Inter-subject, inter-ocular and inter-session repeatability of the photopic negative response of the electroretinogram recorded using DTL and skin electrodes

The photopic negative response (PhNR) has attracted interest as a flash ERG component reflecting inner retinal activity, with investigators adopting various approaches to analysing the response. This study has two principal aims: first to determine the most reliable technique for assessing the PhNR amplitude; secondly to compare the repeatability characteristics of the PhNR recorded using DTL and skin active electrodes. Electroretinograms were recorded in 31 subjects, using both electrode types, in response to a Ganzfeld red stimulus (Lee filter bright red; 1.76 log phot td.s; 4 Hz) presented over a steady blue background (Schott glass filter BG28; 3.9 log scot td). Sixteen subjects returned to assess repeatability. PhNR amplitude was measured from b-wave peak-to-PhNR trough, pre-stimulus baseline to trough, and from peak and baseline to a fixed time-point; a ratio of b-wave/PhNR amplitude was also calculated. Coefficients of variation (CoV), and inter-ocular and inter-session limits of agreement (LoA) were calculated for all measures. The ratio of b-wave/PhNR amplitude showed the lowest CoV (14.3% DTL; 23.2% skin), inter-ocular LoA (22.2% DTL; 25.0% skin), and inter-session LoA (22.8% DTL; 20.3% skin). The peak-to-trough and peak-to-fixed-time measurements were also consistently reliable. Least reliable measures were those measured from baseline. While skin electrode responses were significantly smaller than DTL responses (P < 0.0001), the variability was only slightly increased. This study suggests that peak-to-trough measurements are the most reliable means of measuring the PhNR and ratio calculation further improves repeatability. Skin electrodes provided a viable alternative to DTL electrodes for recording the PhNR.