Effect of 50 Hz Filters on Pattern Electroretinogram.

Abstract

Purpose: Pattern electroretinogram (PERG) is used to evaluate the function of retinal ganglion cells. However, the amplitude of PERG is quite small, thus making the examination challenging to perform. Waveform noise may be minimized by applying various filters. We aimed to investigate the effect of 50-Hz filters on PERG test results.

Methods: This is the retrospective observational study. PERG tests were performed using the RETI-scan system (Roland Consult, Germany) according to the International Society for Clinical Electrophysiology of Vision (ISCEV) guidelines. Three types of 50-Hz filters (soft, middle, and hard) were applied. The differences in parameters (N35 peak time, P50 peak time, N95 peak time, P50 amplitude, N95 amplitude, and N95/P50 ratio) were analyzed. Based on the provided normal range, the changes from normal to abnormal range or vice versa were investigated.

Results: A total of 24 waveforms were analyzed. After filtering, the P50 and N95 amplitudes showed significant reduction of 8-15% (P50 amplitude: without filter, 5.1 ± 2.7 μV; 50-Hz soft filter, 4.6 ± 2.3 μV; 50-Hz middle filter, 4.3 ± 2.1 μV; 50-Hz hard filter, 4.3 ± 2.1 μV. N95 amplitude: without filter, 7.2 ± 4.2 μV; 50-Hz soft filter, 6.6 ± 3.8 μV; 50-Hz middle filter, 6.3 ± 3.6 μV, 50-Hz hard filter, 6.1 ± 3.6 μV). This pattern was more prominent in normal subjects. All latencies except the N35 peak time exhibited no differences between the tests. The N95/P50 ratio was decreased after 50-Hz middle and hard filtering. Considering the normative data, switching between normal and abnormal results was rare.

Conclusions: Although peak time was not significantly affected, amplitude was significantly reduced after using 50-Hz filters. Thus, 50-Hz filters can smoothen the waveform. Nevertheless, caution must be exercised while taking readings.