The effect of filtering on the two-global-flash mfERG: identifying the optimal range of frequency for detecting glaucomatous retinal dysfunction

To study the effects of filtering bandwidth on the two-global-flash multifocal electroretinogram (mfERG) responses in primary open-angle glaucoma (POAG) compared with control subjects.

A two-global-flash mfERG (VERIS 6.06™, FMS III) was recorded in 20 healthy subjects and 22 POAG patients with a band-pass filter (BPF) of 1–300 Hz (103 Hexagons, M-sequence stimulus: Lmax 100 cd/m², Lmin < 1 cd/m², global flash: 200 cd/m²). The root-mean-square average of the central 10° was calculated. Three response epochs were analysed: the response to the focal flash, at 15–45 ms (DC), and the following two components induced by the effects of the preceding focal flash on the response to the global flashes at 45–75 ms (IC1) and at 75–105 ms (IC2). The following BPF settings were analysed: 1–300 Hz, 3–300 Hz, 10–300 Hz, 100–300 Hz, 200–300 Hz, 1–10 Hz, 1–100 Hz and 1–200 Hz.

Filtering at 1–300 Hz showed significantly lower responses in POAG than in control subjects (p < 0.001) for all epochs analysed. At 1–100 Hz, this also held true even though the difference between the groups became smaller. At 1–10 Hz, responses were extremely small and did not differ between POAG and control (p > 0.5). This would suggest a filter setting of 10–300 Hz for mfERG recordings in POAG. However, when a filter setting of 10–300 Hz was compared to 1–300 Hz, with a filter setting of 10–300 Hz, the DC in POAG differed more (p < 0.0001) from normal than with 1–300 Hz (p = 0.0002). For IC1 and IC2, the stronger difference between POAG and control was found with 1–300 Hz (p < 0.0001) rather than with 10–300 Hz (p < 0.0001 and p = 0.0005, respectively). For the ‘oscillatory potentials’ at 100–300 Hz, POAG and control differed significantly in IC1 and IC2 (p < 0.05), but not in DC (p = 0.8). However, filtering at 200–300 Hz did not show a difference between POAG and control (p > 0.5). Thus, we applied a filter setting of 1–200 Hz, which seemed to be most sensitive in detecting glaucomatous retinal dysfunction (p < 0.0001).

A filter setting of 1–200 Hz appears most sensitive to detect glaucomatous damage if using a two-global-flash mfERG: using a band-pass filter a with lower low-frequency cut-off, containing the 10 Hz component, may be especially important in the small induced components that show glaucomatous damage most sensitively. High frequencies of 100–300 Hz also contain information that differentiates glaucoma from normal and thus should be included in the analysis.