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Nabin Joshi, Emma Ly, Suresh Viswanathan; Intensity response function of the Photopic Negative Response (PhNR). Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):487.
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© ARVO (1962-2015); The Authors (2016-present)
The Photopic negative response (PhNR) is a slow negative potential of retinal ganglion cell origin in the cone mediate flash electroretinogram (ERG). We assessed the effect of age and test-retest reliability of the PhNR intensity response parameters in normal healthy subjects.
PhNRs were recorded using DTL fiber electrodes with a ganzfeld system (Diagnosys LLC), from 28 normal healthy subjects (23-71 years) with dilated pupils on two different days. Stimuli consisted of brief (<5 ms) red flashes ranging from -2.2 to 0.8 log phot cd.s/m2 on a 2 scot cd/ m2 blue background. The PhNR intensity response was fitted with Naka-Rushton equation with slope fixed at 1. The effect of age was evaluated on the saturated amplitude (Vmax) and semi-saturation constant (K - intensity at 50% Vmax) parameters derived from the fits. Test-retest reliability of these parameters was assessed with Bland-Altman analysis.<br />
K was positively correlated with Vmax (r=0.57, m=0.019, p=0.002) and both Vmax (r=0.5, m= -0.26, p=0.007) and K (r=0.51, m= -0.009, p=0.006) were negatively correlated with age. Bland-Altman analysis indicated a mean test-retest difference of -0.98 mv for Vmax -0.07 log cd.s/m2 for K that are clinically acceptable. The 95% limits of agreement (1.96 x SD of the test-retest difference) was 15.7 mv for Vmax and 0.7 log cd.s/m2 for K. The test-retest differences were not correlated with age or the magnitude of the measures.
The Vmax and K of the PhNR intensity-response function reduces as a function of age. The reduction in K with age is likely the consequence of the PhNR amplitude saturating at lower stimulus strengths. The average test-retest difference in these response parameters is small suggesting that the they could be potentially useful as an objective measure to explore progressive changes in inner-retinal function in disease states.
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