Towards optimal filtering of standard'' multifocal electroretinogram (mfERG) recordings: findings in normal and diabetic subjects

Aims: To study the effects of two commonly used pre-amplifier filtering bandwidths on normal multifocal electroretinogram (mfERG) responses and their comparative abilities to detect retinal disease. Methods: 103 standard mfERGs were recorded simultaneously in two channels with different pre-amplifier settings ( 10 - 100 Hz and 10 - 300 Hz) from one eye of each of 20 normal subjects, 17 diabetics with nonproliferative diabetic retinopathy (NPDR), and 12 diabetics without retinopathy. Signal to noise ratios (SNR) of the normal subjects' first order mfERGs were compared between channels. All subjects' amplitudes and implicit times were derived using a template stretching'' method. For comparison, implicit time was also measured using a template sliding'' method. mfERG amplitudes and implicit times were compared between the channels and among subject groups. Results: Normal mean amplitudes and implicit times were similar for the two channels. However, normal 10 - 100 Hz recordings had significantly higher SNR and lower intersubject variability than 10 - 300 Hz recordings. In NPDR, the 10 - 100 Hz channel identified significantly more implicit time and amplitude abnormalities. In the diabetics without retinopathy, 10 - 100 Hz filtering identified significantly more implicit time abnormalities than 10 - 300 Hz filtering. For both filter settings, diabetic implicit times were more often abnormal than amplitudes. The 10 - 100 Hz channel was superior for both implicit time measurements. Conclusion: Standard mfERGs recorded from normal eyes and filtered 10 - 100 Hz contain less noise, higher SNR, and less intersubject variability than those filtered at 10 - 300 Hz. This underlies the finding that the 10 - 100 Hz filter setting identifies more retinal dysfunction than the 10 - 300 Hz setting.