Dark-adapted red flash ERGs in healthy adults

Purpose The x-wave of the dark-adapted (DA) ERG to a red flash reflects DA cone function. This exploratory study of healthy adults aimed to investigate changes in the DA red ERG with flash strength and during dark adaptation to optimise visualisation and therefore quantification of the x-wave. Methods The effect of altering red flash strength was investigated in four subjects by recording ERGs after 20 min dark adaptation to red flashes (0.2-2.0 cd s m(-2)) using skin electrodes and natural pupils. The effect of dark adaptation duration was investigated in 16 subjects during 20 min in the dark, by recording DA 1.5 red ERGs at 1, 2, 3, 4, 5, 10, 15 and 20 min. Results For a dark adaption period of 20 min, the x-wave was more clearly visualised to weaker (< 0.6 cd s m(-2)) red flash strengths: to stronger flashes it became obscured by the b-wave. For red flashes of 1.5 cd s m(-2), the x-wave was most prominent in ERGs recorded after 1-5 min of dark adaptation: with longer dark adaptation, it was subsumed into the b-wave's rising edge. Conclusions This small study suggests that x-wave visibility in healthy subjects after 20 min dark adaptation is improved by using flashes weaker than around 0.6 cd s m(-2); for flash strengths of 1.5 cd s m(-2), x-wave visibility is enhanced by recording after only around 5 min of dark adaptation. No evidence was found that interim red flash ERGs affect the dark-adapted state of the normal retina.