2004 has shown patient non-compliance rates of at least 25%, with most studies showing depressingly high non-compliance rates, and researchers have noted that patients are likely to discontinue long-term prescriptions as much as 76% of the time. The Glaucoma Laser Trial compared the efficacy of argon laser trabeculoplasty and timolol maleate 0.5% as initial therapy for primary open-angle glaucoma.13 The initial trial and subsequent follow-up study measured IOP reduction, visual field and optic nerve status in a total of 203 patients. Over an average of seven years of fol- low-up, eyes treated with ALT had 1.2 mmHg greater reduction in IOP (P 0.001), and 0.6 dB greater improvement in visual field (P 0.001). There was slightly more deterioration in the cup- to-disk ratio (P 0.005) for eyes initially treated with topical medication. In a 2-year study comparing the efficacy of ALT and pilocarpine 2% in treating primary open-angle glaucoma, similar results were observed.19

Different studies have yielded various success rates for LTP. In one study (Fig. 31-1), initial IOP reductions of 7–10 mmHg were not maintained through long-term follow-up.At 5 years after treatment, only 30–60% of patients maintained adequate IOP control.

Argon LTP is effective in most forms of open-angle glaucoma.20,21,22 It is rarely effective in cases of trauma or inflammation

and often aggravates inflammation. It may be effective after failed filtering surgery. Laser trabeculoplasty is more effective in older patients with POAG; the effect diminishes in patients younger than 40 years of age.23–29 Dramatically lowered IOP, sometimes greater than 20 mmHg, may occur in patients with pseudoexfoliation glaucoma, but the effect may be brief so these patients continue

to require close monitoring.30 Pigmentary glaucoma also responds unpredictably25,26,28,31–33 and may have lower long-term success rates than POAG.34,35

Retreatment of a previously laser-treated angle

Repeat argon LTP is often not advised.36–38 If considered for patients who have previously received treatment to 360° of the angle, the patient should be warned that filtering surgery may be required soon after the retreatment if it is unsuccessful.39

It is clear that LTP can postpone filtering surgery38 in patients on maximum medical therapy who would benefit from and who achieve a 9–10-mmHg decrease in IOP. However, if a greater

decrease is needed because of advanced glaucomatous damage, filtering surgery should be considered first.32

Selective laser trabeculoplasty

Concept

In contrast to argon, solid-state, and diode laser pulse durations for LTP of 0.1 seconds, recent advances in trabeculoplasty have utilized lasers with short pulse durations of 3–10 ns. Selective laser trabeculoplasty (SLT), based on the principle of selective photothermolysis, relies on selective absorption of a short laser pulse to generate and spatially confine heat to pigmented targets within trabecular meshwork cells.40,41 Selective laser trabeculoplasty uses a Q-switched, frequency-doubled 532-nm neodymium:yttrium-aluminum-gar- net (Nd:YAG) laser. Q-switching of the laser allows for a single, extremely brief, high-power light pulse to be delivered to the target tissue. The short duration of the pulse is critical in preventing collateral damage to the surrounding tissues.42 The energy level of available lasers varies between 0.2 and 1.7 mJ.

An advantage of SLT over LTP performed with larger pulsed lasers is that there is much less thermal damage to the trabecular meshwork. Preserving the trabecular meshwork may become important in the near future as surgical techniques are developed to operate directly on Schlemm’s canal or the juxtacanalicular trabecular meshwork, the region considered responsible for most of the outflow obstruction that causes open-angle glaucoma.Thermal LTP would preclude these patients from the new procedures, as their trabecular meshwork and Schlemm’s canal would be damaged.

Mechanism

The exact mechanism behind SLT remains unclear. Histopathologic evaluation of the trabecular meshwork in eyes treated with both ALT and SLT showed coagulative damage to the trabecular meshwork after ALT but not SLT.43 However, two main theories exist which attempt to explain the IOP-lowering effect. One mechanical theory states that SLT results in a stretching of the trabecular meshwork beams. Another states that the trabecular meshwork beams are separated and their mobility is increased following SLT. The biological theory states that SLT causes the release of chemical mediators and stimulates endothelial cell replication.44 In fact, it is likely that a combination of both mechanical and biological mechanisms causes the IOP decrease seen after LTP.