by having the patient gaze upwards, especially while enlarging the perforation.When the laser energy strikes the iris, a deep pit is produced.These signs indicate that the iris is relatively soft and absorbs the laser energy well so that iridotomy will be easily accomplished.

Initial power settings should be 600–1000 mW with a spot size of 50  m and a shutter speed of 0.02–0.05 second. Repeated applications of the laser in the center of the pit produced by the first shot will result in a 200to 300- m crater in the iris stroma. When the pigment epithelium is penetrated, a cloud of pigment will come out of the pit. Shutter speed can then be reduced to 0.02 second to remove the pigment epithelium from the depths of the pit and create an opening that is at least 0.2 mm in diameter. This is best done by chipping away at the edges of the small initial opening in the pigment epithelium by aiming the laser beam so that two-thirds of the beam is on the pigment epithelium and onethird is in the opening. Again, the surgeon should avoid aiming the laser toward the posterior pole by asking the patient to look up.

This technique usually produces a complete iridotomy with 10–20 shots in a light brown iris.

Dark brown iris

The densely pigmented dark brown iris has a uniform surface with no apparent thin areas. Charring of the surface occurs frequently with exposure times of longer than 0.05 second.This char appears as black shiny material (‘carbon’) at the laser application site. Additional laser applications do not penetrate the char, and instead of forming a coherent single bubble, multiple tiny bubbles spray off the surface after each application. After such charring occurs, it is very difficult, if not impossible, to penetrate that area, and a new location must be chosen. Another option is to use the Nd:YAG laser to perforate the charred site.

To avoid charring, short exposure times of 0.02–0.05 second should be used with initial power settings of 400–1000 mW and a spot size of 50  m. If a reasonable pit develops in the iris, these settings can be continued, striking the same spot and slowly advancing the focal point until perforation of the pigment epithelium is recognized by formation of the typical pigment cloud.The hole is then enlarged in the same manner as with light brown irides.

If a pit does not develop or is very small, power can be increased in 200-mW increments until an effective power is obtained. It is rarely necessary to go above 1000 mW. Exposure times should not be increased above 0.10 second because charring is very likely with longer exposures. Completion of an iridotomy can usually be accomplished with 20–50 applications.

Light blue iris

Blue or pale grey irides have insufficient stromal pigment to absorb laser energy.The energy can pass directly through the stroma, leaving it intact, and separate the pigment epithelium from the back of the iris. This can be recognized as a transillumination defect in the iris with intact overlying stroma. Subsequent shots simply pass through the iris stroma without creating a hole (Fig. 30-6).

Occasionally a small pigmented area, which will respond much like a light brown iris, may be found in an appropriate site for iridotomy. If there is no pigmented area, longer exposures will generate heat in the pigment epithelium; the heat is then transmitted into the stroma and destroys it. The surgeon’s goal is to create a bubble at the laser site before the pigment epithelium is destroyed. Then, by firing additional shots through the apex of the bubble, the stroma is destroyed, exposing the underlying pigment epithelium.41

The initial setting should be a 200-  m spot, 200–400 mW, 0.1 second duration to anneal the pigment epithelium to the stroma.

Fig. 30-6  Argon laser peripheral iridotomy in a light blue iris. The argon beam may pass directly through the light blue stroma and blast the pigment off the back of the iris. Subsequent laser applications will pass through the stroma without affecting it because there is no adjacent pigmented material to absorb the energy.

Then the spot size is reduced to 50  m and the power increased to 600–1000 mW at 0.02–0.1 second to perforate. If the stroma is clearly being treated, as evidenced by its clumping and opacification, then these settings can be continued. If penetration has not occurred in 20–40 shots, then a new spot should be treated with an alternative technique.

One alternative technique requires higher energy (1200– 1500 mW), with exposure times of 0.3–0.4 second and a 50-  m spot size.The shutter speed is set at 0.5 second. As the firing pedal is depressed, a bubble will form.When the bubble is about 0.5 mm in diameter, the pedal is released. Before the bubble can float away, a second laser application is fired directly through the apex of the bubble (Fig. 30-7). Occasionally a third such application is required. These initial high-energy shots will create a crater whose base is the pigment epithelium. It is then a simple matter to remove the pigment epithelium by using shorter exposures (0.05–0.1 second) at lower energies (400–600 mW), as described for brown irides.

Another alternative uses low-energy ‘stretch’ burns of 200 mW and a 200-  m spot size for 0.1–0.2 second on either side of (or surrounding) the site to be perforated. The concept behind this technique is to tighten the iris between the stretch burns, making iris perforation easier. Most iridectomies can be performed without this additional trauma.

Complications of laser iridotomy

Iritis

Some degree of iritis always follows laser iridotomy. Iritis can be minimized by topical corticosteroid eyedrops (e.g., prednisolone acetate 1% hourly until bed time on the day of laser treatment and four times daily for the following day or two), but most patients do quite well without treatment. Sometimes, following argon or diode laser iridotomy, posterior synechiae develop. Occasionally patients develop recurrent iritis after laser iridotomy.This represents aggravation of a pre-existing condition in most patients but rarely arises de novo after the laser therapy. Hypopyon is rare.

Pressure elevation

Preventive therapy to limit a potential rise in IOP is recommended. Intraocular pressure elevation commonly occurs 1–4 hours after laser iridotomy and is usually self-limited. Elevations greater than 10 mmHg are seen in roughly 17–27% of patients.42,43 Treatment

options include the use of an -adrenergic agonist (Iopidine 0.5–1%,42 Alphagan 0.2%), a -blocker (Timoptic 0.5%, Betoptic 0.5%), carbonic anhydrase inhibitors (Trusopt 2%, Diamox, Neptazane), or an osmotic agent (Ismotic). Most recommend either topical apraclonidine or topical brimonidine, one drop before treatment, and some add one drop after.

Usually the IOP returns to pretreatment or lower levels within 24 hours. In combined-mechanism forms of glaucoma in which the outflow is abnormal before laser therapy, however, the IOP elevation can be severe and sustained. Such patients should be warned before undergoing laser surgery that filtering surgery may be required to control this pressure elevation. This complication is more likely if high IOP is associated preoperatively with a chamber angle that, although quite narrow, is open, so that aqueous has access to the trabecular meshwork. These findings indicate that although the incomplete angle closure may be contributing in part to the IOP elevation, the trabecular meshwork is not functioning adequately.The trauma of the laser iridotomy and the pigment released may further tax the meshwork, precipitating the need for filtering surgery.

Cataract

Cataract formation can occur after surgical iridotomy. Follow-up of patients who had argon laser iridotomies indicates that the laser

treatment has no greater likelihood of causing cataract than does surgical iridectomy.26,44–48

Argon laser iridotomy frequently causes localized injury to the lens beneath the iridotomy site. The injury can be seen as a whitening of this area of the lens. Fortunately, long-term follow-up has shown that these focal opacities do not progress. Nd:YAG iridotomy has caused lens capsule perforation in monkeys.49 Obviously the Nd:YAG laser can rupture the capsule in humans as well, thus focus is critical.This complication is rare.

It is reasonable to assume that iridotomy (surgical or laser) is a traumatic experience to the eye. Mild postoperative iritis is common and may be responsible for the metabolic lens changes that accelerate cataract formation.

In some patients undergoing extracapsular cataract surgery, zonular weakness in the area of the argon laser iridotomy has been noted. It is conceivable that the laser peripheral iridotomy may have damaged these zonules.

Hyphema

Hyphema is a rare occurrence with argon laser iridotomy50 but is common after Nd:YAG laser iridotomy. The bleeding is usually self-limited and of little consequence except that it may contribute to transient IOP elevation and reduced vision.51 Temporary IOP elevation created by pressing on the contact lens is usually adequate to control the bleeding site.

Corneal epithelial injury

Corneal epithelial injury occurs commonly with laser iridotomy. If high energy levels are used, if a chromophore such as fluorescein is on the corneal surface, or if the epithelium is slightly edematous, small white burns may appear. All evidence of any topical dye should be gone before attempting laser iridotomy with argon or diode lasers.When burns do occur, they disappear after a day or so without long-term effects. Use of a contact lens helps minimize corneal burns in most patients.

Endothelial damage

Endothelial damage also occurs with argon, solid-state, and Nd: YAG laser iridotomy (Fig. 30-8).52–54 If the iris is very near or

touching the cornea, both types of laser can cause burns. If a bubble is pressed against the endothelium at the point of laser impact or if the laser is focused poorly, an endothelial burn will occur with photothermal laser energy.

Epithelial and endothelial burns reduce both corneal clarity and the amount of laser energy reaching the iris, making the procedure more difficult. If burns occur, power or exposure time should be reduced. If it is not possible to avoid the burn by changing the approach, a new site should be chosen. Extensive endothelial burns can lead to persistent corneal edema. Nd:YAG laser treatments close to the endothelium can also cause tiny glass-like cracks at Descemet’s membrane.