To systematically evaluate the effects of laser beam size, power, and pulse duration of 1 to 100 milliseconds on the characteristics of ophthalmoscopically visible retinal coagulation lesions.
A 532-nm Nd:YAG laser was used to irradiate 36 retinas in Dutch Belt rabbits with retinal beam sizes of 66, 132, and 330 μm. Lesions were clinically graded 1 minute after placement, their size measured by digital imaging, and their depth assessed histologically at different time points.
Retinal lesion size increased linearly with laser power and logarithmically with pulse duration. The width of the therapeutic window, defined by the ratio of the threshold power for producing a rupture to that of a mild coagulation, decreased with decreasing pulse durations. For 132- and 330-μm retinal beam sizes, the therapeutic window declined from 3.9 to 3.0 and 5.4 to 3.7, respectively, as pulse duration decreased from 100 to 20 ms. At pulse durations of 1 millisecond, the therapeutic window decreased to unity, at which point rupture and a mild lesion were equally likely to occur.
At shorter pulse durations, the width and axial extent of the retinal lesions are smaller and less dependent on variations in laser power than at longer durations. The width of the therapeutic window, a measure of relative safety, increases with the beam size.
Pulse durations of approximately 20 milliseconds represent an optimal compromise between the favorable impact of speed, higher spatial localization, and reduced collateral damage on one hand, and sufficient width of the therapeutic window (> 3) on the other.