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Y. M. Paulus, H. Nomoto, A. Jain, C. Sramek, R. F. Gariano, G. Schuele, D. Palanker; Selective Retinal Therapy With Microsecond Exposures Using a Continuous Line Scanning Laser. Invest. Ophthalmol. Vis. Sci. 2009;50(13):199.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal laser therapy with microsecond exposures enables selective targeting of retinal pigmented epithelium (RPE) and photoreceptors. Using a continuous line scanning laser, we evaluate the effects of scanning rate and power on spatial confinement, safety, and healing of the retinal lesions.
A 532 nm frequency-doubled Nd:YAG laser (PASCAL ®) with retinal beam diameters of 40 and 66 µm was applied to 48 eyes of 24 Dutch-Belted rabbits in vivo. The duration of retinal exposure (dwell time), defined as the diameter of the beam divided by the scanning velocity, varied from 15 to 60 µs. Lesions were assessed acutely ophthalmoscopically and with fluorescein angiography (FA), and flatmounts of the RPE, choroid, and sclera were stained with live-dead fluorescent assay (LD). Histological analysis was performed at 1 hour, 1 day, 3 days, 1 week, 2 weeks, 1 month, and 2 months after laser application.
Histological analysis demonstrated that ophthalmoscopic visibility (OV) of the lesions corresponded to photoreceptor damage. In sub-visible lesions, the FA and LD staining yielded similar thresholds of RPE damage, which was also visible histologically. The ratio of the thresholds of rupture and OV to FA visibility (measures of safety and selectivity) increased with decreasing duration and beam size. For 15 µs exposures and a beam diameter of 66 µm, these ratios were 7 (SD 1.23) and 2.8 (SD 0.72), respectively. For 40 µm beam diameter, they increased to 10.5 (SD 0.82) and 4 (SD 1.99), respectively. Above the threshold of OV, histology at one day showed RPE damage and drop-out of photoreceptors without effects in the inner retina. By one week, RPE proliferation and photoreceptor migration from adjacent unaffected areas into the damaged zone restored continuity of the photoreceptor and RPE layers. By 1 month, photoreceptors appeared normal without scarring. Some hypertrophy and hyperpigmentation of the renewed RPE persisted at 2 months.
Retinal therapy with a fast scanning continuous laser achieves selective targeting of the RPE and, at higher power, of the photoreceptors. The safety window and RPE selectivity increases with decreasing exposure time. Continuous scanning laser can treat large retinal areas within the eye fixation time. For example, 100 mm2 can be treated within 350 ms using a 66 µm beam and a scanning velocity of 4.4 m/s.
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