Rod electroretinograms elicited by silent substitution stimuli compared with ISCEV standard full-field ERG

Abstract

Purpose : Rod-isolated electroretinograms (ERGs) generated by silent substitution stimuli require no dark adaptation [Aher et al. Doc Ophthalmol. 2018;136(1)]. The aim was to assess whether rod-isolated ERGs can detect rod dysfunction earlier than International Society for Clinical Electrophysiology of Vision (ISCEV) standard full-field dark-adapted ERG in patients with Stargardt disease.

Methods : ERG responses were obtained from both protocols in the same session in 6 patients with Stargardt disease (29–69y; 3 males) and 14 healthy controls (23–74y; 6 males). The visual stimuli were presented on a 6-primary LED Ganzfeld stimulator. ISCEV standard full-field ERGs were recorded after dilation, with a conductive fibre electrode. Amplitudes of dark- (DA 0.01) and light-adapted (LA 3.0) b-waves were extracted. Subsequently, ERGs elicited by rod-isolating 8 Hz sinusoidal flicker stimuli were measured in one eye for 25% rod contrast and 40-degrees field size, and the first harmonic (f₁) of the Fourier-transformed rod-driven response was extracted. Ocular axial length (AL) was measured with Zeiss IOLMaster 700, and red-green (RG) and blue-yellow (BY) colour vision deficiencies (CVD) were assessed with Hardy-Rand-Rittler 4^th ed.

Results : Amplitudes of LA and rod-isolated f₁ were adjusted for individual differences in AL, to account for negative correlations between amplitudes and AL. In controls, the median (range) amplitudes of LA, DA, and rod-isolated f₁ were 155.3 (106.4–245.5) μV, 145.4 (82.2–551.6) μV, and 6.61 (4.06–8.65) μV. The amplitudes of LA and rod-isolated f₁ were significantly lower in those with Stargardt disease compared with controls (Wilcoxon; W=72 and 76, p=0.011 and 0.0033), with no differences between the groups for DA amplitudes (p>0.05). In the group with Stargardt disease, two patients (29 and 34y) were within the normal range for all ERG measures, and two (53 and 69y) had reduced LA amplitudes only; all had mild-moderate RG CVD. One patient (42y) had reduced LA and rod-isolated f₁, and one (35y) had reduced LA, DA, and rod-isolated f₁ amplitudes; both had severe BY and RG CVD.

Conclusions : The results indicate that the two ERG protocols appear to differentiate three stages of progression, with rod-isolated ERG being affected earlier than DA, revealing an intermediate stage compared with LA and DA full-field ERGs.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.