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J.N. Ver Hoeve, C.B. Y. Kim, T.M. Nork; mfERG and mfVEP Following Laser Axotomy in CynomolgusMonkeys . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3513.
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Purpose: To determine the effectiveness of retinal ganglion cell axotomy produced by laser ablation delivered in a ring around the optic nerve head. Methods: Two adult male monkeys (one cynomolgus and one rhesus) underwent unilateral laser axotomy. A ring of laser burns was placed 0.5 disk diameters from the rim of the optic nerve. The burns were produced by a 532–nm diode laser 200–500 mW, 200–500 microns @ 500 ms delivered through a slitlamp and condensing lens. Burns were contiguous except for small regions occupied by blood vessels. The clinical appearance was consistent with a full–thickness burn. Retinal function was assessed by multifocal electroretinography (mfERG). As a measure of retinal ganglion cell function, a multifocal visual evoked cortical potential (mfVEP) was also obtained using a large seven–element a hexagonal stimulus array. Animals were anesthetized with pentobarbital, refracted for viewing distance. First (K1) and second–order, first slice (K2.1) mfERGs and mfVEPs were extracted using VERISTM 4.1 software. Electrophysiologic testing was performed just prior to, immediately (<2 h_after laser axotomy, and for a follow–up period of up to 5 months. Results: Immediately following laser axotomy the mfERG in the region surrounding the optic nerve was reduced in amplitude, particularly of the N2–P2 component of the K1 mfERG. In contrast, the overall mfERG amplitude of the macular region, which was not directly lasered, increased above baseline measures. The mfVEP of the lasered eye, was indistinguishable from noise immediately following laser axotomy. Within 3 weeks the K1 mfERG from the retina regions that were lasered remained depressed and the areas of enhanced response in the macular region returned to normal levels. The positive waves of the K2.1 mfERG (P1–N1–P2 complex) were also selectively reduced in all regions. The mfVEP in the laser axotomy eye remained flat whereas the mfVEP from the fellow eye was consistently robust. Conclusions: Laser axotomy avoids risky orbital surgery and achieved a nearly complete destruction of ganglion cell axons as evident by the absent mfVEP. However, laser axotomy does destroy a small portion of the retina and the need to avoid vessels may result in less than less than total destruction of ganglion cell axons. Changes in the mfERG include both local changes in the region of the laser and remote changes as evidenced by the enhanced macular response and eventual reduction of the N2–P2 component of the K1 mfERG. The mechanisms underlying these changes are not known and could include removal of ganglion cell influence as well as initiation of excitatory/excitotoxic responses.
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