Factors Affecting on Determination of Laser Power in Transpupillary Thermotherapy

ABSTRACT

PURPOSE: To evaluate whether the degree of melanin pigmentation of fundus or intraocular pressure would have influence on the determination of laser power for effective transpupillary thermotherapy(TTT). METHODS: TTT was performed on 6 eyes of each of the brown, yellow and white rabbits. Immediately after diode laser irradiation to posterior pole, retinal periphery, and iris with various power(2 mm, 60 or 120 sec), macular threshold power(MTP), peripheral threshold power(PTP), and iris threshold power(ITP) were defined as a maximum power not causing any ophthalmoscopic change on the respective laser site. To assess the influence of intraocular pressure, TTT was performed with the eyeballs compressed as much as retinal arterial pulsation disppeared. Three weeks after laser irradiation, funduscopic and fluorescein angiographic appearance were correlated with histopathologic findings. RESULTS: We found no difference of MTP between brown and yellow rabbit but their MTP were smaller than that of white one(P<0.05). There was positive correlation between MTP, PTP and ITP in all experimental species. ITP/MTP and PTP/MTP were 3.45+/-0.68, 1.09+/-0.10 in brown rabbit, 3.84+/-0.79, 1.12+/-0.13 in yellow rabbit and 4.13+/-0.43, 1.16+/-0.14 in white rabbit respectively. When TTT was performed with the eyeball compressed, the laser spot looks whiter in all cases than without. Microscopic examination revealed occlusion of choriocapillaris and photoreceptor degeneration in the laser spots formed by MTP. CONCLUSION: When performing TTT, one should reduce the laser power according to the amount of melanin pigmentation of fundus and intraocular pressure. Empirical laser burn to the retinal periphery will be a helpful mean to determine optimal laser power for TTT.