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G. Stifano, L. Ziccardi, L. Montrone, S. Moreni, G. Iarossi, L. Buzzonetti, A. Fadda, B. Falsini; Phase Changes of the Macular Focal ERG as a Function of Light Adaptation in Retinitis Pigmentosa . Invest. Ophthalmol. Vis. Sci. 2004;45(13):5102.
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Purpose:Previous studies1have shown threshold amplitude abnormalities of the macular focal electroretinogram (FERG) of retinitis pigmentosa (RP) patients under different light adaptation levels. FERG phase light adaptation, however, has not been evaluated in RP patients. Aim of this study was to quantify both amplitude and phase changes of macular FERG at increasing levels of light adaptation in RP. Methods:Macular (18 degrees) FERGs, elicited by 41 Hz sinusoidal uniform field flicker (modulation depth: 95%) on a steady background, were recorded at different levels of adapting mean retinal illuminance (RI) of both stimulus and background (2.8–3.7 Log photopic trolands, PhTr) in 14 typical RP patients (visual acuity: 20/40–20/20; visual field by Goldmann V/4e > 20 degrees) and 12 age–matched normal controls. Amplitude and phase of the Fourier–analyzed response fundamental were measured. Amplitude was quantified as gain ratio (amplitude/PhTr).2 Results:In control subjects, log amplitude gain ratio decreased linearly (mean slope = 1 log m V/log PhTr, SD = 0.04, r=0.99), and phase advanced progressively (mean slope = 50 deg/log PhTr, SD = 10, r = 0.98) with log RI. In RP patients, log gain ratio and phase displayed a shallower than normal slope (gain ratio, mean= 0.8 log m V/log PhTr, SD= 0.02; phase, mean 45 deg/log PhTr, SD= 15), with log RI. In seven out of 14 patients, phase function was substantially shallower (< 15 deg/log PhTr) compared to control values, or even flat. These phase changes were due to an advance, compared to control values, at lower RI levels, and could not be mimicked by conditions simulating a reduced number of photoreceptors or a loss in their quantum catching ability. Conclusions:Results suggest that, at least in some RP patients, adaptation abnormalities in the macular cone system cannot be explained simply by loss of photoreceptors or their reduced quantum catching ability. Other factors, including an increase in intrinsic background activity (intrinsic "noise") at photoreceptor level, and gain control abnormalities in post–receptoral retina, could account for these changes. 1.Seiple et al, IOVS 1993; 34:2638–2645; 2.Wu & Burns, JOSA 1996; 13: 647–657.
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