The underlying mechanism is selective photothermalysis that enables the laser to precisely target intracellular melanin granules to activate individual cells while not disturbing adjacent non-pigmented cells. The activated cells release cytokines that trigger a targeted macrophage response to the trabecular meshwork cells. The macrophages reactivate the meshwork reducing fluid outflow resistance and lowering intraocular pressure.
For 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.
The target tissue is melanin granules within individual trabecular meshwork cells. Based on the size of the pigment granules, it is necessary to deliver the light energy within 1 microsecond. The Q switched laser pulse width is 3 nanoseconds, well within the required time interval to contain the energy and temperature rise to the pigment granules. Mark Latina, M.D. inventor of SLT did cell culture and animal experiments to determine the specific energy range for selective cell damage. (Click here for Latina paper) Through his investigation parameters were determined which would activate certain pigment containing meshwork cells while sparing immediately adjacent non-pigmented cells. Further human studies showed the pressure lowering effects of Selective Laser Trabeculoplasty without observable damage to the trabecular meshwork cells.
ALT spot size (left arrow) vs. SLT spot size (right arrow) on TM
* Source: Review of Ophthalmology, April 2001, David A. Lee, MD & Peter A. Netland, MD, PhD, pgs. 1-2
Comparison table ALT vs. SLT parameters
Comparison of the Morphologic Changes after Selective Laser Trabeculoplasty and Argon Laser Trabeculoplasty in Human
Eye Bank Eyes
Lumenis part number 5462.
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PB-1004787 Rev A
Physicians and patients are benefiting from the SLT procedure in the following ways:
Preoperatively, careful goinoscopy should be done to visualize the trabecular meshwork and plan the treatment area. Preoperative medications consists of a drop each of lopidine or Alphagan and topical anesthesia. A SLT laser gonio or zero magnification lens is placed on the eye with methylcellulose. The aiming beam is focused onto the pigmented trabecular meshwork.
Treatment is done in single-burst mode, placing 50 ± 5 contiguous, but not overlapping, 400-micrometer laser spots along 180 degrees. Bubble formation is monitored with each pulse. In cases with significant variation in trabecular pigmentation, the pulse energy is decreased if bubble formation occurred as described above.
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