Impact of notch filter use on waveforms of First- and Second-Order-Kernel responses from multifocal ERGs

Comparison of DTL and gold cup skin electrodes for recordings of the multifocal electroretinogram

OBJECTIVE

To compare mfERG recordings with the Dawson-Trick-Litzkow (DTL) and gold cup skin electrode in healthy young and old adults and to test the sensitivity of both electrodes to age-related changes in the responses.

METHODS

Twenty participants aged 20-27 years ("young") and 20 participants aged 60-75 ("old") with a visual acuity of ≤ 0 logMAR were included. The mfERG responses were recorded simultaneously using DTL and skin electrodes. P1 amplitudes, peak times and signal-to-noise ratios (SNRs) were compared between both electrodes and across age groups, and correlation analyses were performed. The electrode's performance in discriminating between age groups was assessed via area under curve (AUC) of receiver operating characteristics.

RESULTS

Both electrodes reflected the typical waveform of mfERG recordings. For the skin electrode, however, P1 amplitudes were significantly reduced (p < 0.001; reduction by over 70%), P1 peak times were significantly shorter (p < 0.001; by approx. 1.5 ms), and SNRs were reduced [(p < 0.001; logSNR ± SEM DTL young (old) vs gold cup: 0.79 ± 0.13 (0.71 ± 0.15) vs 0.37 ± 0.15 (0.34 ± 0.13)]. All mfERG components showed strong significant correlations (R² ≥ 0.253, p < 0.001) between both electrodes for all eccentricities. Both electrodes allowed for the identification of age-related P1 changes, i.e., P1-amplitude reduction and peak-time delay in the older group. There was a trend to higher AUC for the DTL electrode to delineate these differences between age groups, which, however, failed to reach statistical significance.

CONCLUSIONS

Both electrode types enable successful mfERG recordings. However, in compliant patients, the use of the DTL electrode appears preferable due to the larger amplitudes, higher signal-to-noise ratio and its better reflection of physiological changes, i.e., age effects. Nevertheless, skin electrodes appear a viable alternative for mfERG recordings in patients in whom the use of corneal electrodes is precluded, e.g., children and disabled patients.

Removing mains interference from the mfERG by applying a post-processing digital notch filter: for the good or the bad?

Purpose

Ideally, the multifocal electroretinogram (mfERG) is recorded without noticeable intrusion of mains interference. However, sometimes contamination is difficult to avoid. A post-processing digital notch filter can help to recover the retinal response even in severe cases of mains interference. While a digital filter can be designed to have little to no impact on peak times, filtering out mains interference also removes the retinal signal content of the same frequency, which may result in a change of amplitude. The present study addressed this issue in the standard first order kernel mfERG.

Methods

In 24 recordings from routine exams with no perceivable mains interference, the effects of 50-Hz and 60-Hz non-causal digital notch filters on amplitude and peak time were assessed. Furthermore, the effect of filtering on contaminated traces was demonstrated and simulated mains interference was used to provide an example of nonlinear superposition of retinal signal and mains interference.

Results

mfERG amplitudes were reduced by 0%–15% (median 6%) with the 50-Hz filter and remained virtually unaffected with the 60-Hz filter. Simulations illustrate that spurious high-frequency components can occur in the filtered signal if a strongly contaminated signal is clipped due to a limited input range of the analog-to-digital converter.

Conclusion

The application of a 50-Hz digital notch filter to mfERG traces causes a mild amplitude reduction which will not normally affect the clinical interpretation of the data. The situation is even more favorable with a 60-Hz digital notch filter. Caution is necessary if the assumption of linear additivity of retinal signal and mains interference is violated.

Impact of a Digital Power Line Filter in the 2-Global-Flash Multifocal Electroretinogram of Glaucoma Patients Compared to Controls

PURPOSE

To assess the effect of a 50 Hz power line digital filter on the response to a 2-global-flash multifocal electroretinogram (mfERG) in primary open angle glaucoma (POAG) compared to control.

MATERIALS AND METHODS

A 2-global-flash mfERG (VERIS™) was recorded (23 control, 34 POAG). Eight recordings were noise contaminated: 4 control, 4 POAG. Response averages from the central 10° and 7 surrounding groups were analyzed with and without a 50 Hz digital filter for the following mfERG response epochs: direct component (15-45 ms), 1st (45-75 ms) and 2nd (75-105 ms) induced components.

RESULTS

A digital 50 Hz filter had little effect on uncontaminated with noise waveforms but, in noisy recordings, changed the waveform dramatically to resemble uncontaminated waveforms. In controls' 50 Hz-filtered uncontaminated with noise mfERGs differed significantly from unfiltered responses in induced components. Uncontaminated with noise recordings from glaucoma patients did not differ with or without the notch filter (p > 0.1 for all three epochs of mfERG). The mfERG response in the central 10°in glaucoma patients differed significantly from controls, whether the notch filter was used or not (p < 0.001).

CONCLUSIONS

A 50 Hz notch filter allows grossly contaminated waveforms to be analyzed in a meaningful manner. With a 50 Hz filter, glaucoma patients still differed significantly from normal.

The clinical applications of multifocal electroretinography: a systematic review

Multifocal electroretinography (mfERG) is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG therefore allows topographic mapping of retinal function in the central 40-50 degrees of the retina. The strength of mfERG lies in its ability to provide objective assessment of the central retinal function at different retinal areas within a short duration of time. Since the introduction of mfERG in 1992, mfERG has been applied in a large variety of clinical settings. This article reviews the clinical applications of mfERG based on the currently available evidence. mfERG has been found to be useful in the assessment of localized retinal dysfunction caused by various acquired or hereditary retinal disorders. The use of mfERG also enabled clinicians to objectively monitor the treatment outcomes as the changes in visual functions might not be reflected by subjective methods of assessment. By changing the stimulus, recording, and analysis parameters, investigation of specific retinal electrophysiological components can be performed topographically. Further developments and consolidations of these parameters will likely broaden the use of mfERG in the clinical setting.

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 non-proliferative 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.