Selective photoreceptor changes after ultrashort pulse laser exposure in the infrared

Abstract

For two-photon excited fluorescence (TPEF) retinal imaging, infrared (IR) ultrashort pulse lasers are used to excite fluorophores such as all-trans-retinol and NADH in the ultraviolet (UV), a spectral range that is usually blocked by the ocular media. Previously, we have shown that in vivo TPEF retinal imaging can test photoreceptor function at light levels below the 2014 ANSI MPE without detectable damage (20.4 J/cm², ~0.26× ANSI). Here, our goal was to investigate which photoreceptor class was most susceptible to ultrashort pulse laser exposures in the IR that were much higher (856 J/cm², ~5× ANSI). Following dark adaptation for 15 min, the photoreceptor layer of two macaques was exposed to a 55 fs, 730 nm pulse laser with an adaptive optics scanning light ophthalmoscope. Reflectance videos and TPEF, prevailing from retinol, were recorded simultaneously and the TPEF time course of rods and cones was tracked separately. After repeated exposures, a subset of cones (11–15%) emitted ~3× less TPEF and appeared atrophied in corresponding reflectance images. These changes persisted for several weeks without noticeable recovery. Structure and TPEF responses of the remaining photoreceptors were largely preserved. No changes could be detected after the same exposures when the likelihood of a two-photon event was decreased by temporally broadening the pulse duration. Retinal exposures with IR ultrashort pulsed light can cause selective photoreceptor damage. The relative quantity, the scattered but regular distribution and the distinct stimulation by the imaging laser suggest that the affected cones are S cones. Interestingly, S cones are the receptor class with the lowest sensitivity in the IR but are known to be particularly susceptible to UV and blue light. This effect of intense IR light is probably not a consequence of photopigment bleaching in the affected cones, but is conceivably a nonlinear effect that has not been considered in the safety standard for ultrashort pulse exposures.

Meeting abstract presented at the 2016 OSA Fall Vision Meeting

Footnotes

 Supported by NIH grants R44 AG043645, P30 EY001319, R01 EY022371, grants BRP-EY014375 and K23 EY016700, and the Arnold and Mabel Beckmann Foundation.