Transpupillary thermotherapy threshold parameters: Effect of indocyanine green pretreatment

Purpose: To evaluate the effect of combined treatment with systemic indocyanine green (ICG) on threshold fluence levels of transpupillary thermotherapy (TTT) in rabbits. Methods: Four pigmented rabbits and 13 nonpigmented rabbits were studied. TTT was performed on normal rabbit choriocapillaris using an 810-nm diode laser via slit-lamp biomicroscope delivery through a Goldmann macular lens. Laser spot size, power, and duration of laser exposure were varied to achieve a range of TTT fluences for threshold testing in both albino and pigmented rabbit fundi. Intravenous ICG pretreatment at doses of 0.41 to 10 mg/kg was initiated at varying times before TTT treatment. After the experiment, the eyes were enucleated under deep anesthesia, the animals were killed, and the eyes were prepared for light microscopy. Results: When intravenous ICG pretreatment was employed, there was a dose-dependent decrease in the TTT fluence threshold as compared with known threshold values. At threshold fluences, histopathologic sections revealed damage to all layers of the retina in addition to choriocapillaris damage. Conclusion: Intravenous ICG pretreatment can be used to lower the TTT threshold fluence and irradiance required to create angiographically visible lesions in the normal rabbit choriocapillaris. Damage was seen in all layers of the retina and choriocapillaris at threshold levels when TTT was used alone or in combination with ICG pretreatment.