Retinal damage threshold of ophthalmic Q-switched Nd-YAG laser in monkey eyes

Q-Switched Nd-YAG laser was irradiated to the ocular fundus of 24 eyes of 12 monkeys (Macaca fascicularis) using MICRORUPTOR II (LASAG AG, Thun, Switzerland) through the Goldmann type three-mirror contact lens. Multimode irradiation conditions were used; the pulse duration of 12 nanoseconds, spot size of 80 micrometers, convergence angle of 16 degrees and with various energy levels from 0.05 to 2.8 mJ. After irradiation, the fundus changes were observed by ophthalmoscopy and color fundus photography at 24 hours, 48 hours, 1, 2 and 5 weeks, and by fluorescein angiography at 24 hours, 1 and 5 weeks. Immediately after the irradiation and at 2 and 5 weeks, eyeballs were enucleated and histological observations were carried out. The fundus changes were more evident at 24 hours than at the immediate period after irradiation. At 24 hours, findings by fundus photography and fluorescein angiography were compared: both findings were in agreement. Consequently the retinal damage threshold was determined from the 24-hour findings; the threshold in ED50 was 296.3 microJ with 95% confidence limits from 249.8 to 351.1 microJ. These values corresponded to the retinal energy density of 3.5 J cm-2. Histological observations revealed that the site of damage was mainly in the retinal pigment epithelium when the irradiation was below 1.4 mJ. When the energy was increased to 1.9 mJ, the tissue damage extended to the middle layer of the neuroretina and also disruption of the Bruch's membrane occurred, leading to subretinal hemorrhage. With the energy of 2.8 mJ, destruction extended over wide areas including whole layers of the choroid and retina, leading to vitreous hemorrhage. Over the period of 5 weeks, proliferation of the retinal pigment epithelial cells took place and replaced the damaged retinal tissues.