Laser photocoagulation of the feeder vessels of age-related macula degeneration-related choroidal neovascularization (CNV) membranes is a compelling treatment modality, one important reason being that the treatment site is removed from the fovea in cases of sub- or juxtafoveal CNV. To enhance the energy absorption in a target feeder vessel, an indocyanine green dye bolus is injected intravenously, and the 805nm wavelength diode laser beam is applied when the dye bolus transits the feeder vessel; this tends to reduce concomitant damage to adjacent tissue. A 3D theoretical simulation, using the Pennes bioheat equation, was performed to study the temperature distribution in the choroidal feeder vessel and its vicinity during laser photocoagulation. The results indicate that temperature elevation in the target feeder vessel increases by 20% in dye-enhanced photocoagulation, compared to just photocoagulation alone. The dye bolus not only increases the laser energy absorption in the feeder vessel but also shifts the epicenter of maximum temperature away from the sensitive sensory retina and retinal pigment epithelial layers and toward the feeder vessel. Two dominant factors in temperature elevation of the feeder vessel are location of the feeder vessel and blood flow velocity through it. Feeder vessel temperature elevation becomes smaller as distance between it and the choriocapillaris layer increases. The cooling effect of blood flow through the feeder vessel can reduce the temperature elevation by up to 21% of the maximum that could be produced. Calculations were also performed to examine the effect of the size of the laser spot. To achieve the same temperature elevation in the feeder vessel when the laser spot diameter is doubled, the laser power level has to be increased by only 60%. In addition, our results have suggested that more studies are needed to measure the constants in the Arrhenius integral for assessing thermal damage in various tissues.
Retinal pigment epithelial detachments in the elderly: a controlled trial of argon laser photocoagulation.
