LASER IRIDOTOMY

Laser iridotomy is now the preferred method for managing a variety of angle-closure glaucomas that have at least some component of pupillary block. Although laser iridotomy was first described with the argon laser, iridotomy with the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser has now become the preferred procedure of most surgeons. This is due to the ready availability of these lasers for posterior capsulotomy and a simpler, more rapid iridotomy protocol with less chance of subsequent iridotomy closure. Complications include iris bleeding with the Nd:YAG laser, iridotomy closure with the argon laser, and pressure spikes and inflammation with both. The impact of the latter are significantly reduced by medical treatment with alpha2-agonists and postoperative corticosteroids.

BACKGROUND

Angle-closure glaucoma caused by relative or absolute pupillary block is treatable by creating a full-thickness opening in the iris, therefore bypassing the pupil. Prior to the advent of lasers, this was achieved with a surgical iridectomy. Although successful, this procedure was associated with many complications, such as hemorrhage, infection, wound leak, flat anterior chamber, cataract, and problems related to anesthesia.1 Frequently, patients falsely associated poor visual results with the surgery and not with their delay in seeking therapy, nor with cataract, corneal decompensation, or long-term glaucoma damage. Many patients with acute angle-closure glaucoma in one eye and an occludable angle in the fellow eye were therefore often reluctant to seek treatment for their fellow asymptomatic eye, leaving them vulnerable to angle-closure attacks or chronic angle closure.

In 1956, Meyer-Schwickerath2 reported the use of light energy with a xenon arc photocoagulator to create a patent iridotomy. Not a true laser, the xenon arc was asso-

ciated with corneal and lenticular damage because of the large amounts of total, noncollimated energy required.2

In the early 1970s the argon laser attached to a slitlamp delivery system became commercially available.3,4 By the mid to late 1970s, L’Esperance,5 Abraham,6 and Pollack7 all reported successful iridotomy formation using this laser. Because the argon laser was easy to use, convenient for the ophthalmologist and the patient, and associated with fewer complications than surgical iridectomy, it replaced iridectomy as the initial procedure of choice for various angle-closure glaucomas by the early 1980s.8 During this time, ophthalmologists began evaluating the advantages of the Nd:YAG lasers over the argon. The Nd:YAG laser required fewer pulses and less total energy and had a diminished rate of iridotomy closure. Today it is the laser of choice for iridotomy.

MECHANISM

Histological evaluation of the iris following a successful iridotomy demonstrates a through and through hole. Posterior synechiae are often evident surrounding the border of the hole. The argon laser primarily acts by producing a thermal burn and depends on energy uptake by pigmented tissues. Following argon laser iridotomy, there is coagulative necrosis of the adjacent stroma, migration of pigment-laden macrophages into the stroma, and loss of pigment epithelium. In time, iris atrophy may develop around the hole (Fig. 41–1A).

By contrast, the Nd:YAG laser creates a plasma of free ions and electrons at the site of optical breakdown (Chapter 39). This photodisruption releases shock waves that mechanically rupture the tissue without depending on uptake of laser energy by tissue pigment. Histology demonstrates disruption of the iris pigment epithelium surrounding the iridotomy site. In contrast to the argon

laser, there is no coagulative effect in the adjacent stroma, and cell migration is also less likely (Fig. 41–1B).

INDICATIONS FOR LASER IRIDOTOMY

Laser iridotomy is the procedure of choice in all forms of angle-closure glaucoma (Chapter 16) in which there is a component of pupillary block.8 It is also helpful in distinguishing pupillary block from both plateau iris syndrome and ciliary block glaucoma. An example of the latter might be an eye with a flat chamber and elevated intraocular pressure (IOP) 1 day after cataract surgery. It is not indicated in cases of secondary angle closure where pupillary block is not a contributing factor.

ACUTE ANGLE-CLOSURE GLAUCOMA

It is easiest to create a full thickness iridotomy in an eye with clear media and with the iris as thin as possible. Because of this, laser iridotomy is best performed 24 to 48 hours after the acute attack is controlled with medical therapy, when the media is clear and inflammation is resolving. In some cases, medical therapy fails to break the acute attack. Here, some surgeons prefer to use the argon laser first, to minimize iris bleeding in these inflamed eyes with engorged blood vessels.9 However, medications that cause iris vasoconstriction, such as apraclonidine or brimonidine, can minimize this complication.

CHRONIC ANGLE-CLOSURE GLAUCOMA

Eyes with chronic angle-closure glaucoma are at risk of developing progressive trabecular meshwork damage and elevated IOP, as well as an attack of acute angle-closure glaucoma.8 In such eyes, eliminating pupillary block by laser iridotomy can minimize these potential complications.10,11 In addition, by allowing the angle to open to its full potential, iridotomy permits the clinician to evaluate

the state of the angle by gonioscopy and determine if further medical or surgical therapy is indicated.

APHAKIC OR PSEUDOPHAKIC PUPILLARY BLOCK

Laser iridotomy has been successfully used to relieve aphakic and pseudophakic pupillary block.12–15 Persistence of angle closure despite a patent iridotomy may indicate the presence of malignant (aqueous misdirection or ciliary block) glaucoma, which requires disruption of the anterior hyaloid face or vitrectomy.16,17

MALIGNANT GLAUCOMA

(AQUEOUS MISDIRECTION, CILIARY BLOCK)

When this condition is present, bilateral prophylactic laser iridotomies may protect the fellow eye against both acute angle-closure glaucoma and malignant glaucoma that could be triggered by subsequent intraocular surgery.8

PROPHYLACTIC LASER IRIDOTOMY

Prophylactic laser iridotomy is indicated in the fellow eye of patients with either acute or chronic angle-closure glaucoma, particularly if the eye has a relatively narrow angle and is thought to be capable of occlusion.8 Eyes with spontaneous appositional closure on darkroom gonioscopy and eyes with narrow angles and positive provocative tests should also have a prophylactic iridotomy (Chapter 16).

NANOPHTHALMOS

Eyes with nanophthalmos are characterized by a short axial length and are associated with narrow, crowded anterior chamber angles. Because of these anatomic features, these eyes are at considerable risk for developing angle-closure glaucoma and often require prophylactic laser iridotomies. However, bilateral nonrhegmatogenous

retinal detachments have been reported following laser iridotomy in eyes with nanophthalmos.18

PIGMENT DISPERSION SYNDROME

In recent years, reverse pupillary block, where aqueous humor becomes paradoxically trapped in the anterior chamber and causes posterior bowing of the iris, has been recognized as a factor in pigment dispersion syndrome (Chapter 19). Laser iridotomy bypasses this relative pupillary block and may eliminate the pressure differential across the iris.19,8 By eliminating the resulting iris concavity, an iridotomy can limit pigment release and the wide fluctuations in IOP seen in some patients with this condition, particularly those with marked iris concavity and very deep anterior chambers. However, many eyes with this condition simply become easier to control with time, whereas others respond well to medical management, and others to argon laser trabeculoplasty. Thus, the role of laser iridotomy for eyes with pigment dispersion and pigmentary glaucoma is not well established.

CONTRAINDICATIONS

Relative contraindications to laser iridotomy include poor visualization of the iris because of moderate corneal edema or corneal opacification, flat anterior chamber, and angle closure caused by peripheral anterior synechiae (PAS) [i.e., uveitis, neovascular glaucoma, or the iridocorneal endothelial (ICE) syndrome].8 Additional contraindications are children and patients who cannot keep their head still, cannot sit comfortably at the laser, or are otherwise unable to comply with the treatment protocol. In some cases, peribulbar anesthesia will permit successful treatment. Some lasers are designed to allow treatment of the patient in the supine position, and will permit treatment under general anesthesia.

SURGICAL TECHNIQUE

PREOPERATIVE CONSIDERATIONS

Informed consent for either argon or Nd:YAG therapy should include a discussion of several potential complications. These include the possibility of incomplete iridotomy and the need for a subsequent treatment session, late closure of the iridotomy, IOP elevation during the immediate postoperative period, iris bleeding (Nd:YAG), and continued IOP elevation requiring medical or surgical intervention. Patients should also be warned of a loud noise or thumping sound in the eye (Nd:YAG) and postoperative inflammation with possible headache, photophobia, hyperemia, and blurred vision.

MEDICAL THERAPY

All patients should be pretreated with topical 1 or 2% pilocarpine approximately 30 to 60 minutes prior to the

CHAPTER 41 LASER IRIDOTOMY • 441

iridotomy. This stretches the peripheral iris, thus making it thinner and easier to penetrate. In addition, its miotic effect helps fixate the iris and minimizes the tendency of the pupil to peak toward the iridotomy site, a problem that is more common with the argon laser.1 Higher concentrations of pilocarpine should be avoided because they may narrow the angle, making it more difficult to create an iridotomy without causing a corneal opacity.

One drop of apraclonidine HCl 0.5 or 1.0% or brimonidine tartrate 0.15% or 0.2%, administered at least 30 minutes prior to laser therapy, decreases the incidence of large postoperative rises in IOP from 30% to less than 5% for both argon and Nd:YAG laser iridotomy.20–22 As a further advantage in eyes undergoing Nd:YAG laser iridotomy, the vasoconstrictive alpha1 action of apraclonidine (brimonidine has minimal alpha1 effect) minimizes bleeding, improving the safety and ease of completing the iridotomy.

CONTACT LENS

A contact lens, following topical anesthesia with proparacaine HCl 0.5%, offers several advantages for performing a laser iridotomy.23 The lens concentrates the laser energy at the level of the iris, increasing the power density at the iris surface and decreasing it at the cornea and the lens. It also acts as a heat sink, minimizing the number of corneal epithelial burns. In addition, the lens magnifies the target site with less loss of depth of field than would occur if magnification is simply increased with the slit-lamp controls. Finally, a contact lens acts as a speculum; it helps separate the lids and minimizes fine eye movements.

Although several different contact lenses have been developed for laser iridotomy, the Abraham lens and the Wise lens are the most commonly used. The Abraham lens24 is a modified Goldmann-type fundus lens with a flat glass plate bonded to its anterior surface. The glass plate has a + 66.0 diopter planoconvex button bonded into a decentered 8 mm hole (Fig. 41–2). Antireflective

FIGURE 41–2 Abraham laser iridotomy lens.

442 • SECTION VI LASER THERAPY OF GLAUCOMA

coating on the front surface of the lens improves energy transmission and slightly increases brightness and contrast. When the laser beam is directed through the Abraham lens its diameter is doubled at the cornea and halved at the iris, which increases the concentration of energy at the iris and decreases it at the cornea. Posterior to the site of focus, the beam is more rapidly defocused, decreasing potential injury to the posterior segment.25,26

The Wise lens has a + 102.0 diopter button, permitting higher energy density.27 This lens is more difficult to use than the Abraham lens because it provides a high amount of magnification and limited depth of focus.

When using either lens, it should be held between the thumb and first finger, allowing the fourth and fifth fingers to rest on the patient’s cheek or temple to improve stabilization. Holding the lens with the left hand for the right eye, and the right hand for the left eye will allow the patient to fixate with the fellow eye. The plane of the lens must always be oriented parallel to the iris plane and the laser spot centered within the button.

SELECTION OF IRIDOTOMY SITE

Ideally, an iridotomy should be placed beneath the upper eyelid to minimize the chances of postoperative glare or diplopia, and located slightly temporal to minimize the risk of accidental macular damage. Avoiding the 12 o’clock position will decrease the chance of the iridotomy site becoming obscured with gas bubbles during the procedure, especially with the argon laser. Most surgeons choose an iris crypt, or other naturally thin area, to facilitate iris penetration. Avoiding obvious iris blood vessels, particularly with the Nd:YAG laser, will minimize the chance of bleeding.

The iridotomy should be made in the mid or far peripheral iris, just anterior to any arcus senilis. At this position, the anterior convexity of the crystalline lens has begun to curve away from the iris. This usually represents a compromise between keeping the iridotomy away from the lens capsule, but not too close to the corneal endothelium. In addition, this location maximizes the chances of visualizing the iridotomy after surgery and minimizes the chances of postoperative diplopia.

SPECIAL CONSIDERATION

The laser iridotomy should be positioned in the mid to far peripheral iris. This minimizes the risk of damage to the lens capsule and the corneal endothelium.

LASER FOCUS

Adjusting the aiming beam to a crisp, round (not oval) spot on the iris insures a precise laser focus. Because the iris is thick, the surgeon must continually adjust the focus,

especially when drilling through an iris with the argon laser. With both the YAG and the argon laser, the laser beam must be centered within the convex button with the contact lens held parallel to the surface of the iris.

If a corneal opacity does develop and inhibits the view of the iris, the surgeon can still proceed by asking the patient to look up, or in another direction. This allows the laser energy to travel through a different area of the cornea toward the iris and bypass the clouded area. When using the argon laser, keeping the patient’s gaze nonparallel to the laser path will also minimize the chance of inadvertent macular damage.

ARGON LASER IRIDOTOMY

Three basic techniques have been described for creating an argon laser iridotomy: the direct technique, the “hump” method,28 and the “drumhead” technique.29,30 For all of these methods, the basic approach is first to direct the laser beam at the iris stroma and then at the iris pigment epithelium.31 These techniques are not ideal for all situations, and the argon laser iridotomy technique may have to be adjusted, depending on the iris color.

Many surgeons generally use the direct technique to perform an argon laser iridotomy. This consists of simply directing repeated laser energy pulses at the same iris location until the anterior lens capsule is visible. Transillumination defects alone do not guarantee a patent iridotomy. Remember that similar defects are commonly seen in pigmentary dispersion syndrome, even though the iris stroma is intact.

PITFALL… Transillumination defects alone do not guarantee a patent laser iridotomy. Treatment should be continued until the anterior lens capsule is visible.

This technique uses 50 m spots for 0.2 sec duration to maximize the energy density at the site of treatment. The smaller the spot size, the greater the energy or power density. A power setting of 1 W is usually ideal for most eyes. This maximizes the iris burn, with minimal surrounding char, and with minimal damage to the cornea and lens on either side of the iris.

Occasionally, an air bubble develops and blocks the view of the iridotomy site. This can often be dislodged by aiming a 50 m spot with 300 to 500 mW power at the inferior margin of the bubble. However, aiming at the center of the bubble may also reflect the laser energy back toward the cornea and cause a corneal burn.

For medium-brown irides, some surgeons will initially create a crater in the iris stroma, and then penetrate the iris pigment epithelium through the crater’s center. The approximately 500 m diameter crater is made using several laser applications with spot size 500 m, power 700 to 1000 mW

(average 1000 mW) and duration 0.2 sec.31 Further laser applications are directed at the bed of the crater until a cloud of pigment develops. This suggests that the iris stroma has been eliminated and only iris pigment epithelium remains.

At this stage, the laser settings can be changed to a spot size/power ratio of 100 m>500 to 700 mW, or 50 m>200 to 600 mW. Higher energy levels can dislodge adjacent iris pigment epithelium (cascade phenomenon), resulting in closure of the iridotomy.31 The same laser settings are used to remove the iris pigment epithelium layer regardless of iris color. This two-staged technique for the medium brown iris takes 30 to 60 laser applications. The laser procedure is considered completed when the anterior lens capsule is seen through the iridotomy site.32

Patients with a dark brown iris may be more difficult to penetrate, due to a thicker iris stoma and fewer crypts. Thick brown irides also tend to char more easily. This char has an aluminum-like appearance and characteristic that reflects further laser energy, making it more difficult to penetrate. To prevent this problem, some surgeons advocate a “chipping” technique.28,33,34 Here, the duration is decreased to 0.02 to 0.05 sec and the spot size and power remain the same (using a 50 m spot size and 700 to 1000 mW power level). This technique often requires many more laser applications (200 to 300) to penetrate the stroma31 and can be much more tiring for both the patient and the physician. Once the stromal crater has been eliminated, the iris pigment epithelium can be removed by the previously described method.

PITFALL… Penetrating a light blue iris with the argon laser is more difficult because of poor energy absorption by the relatively depigmented iris stroma.

Conversely, a light blue iris may be more difficult to penetrate because there is less stromal absorption of the laser energy due to the decreased pigment.31,32 Stetz et al. have recommend a two-step technique for eyes that lack any area of increased pigmentation.35 First, long (0.2 to 0.5 sec) overlapping burns with a large spot size (500 m) and 200 to 300 mW power are used to create a localized, tan-colored area of increased stromal density. Next, the laser settings are changed to spot size 50 m, power 500 to 700 mW and duration 0.1 sec to create a full-thickness hole in the stroma, as already described. Alternatively, Kolker34 and Hoskins and Migliazzo36 use two to three applications with spot size of 50 m, power 1000 to 1500 mW and duration 0.5 sec to create a hole in the stroma. In either case, the iris pigment epithelium layer is then eliminated using the previously described technique.

NEODYMIUM:YAG LASER IRIDOTOMY

Over the last decade, the Nd:YAG laser has become the preferred method of creating an iridotomy. This is due to

CHAPTER 41 LASER IRIDOTOMY • 443

the increased popularity of extracapsular cataract surgery with posterior chamber intraocular lenses, and the concomitant increased availability of Nd:YAG lasers for posterior capsulotomies. In addition, the Nd:YAG laser iridotomy is both easier and safer for the patient and the physician. The iridotomy rarely closes following Nd:YAG laser iridotomy, and it usually can be completed with at least one tenth the number of laser pulses required for an argon iridotomy.

In general, a higher energy setting with the Nd:YAG laser increases the likelihood of penetrating the iris in one treatment setting. However, increasing the energy also increases the chance of iris bleeding. Treatment typically begins with approximately 7 to 10 millijoules (mJ) of energy.8,31 If well focused, 7 to 10 mJ might penetrate 75% of the time with the first pulse. In thick brown irides, or otherwise difficult eyes, increasing the energy to 10 mJ or the burst mode to two shots per pulse may help.

The main disadvantage of the Nd:YAG laser is that it has minimal thermal effects and does not cauterize the tissue as it penetrates. Instead, it disrupts the tissue, and this can cause bleeding from vessels that lie deep within the stroma.

Typically, the surgeon can control bleeding at the slitlamp by pressing on the contact lens. This temporarily increases the IOP and collapses the iris blood vessels. After approximately 30 seconds, the pressure is released while watching the iridotomy site. If bleeding begins again, the pressure is resumed. If not, the surgeon can decide whether to continue the procedure, avoiding obvious blood vessels, or quit if the iridotomy is adequate. In general, if an initial treatment session is unsuccessful, the iridotomy can be easily completed 1 to 3 weeks later. If an argon laser is available, the bleeding vessel can be promptly and easily cauterized using 50 to 100 m spots for 0.2 sec.

POSTOPERATIVE MANAGEMENT

The IOP should be determined 30 to 60 minutes after the procedure. If there is any increase from baseline, the IOP should be rechecked after another 30 minutes to rule out any further increase that may need medical treatment. If there is a postoperative IOP spike of 8 mm Hg or more, or if the patient has extensive disk cupping, it is advisable to reevaluate the patient in 1 day. Significant rises in IOP are treated with standard medical glaucoma therapy. Most surgeons routinely treat patients with topical corticosteroids for several days (Table 41–1) and perform a follow up evaluation 1 week after the procedure.

Once a patent iridotomy is confirmed, the eye should be dilated, to minimize posterior synechiae and examine the fundus. This is especially important following argon laser iridotomy. If posterior synechiae do exist, pharmacological stretching of the synechiae may cause bleeding at the iris-lens border. Although this bleeding is self-lim- ited, the patient should be warned about the possibility of a temporary decrease in vision.

Put iris on stretch Minimize IOP rise Minimize IOP rise

Reduce iris bleeding (Nd:YAG) Minimize IOP rise

Reduce inflammation and decrease the risk of iridotomy closure (argon)

COMPLICATIONS

Complications of laser iridotomy include iris bleeding, focal cataracts, increased IOP, inflammation, and late closure of the iridotomy. Iris hemorrhage is restricted to Nd:YAG iridotomies. Iris bleeding can obscure the iridotomy site, interfere with successful completion of the iridotomy, and encourage postoperative closure of a patent iridotomy. In addition, bleeding can produce a layered hyphema and cause transient elevation of IOP (Fig. 41–3). Lens opacities generally occur if the iridotomy site is located too close to the pupil, where the iris is still in contact with the lens capsule. They can result from heat buildup during argon laser iridotomy, or from direct tissue disruption by the Nd:YAG laser (Fig. 41–4).

A transient, large postoperative IOP elevation can occur in approximately 30% of eyes undergoing iridotomy, regardless of the type of laser used.20–22 As stated above, routine treatment with apraclonidine or brimonidine significantly reduces the risk of this complication.

Nearly all eyes develop inflammation following laser iridotomy. If topical corticosteroids are not used, this can produce moderate discomfort. There appears to be no need for systemic corticosteroids, nor is there any proven benefit of topical nonsteroidal anti-inflammatory agents. Four to 7 days of topical therapy is sufficient for most cases.

Posterior synechiae can often develop after the iridotomy. In some, this may be severe enough to limit the pupil size, making subsequent perimetry, ophthalmoscopy, and cataract surgery more difficult. Avoiding postoperative pilocarpine, using frequent topical corticosteroids, and dilating the eye as soon as possible after the treatment can all minimize this complication.

Closure of the iridotomy appears limited, on the whole, to argon laser iridotomies and occurs in approximately 30% of cases. Most occur within the first month after therapy. Beyond this time, iridotomies will rarely close if there is no underlying inflammation. If closure does occur, another burst of laser energy will generally easily reopen the iridotomy.

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