• Is the use of a Photocoagulator unsuitable for SLT?
• Does a YAG Capsulotomy Laser have the wrong parameters for SLT?
• How does SLT fit into my practice if I already own an Argon Laser?
• If it is clinically equivalent to ALT, why is SLT a better option?

A: If low energy is the key to preventing tissue damage, why not simply turn down the power on a standard continuous wave (cw) laser and achieve the same result. While the power can be reduced, the shortest pulse width for most laser photocoagulators is .01 seconds, compared to the 3 x 10-9 seconds for the SLT laser. The work of Jim Wise, M.D., (click here for Wise article PDF) who developed ALT, showed that to obtain a reliable pressure lowering effect it was necessary to utilize a high level of power and a small spot size. Normal parameters for ALT are 800 milliwatts for 0.1 seconds resulting in 80 millijoules being delivered in a 50 micron spot. This compares to typically less than 1 millijoule in a 400 micron spot for SLT. Simply turning down the power and reducing the exposure time on a photocoagulator results in no pressure lowering effect.

A: If a short pulse width is important, why not use an existing Nd:YAG capsulotomy laser for SLT? While energy levels and pulse widths of YAG lasers might seem appropriate, the small focused spot size of 6 to 10 microns makes the fluence far too high even at the lowest energy settings when compared to the 400 micron spot size of the SLT laser. In addition experiments by Mark Latina, showed that the reduced melanin absorption of the 1.06 micron wavelength YAG laser reduced the selectivity of the effect when compared to 532 nm green light which is more highly absorbed by melanin. Historically Q-switched lasers have been used to attempt trabecular punctures to connect the anterior chamber and Schlemm's canal to facilitate outflow. Both ruby and YAG laser trapeculoplasty have been reported. Only ALT and now SLT have shown sustained pressure lowering effect from laser trabeculoplasty.

A: The Selecta® laser system was specifically developed to perform selective laser trabeculoplasty, a whole new approach to treat glaucoma. In a similar way, the argon laser was developed to treat diabetic retinopathy and Nd:YAG laser to treat secondary cataracts. More recently the excimer laser was developed for refractive surgery. And even among photocoagulators, different wavelengths have been found more effective for different procedures. While it might be nice to have a universal laser, the variations in applications and tissue effects make it impossible.

A: For open angle glaucoma patients who maximally tolerated medical therapy, which are some of the most severe glaucoma patients, SLT is equally effective in reducing and maintaining low intraocular pressure (IOP) as ALT. However, the Selecta laser does not cause a burn in the trabecular meshwork like ALT, which is causing physicians to reevaluate how laser treatment fits into the glaucoma therapy hierarchy.