The Area Under the Waveform as an Alternative Measure of the Photopic Negative Response

Purpose: To explore how the photopic negative response (PhNR), measured as an area defined by the ERG waveform can be used as complementary or an alternative measure to the traditionally measured PhNR amplitude. Methods: A retrospective chart review and data analysis of light-adapted 3.0 ERG records in normal subjects patients aged 18 and older undergoing routine ERG testing was conducted. A new measure was analyzed: area defined by the PhNR curve. It was obtained in two ways: as an area under the ERG waveform (AUW) and an area above the ERG waveform (AAW) with starting points defined by the b-wave peak. A linear regression was conducted between PhNR1 amplitude (trough before i-wave), PhNR2 amplitude (trough after i-wave), PhNR AUW, and PhNR AAW. Furthermore, a prediction analysis based on AUW/AAW was conducted where the strongest correlated measures were used to predict the PhNR2 amplitude. Results: The ERG recordings of 70 patients/135 eyes (52F/18M,average age: 49.2 ± 15.3 years) and six healthy subjects (1F/5M, age between 22 and 58) were included in this study. 26 patients had well-defined PhNR2 troughs and were used for prediction analysis. There was good correlation between PhNR1 and PhNR2 amplitudes (r² 0.9755-0.9759 when amplitude was measured from the b-wave peak and 0.7265-0.6413, when amplitude was measured from baseline). The best predictors for PhNR2 amplitude were areas of 35 ms duration: AAW starting 30 ms after b-wave peak and AUW starting at 35 ms after the peak, when amplitude was measured from b-wave peak or baseline, respectively. Similar results were obtained by conducting similar prediction analysis on a small number of healthy volunteers. Conclusions: Determining and using AAW/AUW for PhNR2 prediction could be a valuable method in cases where the PhNR2 peak is not well defined.