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Jennifer J Hunter, Benjamin D Masella, William Fischer, Ethan A Rossi, David R Williams; Long-term reduction of infrared autofluorescence caused by infrared light below the maximum permissible exposure. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2172.
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Low-level infrared (IR) light exposure, often thought to be eye safe, is used in many retinal imaging instruments. However, we have discovered that exposure to IR illumination causes a long-lasting reduction in the intensity of retinal infrared autofluorescence (IRAF), which originates from fluorophores in the retinal pigment epithelium and choroid. We have investigated the irradiances required to generate IRAF reduction and the time course of its recovery.
A scanning laser ophthalmoscope (Heidelberg Spectralis HRA + OCT) was used to take IRAF images (ex: 785 nm, em: 805 - 840 nm) of two macaques before and after IR illumination (790 nm Δ17 nm) with retinal radiant exposures (RREs) of 15 - 450 J/cm2. These RREs ranged from 22% to 53% of the maximum permissible exposure (MPE, ANSI Z136.1-2007). The IR exposures covered rectangular areas approximately 2° per side and were administered by either raster-scanning a small spot or with uniform illumination. IRAF images were also recorded in two human subjects before and after a RRE of ~190 J/cm2 at 790 nm. Changes in IRAF intensity post-exposure were quantified by calculating the ratio of signal intensity in exposed to unexposed regions and normalizing to the pre-exposure ratio.
All 790 nm exposures caused a significant decrease in IRAF ratio (p < 0.001). The magnitude of this reduction increased with RRE and ranged from 4.4% to 13.6% in macaque. Human subjects showed a decrease in IRAF ratio that was consistent with macaque results. Follow-up imaging showed partial recovery after 1 month in macaque and human (p < 0.02), and full recovery within 21 months in the macaque (p > 0.17). The IRAF decreases caused by raster and uniform illumination were not significantly different (p = 0.85). No retinal changes were observed using any other imaging modality. Microperimetry and multifocal ERG in the human subjects showed no deficit in visual function in the exposed retinal regions.
This study demonstrates a long-lasting effect of IR illumination on the retina that occurs at levels well below the MPE. The lack of dependence on light delivery method suggests a photochemical mechanism that reduces the fluorophore’s efficiency or shifts its emission spectrum. This could be benign, such as isomerization, or potentially harmful, such as photooxidation.
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