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Christina Schwarz, Robin Sharma, Matthew Keller, David R Williams, Jennifer J Hunter; Intense ultrashort pulsed light in the infrared selectively damages putative S cones. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4301.
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© ARVO (1962-2015); The Authors (2016-present)
Infrared ultrashort pulsed light is used for two-photon excited fluorescence (TPEF) ophthalmoscopy to excite naturally occurring retinal fluorophores such as retinoids and NADH in the ultraviolet. Previously, we have shown that the visual cycle can be tracked at light levels that do not cause detectable damage. However, safety standards are not well established. Here we explore damage thresholds for this pulse regime in the living macaque eye.
Following dark adaptation, the photoreceptor layer of 3 macaques was exposed at several retinal locations between 2 and 15° eccentricity from the fovea with a 730 nm, 55 fs pulsed laser. Tested retinal radiant exposures were 214, 428, and 856 J/cm2. Reflectance videos and TPEF were recorded simultaneously with an adaptive optics scanning light ophthalmoscope, and the TPEF time course of rods and cones was tracked. Locations were followed up immediately after the exposure and over several weeks.
Only the highest tested exposure (5x ANSI) resulted in detectable changes. During the immediate follow up, a regularly spaced subset of cones emitted ~3x less TPEF and showed an altered TPEF time course. The relative number of affected cones was 11-15%, consistent with the occurrence of S cones in macaque. Over several weeks, the diameter of these cones decreased. Ten weeks after the initial exposure, their space was occupied by cells with the appearance of rods. Other photoreceptors in the exposed area appeared to be unaffected. No changes in structure and TPEF time course were detected when the likelihood of two-photon events was decreased by broadening the pulse duration, while keeping the time-averaged exposure the same.
Retinal exposures to 856 J/cm2 with infrared ultrashort pulsed light can cause selective photoreceptor damage. The density and distribution of the affected cells suggest that they are S cones. While S cones have the lowest sensitivity in the infrared of any receptor class, they are known to be particularly susceptible to ultraviolet and blue light. The effect appears to be due to a multiphoton event and distinct from pure thermal and mechanical mechanisms often associated with ultrashort pulse exposures.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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