Light-Induced Thickening of Photoreceptor Outer Segment Layer Detected by Ultra-High Resolution OCT Imaging

Abstract

PurposeWe examined if light induces changes in the retinal structure that can be observed using optical coherence tomography (OCT).MethodsNormal C57BL/6J mice (age 3–6 months) adapted to either room light (15 minutes to ∼5 hours, 50–500 lux) or darkness (overnight) were imaged using a Bioptigen UHR-OCT system. Confocal histologic images were obtained from mice killed under light- or dark-adapted conditions.ResultsThe OCT image of eyes adapted to room light exhibited significant increases (6.1 ± 0.8 μm, n = 13) in total retina thickness compared to the same eyes after overnight dark adaptation. These light-adapted retinal thickness changes occurred mainly in the outer retina, with the development of a hyporeflective band between the RPE and photoreceptor-tip layers. Histologic analysis revealed a light-evoked elongation between the outer limiting membrane and Bruch's membrane from 45.8 ± 1.7 μm in the dark (n = 5) to 52.1 ± 3.7 μm (n = 5) in the light. Light-adapted retinas showed an increase of actin staining in RPE apical microvilli at the same location as the hyporeflective band observed in OCT images. Elongation of the outer retina could be detected even with brief light exposures, increasing 2.1 ± 0.3 μm after 15 minutes (n = 9), and 4.1 ± 1.0 μm after 2 hours (n = 6). Conversely, dark-adaptation caused outer retinal shortening of 1.4 ± 0.4 μm (n = 7) and 3.0 ± 0.5 μm (n = 8) after 15 minutes and 2 hours, respectively.ConclusionsLight-adaption induces an increase in the thickness of the outer retina and the appearance of a hyporeflective band in the OCT image. This is consistent with previous reports of light-induced fluid accumulation in the subretinal space.

In vivo optophysiology reveals that G-protein activation triggers osmotic swelling and increased light scattering of rod photoreceptors

“…Previous investigations have reported light-induced changes in the length of photoreceptor OSs measured in vivo (32), including recently from human cone OSs (10). Three lines of evidence presented in this work support the hypothesis that the elongation reported here results from osmotically induced swelling triggered by excess osmolytes generated by phototransduction reactions at or downstream of Gα t activation.…”

The Effect of Retinal Melanin on Optical Coherence Tomography Images

“…This change could be due to changes in the intracellular milieu 38 or shortening of the photoreceptor outer segment 39 (increasing the distance between outer segment tips and RPE melanin) following light activation. Indeed, zebrafish have been shown to exhibit shortening of the rod outer segments and lengthening of cone outer segments with dark adaptation, 37 and Li et al 40 also have shown outer segment shortening with dark adaptation in mice. In contrast, Abràmoff et al 39 describe apparent outer segment shortening with light adaptation in humans.…”

“…37 With dark adaptation, the pigment is displaced from the region of the the outer segments, which would allow better distinction of the outer segment regions from the RPE in OCT. 37 However, separation of photoreceptor bands and RPE is more distinct in the light-adapted state compared to dark adaptation in mice. 40 As such, it is possible that in addition to changes in the amount of melanin pigment, the relative location and distribution of melanin within the RPE could affect the appearance of outer retina in OCT images, with differences across species.…”

Hexokinase 2 is dispensable for photoreceptor development but is required for survival during aging and outer retinal stress

“…OSEL is defined as the length from the ends of the inner segment boundary to the surface of the RPE. 7,48…”