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

first creating a stromal crater with short-duration argon laser burns and then penetrating the iris with low-energy, single-pulse Nd:YAG applications (230, 231 and 232). This has an advantage of minimizing bleeding by first coagulating iris vessels. It is especially useful in eyes with thick, dark iris stroma, in which the Nd:YAG laser energy may cause considerable disruption and dispersion of stromal tissue before penetrating the iris. Another technique involves multiple lowenergy (1.0 to 1.7 mJ) applications in a line across the radial iris fibers to create an iridotomy of larger, more controllable size, which was thought to be safer than a similar approach with the argon laser or the standard, higherenergy Nd:YAG technique (233, 234). Iridotomies have also been created experimentally

P.462

with the transscleral application of longer-duration thermal Nd:YAG laser burns via a fiber-optic system (235).

Figure 36.7 A: Argon laser iridotomy with patency demonstrated by visualization of anterior lens capsule. B: Typical appearance of peripheral iridotomy created with an Nd:YAG laser. Techniques with Other Lasers

Pulsed Argon Laser

This instrument emits laser energy in a chain of very short pulses, rather than in a continuous wave, which vaporizes the absorbing tissue with minimal heat loss and destruction to the surrounding area. These features provide some advantage over continuous-wave argon lasers for producing iridotomies in that more energy is used in penetrating the iris, with less distortion and disruption of the surrounding tissue (202).

The basic technique is similar to that for continuous-wave argon laser iridotomy. The settings, however, differ considerably for the pulsed argon laser unit. The perforating mode is used, and the power setting is 20 to 25 W. The usual parameters are 50 µm, 0.2 second, and 300 pulses/sec, adjusted according to tissue response (the individual pulse is fixed at 128 microseconds). With these settings, the number of exposures to achieve an iridotomy varies from 2 to 250, depending on the type of iris (202). Neodymium: Yttrium Lithium Fluoride (Nd:YLF) Laser

This 1053-nm laser can create iridotomies of precise size and shape with minimal thermal damage to the surrounding tissue because of low energy per pulse levels, with a short pulse duration in picoseconds, and a high repetition rate. Optimal settings, established in a series of cadaver eyes, included a rectangular cutting pattern of 0.3 × 0.3 mm, 500-µm cutting depth, 50-µm spot separation, and 200 to 400 pulses/sec (236, 237).

Semiconductor Diode Laser

As noted in Chapter 35, the semiconductor diode laser has several distinct advantages over other lasers, including the small, portable size, the solid-state construction, which provides durability and relatively low maintenance requirements, and the need for only a standard electric outlet and no water cooling.

With a wavelength of approximately 805 nm and operation in the continuous-wave mode, the mechanism of iridotomy is like that of the argon laser— that is, absorption by melanin, resulting in

photocoagulation, rather than the electromechanical disruption of the Nd:YAG and Nd:YLF lasers. In rabbit studies and preliminary clinical trials, the settings and the clinical and histopathologic results were all similar to those noted in this chapter for the argon laser (238, 239 and 240).

Other Lasers

As noted previously, the krypton laser was found to be effective in creating iridotomies (203). A Q- switched ruby laser was also found in monkey studies to be suitable for producing iridotomies (241), and dye lasers have been used clinically to create iridotomies with a single pulse (242). The continuouswave, frequency-doubled Nd:YAG laser with a wavelength of 532 nm, pumped by a diode laser, successfully created patent iridotomies in rabbit eyes with the thermal damage zones comparable to the argon laser (243).

Results of Argon and Nd:YAG Laser Iridotomies

When laser iridotomy is performed for an acute angle closure, the IOP decreases and remains stable without requiring additional surgery in approximately two thirds of white patients and half of Asian patients (201, 244, 245 and 246). The difference may be explained by mechanisms other than pupillary block, such as changes in the angle morphologic characteristics, a longer duration, and severity of the attack (245, 247).

In chronic angle-closure glaucoma, despite widening of the angle in 73% to 97% of eyes (246, 248), eyes that have optic disc and visual field damage require filtering surgery in approximately half of the patients, despite the presence of a patent iridotomy (249, 250). The outcomes appear to be similar among white and Asian patients (250). The effect of iridotomy on one eye is predictive of the effect on the fellow eye (248).

Prophylactic laser iridotomy prevented IOP elevation in 88.8% of fellow eyes in patients with acute angle closure within 4 years of follow-up and is recommended for the treatment of fellow eyes of patients with acute angle closure (250, 251). However, because some of the fellow eyes may experience IOP elevation within 6 to 12 months, despite the presence of a patent iridotomy, close follow-up is recommended (245, 252).

P.463

Comparison of Argon and Nd:YAG Laser Iridotomies

Histologic studies have shown that iridotomies created with an argon laser have more extensive early edema and tissue destruction at the margins of the treatment site than iridotomies created with the Nd:YAG laser, in which the lesions are more circumscribed with limited tissue alterations at the margins (202, 209, 211, 253). However, freeze-frame analysis of highspeed cinematography in ox eyes showed particles traveling over 8 mm from the Nd:YAG treatment site at speeds in excess of 20 km/h (210), and the shock waves affected the trabecular meshwork and corneal endothelium of monkey eyes when the Nd:YAG application was within 0.8 mm of the limbus (254).

Argon and Nd:YAG laser iridotomies were compared in human autopsy eyes using a high-magnification video recording system that allowed real-time observation of the posterior iris during the laser procedures (255). With argon laser iridotomy, gradual mounding up of iris pigment epithelium occurred with each successive energy application before final penetration. In contrast, Nd:YAG laser iridotomy caused a complete disruption and dispersal of the pigment epithelium with a single pulse of energy. These observations may explain the tendency for argon laser iridotomies to become obstructed with pigment epithelium, which is rarely seen with Nd:YAG laser iridotomies.

In clinical comparisons of the two surgical approaches, the Nd:YAG laser iridotomies had the disadvantage of frequent bleeding, although this usually stops spontaneously or by applying pressure to the eye with the contact lens and rarely leads to significant complications (206, 207, 256, 257). Disadvantages of argon laser iridotomy, on the other hand, include more iritis, pupillary distortion, and late closure of the iridotomy. When an iridotomy could not be created with the argon laser, a patent iridotomy was be achieved in all eyes with the Nd:YAG laser in single sessions (258). Nd:YAG laser iridotomies, in general, require considerably fewer total applications, with a marked reduction in total energy delivery, compared with argon laser iridotomies.

Prevention and Management of Complications

As with laser trabeculoplasty, a transient IOP rise and a mild anterior uveitis are common early postoperative complications. Other potential complications include closure of the iridotomy, corneal damage, hyphema, cataract formation, retinal burns, malignant glaucoma, and monocular blurring. Transient Intraocular Pressure Rise

This is one of the most common serious complications in the early period after argon or Nd:YAG laser iridotomy (259, 260). It was reported in 24% of the eyes undergoing Nd:YAG iridotomy (246). The IOP rise is caused by reduced outflow facility, with an actual decrease in aqueous production (261). A biphasic IOP response has been seen in rabbits, in which the initial IOP rise of 0.5 to 2 hours' duration is followed by a prolonged IOP reduction lasting 6 to 24 hours (262). Studies in rabbits also suggest that this pressure response is related to a release of prostaglandin and prostaglandin-like substances into the aqueous with a breakdown in the blood-aqueous barrier and an accumulation of blood plasma and fibrin in the anterior chamber angle (262, 263, 264, 265, 266, 267 and 268). A histopathologic study in monkey revealed a rapid accumulation of particulate debris in the angle (269), which may also contribute to the transient IOP elevation.

Clinically, the risk for a transient IOP rise was found in one study to be related to the total energy delivered, but not to the presence of chronic angle-closure glaucoma (260), whereas another study found no correlation with total laser energy but did find that the preoperative outflow facility was directly related to the maximum postoperative IOP elevation (261). As previously noted, 1 drop of apraclonidine, 0.5% to 1%, 1 hour before or immediately after the laser surgery has a profound effect on minimizing this complication (114, 218).

Pretreatment with latanoprost was associated with an increase in IOP within the first 2 hours following iridotomy (270). This was likely due to the short time interval between drug instillation and laser treatment, which prevents the medication from achieving its peak effect and limits the effectiveness of latanoprost as a prophylactic medication in anterior segment laser surgery (270).

Anterior Uveitis

Some degree of transient iritis occurs after laser iridotomy in all eyes, which is associated with the blood-aqueous barrier breakdown noted in animal studies (264, 266, 267 and 268). Topical steroids for the first 3 to 5 postoperative days are sufficient to control this mild complication in most cases. In rare cases, however, an eye may have a marked inflammation, sometimes occurring days or weeks after the procedure, with associated hypopyon (271, 272). Granulomatous endophthalmitis was reported following laser iridotomy, associated with several large tears in the anterior lens capsule of a blind eye with a mature cataract (272). A case of prolonged iritis with transient cystoid macular edema has also been described (273), and two cases have been reported in which postoperative inflammation and longterm miotic therapy were thought to be responsible for occlusion of the pupil with a pigmented pseudomembrane (274).

Closure of Iridotomy

The iridotomy may close during the first few weeks, especially with argon laser iridotomy, due to accumulation of pigment granules and debris. It may be advisable, therefore, to continue use of pilocarpine for the first 4 to 6 postoperative weeks. If the iridotomy remains patent, stopping use of the miotic after this time is usually safe, unless it is needed to control a chronic pressure elevation. Some authors suggested that a mydriatic provocative test should be used after stopping use of the miotic to confirm the functional reliability of the iridotomy (201). Late closure is rare with Nd:YAG laser iridotomies. In one series of 200 cases, the two late closures were in eyes with preexisting chronic uveitis (275).

As discussed earlier, the minimum diameter of a laser iridotomy that is needed to prevent further attacks of angle-closure glaucoma is yet to be determined and probably differs from one patient to the next. Cases have been reported in which

P.464

angle closure recurred despite patent but small iridotomies, and as previously noted, a minimum

diameter of 150 to 200 µ m has been recommended (228, 276). In some eyes following argon laser surgery, the laser iridotomy spontaneously enlarges over months or years (277), although this should not be relied on in borderline situations, in which case the opening should be further enlarged.

Patency of the iridotomy is best confirmed by visualizing anterior lens capsule or vitreous face through the opening (Fig. 36.7A). Transillumination can also be used, although this is sometimes misleading, especially with a blue iris, in which dislodged pigment epithelium can produce a transillumination defect despite an intact overlying stroma, which is impermeable to aqueous flow.

Corneal Damage

Focal epithelial and endothelial burns of the cornea are not uncommon when larger amounts of laser energy are used, although these usually heal quickly with no apparent sequelae. In monkey eyes, laser iridotomy was not associated with significant endothelial cell damage (278). In several clinical trials, pachymetry has revealed no significant difference in corneal thickness before and after laser iridotomy (279, 280 and 281). Specular microscopic studies have been less conclusive, however, with some showing no significant difference in endothelial cell count (279, 280), whereas others have revealed a loss of endothelial cells or an increase in cell size (105, 281, 282 and 283). Generalized corneal decompensation has been reported in several series, nearly all of which involved argon laser iridotomy (283, 284, 285, 286 and 287). This often begins with focal corneal edema overlying the iridotomy site, followed by generalized corneal decompensation, which may not appear until months to years after the laser surgery. These cases frequently require penetrating keratoplasty, histology of which typically reveals abnormalities characteristic of Fuchs endothelial corneal dystrophy (285, 287). Factors that may predispose to this complication include episodes of angle-closure glaucoma with pressure elevations and inflammation, cornea guttata, diabetes, and high total laser energy (284, 285, 286 and 287). Descemet membrane detachment after laser iridotomy was also reported (288).

Hyphema

As previously noted, a small amount of bleeding from the iridotomy site is common following Nd:YAG laser iridotomy but is rarely serious (204, 205, 206, 207 and 208). Persistent bleeding from the treatment site can usually be stopped by applying pressure to the eye with the contact lens for a few seconds to a minute. Hyphemas are uncommon after argon laser iridotomies but may occur (289, 290), especially in eyes with rubeosis iridis or uveitis.

Cataract Formation

Focal anterior lens opacities are common beneath an iridotomy produced with argon laser energy (206, 208, 291). Most of these are nonprogressive, although reduced visual acuity due to cataract progression has been documented (291). The rate of progression is similar to that following incisional surgical iridectomy (201), and a clear cause-and-effect relationship between either surgical approach and cataracts has not been established. Lens changes are much less common with Nd:YAG laser iridotomies (206, 207 and 208), although capsular damage with rare cataract formation has been reported (292, 293 and 294). In two rabbit studies, no lens damage was seen with either argon or Nd:YAG laser iridotomy, even when additional laser applications were placed through patent iridotomies (295, 296). A study in monkeys, however, suggested a threshold for lens damage with Nd:YAG laser iridotomy, with no damage at 6 mJ or less and one to two pulses per burst, but local damage with higher energies or three pulses per burst (297).

Retinal Injuries

Most visual function studies have shown no adverse effect from argon laser iridotomy (298), and the same is presumably true for Nd:YAG laser iridotomy. One study, however, did reveal static perimetric and fluorescein angiographic evidence of focal retinal damage in the quadrant of treatment 6 months after argon laser iridotomy (299). Retinal damage is best minimized by always aiming the laser beam toward peripheral retina. Failure to do so may result in serious retinal burns, and acute permanent loss of vision due to inadvertent foveal photo-coagulation during argon laser iridotomy has been reported (300). Macular injuries from an Nd:YAG iridotomy have also been reported (301). The final visual acuity in these cases depends on the distance between the injury and the fovea (301). The risk is also reduced but not eliminated by using an Abraham lens (302). One case of temporary bilateral serous choroidal and

nonrhegmatogenous retinal detachment following Nd:YAG laser iridotomy has also been reported (303). Malignant Glaucoma

Cases of possible malignant glaucoma have also been reported following laser iridotomy for acute or chronic angle-closure glaucoma (304, 305, 306 and 307), one of which was a simultaneous bilateral case 4 weeks after bilateral laser iridotomy (308).

Monocular Blurring

If the iridotomy is not fully covered by the upper lid, the patient may report monocular blurring, diplopia, or “ghost images.” Diplopia, or “ghost im ages,” may occur when the upper lid and associated tear film bisect the light path through the patent iridotomy. In some patients, diplopia, often alleviated when the lid is lifted away from the eye, may result despite an iridotomy that is well covered by the upper eyelid. This may result from a prism-like effect of the tear meniscus along the margin of the upper eyelid. If tinted glasses or sunglasses fail to relieve symptoms, a cosmetic contact lens can be helpful in unusually symptomatic cases. Some investigators report that diplopia occurs less frequently when the iridotomy is placed in the horizontal axis (3- or 9-o'clock positions) (309).

LASER PERIPHERAL IRIDOPLASTY (GONIOPLASTY)

There are times in which a patent iridotomy may fail to relieve angle closure, as with a microphthalmic or nanophthalmic eye, the swelling (e.g., sulfonamide-induced uveal effusion) or

P.465

forward rotation of the ciliary body (i.e., plateau iris syndrome), or the presence of peripheral anterior synechiae. In each of these situations, the anterior chamber angle may be opened by applying lowenergy argon laser contraction burns to the peripheral iris. The procedure has been referred to as laser peripheral iridoplasty, gonioplasty, or peripheral iris retraction.

Mechanisms of Action

The mechanism of action is a tightening of the peripheral iris, which pulls it posteriorly from the trabecular meshwork. The histopathology of eyes treated with peripheral iridoplasty revealed contraction furrow formation, proliferation of fibro-blast-like cells, collagen deposition on the iris surface, denaturation of stromal collagen, and coagulative necrosis of blood vessels in the anterior two thirds of the iris stroma (310). These findings are believed to suggest that the immediate, short-term mechanism of peripheral iridoplasty is heat shrinkage of collagen, whereas the long-term effects may be related to contraction of a fibroblastic membrane. The observation of coagulative necrosis of iris blood vessels also provides a note of caution that overtreatment may lead to iris necrosis.

Techniques

Suggested argon laser settings for peripheral iridoplasty vary considerably, with ranges of 50to 500-µ m spot size, 0.5-second duration, and 150 to 1000 mW of power (217, 311, 312 and 313). In general, however, a laser application of relatively large area, long duration, and low power is preferable, and reasonable initial settings are 200 µm, 0.2 second, and 400 mW. The power or duration should be increased if no contraction is produced but reduced if pigment liberation is produced by the laser application. The recommended number of applications also varies. Approximately 10 to 15 burns are usually applied to peripheral iris in each quadrant, and additional applications can be placed in a row adjacent to the first burns if necessary (Fig. 36.8). It is usually advisable to treat no more than 180 degrees of an angle in a single session. Gonioplasty with a diode laser was also reported for the treatment of chronic angleclosure glaucoma and acute angle closure (314, 315).

The most commonly used technique for applying the laser burns is with a Goldmann three-mirror or single-mirror goniolens, which causes the laser beam to strike the iris tangentially. It is important with this technique to ensure that a portion of the laser beam does not strike exposed angle structures. An alternative is to apply the laser burns directly through peripheral cornea, which is usually best done through the flat surface of a laser contact lens (e.g., peripheral Abraham lens). When using this technique, the spot size should be larger and the power lower than with the tangential approach, because the direct approach creates smaller burns with higher energy per unit area.

Indications

As noted earlier, peripheral iridoplasty may be useful in opening a functionally closed anterior chamber angle, as with pupillary block glaucoma, when corneal edema prevents adequate laser energy delivery to create an iridotomy (217, 316, 317). In other cases, a patent iridotomy may fail to relieve the angle closure because of “crowding” of the angle, as with a small, microphthalmic or nanophthalmic eye; an eye with plateau iris; iris cysts; or forward rotation of the ciliary body due to various mechanisms, including retinal detachment surgery (318, 319 and 320). Laser peripheral iridoplasty has been successful in opening the anterior chamber angle in many of these cases. Another reason for failure of a patent iridotomy is the presence of extensive peripheral anterior synechiae. Gonioplasty can open the angle in some of these cases if the laser energy is applied to the base of the synechiae (311, 312). The success rate is higher if the duration of synechial closure is short. It has been suggested that gonioscopy should be performed immediately after an iridotomy for pupillary block glaucoma and for any synechial closure (311). However, success has also been reported after several years of synechial closure (312). Another reported indication for gonioplasty is to open areas of persistent or recurrent synechial closure after incisional goniosynechialysis (313). In addition, peripheral iridoplasty can be used to deepen the anterior chamber angle to facilitate laser trabeculoplasty.

Figure 36.8 Laser peripheral iridoplasty (right) deepens the anterior chamber angle with low-energy contraction burns to the peripheral iris, while laser pupilloplasty (left) dilates the pupil with low-energy contraction burns to the more central iris.

Complications

Peripheral iridoplasty may be complicated by further elevation of the IOP. This is usually transient, but it may be chronic if the outflow structures are further compromised by the laser applications. A mild, transient iritis is a consistent finding and should be treated with several days of topical steroid therapy. Other potential complications include corneal endothelial burns, distortion of the pupil, and focal iris atrophy.

P.466

LASER PUPILLOPLASTY

Laser pupilloplasty is a technique by which the pupil can be partially dilated by applying contraction burns near the pupillary portion of the iris. Suggested argon laser settings are 200 to 500 µm, 0.2 to 0.5 second, and 200 to 500 mW (217). Several rows of laser energy are applied to the sphincter portion of the iris, starting at the pupillary border and working peripherally. One technique is to use a smaller spot size near the pupillary border and then enlarge the spot size for more peripheral burns. The contraction of stroma with each application tents the pupil in the direction of the treatment site. Radial rows of contraction burns can be applied for 360 degrees to create symmetric pupillary dilatation or in one quadrant to create focal dilatation (Fig. 36.8).

One indication for laser pupilloplasty is as another alternative treatment of pupillary block when a laser iridotomy is not possible, as with a cloudy cornea. It is an especially useful technique for pupillary block glaucoma in aphakia or pseudophakia (321, 322 and 323). By peaking the pupil in one quadrant, the iris may be retracted away from an area of vitreous contact or beyond a point of apposition with the intraocular lens implant, thereby reestablishing communication between the anterior and posterior chambers. This usually works only when the amount of lens-iris contact is minimal, because the degree of pupillary retraction is small. When pupilloplasty is not effective in these cases, combined therapy with peripheral iridoplasty may be effective (217).

Pupilloplasty may also be used to dilate a chronically constricted pupil, although the amount of dilatation is usually small and often temporary. The procedure in this situation may be complicated by a significant IOP rise. Transient iritis is also a consistent complication and should be treated with several days of topical steroid therapy.

IRIS SPHINCTEROTOMY

A technique has been described in which the pupil can be enlarged, reshaped, or repositioned by making a linear cut across the iris with an argon laser set at 0.01 to 0.05 second, 50 µm, and 1.5 W, allowin g the intrinsic tension of the iris to spread the cut apart (324). A pupil can be also “created” by opening pupillary membranes with the Nd:YAG laser (325).

INCISIONAL IRIDECTOMY

Laser Iridotomy versus Incisional Iridectomy

The incisional surgical iridectomy is one of the safest, most effective operations for glaucoma. For the laser iridotomy to replace it as the procedure of choice, however, significant advantages had to be demonstrated. Long-term follow-up studies have shown that laser iridotomies are similar to the incisional procedures in terms of efficacy and safety (201, 244, 326, 327). However, in treatment of acute angle closure, filtering surgery is more likely to be required after laser iridotomy than after incisional peripheral iridectomy, particularly after longer duration of the angle-closure attack (328). Laser iridotomy has become the procedure of choice for most cases of angle-closure glaucoma. There are situations, however, when the incisional approach is still required. Some patients cannot sit at the slitlamp or cooperate sufficiently for laser therapy. At other times, the cornea may be too cloudy or the iris may be too close to the cornea to allow laser iridotomy. There is also the rare case in which a patent iridotomy cannot be achieved with laser treatment, or in which the opening repeatedly closes postoperatively. The latter is particularly common in eyes with marked uveitis. For these reasons, the surgeon must still be familiar with the time-honored procedure of incisional iridectomy.

Techniques Peripheral Iridectomy Basic Technique

In the technique described by Chandler (329), a small conjunctival flap is prepared in one of the superior quadrants with either a fornix or a limbus base. A 3- to 4-mm incision is made into the anterior chamber, beginning approximately 1 to 1.5 mm behind the corneolimbal junction (Fig. 36.9).

If the iris prolapses, it is lifted up with iris forceps and a small section is excised using iris scissors that are held parallel to the limbus. If the iris does not spontaneously come through the limbal opening, a slight pressure on the posterior margin of the incision may cause the prolapse. Factors that may prevent the peripheral iris from prolapsing through the limbal incision include (a) inaccurate placement of the incision, (b) a hypotonus eye, (c) peripheral anterior synechiae, (d) a hole elsewhere in the iris, and (e) ciliary-iridial processes (attachments between the posterior peripheral iris and the ciliary body).

When iris prolapse cannot be achieved, the iris is grasped with forceps and brought up through the incision to make the iridectomy. The iris is then repositioned by a gentle stroking action across the cornea in a direction away from the incision, using a blunt instrument, such as a muscle hook.

In closing the wound, a single suture may be placed through both the limbal wound and the conjunctiva if a fornix-based flap was used. With a limbus-based flap, closure of the conjunctiva alone may be sufficient, if the limbal incision was beveled slightly to achieve spontaneous apposition.

Modifications

Some surgeons prefer to make the incision into the anterior chamber through clear cornea adjacent to the corneolimbal junction (330, 331 and 332). The main advantage is that the undamaged conjunctiva remains available for future filtering surgery if required. The incision is usually placed perpendicular to the limbus to reach peripheral iris, and suture closure is generally necessary. However, some surgeons believe suturing is not essential (330), especially if the incision is beveled posteriorly (332).

Another modification of the peripheral iridectomy is transfixation, in which the anterior chamber is entered at the limbus with a narrow blade, which is then passed across the anterior segment of the eye, piercing first the peripheral iris and then the iris near the sphincter muscle. This approach has been P.467

favored by some surgeons for the management of iris bombé in an inflamed eye, in the hope that it would reduce the danger of hemorrhage. However, one study showed that a conventional peripheral iridectomy does not have a greater incidence of bleeding (333).

Figure 36.9 Peripheral iridectomy. A: Incision into anterior chamber may be placed (1) behind the corneolimbal junction or (2) in peripheral cornea (note the slant of each incision). B: Peripheral iris is grasped with forceps and excised with iris scissors. C: Remaining iris is repositioned by a gentle stroking action across the cornea (arrow).

A procedure called pigment vacuum iridectomy was described for phakic refractive lens implantation, in which the stromal layer is initially removed by surgical excision, and the pigment layer is removed by gentle vacuum aspiration with a 25-gauge cannula to ensure a proper basal iridectomy (334).

Sector Iridectomy

A sector iridectomy may have advantages over a peripheral iridectomy in some situations. Such situations include the need to enlarge the optical opening, to minimize total posterior synechiae, and to provide a better view of the fundus when retinal disease is suspected. In the technique described by King and Wadsworth (335), a limbal incision larger than that for the peripheral iridectomy is required so that the iris can be grasped within 1 to 2 mm of the pupillary margin and brought well out through the wound. A radial cut is then made across the iris at one side of the exposed portion, the iris is torn at its root, and a second incision is made across the other side of the exposed tissue. This creates a truly basal iridectomy. An alternative approach is to grasp the midperipheral iris, withdraw it until the pupillary

margin is exposed, and excise the tissue with a single cut. Prevention and Management of Complications Intraoperative Complications

Hemorrhage

Cut edges of the iris normally do not bleed. However, hemorrhage may occur, especially if inflammation or neovascularization is present. To minimize bleeding in the latter situations, the use of bipolar cauterization of the iris surface before cutting the iridectomy has been suggested (336, 337). Brisk bleeding is especially likely to occur if the ciliary body is inadvertently cut. Hemorrhage from either iris or ciliary body can usually be

P.468

stopped by placing a large air bubble in the anterior chamber for several minutes. Incomplete Iridectomy

It is possible to cut only the stroma of the iris, leaving intact pigment epithelium, which prevents a successful operation. This complication should be avoided at the time of surgery by checking the iridectomy specimen for the dark pigment epithelium and by noting transillumination through the iridectomy if there is any doubt about its patency. If the complication is discovered postoperatively, it is best managed by penetrating the epithelial layer with the argon laser (338). Low energy settings of 300 to 400 mW, with a 100-µm spot size and a 0.1-second duration of exposure, are sufficient in most cases, and the pigment epithelium is usually eliminated with a few applications.

Injury to the Lens

Injury to the lens or disruption of the lens zonules with possible dislocation of the lens and vitreous loss should be avoided by gentle surgical manipulation. Intralenticular hemorrhage has also been reported as a rare complication of iridectomy (339).

Postoperative Complications

Intraocular Pressure Elevation

If the central anterior chamber is flat and the IOP is elevated, malignant (ciliary block) glaucoma should be suspected. This is an uncommon complication, with only one such case encountered in one series of 155 eyes (340). However, the consequences can be devastating. (The management of this situation is discussed in Chapter 26.) An incomplete iridectomy is another cause of high pressure and a flat anterior chamber, with the distinguishing feature from malignant glaucoma being more central anterior chamber depth with a bombé configuration of the iris. Thissituation can be managed (as described previously) by completing the iridotomy with the argon laser. A formed anterior chamber and patent iridectomy with an elevated pressure suggest that chronic obstruction of the trabecular meshwork may be present. The latter should initially be managed with IOPlowering medications, although laser trabeculoplasty or a filtering procedure may be required if medical therapy is inadequate.

Hyphema

Hyphemas should be handled conservatively with elevation of the head and limited activity. Cataract Formation

Cataracts. The frequency with which peripheral iridectomies lead to cataract formation is somewhat controversial. However, several studies indicate that some degree of lenticular opacity occurs in up to half of cases with an acute angle-closure glaucoma attack and in one third of eyes treated prophylactically (341, 342, 343 and 344). The mechanism of this complication is uncertain, although the frequency increases with age.

Endophthalmitis

As with any intraocular procedure, infection is a potential complication. TRABECULOTOMY

The basic principle of this operation is the creation of an opening in the trabecular meshwork to establish a direct communication between the anterior chamber and the Schlemm canal. It is generally performed with incisional surgical techniques, although laser techniques are also being evaluated. (Trabeculotomy in children is discussed in detail in Chapter 40.)