What is a laser?
A laser is a device that creates a highly focused beam of light. Lasers are used for a variety of reasons in the treatment of eye diseases, including retinal disorders.
How does a laser work?
When a laser beam is focused on the retina, it causes a burn. As the burned tissue heals, it becomes a scar. This is the main goal of most laser treatments for retinal diseases. Burns of varying size and intensity can be created, and these parameters are adjusted by the doctor depending on the purpose of the laser.
When is laser used?
Below is a summary of some rather common reasons to use laser in the treatment of retinal diseases:
1. To reduce swelling in the retina
Some retinal disorders cause the blood vessels in the retina to become abnormally permeable and leak fluid into the retina. As a result, the retina becomes swollen, much as a sponge swells when it soaks up water. The retina does not function normally when it is swollen. If swelling affects the central part of the retina (the macula), the vision becomes blurred. Laser treatment can be used to create small burns of low intensity in the area of swelling, in an effort to reduce the swelling. A number of different diseases can cause swelling in the macula and may benefit from laser treatment. The most common of these are diabetic retinopathy and branch retinal vein occlusion.
2. To reduce abnormal blood vessel growth on the retina
When the blood supply to the retina is low, the eye senses the need for more blood vessels. In response to this need, the eye actually makes new vessels. Unfortunately, the new vessels do not provide more blood to the retina because they are abnormal. Instead, they often cause loss of vision. Bleeding, retinal detachment and severe glaucoma (high pressure inside the eye, which can cause severe pain and visual loss) can result from these vessels without treatment. The strategy for this type of laser treatment is to create many (often more than 1,000) burns of relatively high intensity in the peripheral part of the retina (outside the macula). These burns actually “kill” some of the peripheral retina, “tricking” the eye into thinking that there is less retina to supply with blood. In response, the eye no longer feels the need to create the abnormal blood vessels, and they often “shrivel up”. This kind of laser treatment is performed for a wide variety of retinal disorders, but some of the more common ones include diabetic retinopathy and retinal vein occlusion.
3. To prevent retinal detachment
The retina is similar to wallpaper, in the sense that it lines the inner wall of the eye. It must be flat against the eye wall in order to function properly. In some instances, a tear develops in the peripheral retina. Just as someone could put his/her hand through a tear in wallpaper and pull it off of a wall, the fluid inside the eye (called the vitreous) can go through a retinal tear and pull the retina off of the eye wall. This is how a retinal detachment occurs. A retinal detachment must be repaired surgically. If, however, the retina has torn but has not yet detached, laser can be done to prevent the retina from detaching. Tiny laser burns are placed around the tear in order to create a scar that “spot welds” the retina to the eye wall and prevents the vitreous fluid from going through the tear and detaching the retina. You can think of the laser like a staple gun. The scar created by the laser is similar to placing staples around a tear in wallpaper to prevent people from putting a hand through the tear and pulling the wallpaper off of the wall.
4. To reduce abnormal blood vessel growth under the retina
Abnormal blood vessel growth under the retina can cause vision loss if it occurs under the central part of the retina (the macula). A variety of diseases can cause this abnormal blood vessel growth, but some of the more common ones are age-related macular degeneration and ocular histoplasmosis.
If the abnormal blood vessel growth has not yet reached the very central part of the macula (the fovea), thermal laser treatment (a type of laser that has been used for decades) can be performed to burn the abnormal vessels. The laser damages both the abnormal vessels and the overlying retina, creating a scar. Some patients notice a grey area in the location of the laser scar, but the central vision is usually unaffected by the laser. Unfortunately, the abnormal vessels sometimes grow back after laser treatment. When this happens, the vessels tend to grow under the fovea. Since traditional laser treatment damages the “good tissue with the bad”, it is not useful for abnormal vessels under the fovea.
A less destructive laser treatment, called photodynamic therapy (PDT), involves two steps: 1) injection of a light-sensitive dye into a vein in the arm and 2) shining a low-intensity laser on the abnormal vessels. The dye, which is slowly injected over 10 minutes, collects in the abnormal vessels beneath the retina. The low-intensity laser, which is directed at the abnormal vessels continuously for 83 seconds, “activates” the dye, causing damage to the abnormal vessels. The goal is to close the abnormal vessels, thereby stopping them from bleeding or leaking fluid under the retina, without causing damage to the overlying retina. PDT is by no means a perfect procedure. It does not usually cause the vision to improve; it is intended mainly to prevent further worsening of the vision. In addition, it often needs to be repeated several times in order to stop the leaking from the abnormal vessels. Furthermore, PDT makes the skin very sensitive to sunlight and can cause severe burns if the skin is exposed to the sun, so patients who have had PDT should avoid sunlight for 3 days after the procedure. PDT was once used quite often, but it is used much less frequently these days. Intravitreal injections are often performed, instead. Sometimes, PDT is used in combination with intravitreal injections.