Dose-dependent retinal changes following sodium iodate administration: application of spectral-domain optical coherence tomography for monitoring of retinal injury and endogenous regeneration.

Background: The purpose of this study was to demonstrate the progression of acute retinal injury by correlating histological sections with in vivo spectral-domain optical coherence tomography (SD-OCT) images.

Methods: Male C57BL/6 mice were treated intravenously with two different sodium iodate (NaIO3) doses (35 mg/kg or 15 mg/kg). In vivo SD-OCT was performed up to 3 months post-injury. Ex vivo retinal histology, TUNEL and IsolectinB4 immunostaining were also conducted. Quantitative comparison of histopathological images and SD-OCT images was performed.

Results: SD-OCT examination revealed that administration of 35 mg/kg NaIO3 was associated with progressive and irreversible retinal degeneration. On day 3 post-injury, we found numerous apoptotic cells in the outer nuclear layer (ONL) that strongly corresponded to hyper-reflective areas in the SD-OCT images. At 7 d post-injury, SD-OCT images showed irregular-shaped patterns of hyper-reflectivity in the retinal pigment epithelium (RPE) that corresponded with the accumulation of macrophages phagocytosing melanin granules and cell debris. Additionally, we documented hyper-reflective opacities in the vitreous that were most numerous at 7 d. At 3 months post-injury, the neurosensory retina was significantly thinner, predominantly due to progressive photoreceptor (PR) loss. In contrast, administration of 15 mg/kg NaIO3 did not induce hyper-reflectivity of ONL in SD-OCT images, which indicates a lack of massive PR cell death. At 3 months post-injury, SD-OCT images showed the complete restoration of outer retina lamination and restoration of hyper-reflective structural bands. Histological assessment of retinas acquired after the last SD-OCT imaging session revealed complete regeneration of the RPE and considerable improvement of PR architecture.

Conclusions: Our findings showed the high level of effectiveness of SD-OCT imaging for monitoring dynamic changes in retinal morphology following acute retinal injury. Moreover, we demonstrated for the first time that SD-OCT can be used to non-invasively detect regeneration in the damaged retina.