Histopathologic and ultrastructural findings of photocoagulation lesions produced by transpupillary diode laser in the rabbit retina

Transpupillary retinal photocoagulations were performed on ten eyes of five pigmented rabbits using a diode laser (Nidek Co., LTD, Aichi, Japan) emitting infrared radiation at 800 nm wavelength. A histological and an ultrastructural study on the treated eyes were done at 1, 3, 5, and 7 days after retinal photocoagulations. The purpose of this study was to observe the sequential changes in the retina and the choroid following transpupillary diode laser retinal photocoagulations at the parameters of laser power which produced a grayish white retinal discoloration with distinct white center. It seemed that the lesion was grade 3 retinal photocoagulation by Tso et al's classification. It appeared that the parameters necessary to produce grade 3 photocoagulation lesions were 160 mW power, and 0.2 second duration at 200 microns size. In general, with an agreement to other reports, histologic study of the diode laser lesions showed that the outer retina was damaged more severely than the inner retina. However, on day 1 after laser treatment, the alterations were more profound in the inner retina than in the outer retina and an occasional swelling of the axons in the nerve fiber layer was observed on the ultrastructural study. The results observed have not been found in other previous studies and suggest that the inner retina might be injured directly by 800 nm wavelength diode laser radiations. Thus we could conclude that 800 nm wavelength diode radiation might be absorbed by melanin pigment and also by other chromophores contained in inner retinal tissues. Further studies must follow to verify the laser-tissue interactions in diode laser retinal photocoagulations.