LASER PERIPHERAL IRIDOTOMY
Laser iridotomy has, for the most part, replaced incisional surgical iridectomy. It is safer, achieves similar results, and is preferred by patients. Laser iridotomy is indicated for all forms of angle-closure glaucoma involving pupillary block and as a prophylactic measure for patients with occludable angles. A success rate of almost 100% can be achieved by experienced laser surgeons. Late failure is rare, especially in eyes treated with the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser.
Using light energy transmitted through the cornea instead of a blade incision to create an iridotomy was first demonstrated by Meyer-Schwickerath in 1956 with a xenon light source. Argon and Q-switched Nd:YAG lasers have enabled the creation of iridotomies more safely than by incisional surgical methods because the eye need not be opened. The laser procedure requires only topical anesthesia. In addition, the postoperative recovery period is shorter. This improvement in the risk:benefit ratio has changed the criteria for iridotomy. Surgical iridotomy is currently used only when laser iridotomy is not possible, such as when the cornea is opacified or the anterior chamber is very shallow.
A firm indication for laser iridotomy exists if the patient has pupillary block as evidenced by an anterior bowing of the peripheral iris with occludable angles accompanied by one or more conditions ( Box 30-1 ). Certainly, laser iridotomy should be a strong consideration in any patient who has had symptoms of intermittent blurring of vision accompanied by rainbow-colored haloes around lights, occurring under circumstances that promote pupil dilation and whose examination shows bowing forward of the peripheral iris and very narrow or worse angles on gonioscopy.
Acute angle-closure glaucoma
Chronic angle-closure glaucoma with peripheral anterior synechiae
Intermittent angle-closure glaucoma with classic symptoms of angle closure
Aphakic or pseudophakic pupillary block
Anatomically narrow angles and signs of previous attacks
Narrow-angle eye with acute angle-closure glaucoma in the fellow eye
Incomplete surgical iridectomy
Luxated or subluxated crystalline lens
Anterior chamber lens implant
Pupillary block from silicone oil after vitrectomy
Mixed-mechanism forms of glaucoma when filtering surgery might not be necessary for adequate pressure control
Critically narrow angles in asymptomatic patients
Younger patients, especially those who live some distance from medical care or who travel frequently
Narrow angles with positive provocative test
Iris–trabecular contact demonstrated by compression gonioscopy
Not all patients with narrow angles require iridotomy; most such patients never develop glaucoma. Relative indications for laser iridotomy exist because the procedure is not totally free of complications, and it is not always possible by gonioscopy alone to predict who will develop acute or chronic angle-closure glaucoma.
However, some asymptomatic patients in whom the angles are critically narrow are well served by laser iridotomy. In general, asymptomatic elderly patients who have access to adequate medical care can be counted on to report symptoms typical of angle closure and return for routine monitoring and can be observed safely. However, care must be taken to monitor these patients so that iridotomy can be instituted if progressive narrowing of the angle, formation of peripheral anterior synechiae, or elevated intraocular pressure (IOP) occurs.
Laser iridotomy should be considered in asymptomatic patients whose angles are narrow such that a portion is closed, in patients whose life expectancy will allow their lens to increase in size with subsequent angle narrowing and for whom cataract extraction is not anticipated in the immediate future, and in patients who do not have constant ready access to medical care such as frequent travelers or those who live in remote areas. This procedure is generally preferred over miotic therapy in such patients because, in many cases, miotic treatment does not prevent angle closure. If the choice is not clear and the two eyes are similar gonioscopically, iridotomy can be performed in one eye and the other eye can be observed.
In acute angle-closure glaucoma caused by pupillary block, treatment is initiated with medication to decrease IOP and help restore corneal clarity. Laser iridotomy, which is the definitive treatment, can then be performed more successfully.
TYPES OF LASER
Types of laser commonly used for iridotomy include the photodisruptive Q-switched Nd:YAG laser, the photothermal argon lasers, or the solid state lasers. The Q-switched Nd:YAG laser is preferred by many surgeons because it perforates the iris easily. This is particularly true in dark brown or light blue irides. It is more difficult to penetrate dark brown irides with photothermal argon or solid-state lasers because they have a tendency to char during treatment; light blue irides can be difficult because pale irides do not absorb argon or solid-state laser energy very well. Moreover, Nd:YAG iridotomies may be less likely to close over time.
Because the Nd:YAG laser (unlike photothermal argon or solid-state lasers) has no coagulative effect, bleeding occurs more frequently. Local hemorrhage is usually self-limited and rarely of consequence. Bleeding may be limited by pretreating the proposed iridotomy site with the coagulative energy from an argon laser or a 532-nm frequency-doubled solid-state Nd:YAG laser, but this is rarely performed. However, it may be indicated in those who have a coagulative disorder or who are using anticoagulative medications. In patients with severe coagulative disorders, pretreatment with intravenous blood derivatives to replace a missing factor(s) may help limit intraocular bleeding. The vast majority of the time, if bleeding does occur at the surgical site, it can be stopped by pressing on the contact lens and temporarily raising the IOP ( Fig. 30-1 ).
Miosis, which helps to tighten and thin the peripheral iris and pull it away from the cornea, can be accomplished with a drop of pilocarpine 1% or 2%. Topical anesthesia is achieved with proparacaine hydrochloride 0.5%.
An Abraham iridotomy lens ( Fig. 30-2 ) greatly improves visualization, separates the lids, stabilizes the eye, minimizes epithelial burns because it acts as a heat sink, and increases the power density by concentrating the energy into a smaller spot size at the iris. The Wise lens modification provides a higher power density at the tissue site but causes greater image distortion as a result of the higher magnification. A variety of other lenses that are especially corrected for argon or Nd:YAG wavelengths have been developed for this purpose. One relatively recent one is the Pollack lens (Ocular Instruments, Bellevue, WA) which we have found to be useful for iridotomies, trabeculoplasty, and also for iridoplasty (see below).
The iridotomy should be placed in the periphery of the iris. Such placement reduces the likelihood of lens injury and possible subsequent sealing of the iridotomy by posterior synechiae to the lens. Furthermore, peripheral placement also reduces the likelihood of later ghost images through the iridotomy. If a dense arcus senilis is present, the iridotomy site must be central to it. A site between the 11 and 1 o’clock meridians is preferable because it will be covered by the upper lid. Iridotomies within the palpebral fissure can cause visual disturbances due to polycoria. The 12 o’clock meridian should be avoided because (1) with argon or solid-state laser iridotomy, gas bubbles rise to this area and may obscure the laser site before treatment can be completed, and (2) with Nd:YAG laser iridotomy, a small trickle of hemorrhage may cascade down from the treatment site and obscure the patient’s vision temporarily. Both of these problems will be avoided if the iridotomy is placed closer to the 11 or 1 o’clock positions. Wand (Personal Communication) has advocated placing the iridotomies in the temporal or nasal periphery and reports no visual problems or ghost images with this technique.
With careful examination, a relatively thin region in the iris can often be identified. This area may be located in the depths of a crypt or in an area evidenced by a rather lacy, translucent appearance of the superficial stroma. In blue irises, the dense white radial cords of the iris stroma should be avoided. Perforation of the iris is recognized by release of a pigment cloud billowing forward from the iridotomy site and the posterior movement of the iris deepening the anterior chamber. With Nd:YAG iridotomies a rapid gush of fluid is often indicated by small flecks of pigment racing through the opening. Transillumination through the iridotomy is a reasonably good sign of complete perforation in a brown iris but can be misleading in a light blue or grey iris. Clear evidence of perforation is direct observation of the anterior lens capsule through the iridotomy site. Another useful technique is to direct the aiming beam into the depths of the iridotomy. Be sure the main beam is inoperative. If the opening is through-and-through the iris, the aiming beam will disappear. The aiming beam is also helpful for coherent transillumination. A small but complete peripheral perforation is the ideal end point. It is important to have the patient gaze upward, both to clear a peripheral corneal arcus and to ensure that no laser light will be aimed toward the macula.
ND:YAG LASER IRIDOTOMY
Q-switched Nd:YAG lasers (1064 nm) are very useful for iridotomy ( Figs 30-3 and 30-4 ). Power settings depend on the power 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, 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 block and is potentially more susceptible to later closure by pigment. 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 techniques 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 circumstances. The following discussion outlines a few useful argon and solid-state iridotomy techniques.
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 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 μm 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 200- to 300-μm 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 one-third 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 μm. 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.
The initial setting should be a 200- μm spot, 200–400 mW, 0.1 second duration to anneal the pigment epithelium to the stroma. Then the spot size is reduced to 50 μm 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- μm 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.