease in NRR volume occurred prior to a change in global RNFL area, and the relationship was best described by an exponential model (R2=0.64, p<0.01). The percentage vascular contribution to the RNFL increased with decreasing area (R2 = 0.64, p < 0.01), but the overall vessel area decreased with increasing stages of glaucoma (R2 = 033, p < 0.01). A later decrease in macular volume in a central 3 mm circular zone was linearly correlated with the temporal RNFL area (R2 = 0.61, p<0.01).
Early structural changes in the ONH morphology were correlated to the cumulative IOP, which supports a biomechanical paradigm for the initial neuronal damage in glaucoma. Subsequent structural neuropathologic alterations represent a cascade from NRR volume to RNFL area and macular thinning. These results suggest that different aspects of neuropathy should be assessed with advancing stages in the progression of glaucoma.
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