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J.N. Ver Hoeve, C.B. Y. Kim, T.M. Nork, B. Hennes, P.A. Kaufman, H.T. Whelan, J.T. Eells; Supernormal multifocal ERG after macular grid photocoagulation . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4239.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine functional changes in the retina following macular grid photocoagulation. Methods: A grid pattern of laser burns was created across the macula of one eye in each of six cynomolgus monkeys. The grid consisted of approximately 120 laser spots created by a 530 nm diode laser delivered through a slit lamp and Kaufman–Wallow contact lens. Laser parameters were 100 mW, 75 microns width, and 100 ms duration. Similar grid patterns were applied to all monkeys using retinal vessels as landmarks. Multifocal electroretinograms (mfERGs) were recorded using a 103 element unscaled stimulus array presented at a base rate of 13.3 ms. The mfERGs were grouped into three rings, excluding the foveal element, at eccentricities of 8, 16, and 24 deg from the fovea. The amplitude and implicit times (ITs) of individual first–order kernel waves were measured and the means and standard errors for each of three rings were calculated. Baseline mfERG recordings were made immediately prior to laser in 4 animals and 1 week prior to laser in 2 monkeys. mfERG testing was performed within 1 h following macular laser. Animals were sacrificed and retinal histology performed. Results: Within 1 h following delivery of the macular grid laser, the amplitude of the initial negative wave of the first–order kernel (IT=18–23 ms) increased significantly in each the 6 animals tested. The positive wave (IT=35–42 ms) was enhanced in 5/6 animals. ITs of the negative and positive mfERG waves were prolonged following laser. Subsequent testing of three animals at 4 and 11 days post laser revealed a marked decline in the amplitude of the major waves. ITs remained prolonged relative to pre–laser baseline. Histology of the retina showed that the spots were grade I or II, ablating the photoreceptors while leaving the proximal cell layers intact. Conclusions: Grid laser to the macula produces an initial supernormal mfERG, followed by a loss of response. Supernormal full–field cone ERGs have been reported in cases of retinal vessel obstruction and siderosis. It has been speculated that supernormal responses may reflect excitatory and possibly excitotoxic substances produced in response to the insult. The mfERG changes following laser appear out of proportion to the relatively small areas of photoreceptor loss evident histologically and suggest these burns are having a widespread effect on the retina. Grid macular laser in the non–human primate may be useful as a model for the study of mechanisms underlying transient supernormal retinal electrical response.
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