Feasibility of selective occlusion of choriocapillaries by laser targeted photo-occlusion

Purpose: Photodynamic therapy (PDT) is being considered as a method to non-thermally occlude choroidal neovascular membranes. However, it may not preserve integrity of the RPE. We developed a new method, laser targeted photo-occlusion, which potentially can preserve this essential tissue. The feasibility of occluding the normal choriocapillaris by this method was thus tested in normal rats. Methods: A photosensitizer, encapsulated in heat sensitive liposomes, was administered to pigmented normal rats by intravenous injection. Local release of the photosensitizer was achieved by causing a local mild rise of temperature in the choriocapillaris with the aid of a non-coagulating argon laser beam delivered through the pupil. A diode laser (680 nm) was then used to activate the photosensitizer upon its release and cause localized occlusion. The effect was monitored at 3, 8, 15 and 30 days thereafter with conventional and laser targeted angiography using carboxyfluorescein liposomes. Control experiments to evaluate the potential effects of the diode laser and the low dose of the unencapsulated photosensitizer were performed. Results: The choriocapillaris was occluded at day 3 and remained unchanged for the time of follow-up. Large choroidal vessels remained patent, and the retinal capillaries were clearly unaffected. The lack of changes in the control experiments demonstrated that the effect was not due to heat or to activation of the low dose of free photosensitizer. Light microscopy demonstrated absence of damage to the RPE. Conclusion: Laser targeted photo-occlusion permits selective occlusion of the normal choriocapillaris while minimizing damage to the overlying retina. The method thus has potential as a treatment of choroidal neovascularization that could possibly preserve vision.