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T.M. Nork, C.B. Kim, M.J. Lucarelli, L.A. Levin, P.L. Kaufman, J.N. Ver Hoeve; Origin of the Multifocal Electroretinogram (mfERG) N1 Waveform: Clues From Eyes With Central Retinal Artery Occlusion (CRAO) and Optic Nerve Transection (ONT) . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2698.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To qualitatively compare the mfERG in CRAO and ONT in order to learn more about the origins of the N1 waveform. Methods: A 38 year-old woman underwent removal of an orbital lymphangioma and suffered a CRAO with complete loss of vision in the eye. mfERG testing was performed 6 months after the operation. ONT was carried out on two rhesus monkeys. The first one was uncomplicated, showing good perfusion of the retinal vessels immediately following the procedure. mfERG and full-field ERG testing was done 1 and 6 months later, respectively. The ONT on the second animal was complicated by an inadvertent CRAO, as determined by the observation of acute retinal whitening and immediate reduction of the full-field ERG b-wave performed one hour after the ONT. Subsequent electrophysiologic testing was done at various times on this animal during the year following ONT. One year after the CRAO + ONT, this eye was injected intravitreally with tetrodotoxin (TTX) followed 2 hours later by aminophosphonobutyric acid (APB). Results: In the human eye, the 1st order kernel of the mfERG ring averages showed partial preservation of the N1 component and marked reduction of all later waveforms. The 2nd order kernel was markedly reduced. Similar changes in both the 1st and 2nd order kernels were noted in the monkey with CRAO + ONT. By contrast, the animal with ONT alone showed strong N1 and P1 waveforms. However, this monkey had a markedly reduced N2-P2 complex and a greatly diminished 2nd order kernel response. Intravitreal injection of TTX followed by APB resulted in no appreciable change in the N1 waveform in the CRAO + ONT animal. The full-field ERG of the CRAO + ONT animal showed an a-wave and a greatly truncated b-wave. Prominent a- and b-waves were evident in the ONT-only animal. Conclusions: CRAO is well known to result in loss of the retinal ganglion cells (RGCs) and most of the cells of the inner nuclear layer, whereas the damage following ONT is largely confined to loss of the RGCs and their axons. Therefore, the mfERG responses in CRAO should be owing almost entirely to photoreceptors. Our finding that the N1 waveform remained in both human and monkey CRAO suggests that, as with the a-wave of the full-field ERG, the photoreceptors provide an important contribution to N1.
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