Development and recovery of laser-induced retinal lesion in rats.

Retinal photocoagulation lesions undergo primary and secondary degeneration followed by partial healing. This study follows the sequential changes in laser-induced retinal lesion over a time span of 60 days. Argon laser lesions (514 & 544 nm, 200Pm, 0.1 W, 0.05 sec) were created in 36 pigmented rats. Sections of the retinal lesions were evaluated by light microscopy 1, 24, 48, 72 hours, 20, and 60 days after the injury (six animals at each time point). The diameter of the lesion was equal to that of the laser spot 1h after irradiation and increased by 24h. It decreased later, slightly during the following 48h and significantly by 20 days. The destruction of photoreceptors was most severe after 24-48h. The nuclei in the outer-nuclear layer at the lesion site were pyknotic at one hour and disappeared later. Healing processes began 72h after the irradiation and was completed by 60 days. Filling-in by sliding of near nuclei was observed by the 60th day. Reversible changes were seen also in the retinal pigment epithelium (with formation of a plaque at 72h and its degradation later on) and in the choroid (disorganization of capillaries by 48h with later reorganization). In summary, the development of a laser-induced injury is gradual: photoreceptors are damaged first and than the damage spreads to other layers of retina and to areas adjacent the primary injury site. The expansion of the damage is stopped by 72h and the adjacent tissues start to fill the area of the lesion and remodel the retina.