Bubble formation as primary interaction mechanism in retinal laser exposure with 200-ns laser pulses

Retinal laser photocoagulation is generally performed by laser pulses of a few hundred milliseconds. The tissue interaction mechanism is a pure thermal interaction mechanism. As pulse duration gets shorter, different, non-thermal interaction mechanisms start to appear. The time domain for a change of tissue interaction mechanism seems to be in the ns and micros range. The goal of this study was to characterize the tissue interaction mechanism with 200-ns laser pulses, which approximate the thermal relaxation time of single melanin granules.

The retinas of 19 eyes of 10 rabbits were irradiated by 10 and 500 repetitive laser pulses (wavelength, 532 nm; repetition rate, 500 Hz; pulse duration, 200 ns; per pulse energy, 0-120 microJ; retinal spot size, 100 microm). The effects were evaluated by fluorescein angiography, ophthalmoscopy and by theoretical thermal calculations. Light microscopy and transmission electron microscopy were additionally performed on lesions irradiated by 500 pulses.

Single pulse threshold energies for angiographic visibility were 3.5 microJ (10 pulses) and 2.1 microJ (500 pulses), for ophthalmoscopic visibility 9.0 microJ (10 pulses) vs. 8.6 microJ (500 pulses). At energy levels above ophthalmoscopic visibility macroscopically visible bubble formation inside the retina could be observed. This occurred at energy levels of 35 microJ (10 pulses) vs. 17 microJ (500 pulses). Microscopic evaluation of lesions irradiated with 500 pulses and energies at the angiographic threshold showed a damage primarily to the RPE. Additional outer segment damage of the photoreceptors could be found. A gap between damaged RPE cells and the outer segments could be repeatedly found as well as damaged RPE cells, which were detached from intact Bruch's membrane. Temperature calculation shows that temperatures above 100 degrees C may exist around single melanin granules.

The studies suggest that RPE damage may occur by bubble formation around single melanin granules.