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Jie Zhang, David R Williams, Jennifer J Hunter; RPE disruption with long duration visible light exposure is governed by a photochemical damage mechanism. Invest. Ophthalmol. Vis. Sci. 2018;59(9):666.
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© ARVO (1962-2015); The Authors (2016-present)
Exposure to bright visible light for durations typical in ophthalmic imaging disrupts the RPE mosaic, as revealed with autofluorescence imaging. Using a fluorescence adaptive optics scanning light ophthalmoscope in macaque, we measured the action spectrum for RPE disruption from 460 - 670 nm. Here, we determine whether RPE disruption is photochemical by testing reciprocity between power and duration, ie. that it depends only on the total photon dose delivered.
We tested retinal radiant exposures (RREs) that corresponded to 20% and 80% probabilities of causing RPE disruption in our original dataset using 4 macaques. We assessed reciprocity of exposure power and duration by varying the power at the cornea and the exposure duration while maintaining constant RREs. At 460 nm, the RREs were 29 (10 µW, 497 s; 20 µW, 248 s; 50 µW, 99 s) and 36.4 J/cm2 (10 µW, 624 s; 20 µW, 312 s; 50 µW, 125 s). At 594 nm, the RREs were 267 (114 µW, 400 s; 92 µW, 500 s; 46 µW, 1000 s) and 326 J/cm2 (140 µW, 400 s; 112 µW, 500 s; 56 µW, 1000 s). The exposure calculations assumed a 15 mm focal length. Each combination was tested twice in each of 2 macaques (axial length, AL = 18.20 and 17.17 mm). The RPE disruption was determined using a two-alternative forced choice task comparing the pre- and 2-week post-exposure RPE images.
Overall, the two monkeys showed different probabilities of RPE disruption for the same RRE calculated using the assumed model eye. However, variations in AL alter the actual amount of light delivered to the retina. After compensation for the AL difference, the exposure required to produce the expected percentage of exposure locations with RPE disruption agreed closely across different monkeys. The data were consistent with the hypothesis that different combinations of exposure power and duration produce the same amount of RPE disruption, demonstrating reciprocity.
Over the range of powers and durations tested, RPE disruption is consistent with a photochemical damage mechanism. Therefore, an action spectrum based on the total energy delivered to the retina can be used in a light safety standard revision. Given a typical variation from 16 to 27 mm in human AL (Bhardwaj et al. 2013), AL could be an important factor to consider in light safety guidelines. Sufficient protection could be achieved if safety standards incorporated an additional safety factor of 50%.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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