Abstract
Purpose :
Optoretinograms show light-evoked dynamical photoreceptor changes with a rapid outer-segment (OS) contraction followed by a slower elongation (K. C. Boyle et al., BioPhys. J. 2020). These biomechanical changes are believed to originate in photoisomerism leading to axial contraction followed by osmotic swelling. Here, we revisit this analysis with focus on thermal effects in the OS and the adjacent retinal pigment epithelium (RPE) cells with the purpose of evaluating if photothermal effects have an impact via thermal expansion and thermal refractive index alternations that combined determine the optical path in the confined retinal space.
Methods :
The OS and RPE are modelled using COMSOL® with thermal ray optics and absorption to determine light-invoked geometry and refractive changes that combined determine the optical path being probed with optical coherence tomography (OCT). The dark-adapted retinal temperature is set at 95°F (35°C) with light-invoked heating by up to 3.6°F (2°C) and transient photothermal effects using realistic parameters with OS length and diameter of 30 μm and 2 μm, respectively, and RPE cells with height of 10 μm and sizes in the range of 15 - 60 µm. Light-invoked heating on refractive index and sizes are evaluated numerically for green light at 550 nm wavelength. RPE absorption is up to an order of magnitude higher than in the OS. A Young's modulus of 20 MPa and 60 MPa is used for the RPE and OS, and a 0.48 Poisson ratio. Retinal heat transfer of up to 500 W/m2K is assumed.
Results :
The model shows that thermal effects contribute to optoretinogram data via thermal expansion and alteration of refractive indices. The highest absorption happens in the RPE that initially compresses the OS by up to 10's of nm while the OS gradually expands at a slower pace to reach 100's of nm elongation adding to the slow osmotic swelling to produce the final optoretinogram. Retinal heat transfer scales the observed changes in magnitude but not in appearance.
Conclusions :
We found that photothermal effects of the RPE and outer segment combination plays an essential contribution to the light-evoked signature observed with OCT-based optoretinograms. This shows a high impact of thermal gradients due to strong absorption in the RPE layer that initially will expand at a higher rate than the OS.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.