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K. Merrill, S. N. Truong, W. C. Lloyd, III, L. S. Morse, D. G. Telander; The Comparative Histologic Effects of Subthreshold 530 nm and 810 nm Diode Micropluse Laser on the Retina. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1424. doi: https://doi.org/.
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Conventional laser photocoagulation delivers a continuous wave (CW) laser burn, which will normally cause a full-thickness burn of the retina. New micropulse lasers deliver a pulse train of interrupted laser, which aim to limit thermal damage to the RPE. This study’s purpose was to compare the histologic effects on the retina of a newly developed 530 nm micropulse laser (IRIDEX) to the 810 nm micropulse laser (IRIS) at multiple time points.
Dutch belted rabbits were treated with two wavelengths of the diode micropulse laser (530 nm and 810 nm). Four rabbits, totaling eight eyes for each wavelength were studied. A grid of 6 x 6 laser application spots was made in each eye making 36 burns. Spots were made one burn width apart. The threshold power was determined with CW laser for 200msec using a 400µm diameter aerial laser spot and a standard Mainster lens, thus creating laser spots of 260µm on the rabbit retinas. Subthreshold (invisible) burns were then placed using duty cycles of 5, 10, 20, 40, and 100% of the threshold burns. One-half of the rabbits in each group were sacrificed at 2 weeks post-treatment and the other half sacrificed at 4 weeks post-treatment. The eyes were then enucleated and the retinas isolated and fixed in paraffin for histologic analysis.
Histologic evidence of retinal photocoagulation from subclinical micropulse laser burns at both the 530 nm and 810 nm wavelengths was shown. Micropulse laser was applied at multiple levels (5, 10, 25, 40% duty cycle of threshold power). All levels showed varying degrees of retinal disruption, edema, and pigment migration in several retinal layers. Full thickness changes were not seen with subthreshold laser powers. Micropulse burns from both the 530 nm and the 810 nm lasers showed limited tissue damage to the RPE layer, largely sparing the choriocapillaris and overlying neurosensory retina. The most selectivity of tissue damage appeared to be at duty cycles of 10 - 20%. Control laser (100% threshold power) showed full thickness tissue disruption, the effects of which were consistent among the subjects.
This study finds that the 530 nm, like the 810 nm micropulse laser, can be used to selectively photocoagulate the RPE, and at higher duty cycles the RPE and outer retina. As the 530 nm laser is more commonly used for macular laser photocoagulation, a micropulse laser at this wavelength may be more convenient to the clinician. This subthreshold laser treatment may prove to limit both photoreceptor damage and associated vision loss in the laser treatment of patients with macular edema.
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