الفهرس 
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Abstract
This work aimed to consider the hazardous side effect of Q-switched Nd:YAG laser on the vitreous humor and on the structure of the retina. In addition, to evaluate the protective role of vitamin C against laser photo disruption. Rabbits eyes were exposed to vitreous humor liquefaction with Q - switched Nd:YAG laser using two different energy protocols (5 mJ x 100 pulse and 10 mJ x 50 pulse) with and without vitamin C administration .Methods: The rabbits were divided into five main groups which are a control group ( without any treatment ) and another four groups of nine rabbits each as follows: The 1st group was used as a control group without any treatment. The 2nd group was divided into three subgroups (n=6) and then treated with 5 mJ x 100 pulse (x means times) delivered to the anterior, middle and posterior vitreous humor respectively. The 3rd group was received a daily dose of 25 mg/Kg vitamin C for two weeks then divided into three subgroups and treated with laser in the same manner as the first group. The 4th group was divided into three subgroups (n=6) and then treated with 10 mJ x 50 pulse delivered to the anterior, middle and posterior vitreous respectively. The 5th group was received a daily dose of 25 mg/Kg vitamin C for two weeks then divided into three subgroups and treated with laser in the same manner as the third group. After two weeks of laser treatment, rabbits were decapitated, eyes were enucleated for determination of the protein content, refractive index (RI), the viscosity and SDS- poly acrylamide gel electrophoresis of vitreous humor. Moreover, the histological changes in the retina were investigated to evaluate the protective role of vitamin C against laser photodisruption effect. The results showed that, the anterior vitreous group exposed to of 5mJx100 pulse and supplemented with vitamin C, showed no obvious change. Furthermore, all other treated groups especially for mid-vitreous, and posterior vitreous showed increase in the protein content, refractive index (RI) and the viscosity of vitreous humor. The electrophoresis separation indicates alteration in the composition of the protein of vitreous humor. In addition, histological examination of the retina indicated alteration in retina’s tissues after laser treatment. Application of Q-switched Nd:YAG laser for vitreous humor liquefaction induces deleterious effect on the gel state of vitreous humor and in retina’s layers. Although there were some sorts of improvements in retinas supplemented with vitamin C, it cannot protect them against laser oxidative damage. The present work may provide a model to compare the relative effect of different treatment parameters to help in establish the least adverse effects. Laser tissue interaction after photo disruption with the Q-switched Nd:YAG laser induce optical breakdown, could conceivably collapse the vitreous in a purely mechanical fashion changing its protein content, refractive index, viscosity, and protein composition. The effect of laser on vitreous is site dependence. When it is applied near the retina, the effect will be more pronounced as indicated by histological examination. Oral supplementation of vitamin C two weeks before Q- switched Nd:YAG laser may have inefficient protective effect against laser complications. Future clinical and experimental researches are required to downgrade Q- switched Nd:YAG laser complications.