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RJ Dhyanchand, CB Y Kim, JN Ver Hoeve, PL Kaufman, MJ Lucarelli, LA Levin, TM Nork; Functional Relationships between Test IOP, Duration and Two Features of the mERG Waveform in Experimental Chronic Glaucoma . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3905.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To test an hypothesis of outer retinal ischemia in chronic experimental glaucoma, which predicts that some of the changes found in the mERG waveform are reversible. Methods: Experimental glaucoma was induced in 3 young adult rhesus monkeys by unilateral laser trabecular destruction (LTD). One of these animals had undergone an optic nerve transection (ONT) in the same eye 44 days prior to the first LTD. Median intraocular pressures (IOPs) were 41 mmHg (range, 10-72), 37.5 (14-61) and, for the ONT animal, 29 (13-45). Subtotal cupping developed in both of the animals with glaucoma alone. Periodic multifocal electroretinographic (mERG) testing was done for up to 1.75 years following induction of glaucoma. mERGs were recorded with the use of 103 equal-sized hexagonal elements in a stimulus array, which subtended ± 44 deg about the central visual axis. Mean luminance of the stimulus display was 100 cd/m2. Testing was sequentially monocular with the nontested eye covered with an opaque occluder. Relative latencies for an early (2-12 ms) feature of the first order kernels of all 103 traces (N1 implicit time) and then the variability of the 103 implicit times was determined. The root mean square amplitude of the prominent N1-P1 feature occurring between 9-35 ms (N1-P1 RMS) was calculated. Linear regressions of both waveform features on either intraocular pressure at the time of testing or against the duration of experimental glaucoma were carried out. Results: For the two animals with experimental glaucoma alone, significant positive associations were found between IOP and N1 implicit time variability. In one of these animals there was also an association between IOP and duration. N1-P1 RMS, by contrast, was associated with duration in both animals and with test IOP in one animal. For the one animal with LTD preceded by ONT, a significant association was found between N1-P1 RMS and duration. N1-P1 RMS was also correlated, albeit less strongly, with IOP. Conclusions: A linear association between IOP and implicit time and an amplitude component of the mERG, despite marked fluctuation in IOP over the course of 1.75 years, suggests that some of the mERG waveform alterations in chronic experimental glaucoma are reversible—such as might be caused by outer retinal ischemia. In addition, correlations of the N1-P1 RMS mERG waveform with duration of glaucoma, and following ONT, suggests that there is also a cumulative effect—perhaps owing to dying ganglion cells.
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