Fig. 1. Iridotomy.

Argon laser iridectomy

Laser parameters vary depending on the type of treatment being administered and the color of the iris. For lightly pigmented irides, energy levels of 800–1000 mW are used with a spot size of 50 mm and an exposure time of 0.2 s. For thick, darkbrown irides, 1500–2500 mW of power is used with an exposure time of 0.02–0.05 s and a spot diameter of 50 mm.

Complications

When the iridotomy is done correctly, both intraoperative and long-term complications are very rare. There are reports in the literature of isolated cases with complications, including some that are serious: retinal detachment, cataract, lens luxation, and rupture of Descemet’s membrane (Mastropasqua et al., 1984; Zadok and Chayet, 1999; Liu et al., 2002).

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The most common complications are less severe. They include:

Transient increases in pressure, which occur in approximately one third of treated eyes. They are caused by treatment debris, which impedes outflow of the aqueous humor (Hsiao and Hsu, 2003). This problem can be prevented by the administration of topical timolol and oral acetazolamide (Fleck and Wright, 1997).

Temporary ‘‘fogging.’’

Laser-induced iritis; posterior synechiae (rare); the risk of these events can be reduced with the use of topical steroids.

Diplopia — this occurs when the iridotomy is positioned on the horizontal meridians and not covered by the upper eyelid.

Bleeding.

Damage to the corneal endothelium (Wu and Jeng, 2000).

Incomplete (partial-thickness) iridotomy.

Efficacy

Laser iridotomy is now a valid, effective alternative to surgical procedures for the treatment of narrow-angle glaucoma (Rivera et al., 1985; Fleck and Wright, 1997). It is easy to perform, relatively safe, and rarely associated with complications. The efficacy of this approach has been widely demonstrated over the past 20 years. In 70–85% of the patients treated (Playfair and Watson, 1979; Jiang, 1991; Fleck and Wright, 1997) significant reductions in IOP have been achieved without IOPreducing drug therapy in follow-ups ranging from 1 to 3 years.

The iridocorneal angle remains open in 70–80% of treated patients, and only a small percentage require retreatment (Thomas and Arun, 1999; Ritch et al., 2004).

LASER trabeculoplasty

The first experimental laser treatments of the iridocorneal angle were the ‘‘goniopuncture’’

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procedures performed by Krasnov (1973) to improve aqueous-humor outflow. The method known as ALT was developed laterby Wise and Witter (1979). In the 23 years that have passed since its introduction, ALT has not undergone any substantial modifications. It involves a series of photocoagulations that produce nonperforating thermal lesions uniformly distributed over the trabecular ring.

Treatment technique

Laser trabeculoplasty is frequently used to treat glaucoma, and it produces good long-term results. It is usually done with an argon laser (use of diode lasers is less common) and a gonioscopic contact lens. The most widely used gonioscopy lenses are the Goldmann three-mirror lens and the Ritch gonioprism, which allows better visualization of the trabecular mesh and uses less energy.

The standard settings proposed by Wise and Witter include a spot diameter of 50 mm and an exposure time of 0.1 s. The power varies depending on the response of the tissue and the type of lens (600–1200 mW with the Goldmann lens, 600–800 mW with the Ritch). The spots (20–25 per quadrant) must be uniformly distributed along the entire circumference of the trabecular meshwork. The recommended site of application is the junction of the pigmented and nonpigmented trabecular meshworks. This approach spares part of the filtering trabecula and the posterior trabecular meshwork, thus reducing the incidence of pressure spikes and posttreatment peripheral anterior synechiae (PAS). ALT can be performed in one or two 50-to-60-spot sessions to reduce the risk of laser-induced pressure spikes. The laser produces whitening of the treated zone or the formation of a small bleb in the burst zones.

When an argon laser is used, green light is just as effective as blue-green, and it reduces the risk of damage to the photoreceptors of the operator (Berringer et al., 1989). The operator can also use a contact lens with a metal halide coating to reduce the risk of macular damage caused by reflected light.

Mechanism of action

Various mechanisms of action have been hypothesized; according to the theory proposed by Wise, the thermal effect produced by the laser spots stretches and widens the trabecular meshes and/or the Schlemm’s canal (Wise and Witter, 1979). Other authors maintain that the laser triggers physiological changes in the activities of the endothelial cells with increased phagocytotic activity (Bylsma et al., 1988) or increased cell replication within the trabeculate that promote the outflow of aqueous humor (Wilensky and Weinreb, 1983).

Indications for treatment

The European Glaucoma Society’s guidelines recommend trabeculoplasty in the following cases:

Primary open-angle glaucoma that is refractory to medical therapy.

Patients who are noncompliant or unable to tolerate medical therapy.

Pseudoexfoliation glaucoma.

Pigmentary glaucoma.

Contraindications to treatment

Treatment is contraindicated in patients whose angular structures cannot be explored and/or those with uveal inflammation or congenital malformations involving the trabecular structures, as in the following cases:

Narrow or closed angle.

Corneal opacity that precludes gonioscopy.

Absence of trabecular pigmentation.

Inflammatory or postuveitic glaucoma.

Juvenile glaucoma.

Absence of effects in the contralateral eye.

Patient preparation and postoperative follow-up

Written consent should be obtained from the patient after the objectives of the treatment and its potential risks have been presented and discussed.