Introduction to Selective Laser Trabeculoplasty
In addition to Argon Laser Trabeculoplasty
, there is the newer Selective laser trabeculoplasty (SLT) which has fewer side effects. As mentioned previously with ALT, it is possible to cause thermal injury and coagulative damage to the trabecular meshwork, and this makes repeat procedures difficult. However, with SLT the treatment can be repeated by targeting more degrees of the trabular circumference. Selective laser trabeculoplasty can be a primary treatment of open angle glaucoma or it can be an adjunct to medication therapy. But just like Argon laser trabeculoplasty it is also contraindicated for closed angle glaucoma and should not be performed on patients who did not respond to initial treatment.
In an article, “Comparison of the Morphologic Changes after Selective Laser Trabeculoplasty
and Argon Laser Trabeculoplasty in Human Eye Bank Eyes” researchers Theresa R. Kramer, M.D. and Robert J. Noecker, M.D. Concluded that “SLT applied in vitro to the trabecular meshwork of human eye bank eyes seemed to cause no coagulative damage and less structural damage to the human trabecular meshwork when compared with ALT and, therefore, it is a safer and more repeatable procedure”.
Mechanism of effect
The newer SLT selectively targets only the pigmented cells.The doctor will use a slit lamp microscope and gonio lens to guide the laser to the target drainage cells of the trabecula. Lowering the intraocular pressure is achieved by precisely targeting intracellular melanin granules to activate individual cells while not disturbing adjacent non-pigmented cells. This process is called selective photothermolysis. The macrophages triggered by the cytokines released from activated cells reactivate the meshwork thereby reducing fluid outflow resistance and lowering intraocular pressure.
Unlike Argon laser trabeculoplasty, in selective laser trabeculoplasty the melanin granules within individual trabecular meshwork cells receives 6000 times less energy density (fluence). The lower fluence is a result of the larger spot size and the lower energy required. With SLT the light energy is provided by a specially designed Q-switched, frequency doubled Nd:YAG laser operating at 532nm green with an output of from 0.3 to 1.5 millijoules. Using a specially developed gonio lens, a large 400 micron spot is directed into the angle to minimize beam distortion and provide good visualization of the angle. Unlike ALT, focus is not critical since the large spot has a greater depth of focus than the 50 micron ALT spot. A red aiming beam the same size as the treatment beam shows the area to be treated.
Approximately 80% of people experience lower eye pressure after SLT with the pressure reduction being in the range of 10-20%. The effectiveness of this laser treatment will depend somewhat on the pigment content of the trabecular meshwork, as these are the cells which are targeted for pressure lowering. The effect of the laser is not permanent, wearing off within a few years. Depending on response, the laser may be repeated more than once by targeting different degrees of the trabecular circumference. If taking eye drops for glaucoma, one will most likely still need them after this laser treatment, and if the initial treatment is not successful, the doctor may choose to repeat it. Note: Many respondents to our FitEyes email group have indicated that they either did not receive much of a pressure lowering effect, or that it needed to be redone soon after.
What to expect during the procedure
The entire routine will take approximately one and a half hours, but the actual procedure will last only a few minutes. Numbing drops are used and then a special contact gonio lens will be used to help focus the laser light. The SLT involves sitting at a slit lamp similar to those used for eye exams. People rarely experience discomfort during this procedure. Once it is over and the lens is removed, there can be some temporary blurriness for about an hour, due to the ointment used with the contact lens. Depending on the circumstances, you may need to use special eye drops for several days to prevent inflammation.
Possible risks and/or side effects
Though serious side effects are very rare, there may be a transient rise in intraocular pressure after the treatment which would require additional eye drops. And there may be some temporary inflammation requiring additional anti-inflammory drops. Finally, there may be some slight irritation from scratching of the eye by the lens used in the procedure.