To compare histopathological and apoptotic changes of ophthalmoscopically similar subthreshold laser burns made by a low power-long duration (LD) and a high power-short duration (SD) subthreshold laser treatment.

Ophthalmoscopically invisible subthreshold laser burns with a 3.0 mm spot size were made using an 810 nm diode laser on the rabbit retina. Lasers were applied for 60 seconds in the LD group, and 1 second in the SD group. Laser power was adjusted to achieve ophthalmoscopically invisible burns just below the threshold. The rabbits were sacrificed at 6, 12, 24, and 72 hours, 1, 2, and 4 weeks after laser treatment. The eyes were processed for light microscopic examination using hematoxylin and eosin (H&E), toluidine blue, and TdT-dUTP terminal nick-end labeling (TUNEL) staining. Eyes were also processed for electron microscopic examination.

The changes in the retina were different between the two groups. The LD group showed abundant TUNEL positive cells in all the retinal layers at 6 hours after laser treatment, and distinct histological changes in the outer nuclear layer. Conversely, in the SD group, apoptosis did not occur and histological alteration in the outer nuclear layer was minimal.

Subthreshold laser treatment for 1 second reduced damage of the inner retinal layer and did not result in apoptosis in the neurosensory retina while maintaining a similar effect on the RPE and its adjacent region.