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Anja Palmowski-Wolfe, Margarita Todorova, Andreas Schötzau, Anna Ledolter; Impact of a notch filter on the detection of glaucomatous damage in POAG compared to controls using the 2-global-flash multifocal ERG. Invest. Ophthalmol. Vis. Sci. 2013;54(15):801.
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to assess the effect of a 50Hz notch filter on the electrophysiological response in the 2-global-flash mfERG in patients with primary open angle glaucoma (POAG) compared to control subjects and its impact on the detection of glaucomatous damage.
A 2 global flash mfERG (VERIS 6.06TM, FMSIII) was recorded in 24 healthy control subjects and 34 POAG patients. Recording parameters: bandpass filter (BPF): 1-300Hz, 103 Hexagons, M-sequence stimulus: LMax 100cd/m^2, Lmin <1cd/m^2, global flash: 200cd/m^2. RMS for each subject was calculated with cut-off-frequencies of 1-200Hz with and without a digital 50Hz notch filter (Veris Science, Software 6.2.2 d2). Three response epochs of the central 10° were analysed: the response to the focal flash, at 15-45ms (DC) and the response to the global flashes at 45-75ms (IC-1) and at 75-105ms (IC-2). Statistical analysis: linear mixed effects models in the statistical package R 12.1.
A notch filter of 50Hz had little effect on waveforms uncontaminated by noise. In noisy recordings (4 control, 4POAG), the notch filter changed the waveform dramatically to resemble uncontaminated waveforms. In control subjects' uncontaminated mfERGs the RMS of the DC did not differ with or without a notch filter, while IC1 and IC2 differed significantly. In uncontaminated POAG recordings RMS did not differ with or without the notch filter (DC p=0.1, IC1 p=0.3, IC2 p=0.3). POAG differed significantly from controls whether the notch filter was used or not (p<0.001 for all three epochs). When notch filtered, noisy responses of POAG patients lay within 2 SD of good POAG responses, and noisy responses of healthy subjects within 2 SD of uncontaminated responses of healthy subjects.
Bock M et al. (2000) have shown that in the standard mfERG a 50Hz notch filter primarily affects the second but not the first order kernel. In analogy, in the 2-flash mfERG a 50Hz filter affected primarily the induced components. In good POAG recordings, the notch filter did not alter DC, IC1 or IC2 significantly which may point to a component in this range that is reduced in POAG but not control. Importantly, with a notch filter POAG still differed significantly from normal . Thus a 50 Hz notch filter allows grossly contaminated waveforms to be analysed in a meaningful manner, which can be extremely helpful in a clinical setting.
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