Laser trabeculoplasty (LTP) involves application of laser energy to the trabecular meshwork in discrete spots, usually covering 180°–360° per treatment. The goal of LTP is to increase outflow facility and thus reduce IOP. Different laser wavelengths and delivery systems can be used, including argon laser, diode laser, and Q-switched Nd:YAG laser.
The Glaucoma Laser Trial (GLT) was a multicenter randomized clinical trial that assessed the efficacy and safety of argon laser trabeculoplasty (ALT) as an alternative to topical medical therapy in patients with newly diagnosed, previously untreated POAG. The study was flawed in that one eye was assigned to ALT and the fellow eye was assigned to timolol treatment, which can have an effect on the contralateral ALT eye and confound the study results. Within the first 2 years of follow-up, ALT as initial therapy appeared to be as effective as medication. However, more than half of the eyes treated initially with laser required the addition of one or more medications to control IOP over the course of the study.
Several mechanisms of action were initially proposed for the increased outflow facility that occurs following successful LTP. In ALT specifically, thermal damage to the treated trabecular meshwork causes shrinkage of collagen fibers and therefore stretching and widening of adjacent areas to allow for more outflow. In all forms of LTP, most investigators believe that chemical mediators, specifically interleukin-1β and tumor necrosis factor-α, are released from treated trabecular meshwork cells, increasing outflow facility through induction of specific matrix metalloproteinases. The energy of the frequency-doubled Q-switched Nd:YAG laser used in selective laser trabeculoplasty (SLT) is selectively absorbed by pigmented trabecular cells, sparing adjacent cells and tissue from thermal damage. The number of monocytes and macrophages in the trabecular meshwork increases substantially after SLT, and this increase may play a role in lowering IOP.
Kramer TR, Noecker RJ. Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes. Ophthalmology. 2001;108(4):773–779.
Many clinicians use medical therapy before advancing to LTP, but LTP is a reasonable initial step in the management of glaucoma and ocular hypertension (discussed in Chapter 4). In addition, patients who cannot tolerate or adhere to initial medical therapy may be candidates for LTP.
Multiple prospective studies have demonstrated that ALT and SLT achieve similar pressure lowering. The IOP-lowering effect of LTP is similar to that of prostaglandin analogues, with LTP expected to lower IOP by 20%–25%. LTP effectively reduces IOP in POAG, pigmentary glaucoma, pseudoexfoliation syndrome, and corticosteroid-induced glaucoma. Aphakic and pseudophakic eyes may respond to LTP less favorably than phakic eyes. IOP control is unlikely to be diminished by subsequent cataract extraction. LTP is not effective for treating certain types of secondary glaucoma, such as uveitic glaucoma.
LTP is not advised in patients with inflammatory glaucoma, iridocorneal endothelial syndrome, neovascular glaucoma, synechial angle closure, or developmental glaucoma. LTP can be tried in angle recession, but the underlying tissue alterations may cause the procedure to be ineffective. Another relative contraindication for LTP is the lack of effect in the fellow eye. If an eye has advanced damage and high IOP, LTP is unlikely to achieve the target pressure (see Chapter 7 for discussion of target pressure).
As with all ocular surgery, the preoperative evaluation recommended for LTP includes a detailed medical and ocular history and a comprehensive eye examination. Particular attention must be paid to visual field examination, gonioscopy, and optic nerve evaluation; the trabecular meshwork must be visible on gonioscopy. The degree of pigmentation in the angle determines the power setting: the more pigmented the trabecular meshwork, the less energy required.
In ALT, a 50-µm laser beam of 0.1-second duration is focused through a goniolens at the junction of the anterior nonpigmented and the posterior pigmented edge of the trabecular meshwork (Fig 8-1). Application to the posterior trabecular meshwork tends to result in inflammation, pigment dispersion, prolonged IOP elevation, and peripheral anterior synechiae (PAS) formation. The power setting (300–1000 mW) should be titrated to achieve the desired endpoint, which is blanching of the trabecular meshwork or production of a tiny bubble. If a large bubble appears, the power is reduced and titrated to achieve the proper effect. As LTP was originally described, laser energy was applied to the entire circumference (360°) of the trabecular meshwork. But evidence suggests that a satisfactory reduction in IOP is achieved in many eyes and there is a lower risk of short-term pressure elevation when only half of the circumference is treated, with approximately 40–50 applications over 180° of the meshwork.