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Purpose: To compare the chorioretinal tissue response after different half-strength parameters of photodynamic therapy (PDT) in rabbits., Methods: The study included 4 groups, and each group contained 4 animals. The full dose served as the control group: verteporfin (4 mg/kg) with 600 mW/cm(2) irradiance from a diode laser at 689 nm applied to the retina for 8 s. One parameter was changed to half-strength in the other 3 groups. The HLaser group received half-strength laser irradiance. The HTime group was exposed to photosensitization for half the time, and the HDose group received half the drug dose. Six laser spots were generated in each of the eyes of every rabbit and documented graphically. The lesions were examined on days 1, 7, and 42 after PDT treatment using color fundus imaging, fluorescein angiography (FA), and histopathology analysis., Results: PDT treatment in rabbits caused chorioretinal damage in all 4 groups. FA on day 1 showed that the use of half the laser irradiance, half the drug dose, or half the photosensitizing time tended to decrease the damage to the chorioretinal tissue in terms of the number of occlusions and the area of occlusion, but only the results from half the laser irradiance were significantly different. In addition, the HLaser and HDose groups showed significantly less apoptosis by TUNEL staining on day 1., Conclusions: Among these PDT parameters, decreasing the laser irradiance by half showed the greatest decrease in chorioretinal damage in an experimental animal model.

Selective thermal treatment to retina is induced by short pulsed lasers to denaturize retinal pigment epithelium (RPE) selectively, while sparing the sensitive photoreceptors. The problem associated with the usage of short pulsed laser is the difficulty in determining the correct dosimetry parameters. This study quantifies the influence of laser parameters over the therapeutic range. The laser-tissue interaction is numerically investigated by analyzing the transient temperature in ocular tissues during the treatment. The rate process analysis for thermal injury is employed to estimate the selective damage of retina. The contours of Arrhenius integral value (Omeg/ Omegamax) presented in this study show both the area and magnitude of damage caused by various laser parameters. Results reveal that the 2 micros pulsed laser with green wavelength and Gaussian profile is relatively more effective for selective retinal treatment. The repetition frequency of 100 Hz is found to produce selectively RPE damage, while higher frequencies produce collateral damage to neural retina and choroid located within 2 microm from the RPE interface.

Laser-chorioretinal interactions depend on: non controllable parameters: spectral transmission, absorption of tissue, scattering of laser, distribution of ocular chromophores, thermal conductivity of tissue. They are important determinant of laser tissue effects. parameters that can be monitored by the physician: size of the spot, pulse duration, laser energy and laser wavelengths. Theoretical predictions of laser-tissue interaction with the choroid and the retinal agree with histological and clinical data. The choice of physical parameters of laser must be done in order to prevent adverse effects as to obtain researched therapeutic goal.

A neodymium-YAG laser, operating in its free-running mode with pulse durations of 10 to 20 ms, was used to treat 63 cases of choroidal and retinal diseases. These cases included diabetic background retinopathies, thrombosis of the central retinal vein or branch retinal vein, neovascular membranes under the pigment epithelium, and retinal breaks or degenerations of the peripheral retina. Because the number of diabetic retinopathies (24) and their average follow-up time (25.5 months) are insufficient for the evaluation of irradiation effects, no definite statement as to the merits of this energy modality in such cases can be made. The irradiation of infarcted retina was followed by extensive atrophy of the involved area. No secondary glaucoma was observed during a follow-up period of 16.4 months. The irradiation in four cases of neovascular subretinal membranes was followed by disappearance of the membranes. However, relapse later occurred in all four cases. The irradiation of eight retinal breaks resulted in solid scar tissue. Two choroidal melanomas were destroyed and did not recur during a follow-up period of 22 months.

The effects of the neodymium:YAG laser in the thermal mode on the subhuman primate retina were studied with ophthalmoscopy, intravenous fluorescein angiography, light microscopy, transmission electron microscopy, and scanning electron microscopy using a plastic injection-corrosion technique. Representative samples were examined at 24 hours, 14 days, and 28 days after treatment. The burns appeared to predominantly affect the outer half of the retina and the inner half of the choroid, although with higher energy levels the inner retinal layers were occasionally involved. In most instances Bruch's membrane was disrupted. The injured choriocapillaris underwent a reparative process during a four-week period in which the discrete lesions observed at 24 hours had disappeared.

This paper reviews the historical events leading to the development of the ophthalmic laser. The formation of the argon laser beam and its biological effects on ocular tissues are outlined, with the major emphasis on the retinal and choroidal tissues. The clinical indications, contraindications, and complications of ophthalmic argon laser photocoagulation for the anterior segment as well as the posterior segment structures of the eye are also thoroughly discussed.

The Authors were able to observe histologically the lesions induced by Argon-laser on the retina and choroid of two eyeballs that afterwards were enucleated because they developed choroid melanoma. The observations have been respectively made 10, 20 and 40 days after the beginning of the treatment and a progressive structural overturning of the retina and the choroid due to the obstruction of choriocapillary vessels, to the tendency to fusion of the various nuclear layers as well as to the connection of the layer of nerve fibres, has been observed.