Electroretinograms following short-term chromatic light adaptation in high myopes and non-myopes

Abstract

Purpose: Worldwide, myopia cases are on the rise and the need for finding a definitive mechanism by which myopia develops has become more imperative than ever before. Axial elongation in myopic eyes is linked to short wavelength (λ) light-dependent increases in retinal dopamine (DA). DA is associated with enhanced electroretinogram (ERG) amplitudes. This thesis seeks to examine short-term adaptation to short and long λ light at moderate and strong levels. I hypothesize that short λ light exposure differentially affects full-field ERGs (ffERGs) and central ERGs in myopic eyes compared to controls. Methods: In a multi-visit cross-over design, we compared ERGs before and after 20 minutes of full-field adaptation to long (red LED peak λ (λ 627 nm) or short (blue LED peak λ 448 nm) λ light (Espion ColorDome™, Diagnosys LLC). Human participants (ages 18-30) were healthy high myopes (≤-5 diopters (D)) and emmetropes (+1 D to - 0.25 D). Monocular, light-adapted (LA) ffERGs were recorded using skin electrodes and a handheld system that adjusted for pupil diameter in real time (RETeval™). ERG stimuli were LA standard white flash and flicker (85 Td.s), presented at 2 and 28.3 Hz, respectively. The a-wave, b-wave and flicker ERG amplitudes, and implicit times were compared as a function of pre/post adaption (time), adapting stimulus (λ), and refractive error (RE) group using a mixed model ANOVA. Monocular multi-focal ERGs (mfERGs; 61 hexagons) and pattern ERGs (PERGs; 15° field, 15’ reversing checks) were recorded with natural pupils in keeping with ISCEV standards using DTL electrodes. The Espion™ console and amplifier (Diagnosys LLC) was used for both ERG tests. The primary outcome measures analyzed were the amplitudes and implicit times of the mfERG central wavelet, mfERG average of the surrounding rings and the PERG P50 peak and were compared as a function of pre/post adaption (time), adapting stimulus (λ), and RE group using a mixed model ANOVA. Results: There were no significant differences between controls and myopes prior to adaptation (all p≥0.05) for ffERG tests. The luminance of the light had an effect such that changes in b-wave implicit time decreased with adaptation to 300 cd/m² light in controls (blue 30: -0.451 ± 1.28%; blue 300: -2.28 ± 2.56%; p≤0.001, η2G=0.14) but less so in the high myope group (blue 30: +0.05 ± 1.3%; blue 300: -1.26 ± 2.43%; p=0.01, η2G=0.05). Changes in flicker implicit time decreased with the stronger luminance level but were not different between refractive error groups (i.e. controls: blue 30: 0.15 ± 1.0%; blue 300: -1.05 ± 1.61%, myopes: blue 30: +0.44 ± 2.74%; blue 300: -1.1 ± 1.50%, p≤0.001, η2G=0.13). Stronger light conditions caused b-wave amplitudes to become less negative (smaller) (i.e. controls: blue 30: –5.38 ± 15.6%; blue 300: –3.50 ± 17.7%, myopes: blue 30: –10.9 ± 15.4%; blue 300: +0.85 ± 10.9%, p=0.02, η2G=0.05). Similarly, with v flicker amplitudes, they became less negative (smaller) with stronger luminance adaptation (i.e. controls: blue 30: –4.1 ± 13.4%; blue 300: –0.62 ± 17.1%, myopes: blue 30: –9.9 ± 18.3%; blue 300: +4.7 ± 12.0%, p=0.00, η2G=0.07). When comparing between controls and high myopes or between short and long wavelengths, changes in b-wave amplitudes did not differ. There was a significant difference between RE groups for the PERG P50 amplitude prior to adaptation (p=0.03; Cohen’s d=1.06) such that they were significantly smaller in myopes (2.63 ± 1.0 μV) than controls (3.95 ± 1.5 μV). The N95 amplitudes were also smaller (i.e. less negative) in myopes (-4.39 ± 1.82 μV) than controls (-6.58 ± 1.55 μV; p= 0.01; Cohen’s d=-1.3). The mfERGs showed no significant pre-adaptation RE differences (all p≥ 0.05; all η2G≤ 0.07). For the PERG, in both controls and myopes, the change in N95 amplitudes decreased more with long λ light (controls: -16.6 ± 23.4%; myopes: -24.7 ± 43.4%) than short λ light (controls: +10.4 ± 23.5%; myopes: +1.88 ± 26.7%; p≤ 0.001; η2G=0.541). There were no effects of adaptation on the P50 amplitude or implicit time. For mfERG, the change in N1P1 magnitude was not different between controls and myopes and was not different when comparing λs for any of the five rings (p≥ 0.05). Conclusions: There is no evidence that chromatic adaptation has a differential effect on post-adaptation ffERGs in high myopes. Long λ adaptation, especially with stronger luminance, prolongs ERG implicit times, probably reflecting relatively reduced input from the faster long cone system. There is evidence to suggest smaller central retinal responses with the PERG P50 but not with the central wavelet of the mfERG, indicating altered retinal ganglion cell function but not altered central inner retinal function in young adults with myopia. Further studies should focus on confirming whether altered central retinal function persists in adult myopes and whether longer adaptation times would yield a greater RE difference.