Ординатура / Офтальмология / Английские материалы / Shields Textbook of Glaucoma, 6th edition_Allingham, Damji, Freedman_2010

trabecular meshwork, with approximately 100 applications evenly spaced around the full 360 degrees of the meshwork (8). Complications associated with this basic protocol, however, led to variations in technique. We consider, first, the complications and how they are managed and, then, the variations in technique that have been used to minimize the complications.

With SLT, a total of approximately 50 to 70 adjacent, nonoverlapping spots are placed over 180 degrees of the trabecular meshwork, with energy ranging from 0.5 to 1.2 mJ per pulse, set to prevent bubble formation. Typically, the power is titrated until the appearance of tiny air bubbles are released from the site of the laser burn, termed “champagne bubbles.” After the bubbles are seen, the power is slightly reduced to eliminate their appearance.

Alternative Protocols with Argon Laser

The parameter evaluated most extensively has been the total number of laser applications and the amount of trabecular meshwork treated. Applying 25 burns to 90% of the meshwork is less effective than protocols with larger amounts of treatment (68, 69). However, the application of 50 burns to 180 degrees or 360 degrees has a similar effect on IOP reduction as treatment of 100 burns to 360 degrees of the meshwork (69, 70 and 71). In one such study, the eyes receiving 50 applications over 180 or 360 degrees had a lower probability of requiring subsequent filtering surgery than those receiving 100 applications over 360 degrees (72). A two-stage protocol, in which treatment of the full 360-degree circumference is divided into two sessions, 1 month apart, had the same IOP reduction as the full treatment in one session (73). With the latter technique, most of the pressure reduction is achieved with the first stage of therapy, although some patients may have minimal benefit from the first stage and yet a substantial pressure reduction after the second stage (74). The main advantage of the lower number of laser applications during a single session is a reduction in the transient IOP rise in the immediate postoperative period (69, 70 and 71, 73, 74, 75, 76 and 77). In one study, however, the frequency and magnitude of postlaser IOP increase were the same in groups receiving 360-degree treatment in one or two stages (78). The long-term outcome does not appear to be influenced by which quadrants are treated first. One study randomly assigned patients into initial inferior versus superior halves and found no significant difference between the two groups (72).

Another variation from the basic protocol that appears to minimize the complication of early posttreatment IOP rise is the placement of the laser applications along the anterior portion of the pigmented meshwork (Fig. 36.3) (69, 71, 77). An anterior placement of the laser burns also reduces the complication of peripheral anterior synechiae (79, 80). It may, however, increase the potential complication of cellular proliferation from the corneal endothelium over the trabecular meshwork (37). Complications and Postoperative Management

Transient IOP elevation in the immediate postoperative period is the most serious early complication of ALT (75, 76, 81, 82, 83 and 84). In most cases, the pressure rise is mild and lasts less than 24 hours, causing no long-term problems. In some patients, however, the elevation is marked and sustained and can lead to further loss of vision, especially in eyes with advanced visual field loss before trabeculoplasty. The IOP rise occurs within 2 hours after treatment in most cases, although some eyes may not develop an increase until 4 to 7 hours after therapy (82, 83). Postoperative

P.456

management, therefore, should include a pressure check within the first few hours after the procedure. Patients who have a significant early postoperative pressure rise or who have advanced glaucomatous damage may require an IOP check the following day. However, a pressure rise on the first postoperative day is uncommon, with only 4.2% having a rise greater than 3 mm Hg in one study, and seeing most patients in 1 to 3 weeks postoperatively is considered reasonable (85). With SLT, approximately 25% of patients had a transient IOP elevation of 5 to 6 mm Hg (54, 86, 87), and in one study, some patients had an IOP elevation of more than 10 mm Hg (88).

Figure 36.3 Gonioscopic view of patient following argon laser trabeculoplasty. Note the typical blanched lesions of the pigmented trabecular meshwork, which may persist for several days. Histopathologic studies suggest that the mechanism of posttrabeculoplasty pressure rise after ALT is an inflammatory reaction, with fibrinous material and tissue debris in the meshwork (28, 34, 89, 90). Laboratory studies in bovine eyes indicate that the trabecular meshwork can contract in response to endothelin1, which may be a mechanism in the immediate posttrabeculoplasty IOP elevation (91, 92). This hypothesis is sup ported by the finding of an increased concentration of endothelin-1 in the aqueous humor of rabbit eyes after ALT (93, 94 and 95).

The main patient characteristic associated with the transient pressure rise is meshwork pigmentation (84). Two patients with exfoliation syndrome had a delayed IOP rise during the first postlaser month, associated with inflammatory precipitates on the trabecular meshwork (96). It should also be noted that eyes with active inflammation are at a high risk for marked IOP rise after ALT, and the operation is contraindicated in these eyes.

Figure 36.4 Scanning electron microscopic view of trabeculectomy specimen from eye with failed argon laser trabeculoplasty showing endothelial growth over portions of the intertrabecular spaces (arrows). Iritis is a common early posttrabeculoplasty complication. In one study, by using a laser flare-cell meter, 49% of 71 eyes showed significant inflammation, which peaked 2 days after treatment (97). The inflammation was significantly more frequent in eyes with exfoliation syndrome or pigmentary glaucoma than in those with COAG. Postoperative iritis is usually mild and transient and is easily controlled with a brief postoperative course of topical corticosteroids. A typical protocol for postoperative management of ALT includes prednisolone, 1%, fluorometholone, 0.1%, or the equivalent four times daily for 5 days. Pretreatment with topical steroids or nonsteroidal antiinflammatory agents (98, 99, 100 and 101) has been shown to reduce posttrabeculoplasty inflammation but had no effect on the postoperative IOP elevation (102, 103). There is no definite consensus on the posttrabeculoplasty anti-inflammatory regimen following SLT. Various protocols, ranging from use of topical prednisolone acetate, 1%, to use of a topical nonsteroidal antiinflammatory agent, to use of no anti-inflammatory agents, have been used. However, a greater anterior chamber reaction was seen after the SLT than after ALT in one study (104).

The formation of peripheral anterior synechiae is also a common complication of trabeculoplasty (79). These are typically small and tented, corresponding to the location of the laser applications. Alterations of corneal endothelium after ALT may include a significant increase in cell size (105), although another study showed no statistically significant changes (106). The formation of peripheral anterior synechiae after SLT is rare.

The most serious late-posttrabeculoplasty complication is, at the present time, more theoretical than real. Histopathologic studies, as previously described, show changes in the trabecular meshwork, including an endothelial layer over the inner surface (Fig. 36.4), which could eventually lead to an increase in P.457

resistance to aqueous outflow (28, 29 and 30, 36, 37). One retrospective study evaluated ALT specimens treated with one or more ALT procedures before trabeculectomy and found that eyes treated with argon laser had an increased incidence of membrane formation in the chamber angle. Half of the specimens

had a cellular and collagenous membrane covering the entire trabecular meshwork, which was more common in eyes in which more ALT procedures were performed (107). Whether these structural changes eventually make the glaucoma more difficult to control has never been proven, despite more than 30 years of experience. There is, however, a limit to the amount of laser treatment that an eye can tolerate, and the success is time limited in nearly all patients, as discussed later in this chapter. There has also been concern that laser trabeculoplasty might interfere with the success rate of subsequent filtering surgery, causing a higher rate of encapsulation in eyes with previous ALT (108), although this did not appear to be the case in another study (109).

Pharmacologic Control of Increased Pressure

Topical application of the a2-adrenergic agonist apraclonidine, 1%, at 1 hour before and immediately

after laser trabeculoplasty was shown to have a marked effect on minimizing the postoperative pressure rise (110). When compared with eyes treated with pilocarpine, 4%, timolol, 0.5%, dipivefrin, 0.1%, or acetazolamide, 250 mg, each given 1 hour before and immediately after trabeculoplasty, only 3% of apraclonidine-treated eyes had IOP increases greater than 5 mm Hg, in contrast to 33%, 32%, 38%, and 39%, respectively, with the other treatments (111). A single drop of apraclonidine 15 minutes before or immediately after the laser treatment is as effective as the two doses, and apraclonidine, 0.5%, is as effective as 1% (112, 113, 114 and 115). This has now become a standard part of laser trabeculoplasty for most surgeons. So profound is the benefit of apraclonidine that treatment in two sessions of 180 degrees each may no longer be necessary to avoid the transient IOP rise. In one study, 360-degree trabeculoplasty with perioperative apraclonidine had the same early postoperative IOP course as the 180% treatment without apraclonidine (116). However, caution is advised for patients on long-term a2-

adrenergic agonist therapy, in which case the apraclonidine may be less effective.

The selective a2-adrenergic agonist brimonidine, 0.5%, has been shown to effectively control the

postlaser pressure rise when given either before or after the laser surgery (117, 118). Brimonidine, 0.2%, has also been found to be as effective as apraclonidine, 1.0%, in preventing IOP spikes after ALT (119). Pilocarpine, 4%, alone immediately after ALT was also shown to be effective in minimizing the IOP elevation (120). In a randomized trial, apraclonidine, 1%, was not effective in preventing the IOP spikes in patients on long-term apraclonidine (121). Pilocarpine, 4%, was only slightly less effective in patients on long-term pilocarpine therapy and was at least as effective as apraclonidine, 1%, in post-ALT IOP spike pro-phylaxis. Another study found that adding pilocarpine to apraclonidine therapy further reduced the incidence of postoperative pressure rise (122). Pilocarpine, therefore, can be considered as a first choice for prevention of posttrabeculoplasty IOP spike, especially in patients treated with apraclonidine (121) or possibly with other a2-adrenergic agonists.

Acetazolamide was also shown to reduce the IOP rise following ALT in one study (123), although, as previously noted, it is less effective than apraclonidine (111). As discussed previously, neither corticosteroids (98), nor the prostaglandin synthetase inhibitors indomethacin or flurbiprofen, significantly influenced the postoperative IOP (99, 100, 124, 125). One study showed that patients receiving topical indomethacin had higher pressures after 1 month than those receiving a placebo (124). Prostaglandin synthetase inhibitors also appear to have no influence on the postoperative iritis (100, 126).

Results

Short-Term Intraocular Pressure Control

Most reports show that useful IOP reduction is achieved in approximately 85% of eyes treated with ALT (8, 9, 10 and 11, 22, 75, 127). Some eyes may have a pressure drop within the first few hours after treatment, although days or weeks are usually required to achieve the full response to ALT and SLT, with further IOP reduction rarely occurring beyond 1 month. The magnitude of the final pressure reduction averages 6 to 9 mm Hg, which is usually insufficient to allow discontinuation of all medical therapy, although the medication can occasionally be reduced or eliminated (128). One study suggested that pilocarpine may lose its effectiveness after ALT (129), and it may be advisable to re-evaluate the efficacy of any miotic treatment approximately 1 month after the laser treatment. However, a later study

27 - Principles of Medical Therapy and Management Page 109 of 267

showed no difference between the IOP-lowering effect of pilocarpine, 1%, before and after ALT (130). The IOP reduction after the SLT ranged from 3 to 18 mm Hg (86). Six months after 180-degree SLT, the mean IOP reduction was 4.4 mm Hg, with a success rate of 64.6%. An elevated preoperative IOP was the significant determinant for success, whereas age, sex, history of ALT, and trabecular meshwork pigmentation were not significantly related to success (87). When the trabecular meshwork was treated 360 degrees with the SLT, the IOP was reduced in all eyes by approximately 40% at 6 weeks after the treatment (88). In a prospective study, 50 eyes were treated with SLT, with the mean IOP reduction of approximately 5 mm Hg at 1, 3, 6, and 12 months (131). In another clinical trial of 10 eyes treated with SLT, the IOP was reduced only slightly less in the exfoliative glaucoma than in the COAG (132). In a randomized trial, patients with previously failed ALT had a better IOP reduction with the selective laser than with a repeated argon laser (104). IOP lowering during the first 6 months after the SLT was similar to that of ALT and appears to diminish over the first year of follow-up (133).

Factors Affecting IOP Response

Many factors influence the IOP response to ALT. Eyes with a higher pretreatment IOP tend to have a greater decrease in IOP (134), but a pretreatment IOP greater than 30 mm Hg has been associated with a higher frequency of failure (135, 136), whereas

P.458

eyes with pressures closer to the target IOP may obtain useful pressure reduction after trabeculoplasty (137, 138 and 139).

Another significant factor influencing IOP response to ALT is the type of glaucoma. A particularly favorable response is obtained with COAG, exfoliation syndrome, and pigmentary glaucoma (21, 127, 134, 135, 140, 141, 142 and 143). Success in the latter two conditions is most likely related to the favorable influence of increased trabecular meshwork pigmentation (144). In pigmentary glaucoma, younger patients appear to have a more sustained pressure reduction than older patients with the same condition do (142, 143). Some clinicians have noted that eyes with darkly pigmented trabecular meshwork are at greater risk for an immediate IOP spike following SLT (145). In these eyes, decreasing the power of the SLT is generally recommended.

Other forms of glaucoma that respond to ALT, although less well than those noted earlier, include openangle glaucoma in aphakia or pseudophakia and angle-closure glaucoma after an iridotomy (140, 146). Although eyes that have had multiple operations generally do not do well with ALT (141), those with a single failed trabeculectomy may obtain useful pressure reduction after the laser surgery (147). Other forms of glaucoma that do not respond well to ALT include glaucoma associated with uveitis, anglerecession glaucoma, and congenital or juvenile glaucoma (140, 141).

Some investigators believe that young age has an unfavorable effect on the results of laser trabeculoplasty (127, 136, 148), although one study showed no effect of age (135). As previously noted, young patients with pigmentary glaucoma appear to do better than older patients with the same condition (142, 143).

Race may influence the results of laser trabeculoplasty (149). In the Advanced Glaucoma Intervention Study (AGIS), eyes were randomly assigned to an ALT-trabeculectomy- trabeculectomy sequence or a trabeculectomy-ALT- trabeculectomy sequence. The initial report from this randomized clinical trial recommended the initial use of the ALT for all black patients (150). However, a later report from the AGIS provided only a weak suggestion that an initial trabeculoplasty delays the progression of glaucoma more effectively in black patients than in white patients (151).

Long-Term Intraocular Pressure Control

A major question regarding the results of laser trabeculoplasty is how long the IOP reduction will last. Although a high percentage of patients show an initial favorable reduction in IOP, most patients gradually lose this effect (152, 153, 154, 155, 156, 157, 158 and 159). Failure is most common in the first year, with reported rates of 19% to 23%, and thereafter failure occurs at a rate of 5% to 9% per year (157, 159). As a result, approximately half of the patients will have lost the benefit of the initial trabeculoplasty by 5 years, and two thirds within 10 years, after the procedure (159).

Repeated Trabeculoplasty

If a successful IOP reduction is never achieved after 360-degree ALT, further argon laser treatment is generally not thought to be indicated. When an initial good response to treatment, lasting for approximately 1 year or more, was followed by a return to higher pressures, repeated trabeculoplasty was once common practice. However, most studies have shown a much lower success rate with repeated ALT than with the initial treatment, in the range of one third to one half (158, 160, 161, 162, 163, 164, 165 and 166). In one longterm study, success rates with repeated ALT were 35% at 6 months, 21% at 12 months, 11% at 24 months, and 5% at 48 months (166). Although one study suggested that the ALT can be repeated with good results (167), most surgeons no longer recommend repeated ALT. Some studies have noted a higher incidence of transient IOP rise after repeated ALT (160, 161, 166), and it is probably advisable to perform these in two stages of 180 degrees each, if a repeated procedure is attempted.

SLT caused more significant IOP lowering in patients with previously failed ALT compared with repeated ALT in a randomized trial (104). Repeated SLT may be almost as effective as initial SLT based on a single retrospective study (168). Repeated SLT, after either initial SLT or initial ALT, may become accepted practice, although further long-term experience is needed. Because of the greater preservation of trabecular meshwork with SLT, it has been suggested that SLT may be less likely to interfere with future incisional surgery (169).

Indications

Laser trabeculoplasty may be indicated in the treatment of those forms of open-angle glaucoma in which favorable responses have been reported, including COAG, exfoliation syndrome, pigmentary glaucoma, and open-angle glaucoma in aphakia or pseudophakia. The IOP-lowering effect was more pronounced in pseudophakic than aphakic eyes, and in eyes that had extracapsular surgery rather than intracapsular surgery (170). ALT was also preferred to cyclocryotherapy for the initial treatment of patients with uncontrolled glaucoma after a penetrating keratoplasty (171).

During the first decade of experience with ALT, the procedure was used as a supplement to maximum tolerable medical therapy, and studies have shown it to be effective in this regard (75, 172). The rationale for this approach was based not only on the risk for early postoperative complications, especially the transient pressure rise, but also on the concern that eyes treated with laser trabeculoplasty might eventually become more difficult to control than if they had been left on medical therapy. The histopathologic studies showing proliferation of a cellular layer over the trabecular meshwork have given reason to seriously consider this theoretical complication (28, 30, 36, 37). Nevertheless, short-term and long-term studies of ALT for open-angle glaucoma suggest that the procedure may be a safe and effective initial treatment of glaucoma (172, 173, 174, 175, 176, 177, 178 and 179).

In a multicenter clinical trial (the Glaucoma Laser Trial), 271 patients with newly diagnosed open-angle glaucoma were randomly assigned to initial ALT in one eye and timolol, 0.5%, in the other eye, with the same stepped regimen of additional medical therapy in either eye as required (178). During the first 2 years of follow-up, the laser-treated eyes had a slightly lower mean IOP of 1 to 2 mm Hg, although more than half of these eyes eventually required the addition of one or more medications. In a follow-up study of 203 of these patients, with a mean duration of 7 years, eyes initially treated with laser

P.459

trabeculoplasty had 1.2-mm Hg greater reduction in IOP, 0.6-dB greater improvement in visual field, and slightly less optic nerve head deterioration (179). Although these findings suggest that initial treatment with ALT is at least as efficacious as initial treatment with topical medications that were available at the time of the study, medical therapy is still more commonly used in North America, particularly with the newer, more efficient IOP-lowering topical medications.

A shorter-term study comparing SLT and a topical prostaglandin, latanoprost, found that the two therapies were equally effective over 1 year (180, 181). Randomized, controlled trials comparing treatment of 180 degrees of trabecular meshwork with ALT versus SLT showed no difference regarding effectiveness of IOP lowering up to 5 years (182, 183). However, most clinicians treat 360 degrees of

trabecular meshwork with SLT at one time. Some support in the literature indicates that 360 degrees is more effective than 180 degrees (181, 184). Furthermore, a second treatment of the trabecular meshwork using SLT after SLT or ALT as the first laser trabeculoplasty is effective (168, 185).

LASER IRIDOTOMY

Historical Background

In 1956, Meyer-Schwickerath (186) first reported the use of light energy to create a hole in the iris. Using the xenon-arc photocoagulator, he and others found that a peripheral iridotomy could be produced, but that the amount of heat required damaged the cornea and the lens (186, 187). With the introduction of lasers in the 1960s, investigation of this treatment modality continued, primarily with ruby lasers (188, 189, 190 and 191). However, as with laser trabeculoplasty, laser iridotomy became clinically practical after the advent of argon laser technology in the 1970s. By the mid-1970s, several reports of successful argon laser iridotomy appeared in the literature (192, 193, 194 and 195), and by the end of that decade, laser iridotomy had replaced incisional iridectomy as the surgical procedure of choice for angle-closure glaucomas. During the 1980s, continued study of laser iridotomy techniques led to the popular use of the Nd:YAG laser for this operation.

Figure 36.5 A: Abraham contact lens with planoconvex button bonded to front surface for laser iridotomy. B: Slitlamp view of an iris magnified with the Abraham iridotomy lens.

Techniques

The basic principle of laser iridotomy is the creation of a hole in the peripheral iris with an argon or Nd:YAG laser, which allows equalization of the pressure between the posterior and anterior chambers, deepening of the anterior chamber, and opening of the anterior chamber angle.

Instruments

Several different types of lasers and surgical techniques can be used to create an iridotomy. The unit most commonly used in the early days of laser surgery was the continuous-wave argon laser (192, 193, 194, 195, 196, 197, 198, 199, 200 and 201). Other lasers were also shown to be effective for creating iridotomies, including the pulsed argon laser and the krypton laser (197, 202, 203). However, the pulsed Nd:YAG laser subsequently gained popularity and is the most commonly used unit for creating laser iridotomies today (204, 205, 206, 207, 208, 209, 210 and 211). A portable Nd:YAG laser is effective for use in remote geographic areas (212). Other lasers have also been evaluated for performing iridotomies. Those units and the relative merits of the argon versus Nd:YAG laser iridotomies are considered later in this chapter.

A contact lens is helpful in performing a laser iridotomy, because it (a) keeps the lids separated, (b) minimizes corneal epithelial burns by acting as a heat sink, and (c) provides some control of eye movement. In addition, convex-surfaced contact lenses have been designed to increase the power density on the iris (213, 214 and 215). The most commonly used is the Abraham iridotomy lens, which has a 66-D planoconvex button bonded to the front surface of the contact lens (Fig. 36.5) (213). This lens doubles the laser-beam diameter at the level of the cornea, while reducing it to approximately one

half of the original size on the iris, which reduces the power density at the cornea to one fourth of the original level and increases it on the iris by a factor of four. Another contact lens, the Wise iridotomysphincterotomy lens, has a 103-D optical button decentered at

P.460

2.5 mm, which further reduces the iris focal spot and increases the energy density (215). These principles have their greatest application with the argon laser, although the same contact lenses are also useful with the Nd:YAG laser.

With all lasers and contact lenses, a high magnification (e.g., 40×) should be used in the slitlamp delivery system.

Preoperative Medication

Topical pilocarpine may be instilled before the procedure, which helps to maximally thin and stretch the peripheral iris. If the patient presents with an acute attack of angle-closure glaucoma, it is best to break the attack medically, if possible, and maintain the patient on medication to allow clearing of any corneal edema and to facilitate constriction of the pupil. If significant iritis persists after breaking the attack, it may be advisable to use topical steroids for 24 to 48 hours before proceeding with the laser surgery. However, if the attack does not respond to medical therapy, laser iridotomy (or iridoplasty or pupilloplasty, as discussed later in this chapter) may be effective in breaking the attack (216).

In nearly all cases, only topical anesthesia, such as proparacaine, 0.5%, is required. Only rarely is a retrobulbar injection needed for a patient who has nystagmus or is uncooperative. It has become a standard practice among most surgeons to also use topical apraclonidine to reduce the risk for a postoperative IOP rise (217). In the original studies, apraclonidine, 1%, was instilled 45 to 60 minutes before and immediately after the procedure (218), although a single postoperative drop of apraclonidine, 0.5%, has been shown to be as effective as the 1% concentration in preventing IOP elevation (114). Selection of Treatment Site

Any quadrant of the iris can be used to create the laser iridotomy, although our preference is between 11 and 1 o'clock if the opening will be entirely covered by the lid, and otherwise temporally. The reason for this is to avoid the iridotomy in a location where the lid margin bisects the iridotomy, as this can result in monocular optical symptoms such as transient ghosting of images, blurring, shadows, halos, glare, crescents, or a horizontal line (219, 220). When argon laser iridotomy is performed, the 12-o'clock position is usually avoided, because gas bubbles may collect in that area and interfere with completion of the procedure. One exception to the selection of a superior iris quadrant is the patient with silicone oil in an aphakic eye, in which case the iridotomy should be placed inferiorly to avoid blockage by the oil, which rises to the top of the eye.

Whichever quadrant is used, the slitlamp should always be positioned so that the laser beam is directed away from the macula. The iridotomy is usually placed between the middle and peripheral thirds of the iris. However, if this is not feasible, because of peripheral corneal haze or close proximity between peripheral iris and cornea, a more central location can be used, as long as it is peripheral to the sphincter muscle.

Several features of the iris may facilitate creation of the iridotomy. An area of thin iris or a large crypt is usually easier to penetrate. In lightly pigmented eyes, a local area of increased pigmentation, such as a freckle, may improve absorption of argon laser energy. In addition, the radially arranged white collagen strands in the stroma can be very difficult to penetrate, especially with argon laser, and selecting a treatment site where two strands are more widely separated is helpful (221). The collagen strands may also contain radial vessels, which should be avoided with Nd:YAG laser iridotomy.

Techniques with Continuous-Wave Argon Laser

Several basic techniques have been advocated for producing iridotomies with the continuous-wave argon laser. The “hump” technique involves first cr eating a localized elevation of the iris with a largediameter, low-energy burn, and then penetrating the hump with small, intense burns (222). In the “drumhead” technique, large-diameter, low-energy bu rns are placed around the intended treatment site to put the iris on stretch, and that area is then penetrated with small, high-energy burns (198, 223). A

third, and probably the most commonly used, approach goes directly to penetrating burns (197, 200, 201). This last technique may be modified by using multiple short-duration burns (224, 225). None of these approaches, however, is ideal for all situations, and it is best to tailor the iridotomy technique primarily according to the color of the iris. For irides of any color, the argon laser settings are first selected for the iris stroma and then adjusted for the pigment epithelium.

Medium Brown Iris

This is the easiest iris to penetrate with continuous-wave argon laser, and the following method represents one technique to use in these patients. Protocols for irides of other colors are modifications of this basic technique.

Argon laser settings of 0.1- to 0.2-second duration, 50-µm spot size, and 700 to 1500 mW (average, 1000 mW) are initially used to create a crater in the iris stroma. The first few applications may produce gas bubbles, which usually float up away from the treatment site (Fig. 36.6). If the bubble does not move, it can be dislodged by going through it with the next laser

P.461

application or by placing the beam adjacent to the bubble. A cluster of several contiguous burns is used to produce a stromal crater of approximately 500 µm in diameter. Additional laser applications are then placed in the bed of the crater until the pigment epithelial layer is reached, as evidenced by a cloud of pigment.

Figure 36.6 Argon laser was used to create a crater in the stroma of this medium brown iris. Gas bubbles (shown) can form with the first few laser applications; they usually float away from the treatment site but can be dislodged by subsequent laser applications, if necessary.

When most of the stroma in the crater has been eliminated and only pigment epithelium remains, the laser intensity should be reduced to clean away the remaining tissue. Typical settings for this stage of the procedure are 100 µm and 500 to 700 mW, or 50 µm an d 200 to 600 mW, with a duration of 0.1 to 0.2 second. Higher-intensity burns at this stage of the treatment may dislodge adjacent pigment epithelium, creating a “cascade phenomenon,” which causes furth er obstruction of the iridotomy. These same settings can be used for irides of other colors because the pigment epithelial layer is similar in all eyes.

This two-stage technique for argon laser iridotomy in the medium brown iris normally takes 30 to 60 laser applications to create a patent iridotomy.

Dark Brown Iris

A laser iridotomy is more difficult to achieve in these eyes, partially because of the thick, dense stroma. Standard initial settings (as described previously) often produce a black char in the stromal crater, making the site resistant to further penetration. One way to minimize this complication and achieve a patent iridotomy in the dark brown iris is to use multiple shortduration burns, called the “chipping” technique (222, 224, 226). The important feature of this modification is the short exposure time of 0.02 to 0.05 second, with standard settings of 50 µm and 700 to 1500 mW. With this approach, minute fragments of stroma are “chipped away,” often requi ring 200 to 300 applications to penetrate the stroma. Once the pigment epithelial layer is reached, the settings should be changed to the lower intensity level, as described for the medium brown iris, to complete the procedure.

Blue Iris

These eyes can also be difficult with argon laser iridotomy because the lightly pigmented stroma does not absorb laser light sufficiently to produce a burn through this portion of the iris. The pigment epithelium near the treatment site may be dislodged, leaving intact stroma that is impermeable to aqueous flow. Some surgeons prefer a two-stage approach, in which settings of 500 µm and 200 to 300 mW are first used to create a local tan-colored area of increased stromal density, followed by penetration burns of 50 µm, 500 to 700 mW, and 0.1 second to cr eate a full-thickness hole in the stroma (227). Others have suggested a direct approach, using settings of 50 µm, 1000 to 1500 mW, and a prolonged duration of 0.5 second, which usually burns a hole through the stroma in two to three applications (221, 226). With either technique, the settings should then be changed to those described for the medium brown iris to penetrate or remove the remaining pigment epithelium from the iridotomy site.

Techniques with Nd:YAG Laser

As previously noted, Nd:YAG is now the most commonly used technique for laser iridotomy. The extremely high energy levels and short exposure times of these lasers electromechanically disrupt tissue, independent of pigment absorption and the thermal effect. As a result, they are particularly useful in creating iridotomies in light blue irides but are effective in all eyes. The technique usually involves simultaneous perforation of the iris stroma and pigment epithelium with energy levels in the range of 5 to 15 mJ (204, 205, 206, 207, 208, 209, 210 and 211). The pulse duration is fixed for each instrument, in the range of 12 nanoseconds, but the number of pulses per burst can be adjusted in most units, with surgeons generally preferring 1 to 3 pulses per burst. The spot size is also fixed, although some units provide a choice between a single focal point and multiple focal points, with the latter creating a larger lesion. Because the wavelength of the Nd:YAG laser is beyond the visible spectrum, a helium-neon or diode laser beam is typically used for focusing on the iris. With instruments that allow a selected separation between the focal points of the two laser beams, the setting should be such that they are coincident when performing a laser iridotomy.

The standard technique uses the same criteria as for argon laser iridotomy in selecting the iris site, although it is often possible to place the iridotomy more peripherally with the Nd:YAG laser. The latter is desirable, among other reasons, to avoid injuring the lens. When selecting the treatment site, attention should be given to avoid any apparent iris vessels, because these are more likely to bleed with Nd:YAG than with argon laser surgery. A patent iridotomy can often be created with a single laser application, and rarely are more than two or three required, especially in blue or light brown eyes. However, the iridotomy may be smaller than those produced with an argon laser (Fig. 36.7A,B). Cases have been reported of acute angleclosure glaucoma in eyes with patent, but small, Nd:YAG laser iridotomies, and it has been suggested that an iridotomy should be at least 150 to 200 µm in diameter (228). The iridotomy may change in shape and position and occasionally in area after dilatation (229), and it is good practice to make it large enough initially but also to check it after dilatation. If there is doubt regarding the size of the iridotomy during the procedure and it is difficult to enlarge it, creating more than one iridotomy is advisable.

Several variations in technique have been described. One uses both the argon and Nd:YAG lasers by