Photodynamic therapy (PDT) uses a non-heat-generating laser to treat abnormal blood vessels. Visudyne is injected into the patient's arm and flows through the vessels in the eye. Upon exposure to the laser, a chemical reaction occurs that seals off the leaky vessels. Since the dye is light sensitive, you must stay out of the sun or bright light for several days until the dye has passed from your system. Laser photocoagulation must be done before the abnormal blood vessels cause irreversible damage to the retina. After PDT, the abnormal blood vessels may reopen, so multiple treatments and follow-up appointments may be required.