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Anna Polunina, Sophie A. Kramer, Margarita G. Todorova, Andreas Schötzau, Selim Orgül, Anja M. Palmowski-Wolfe; Effect of Filtering on the 2 Global Flash mf ERG of Glaucoma Patients Compared to Normal Subjects. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3507.
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For a standard multifocal electroretinogram (mfERG), ISCEV guidelines recommend a high pass cutoff between 3-10Hz and a low pass cutoff between 100-300Hz. As these are large possible margins, we studied the effects of filtering on a special 2 global flash mfERG in glaucoma patients compared to healthy subjects.
A 2 global flash mfERG (VERIS 6.0.TM, FMSIII) was recorded in 20 healthy subjects and 20 glaucoma patients (POAG: 15NTG, 5HTG) with a bandpass filter (BPF) setting of 1-300Hz (F1) (103 hexagons, M-sequence stimulus: LMax 100cd/m^2, Lmin <1cd/m^2, global flash: 200cd/m^2). The recorded data was filtered offline with VERIS 6.0. The BPF settings were analyzed and compared to F1: 1-100Hz (F2), 100-300Hz (F3) & 1-10Hz (F4). Focal scalar products were calculated using focal templates derived from the controls. Under these stimulus parameters glaucoma patients differed most from control subjects in the response average of the central 4 degrees but also differed significantly in 7 additional response averages (Kramer et al, unpublished observation). Three response epochs were analyzed: The response to the focal flash, at 15-45 ms (DC) and the following two response components induced by the effects of the preceding focal flash on the response to the global flashes at 45-75 ms (IC-1) and at 75-105 ms (IC-2). Statistical analysis was performed using linear mixed effects models in the statistical package R version 12.1.
When all epochs per patient were pooled together, F1 showed significantly lower responses in POAG than in control subjects in all group averages (p<0.001). With F2 this also held true even though the difference between the groups became smaller. For F3 POAG and control still differed significantly in the central 4 degrees (p<0.001). With F4, responses were extremely small and did not differ between POAG and control (p>0.2). These findings also held true when the individual epoch lengths were analysed.
Our results indicate that most of the important information differentiating POAG from control will be picked up with a BPF of 10-100 Hz. However there is still information contributing to the detection of glaucomatous retinal dysfunction if F3 is used. We therefore recommend a BPF setting of 1-300Hz or 10-300 Hz when looking for glaucomatous retinal dysfunction using a 2 global flash stimulus.
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