Three emerging forms of laser trabeculoplasty compare well with established modalities in terms of safety and efficacy, according to a published report.
Argon laser trabeculoplasty was the preferred form of laser trabeculoplasty for more than 4 decades, but selective laser trabeculoplasty has since become the procedure of choice.
In the British Journal of Ophthalmology, researchers discussed findings in the literature regarding the three newer modalities: micropulse laser trabeculoplasty (MLT), titanium-sapphire laser trabeculoplasty (TSLT) and pattern scanning trabeculoplasty (PLT).
“With the available data from the literature, it seems that MLT, TSLT and PLT have a similar efficacy in terms of IOP and anti-glaucoma medication reduction as compared to SLT or ALT,” Jacky W.Y. Lee, FRCSEd, told Ocular Surgery News. “However, these newer technologies offer better treatment comfort with less pain, less post-laser inflammation and fewer IOP spikes due to reduced laser energy diffusion to the surrounding tissues through various treatment algorithms that are unique to the respective technologies.”
Mechanisms of action
MLT delivers energy in repetitive microsecond pulses with intermittent rest periods, reducing the buildup of thermal energy and preventing apparent coagulative damage to the trabecular meshwork, the authors said.
“In MLT, the duty cycle allows repetitive microsecond pulses followed by periods of rest, allowing for surrounding tissues to cool off, preventing thermal damage,” Lee said.
TSLT uses a 790 nm wavelength laser that emits near-infrared energy in pulses ranging from 5 ms to 10 ms. This is believed to allow deeper penetration, about 200 µm, into the juxtacanalicular meshwork and the inner wall of Schlemm’s canal. Energy is selectively absorbed by pigmented phagocytic cells, preserving the trabecular meshwork tissue.
“Mini-bubble bursts are seen with TSLT, so the energy may be titrated as needed,” Lee said.
PLT provides a computer-guided treatment method to apply a sequence of pattern laser spots onto the trabecular meshwork. Automatic rotation with calculated alignment of each pattern allows consecutive treatment of the entire trabecular meshwork without overlapping or excessive gaps. It is a continuous wave light with a green wavelength of 532 nm and a yellow wavelength of 577 nm.
“PLT is a continuous laser that is thought to reduce cellular response and collateral damage by using shorter pulse duration but in turn uses more spots that are delivered through a computer-rotated treatment arc,” Lee said.
The three modalities are similarly effective, Lee said.
“[MLT, TSLT and PLT] all have very similar reported efficacies of around 20% to 30% IOP reductions, with the longest study duration of 24 months with MLT,” he said. “MLT has one of the highest reported response rates of 75%, while TSLT has the highest IOP reduction of around 32%. However, as each of the studies have different pretreatment IOP, pre-existing anti-glaucoma medications, treatment protocols and study duration, it is not possible to directly compare the efficacy of these three technologies until there is a randomized controlled trial investigating these technologies on a similar study population.”
Advantages and downsides
The three new modalities may be safer than ALT and SLT.
“All these newer technologies do not cause scars and result in clinically negligible collateral damage to the trabecular meshwork after treatment,” Lee said. “The incidences of IOP spikes and post-laser inflammation are much less than ALT or SLT. MLT is an adaptation from the micropulse technology whereby a duty cycle allows the tissue to cool off before the next laser application, making it a safe technology even for subfoveal treatment in macular edema due to diabetic retinopathy, vascular occlusions or central serous chorioretinopathy. For micropulse subfoveal treatment, a 5% duty cycle is used (5% laser treatment time, 95% resting interval), and in MLT, a 15% duty cycle is implemented.”
But some disadvantages exist for MLT, TSLT and PLT.
“For MLT, the number of laser spots and energy are fixed, and there are no visible treatment endpoints, rendering treatment titrations not possible,” Lee said. “In TSLT, only very limited published clinical data is available using a 180° trabecular meshwork treatment protocol. It is uncertain whether 360° treatment may maximize the outcome of the laser. In PLT, though, a shorter pulse duration is implemented, but 10 times more laser spots are delivered to the same treatment area. For those with narrower angles, there may not be enough accessible trabecular meshwork to apply these spots, and if lasers are applied over the iris, this may in theory cause more inflammation. In addition, the repeatability of PLT is uncertain at the moment.” – by Matt Hasson
- Tsang S, et al. Br J Ophthalmol. 2016;doi:10.1136/bjophthalmol-2015-307515.
- For more information:
- Jacky W.Y. Lee, FRCSEd, can be reached at Dr. Dennis Lam & Partners Eye Center, Suite 1515 Central Building, 1-3 Pedder Street, Hong Kong; email: firstname.lastname@example.org.
Disclosure: Lee reports he receives publication funding from Ellex and equipment from Iridex.