Jason Yeh, M.D.
Brian Francis, M.D.
Laser trabeculoplasty (LTP) has been employed as an initial, adjunct or replacement therapy to lower intraocular pressure (IOP) in patients with open-angle glaucoma (OAG). One of the most appealing aspects of the procedure is that it is “physiologic”, in that it attempts to enhance the aqueous outflow facility that has been reduced in OAG. The original procedure was described using argon laser (major wavelength peaks at 488 nm and 514 nm). Large prospective studies have shown that argon laser trabeculoplasty (ALT) is a relatively safe and effective procedure. The Glaucoma Laser Trial showed that in patients with newly diagnosed open-angle glaucoma, ALT was at least as effective as initial treatment with timolol maleate 0.5%, even after 7 years (1,2). However, ALT produces significant tissue disruption and coagulation damage to the trabecular meshwork, hence, possibly contributing to the limited effectiveness of retreatment (3,4).
This shortcoming was addressed with the recent approval of selective laser trabeculoplasty (SLT) by the FDA in 2002 for the treatment of OAG. Using the 532 nm, frequency-doubled, Q-switched Nd:YAG laser, SLT results in the selective absorption of energy by pigmented cells and spares adjacent cells and tissues from thermal energy (5). Compared to ALT, each SLT pulse delivers less than 0.1% total energy and is eight orders of magnitude shorter in duration. This results in milder tissue response and the potential for repeatable treatment, which has only been shown in a handful of meeting abstracts (6). SLT is also easier to perform since the area of the laser spot is 64 times larger than that of ALT and large enough to cover the entire width of the trabecular meshwork. SLT was initially studied as a secondary modality in cases of failure of medical therapy or ALT (7-9). More recently, SLT has been proven effective as primary treatment in OAG with minimal side effects or complications (10,11) and as a replacement for glaucoma medications in medically controlled OAG (12). Reports have shown a similar short- and long-term efficacy between SLT and ALT in patients with OAG (7,8). The reported average IOP decrease by SLT ranges from 18 to 40% over a six to 12-month follow-up period (6). As a result, SLT is increasingly being used in clinical practice while ALT is gradually falling out of favor. Other lasers such as krypton, diode, and continuous wave, frequency-doubled Nd:YAG are also used for LTP and have been shown to reduce IOP to a level similar to ALT (6).
Micropulse laser trabeculoplasty (MLT) is a newer technique using 810-nm diode laser in micropulsed emission mode with short “on” time followed by a “long” off time to create sublethal thermal insult to viable cells in the trabecular meshwork. MLT delivers less energy than ALT or SLT and produces less trabecular meshwork damages. The one-year results of an Italian pilot study presented at the 2007 International Glaucoma Symposium and 2007 World Glaucoma Congress showed a mean IOP reduction of 22% in 24 out of 32 eyes with OAG (13).
The newest modality on the block in LTP is Titanium Sapphire laser. Currently in phase III clinical trial, the efficacy of Titanium Sapphire laser trabeculoplasty (TLT) is being compared to ALT in patients having primary OAG with poorly controlled IOP on maximum tolerated medical therapy and/or prior failed glaucoma surgery. TLT emits flashlamp-pumped, near-infrared energy (790 nm) in pulses lasting five to 10 microseconds (duration between ALT and SLT). TLT has been shown to provide deeper tissue penetration than the other lasers currently in use for LTP without causing damage to the trabecular meshwork and may therefore be repeatable (8). TLT results in a significant opening of the trabecular meshwork with statistically significant decrease in IOP (20 to 30%) and minimal complications (14).
Among all the various LTP modalities, published studies to date have shown similar efficacy in IOP reduction in patients with OAG for first-time treatments. The question we are still waiting to be answered in peer-reviewed literature is the clinical efficacy of repeated treatments and their ability to prolong the IOP-lowering effect. None of the newer modalities have been compared against each other in a randomized trial, but each seems similar in efficacy to ALT. Time will tell which of these will gain widest acceptance. Other research questions that have yet to be answered are which medications are best used in conjunction with LTP. It is also unclear whether the procedure affects the efficacy of emerging trabecular surgery techniques such as Trabectome and canaloplasty.
References:
1. The Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial (GLT) and glaucoma laser trial follow-up study, seven-year results. Am J Ophthalmol. 120:718-31, 1995.
2. The Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial (GLT), 2. Results of argon laser trabeculoplasty versus topical medicines. Ophthalmology. 97:1403-13, 1990.
3. Kramer TR and Noecker RJ. Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes. Ophthalmology. 108:773-9, 2001.
4. Rodrigues MM, Spaeth GL, Donohoo P. Electron microscopy of argon laser therapy in phakic open-angle glaucoma. Ophthalmology. 89:198-210, 1982.
5. Latina MA and Park C. Selective targeting of trabecular meshwork cells: in vitro studies of pulsed and CW laser interactions. Exp Eye Res 60:359-71, 1995.
6. Barkana Y and Belkin M. Selective Laser Trabeculoplasty. Surv Ophthalmol. 52:634-54, 2007.
7. Damji KF, Shah KC, Rock WJ et al. Selective laser trabeculoplasty vs. argon laser trabeculoplasty: A prospective randomized clinical trial. Br J Ophthalmol. 83(6):718-722, 1999.
8. Juzych MS, Chopra V, Banitt MR et al. Comparison of long-term outcomes of selective laser trabeculoplasty versus argon laser trabeculoplasty in open angle glaucoma. Ophthalmology. 111(10):1853-9, 2004.
9. Latina MA, Sibayan SA, Shin DH et al. Q-switched 532-nm Nd:YAG laser trabeculoplasty (selective laser trabeculoplasty): A multi-center, pilot, clinical study. Ophthalmology. 105(11):2082-90, 1998.
10. Nagar M, Ogunyomade A, O’Brart DP et al. A randomized, prospective study comparing selective laser trabeculoplasty with latanoprost for the control of intraocular pressure in ocular hypertension and open angle glaucoma. Br J Ophthalmol. 89(11):1413-7, 2005.
11. Melamed S, Simon B, Levkovitz-Verbin H. Selective laser trabeculoplasty as primary treatment for open-angle glaucoma: a prospective, non-randomized pilot study. Arch Ophthalmol. 121(7):957-60, 2003.
12. Francis BA, Ianchulev T, Schofield JK, Minckler DS. Selective laser trabeculoplasty as a replacement for medical therapy in open angle glaucoma. Am J Ophthalmol. 140(3):524-5, 2005.
13. Fea AM, Bosone A, Rolle T et al. Micropulsed laser trabeculoplasty: a pilot study. Presented at 2007 International Glaucoma Symposium, Athens, Greece and 2007 World Glaucoma Congress, Singapore, Singapore.
14. Simon G and Lowery JA. Comparison of three types of lasers in laser trabeculoplasty in human donor eyes and clinical study. Abstract presented at 2007 ASCRS Symposium, San Diego, USA.