Cone-driven electroretinogram responses to flashes delivered on a dim rod-saturating background: prevalence of negative waveforms

Abstract

Purpose : The dark-adapted electroretinogram (ERG) response to a standard bright flash contains rod and cone components. Negative waveforms usually indicate loss of post-receptoral signals. However, when rod responses are lost completely, the waveform reflects the dark-adapted cone system response (usually not seen due to a larger superimposed rod system response). In order to explore the possible form of the dark-adapted cone system response, we recorded ERGs to flashes delivered on a dim rod-saturating background.

Methods : ERGs to white xenon flashes (13 photopic cd m^-2 s, similar to the standard DA 10 stimulus), delivered on a dim blue background (1.0 photopic and 30 scotopic cd m-2), were recorded in 211 healthy adults from the TwinsUK cohort, as part of a wider study. Recordings were made with conductive fibre electrodes in the lower fornix following mydriasis. The background was designed to achieve rod saturation, but minimal cone desensitisation, so that the cone system might resemble the dark-adapted state. The standard ISCEV background is much brighter (30 photopic cd m-2) and significantly light-adapts the cones. A-wave and b-waves were extracted, and the b:a amplitude ratio calculated. Ratios <1 indicated a negative waveform.
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Results : Mean (SD) age was 62.5 (11.3) years. 93% were female. Following exclusion of traces with noise artefact, ERGs from 199 right eyes and 208 left eyes were included. Mean (SD) b:a ratio was 1.22 (0.27) and 1.20 (0.30) for right and left eyes respectively; medians were 1.19 and 1.18. Coefficient of inter-ocular correlation was 0.70. (p<0.0001). No significant correlation with age was found. Proportions of eyes with negative waveforms were 22% and 26% for right and left eyes respectively.

Conclusions : Over 20% showed negative waveforms, consistent with this being a feature of the dark-adapted cone system response to standard bright flashes in many people. The majority did not show negative waveforms. In patients with complete loss of rod function, a negative ERG (with reduced a-wave) could reflect the normal dark-adapted cone system response. Our study suggests that it might also be possible for complete loss of rod function to occur without a negative ERG. A limitation is that the blue background might elicit some cone system adaptation, and this will affect our estimate of the true dark-adapted cone-driven ERG.

This is a 2021 ARVO Annual Meeting abstract.