Fig. 30-4  Magnified view of Nd:YAG laser iridotomy showing the characteristic ragged edge.

density produced by the individual laser. It is important that the surgeon be familiar with the power characteristics of the laser being used and to pretest the laser for focus in an appropriate test chamber before treating the patient.

For lasers that provide multiple bursts, an initial trial at 2–3 shots/burst using approximately 1–3 mJ/burst will be effective in most irides. If a single burst is used, slightly higher power is usually necessary. High-power settings (2–5 mJ) are needed for some particularly thick, velvety brown irides, in Asian patients.

Careful focusing is critical. Because the shock wave travels toward the surgeon from the point of focus,35 it is ideal to have the focal point within the iris stroma. This can be accomplished by focusing precisely on the surface of the iris and then offsetting the Nd: YAG beam so that it converges behind the aiming beam focal point, 0.1 mm in the iris stroma. Because the shock wave propagates toward the surgeon, this approach is hazardous to the cornea when the chamber is very shallow. Enlarging Nd:YAG iridotomy is hazardous because of the risk of lens injury. A tiny iridotomy ( 0.1 mm) may be inadequate for preventing subsequent pupillary block36 and is potentially more susceptible to later closure by pigment.37 If the surgeon is unsure of the adequacy of the iridotomy created, it may be preferable to choose another site and use somewhat higher energy levels for a second attempt, rather than to try to enlarge the first opening, or to consider subsequent enlargement with an argon laser.

If the initial attempt at Nd:YAG iridotomy fails, repeat treatments can be directed to the same site using fewer shots per burst and/or less energy for additional treatments. Repeat attempts are most effective shortly after the initial attempt because pigment debris created by the first attempt may cloud the anterior chamber and reduce the amount of laser energy reaching the iris. After several failed attempts, anterior chamber clarity can be compromised, and further treatments are fruitless. In this situation, it is helpful to have the patient sit quietly for 10–15 minutes to allow the optical pathway to clear of pigment. Once anterior chamber clarity has improved, the treatment can be completed. In particularly resistant cases, the patient can be asked to return several days later. By this time, localized iris atrophy will have thinned the iridotomy site, and the treatment can almost always be completed.

Argon or solid-state laser iridotomy

Argon and solid-state lasers (Fig. 30-5) produce coagulative effects with lower energies at longer exposures or explosive effects due to rapid vaporization when higher energies are used.

Photocoagulative lasers act differently with tissues that have different amounts of pigmentation. Because of these variables, iridotomy with photocoagulative lasers requires more adjustments in technique and a greater variety of techniques38 than does Nd:YAG laser iridotomy.The fact that the Nd:YAG laser is easier and more effective explains its greater popularity over argon and solid-state lasers. Photocoagulative lasers remain good choices in many cir-

cumstances. The following discussion outlines a few useful argon and solid-state iridotomy techniques.7,9,10,24,39,40

Iris color (pigment density) is the most influential factor in the outcome of photothermal laser iridotomy. The chromophore (energy absorber) for laser iridotomy is in the iris pigment epithelium posterior to the stroma. For this discussion of laser iridotomy, the iris color can be divided into three categories: light brown, dark brown, and blue. Light brown irides are the most easily perforated and are discussed first.

Light brown iris

The surgeon can usually locate a thin area in the anterior stroma, often in the base of a crypt, and can actually see into the depths of the iris.The laser beam should be aimed away from the posterior pole