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CILIARY BODY PDT IN PIGMENTED RABBITS EYES: EFFECT OF SINGLE AND REPEATED TREATMENT. M K Tsilimbaris, M Daskalakis, T L Naoumidi, I I Naoumidi and I G Pallikaris Vardinoyannion Eye Institute,University of Crete Medical School,Greece Abstract We investigated the hypotensive effect of ciliary body PDT in the eyes of 30 pigmented rabbits after single (n=25) and repeated (n=5) treatment. We used chloroaluminum sulfonated phthalocyanine as photosensitizer and a diode laser (670 nm) as the light source. Photosensitizer was administered by means of continuous intravenous infusion so as to achieve stable plasma levels during PDT. Ciliary body was irradiated transsclerally by means of an optic fiber applied on the corneoscleral limbus. In all cases 14 - 16 laser applications were performed to cover 360° of the ciliary body. Animals were followed for a maximum of 30 days by means of tonometry and biomicroscopy. Re treatments were performed on day 13 using the same irradiation protocol. At the end of the follow up time animals were sacrificed and their eyes prepared for light and electron microscopy. Transscleral ciliary body PDT resulted in significant but temporary reduction of IOP in all cases. The effect lasted about two weeks. Re treatment led to a new significant drop of the IOP which lasted about two weeks again. In histological examination the initial effect of PDT was vascular thrombosis which was followed by significant alteration at the level of ciliary epithelium. Edema and disintegration of the ciliary epithelial layers were characteristic findings the first days after treatment. All changes, however were temporary; after 15 days the appearance of the ciliary body had returned to normal both in single treated and retreated animals. Contact transscleral PDT with the treatment parameters used in this study results in significant but temporary functional and morphological alteration in pigmented rabbits ciliary body. Further investigation may lead to a new PDT mediated cyclodestructive technique with possible clinical applications in glaucoma treatment.