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Ralf Brinkmann, Keiji Inagaki, Ben Schmarbeck, Alessa Hutfilz, Katharina Bliedtner, Kishiko Ohkoshi, Yoko Miura; Comparison and analysis of micropulse and continuous wave laser application on retinal pigment epithelium (RPE) ex vivo. Invest. Ophthalmol. Vis. Sci. 2018;59(9):6199.
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© ARVO (1962-2015); The Authors (2016-present)
Micropulse (mp) and continuous wave (cw) lasers are both used for the treatment of diabetic macular oedema and other retinal disorders. The aim of this work is to measure the onset and area of RPE damage as well as the induced temperatures for both modalities, respectively, using porcine RPE. The data are analyzed using temperature and Arrhenius damage calculations.
A retinal laser system (IQ 577, Iridex Corp.) at a wavelength of 577 nm was used in cw and 500 Hz mp mode with duty cycles (DC) of 5% and 15% on porcine RPE-sclera organ culture. Irradiation diameter of 200 µm and overall irradiation time of 200 ms were chosen. The average laser power was varied between 10 - 90 mW, respectively, for all three modi. This corresponds to peak powers of 200 - 1800 mW and 66 - 600 mW for the 5% and 15 % micropulse DC, respectively. The temperature increase during irradiation was measured in real-time with 1 kHz resolution by optoacoustics using 523 nm, 75 ns pulses. Three hours after irradiation, RPE cell viability was investigated with the fluorescent dye Calcein-AM. The area of dead non-fluorescent cells was evaluated.
We found an onset of RPE damage around 25 mW and an almost linear increase of the damage area over average power up to 50 mW with a slope of 2000 µm2/mW for all three laser modi without any significant deviations. Exceeding 50 mW, however the slope of the 5% DC drops to about 500 µm2/mW, likely owing to the onset of vaporization, since bubble formation were observed ophthalmoscopically and by optoacoustics. The measured temperatures rise monotonously (cw) and step wise (mp), however, the average temperature courses were almost identical, leading to final temperatures of 66.6 °C and 69.6 °C after 200 ms and 50 mW average power, for the cw and 5% DC mp, respectively, averaged over 5 spots. This corresponds to an RPE absorption of 40 % when assuming an absorption coefficient of 830 cm-1 over a 6 µm absorbing layer.
There are almost no differences in the damage ranges for mp and cw, except for very high peak powers of 1200 mW (60 mW average) and more in the 5 % DC case. We therefore hypothesise that the therapeutic outcome will be virtually identical for continuous wave and micropulse laser application when using the same average laser power over the same overall irradiation time, which, however, needs to be proved in clinical trials.
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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