The double trough a-wave response in the DA 3.0 electroretinogram: possible contributions from cone-driven pathways

Abstract

Purpose : In the dark-adapted electroretinogram (ERG), a double trough can sometimes be seen in the response to the 3.0 cd m^-2 s flash (DA 3.0), which can affect quantification of a-wave amplitude and peak time. We explored whether this might be related to cone-driven pathways. We analysed recordings from healthy volunteers in response to flashes delivered in the dark and then in the presence of a dim rod-saturating blue background (so that the dark-adapted cone system response could be recorded). We also analysed similar recordings from a patient with achromatopsia in whom the cones were not functional.

Methods : Over 200 healthy adult participants were recruited from the TwinsUK cohort. Xenon white flashes were delivered in the dark and then in the presence of a dim blue background (30 scotopic cd m^-2; 1.0 photopic cd m^-2). Pupils were pharmacologically dilated and ERGs were recorded using conductive fibre electrodes placed in the lower conjunctival fornix. Responses recorded on the blue background were subtracted from those recorded in the dark to estimate the rod driven response. A 34 year old male with bi-allelic mutations in CNGA3 underwent similar recordings.

Results : Mean (SD) age of the twin cohort was 62.5 (11.5) years. A double trough was frequently observed in the dark-adapted recordings. In the responses obtained on the blue background, a single trough was seen, with timing similar to the first trough in the dark. Subtraction removed a large part of the first trough. The patient did not exhibit a clear double trough in the dark, and the responses on the blue background were undetectable.

Conclusions : These findings suggest that the cone system contributes to the morphology of the DA 3.0 a-wave, and might contribute to the first component of the double trough. The dim blue background, having a low photopic intensity, might have advantage over the ISCEV standard photopic white background when attempting to subtract out the cone system contribution to dark-adapted flash responses.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.