Measurements of retinal temperature increase during photodynamic therapy for choroidal neovascularization

Abstract

To study the risk of retinal thermal injury from 532 nm laser during photodynamic therapy (PDT) for choroidal neovascularization (CNV) by measuring the retinal temperature increase of rabbit eyes. A microthermocouple technique was developed to measure retinal temperature increase during PDT in pigmented and non-pigmented rabbit eyes. The 532 nm laser exposures were performed with 100-s duration, 2-mm spot size, and retinal irradiance ranging from 400 to 1600 mW/cm². A K-type microthermocouple was inserted through the sclerotomy and advanced until the tip reached the retina at the posterior pole. The thermocouple was connected a computer that recorded and analyzed retinal temperature data. The results showed that the retinal temperature increase during laser exposure was proportional to retinal irradiance with a particular spot diameter, exposure duration, wavelength, and fundus pigmentation. And the measured retinal temperature increases in pigmented rabbits were a little higher than those in albino rabbits under the same radiant condition. Retinal threshold irradiance required for visible lesions at laser wavelength of 532 nm with 2.0-mm spot size and 100-s duration was 1657 mW/cm² in albino and 1003 mW/cm² in pigmented rabbits, respectively, corresponding to retinal temperature increase of about 8 °C and 6 °C. The measured temperatures in albino and pigmented rabbit eyes were both lower than the model predictions, especially in pigmented rabbits. Therefore, further parameter modifying should be performed to obtain accuracy prediction of retinal temperature.