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D. V. Palanker, A. Jain, Y. Paulus, D. E. Andersen, M. S. Blumenkranz; Rapid and Confined Retinal Photocoagulation Using Millisecond Pulses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4168.
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The rapid application of short pulses for patterned retinal photocoagulation improves comfort, speed, and precision while decreasing pain and fatigue. We evaluate the size, characteristics, and safety of retinal burns produced with pulse durations in the 1-100 millisecond range.
A 532nm Nd:YAG laser was used to irradiate retinas in Dutch-Belt rabbits with retinal beam sizes of 132 and 330µm (200 and 500 µm aerial). Burns were clinically graded 1 minute after placement, their apparent size was measured by digital imaging, and the lesion structure was assessed histologically at 1 hour, 1 day, 1 week, and 1 month after treatment.
Burn size increased linearly with laser power and logarithmically with pulse duration. Shorter pulses prevented "thermal blooming" of the lesions and confined the coagulation zone to the outer retina. Decreasing pulse duration was also associated with a decrease in the width of the therapeutic window for visible burns, as defined by the ratio of the threshold power for producing a rupture to that of a light burn. For 132 and 330 µm retinal laser spot sizes, the therapeutic window declined from 3.9 to 3.0, and 5.4 to 3.7, respectively, as the pulse duration decreased from 100 to 20 ms. As pulse durations approached 1 ms, the therapeutic window for visible burns decreased to unity, at which point rupture and light burn were equally likely. Ophthalmoscopically invisible lesions affecting only the RPE and photoreceptors could be safely produced with pulse durations down to 3 ms.
Retinal burn size, depth, and safety are strongly influenced by laser spot size, pulse duration, and power. For visible burns, a compromise between the favorable impacts of improved speed, reduced collateral damage, and decreased pain, while still maintaining a sufficient therapeutic window (≥3) is achieved at pulse durations of 10-20 ms. Selective damage to the RPE and photoreceptors can be safely produced by a single pulse in the millisecond range, creating ophthalmoscopically invisible lesions.
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