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R. Malik, P. G. Schlottmann, A. Carter, C. R. Hogg, G. E. Holder, D. F. Garway-Heath; The Pattern ERG and Photopic Negative Response of the Flash ERG For the Identification of Glaucomatous Optic Neuropathy. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1554.
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The Pattern ERG (PERG) and, more recently, the Photopic Negative Response (PhNR) of the flash ERG are indicators of inner retinal function and mean amplitudes in glaucomatous eyes are reduced compared to normal eyes. In contrast to the PERG, the recording of the PhNR does not require meticulous refractive correction, measurements can be obtained in patients with poor fixation and the signal to noise ratio is higher. The aim of the present study was to compare the sensitivity of the Steady-State (SS) PERG and the PhNR for identifying functional loss associated with glaucomatous optic neuropathy (GON).
One eye each of 30 patients with GON and 23 healthy volunteers was tested. GON was defined on the result of the Moorfields Regression Analysis (MRA) of the HRTII as abnormally low rim area (p>0.01) in at least one of the pre-defined optic disc sectors and raised Intra-ocular Pressure (IOP> 21 mmHg). The SS PERG was recorded with a large field size (36 by 27 degrees) and 2 different check sizes (0.8 degree and 13 degree) at a stimulus frequency of 16 reversals per second. The PERG ratio (PERG amplitude for 0.8 degree check/ PERG amplitude for 13 degree check), which has been recommended for the identification of glaucoma (Bach 2001. Electrophysiological approaches for the early detection of glaucoma. Eur J Ophthalmol.11), was computed. For the ERG, a red flash was presented on a blue background following pupillary dilatation and light adaptation. The PhNR was measured from the baseline to the trough of the negative response following the b-wave. Receiver Operator Characteristic (ROC) curves for the detection of GON were plotted for SS PERG ratio and the PhNR.
Both the mean (± SD) SS PERG ratio and the mean absolute PhNR amplitude were reduced (p<0.001) in glaucomatous eyes (PERG ratio = 0.88 ± 0.22, PhNR = -17.3 ± 12.8) compared to healthy eyes (PERG ratio = 1.10 ± 0.28, PhNR = -28.4 ± 10.3). The total ROC area was similar (p=0.61) for the SS PERG ratio (0.73) and PhNR (0.77). However, the partial area under the ROC (for specificity 80-100% inclusive) was greater (0.11 for the PhNR and 0.04 for PERG ratio) and the sensitivity to detect GON at a fixed specificity of around 90% was higher (p<0.05) for the PhNR (57%) than the SS PERG ratio (17%).
For the identification of GON, the PhNR is a more sensitive indicator of functional loss at higher specificities. Given the relative ease of recording the PhNR, we recommend measurement of the PhNR in patients referred for electrophysiological evaluation of GON.
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