Light-induced modifications of the outer retinal hyperreflective layers on spectral-domain optical coherence tomography in humans: an experimental study

Abstract

Purpose: Numerous small hyperreflective dots (HRDs) can be seen within the hyporeflective layer between the ellipsoid zone (EZ) and the interdigitation zone (IZ) on C-scan spectral-domain optical coherence tomography (SD-OCT) with a yet unknown variation under light conditions. The aim of this study was to explore light-induced SD-OCT changes in these HRDs.

Methods: The study subjects were randomly assigned to two groups: Group 1 experienced a dark adaptation protocol followed by intense retinal photobleaching, while Group 2, serving as the control group, was exposed to constant ambient light without any variation. The number of HRDs was automatically counted.

Results: Twenty healthy volunteers were prospectively included. The number of HRDs differed significantly over time (p = 0.0013). They decreased in Group 1 after dark adaptation and retinal photobleaching before returning to baseline levels 30 min later; conversely, they remained relatively constant in Group 2 throughout the study (p < 0.001). Light-skinned subjects had less HRD than dark-skinned subjects.

Conclusion: We observed light-induced modifications in the space between the EZ and the IZ. We hypothesize that the HRDs visible in this zone correspond to melanosomes that are mobilized during the light stimulation protocol. Larger studies are recommended to further evaluate and confirm light-induced SD-OCT changes under physiological and pathological conditions.

Keywords: melanosome; optical coherence tomography; outer retinal layers; phagosome; retinal pigment epithelium.

Fig. 1

Spectral‐domain optical coherence tomography (SD‐OCT) B‐scan (A) with a red dotted line drawn between the ellipsoid and interdigitation hyperreflective zones. This segmentation was obtained by placing the line 14 µm below the ellipsoid zone. The corresponding C‐scan (B) revealing numerous hyperreflective dots (arrows).

Fig. 2

Software protocol for counting the hyperreflective dots (HRDs): before (A) and after (B) the automatic counting. Inset: close‐up view corresponding to the white box. HRDs were summed across the visual field along the x‐axis per 0.5 degree of the fovea and plotted their number below the panel B.

Fig. 3

Study protocol for the two groups. Subjects in Group 1 (light stimulation protocol) experienced 20 min of dark adaptation (dark) followed by 2 min of retinal photobleaching (bleach) and then two periods of 15 min of ambient light (ambient). Group 2 (no‐light stimulation protocol) underwent optical coherence tomography imaging at each time‐point in ambient light.

Fig. 4

Number of hyperreflective dots (HRDs) with time between the light stimulation (blue, Group 1) and no‐light stimulation (red, Group 2) groups. Error bars represent the 95% confidence interval. DA = dark adaptation, PB = photobleaching.

Fig. 5

Comparison of the mean distance from a hyperreflective dot (HRD) to its nearest neighbour between the light stimulation (blue, Group 1) and no‐light stimulation (red, Group 2) groups. Error bars represent the 95% confidence interval. DA = dark adaptation, PB = photobleaching.

Fig. 6

Diagram showing the hypothetic correlation between hyperreflective dots (HRDs) and melanosome movements in ambient light, dark adaptation or photobleaching. Melanosomes are recruited through the retinal pigment epithelium (RPE) apical processes to protect the photosensitive outer segments from light exposure. After dark adaptation, melanosomes are confined in the basal area of the processes.