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Jenny Wang, Yi Quan, Roopa Dalal, Daniel V Palanker; Optimizing Non-Damaging Retinal Laser Therapy: Tissue Response to Micropulse Modulation. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5981.
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© ARVO (1962-2015); The Authors (2016-present)
Recent progress in retinal laser therapy has centered upon using sub-lethal thermal stress or selective lethality of the retinal pigment epithelium (RPE) layer as a stimulus for repair of retinal disorders. Temporal modulation of the laser pulse, including micropulse, is believed to increase the selectivity of laser treatment but this effect has not been carefully evaluated. In this study, we measure the tissue response to different duty cycles of micropulse laser and calculate the therapeutic window to evaluate the advantages and drawbacks of temporal modulation.
A 577 nm laser was used to deliver 140 μm laser spots onto the retina in Dutch-belted rabbits (n = 11) with overall pulse envelopes of 20 or 200 ms. The micropulse modulation was done with 500 Hz repetition rate and duty cycles of 3%, 5%, 9%, 15%, 25%, 47%, or 100% (continuous-wave, CW). Each laser lesion was given a binary visible-or-invisible grade for 3 different levels of tissue response: ophthalmoscopic visibility, which is indicative of photoreceptor damage (a) immediately after laser (IV), (b) delayed by 1-5 min (DV), and (c) in fluorescein angiography (FA), which indicates RPE damage. The thresholds were determined as the 50% damage points in probit model fit and were compared to an Arrhenius integral of the modeled tissue response.
Micropulse modulation did not affect the average power for IV and DV thresholds, except at 20 ms, 3% duty cycle. The average power for FA thresholds was slightly reduced by micropulse modulation, with increasing effect for shorter pulse envelope and shorter duty cycles. For 5% duty cycle, FA threshold was lower than with CW exposure by 35 ± 21.5% for 20 ms and by 22 ±15% for 200 ms envelopes. These results validated our computational model of tissue response, which was then used for extrapolation to the non-damaging regime. Our model predicts that micropulse modulation reduces the therapeutic window between the onset of heat shock protein expression and RPE damage at 5% duty cycle vs CW by 16% and 5% for 20 and 200 ms envelopes, respectively.
Micropulse modulation can confine thermal stress to the light-absorbing RPE and choroid when employed with sufficiently short pulse envelopes and duty cycles. However, it does not offer any advantages for non-damaging therapy and actually reduces the non-damaging therapeutic window due to higher temperatures in the pigmented layers.
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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