Photodynamic therapy (PDT) is a treatment option for Age-Related Macular Degeneration (AMD), a condition that affects the macula, the small central part of the retina responsible for sharp, detailed vision. This procedure is specifically used for the less common but more aggressive form of the disease, known as wet AMD. In wet AMD, abnormal blood vessels grow beneath the retina, which leak fluid and blood, causing rapid damage to central vision. PDT targets these abnormal vessels to slow down the progression of vision loss.
Defining Photodynamic Therapy
Photodynamic therapy is a precise, two-step medical procedure that utilizes a light-activated drug and a non-thermal laser to treat the underlying cause of wet AMD. The treatment combines a photosensitizing agent with a specific wavelength of light to selectively destroy the unwanted new blood vessels. This method is distinct from older, thermal laser treatments because it does not rely on heat to seal the vessels, which minimizes damage to the surrounding healthy retinal tissue.
The photosensitizing drug used in this therapy is Verteporfin, commonly known by the brand name Visudyne. This medication is engineered to accumulate preferentially in the rapidly growing, abnormal blood vessels—a process called choroidal neovascularization (CNV)—that are characteristic of wet AMD. Once the drug is concentrated in the target vessels, a low-intensity, non-thermal laser is applied to activate the medication. The drug is essentially inactive until it is exposed to this specific wavelength of light, which is typically 689 nanometers.
This targeted approach allows the treatment to be focused primarily on the abnormal vessels that are leaking and causing vision loss. While Photodynamic Therapy was once a primary treatment for wet AMD, it is now often reserved for specific subtypes of the condition or used in combination with newer injectable medications, such as anti-VEGF drugs.
The Step-by-Step Treatment Process
The procedure begins with the preparation of the patient, including the dilation of the pupils with eye drops and the insertion of a small cannula into a vein in the arm or hand. The photosensitizing drug, Verteporfin, is then administered as a slow intravenous infusion through this line. This infusion process typically takes about ten minutes to complete.
Following the injection, a short waiting period is necessary to allow the Verteporfin to circulate throughout the body and accumulate within the abnormal blood vessels beneath the macula. This incubation time is generally about fifteen minutes from the start of the infusion. Once sufficient time has passed, the eye is prepped for the light application, which may involve placing anesthetic drops and a specialized contact lens on the eye.
The final step is the application of the non-thermal laser light directly to the affected area of the retina. The doctor carefully directs the light through the contact lens onto the region containing the abnormal vessels. The laser treatment itself is very brief, lasting for a precise duration of 83 seconds, which is just enough time to photoactivate the drug.
How PDT Works to Treat Macular Degeneration
The therapeutic effect of Photodynamic Therapy stems from a localized photochemical reaction that occurs when the photosensitizer is struck by the laser light. Once the Verteporfin molecule absorbs the light energy, it transitions into an activated state. This energy is then quickly transferred to oxygen molecules present in the surrounding tissue and blood.
This transfer creates highly reactive oxygen species, often referred to as free radicals. These short-lived, toxic molecules are the agents of the treatment, as they cause localized damage to the endothelial cells that form the inner lining of the abnormal blood vessel walls. The resulting cellular damage triggers a process called thrombosis, which means the formation of a clot inside the vessel.
The clotting leads to the occlusion of the targeted choroidal neovascularization, effectively sealing off the leaky blood vessels. By stopping the leakage of fluid and blood, the treatment prevents further damage to the macula and helps to stabilize the patient’s central vision.
Essential Post-Treatment Safety Measures
A consequence of using a photosensitizing drug is that the patient becomes temporarily highly sensitive to all forms of light. The Verteporfin remains in the body and skin for a period after the procedure, making exposure to direct sunlight or bright indoor light a significant risk. Patients must strictly avoid this exposure for at least 48 hours following the treatment to prevent severe skin burns and blistering.
During the initial 48-hour period, every effort must be made to cover all exposed skin, even when simply walking from the treatment room to a car. This includes wearing long sleeves, trousers, a wide-brimmed hat, and gloves. Dark, protective sunglasses must also be worn to shield the eyes from bright light.
Patients are advised to remain indoors during daylight hours for the first two days, keeping curtains drawn to minimize light entering the home. While bright light is dangerous, darkness is not required; exposure to normal, dim indoor lighting is actually helpful, as it assists the body in breaking down the remaining photosensitizer. After the 48-hour period has passed, the photosensitivity subsides, and patients can typically resume their normal outdoor activities without special light precautions.