LASER TRABECULOPLASTY

Argon laser trabeculoplasty (ALT) lowers intraocular pressure (IOP) either by causing focal contraction of the trabecular meshwork beams and widening of adjacent intertrabecular spaces, or by altering endothelial cell function to decrease the resistance to aqueous humor outflow. Experience over the past 20 years has demonstrated an overall initial success rate ranging from 60 to 97%. Primary open-angle glaucoma, pseudoexfoliation glaucoma, and pigmentary glaucoma typically respond best to this procedure.

The primary advantages of ALT are the extremely favorable risk:benefit ratio, offering the prospect of improved pressure control without the risks of filtration surgery. Disadvantages include the potential for postoperative pressure spikes and inflammation, both of which are usually self-limited. The major disadvantage lies in the potential for failure and the long-term loss of efficacy, approximately 10% per year. Recent work suggests that trabeculoplasty in eyes with early glaucoma may prove as effective as, if not superior to, medical therapy when used as primary glaucoma therapy. Trabeculoplasty with other lasers, including the diode and the Q-switched, fre- quency-doubled neodymium:yttrium-aluminum-garnet (Nd:YAG) laser present unique features that may provide additional advantages for glaucoma treatment.

BACKGROUND

The prospect of treating glaucoma with a laser that did not make an entry tract into the anterior chamber was first described by Krasnov in 1973.1 Termed laseropuncture, this consisted of using a Q-switched ruby laser to puncture the trabecular meshwork, producing a hypotensive effect that lasted for several months.2 Although similar work was later done by Robin and Pollack, the instrument was unstable, required frequent repairs, often

caused hemorrhages in the angle, and was unsuitable for wide clinical application.3

In 1974, Worthen and Wickham demonstrated that the argon laser, used to treat the angle much as we do today, could successfully lower IOP. However, the parameters of the argon laser were so varied that their results were somewhat unpredictable.4,5 At the same time, Gaasterland demonstrated that heavy treatment of the trabecular meshwork with the argon laser produced elevated IOP in monkeys.6 Because of the fear that common application of laser therapy to the anterior chamber angle could cause, rather than eliminate, glaucoma, many individuals suspended their research into laser therapy for open-angle glaucoma.

In 1979, Wise and Witter published the first pilot work demonstrating that argon laser trabecular therapy could produce consistent, relatively long-term reduction of IOP in patients with glaucoma.7 Their success, due in part to standardized laser protocol parameters, inspired many others to duplicate their work, and improved our understanding of the indications and success rates for this procedure.

Over the years, many different wavelengths and laser types have been used to treat the trabecular meshwork. Due to the initial ready availability of the argon laser, many individuals termed the procedure argon laser trabeculoplasty, or ALT. It was later shown that trabeculoplasty could be effectively performed with the krypton (KLT), Nd:YAG (YLT), diode (DLT), and frequency-dou- bled Nd:YAG (SLT) lasers. Because of this, laser trabeculoplasty (LTP) has also been used as a descriptive term for this procedure.

Developed at a time when partial thickness filtration surgery was in its infancy, ALT was initially viewed as an alternative to incisional surgery, and only used when maximal medical therapy was unable to control the progression of glaucoma. As experience with ALT grew, it

became an adjunct to medications as it met wider acceptance. Finally, ALT was soon considered as a first-line therapy in lieu of medications, and as its safety and efficacy were confirmed. The Glaucoma Laser Trial legitimized ALT as a potential first-line therapy.8,9

MECHANISM OF ALT

Investigators have found that ALT can increase the facility of outflow by about 50%.10–12 There are two main theories by which this is accomplished. Although the laser energy does not penetrate the full thickness of the trabecular meshwork, it does cause a thermal burn on its surface, the loss of endothelial cells, and disruption of the trabecular beams (Fig. 40–1).

The first theory proposes that the resulting scarring and shrinkage of the treated trabecular beams puts tension on the adjacent beams, opening the intertrabecular spaces and increasing the outflow of aqueous humor.7,11,13 This theory is supported by pathological studies of human and nonhuman primate eyes following trabeculoplasty.14,15 Both show disruption of trabecular beams and cellular necrosis, with later shrinkage of meshwork collagen.

The second theory proposes that the laser in some way alters the biochemical nature of the trabecular meshwork and reduces the resistance to aqueous humor outflow.16 This is supported by pathological observations that trabeculoplasty initially reduces the number of trabecular endothelial cells in the areas of treatment, followed by increased cell division and phagocytic activity by surviving trabecular cells.17,18 Studies with radioactive tracers have confirmed that ALT is followed by cell division in the anterior meshwork. The cells then migrate into and repopulate the treated regions.19 Subsequent work has shown that trabecular meshwork tissues treated by ALT respond with increased production of matrix metalloproteinases, which increase the turnover of extracellular

FIGURE 40–1 Scanning electron microscopy of a trabecular meshwork specimen following argon laser trabeculoplasty. Note the matted appearance of the scarred trabecular beams and loss of trabecular endothelial cells.

CHAPTER 40 LASER TRABECULOPLASTY • 431

matrix materials.19a,19b This, in turn, would be expected to diminish aqueous outflow resistance and lower IOP.

Both theories rely on a cellular and tissue response to the laser treatment. Thus they are consistent with the wellaccepted observation that the final IOP lowering does not occur until 3 to 46 weeks following the procedure.

EFFICACY OF ALT

The experience of the past 25 years has helped define the efficacy of ALT in combination with medications in a wide variety of glaucomas, as well as its role as primary therapy for newly diagnosed glaucoma.

ARGON LASER TRABECULOPLASTY AFTER

MAXIMAL MEDICAL THERAPY

Since the pilot study by Wise and Witter,7 ALT has been an important and viable option for the presurgical patient on maximal tolerated medications and uncontrolled glaucoma. However, the initial enthusiasm for ALT that resulted from the good short-term success rates7,11,20 was later tempered by reports of diminished success several years after the procedure.21,22 Some of this may result from the fact that early experience with ALT was in patients who had far advanced disease and were more likely to progress over time. In contrast, patients treated earlier in their disease could conceivably have better longterm success. In addition, many of these earlier studies used a narrow definition of success, usually an IOP less than 20 to 22 mm Hg. Thus, changes in practice patterns and technique over the years make it hard to assign accurate figures to the efficacy of ALT among the different types of glaucoma.

For all glaucomas, the initial success of ALT ranges from 60 to 97%.7,10,11,12,23 However, phakic patients with primary open-angle glaucoma or pseudoexfoliation generally respond better to this procedure than do those with other types of glaucoma.

Primary Open-Angle Glaucoma

In the short term, the success of ALT in primary openangle glaucoma ranges from 80 to 97%.7,10,11,12,23 In addition, it is possible for some or all medications to be discontinued because the effect of ALT can be dramatic in some cases. Lieberman reported that 58% of patients could reduce their medical regimen and 5% eliminated all medications.23

Unfortunately, the effect of ALT wanes over time. At 1 year, the success of ALT is 77 to 81%, with a subsequent yearly failure rate of 7 to 10%.21,24 After 5 years, the probability of success was around 57% in one study 24 and 35% in another,21 with an overall success rate of 35 to 44%. However, ALT can still be successful in up to 32% of patients after 10 years.25

432 • SECTION VI LASER THERAPY OF GLAUCOMA

SPECIAL CONSIDERATION

The failure rate for ALT in primary open-angle glaucoma is approximately 10% per year.

The Advanced Glaucoma Intervention Study (AGIS) was designed to compare the effectiveness of ALT to trabeculectomy in patients with worsening glaucoma despite maximal medical therapy.26 It demonstrated improved preservation of visual function, as determined by visual acuity and perimetry, in black patients, despite higher IOPs in patients with ALT as compared with trabeculectomy. White patients, on the other hand, fared marginally better when trabeculectomy instead of ALT was initially performed. Although this outcome difference between algorithms appeared to diminish with time in both racial groups, ALT was not found to be detrimental in either one.

Pseudoexfoliation

In short-term evaluations, ALT is effective in lowering IOP in pseudoexfoliation. The success rate in the first few months is 97%, with the extent of IOP lowering ranging from 12 to 17 mm Hg.23,27,28 In addition, medications may be discontinued in some patients postoperatively. Here, too, the effect of ALT can quickly diminish, and at 1 year the “IOP-lowering success” is in the range of 50 to 70%.21,23 Finally, in some cases of pseudoexfoliation, the final IOP can be much higher than baseline, leaving the patient in need of an urgent trabeculectomy.

Pigmentary Glaucoma

ALT appears to work in a number of eyes with pigmentary glaucoma. Adequate IOP lowering is reported in 44 to 80% at 1 year, followed by a gradual decline to about 45% at 6 years.23,29–31 Unlike patients with primary openangle glaucoma, younger patients with pigmentary glaucoma may respond better to ALT than their older counterparts, with a longer-lasting effect. Although the role of iridotomy in pigmentary glaucoma remains controversial, the preferred laser therapy for most of these eyes remains trabeculoplasty, when indicated.

PEARL… Younger patients with pigmentary glaucoma may respond better to ALT than their older counterparts.

Aphakic and Pseudophakic Glaucoma

Aphakic eyes typically respond less well to ALT, although some individual cases may have adequate effect. Although success rates range from 60 to 85%, mean IOP lowering is only around 6 mm Hg.7,10,23 In addition, these patients

cannot be tapered off medical therapy after ALT, and the IOP lowering may be transient.7,28 If vitreous is present in the anterior chamber, ALT is almost certain to fail.7

Although the exact relationship between the cataract surgery and glaucoma history and the success of ALT is unknown, trabeculoplasty may have a better chance of working if the glaucoma developed prior to cataract surgery. If the glaucoma is due to complications associated with cataract surgery, ALT is far less likely to be effective. Finally, it is still unknown if intracapsular or extracapsular cataract extraction, or phacoemulsification surgery with or without clear corneal lens extraction, has an impact on laser success.

Prior Trabeculectomy

ALT can be successful in eyes with prior trabeculectomy in up to 70% of patients.10,28,32 The Advanced Glaucoma Intervention Study appeared to confirm this, although it still found that the IOP decrease was greatest with filtration surgery.26

Angle Recession, Uveitis, Narrow-Angle Glaucoma

In general, ALT is ineffective if the trabecular meshwork is not visible, such as due to permanent peripheral anterior synechiae, a recessed angle, or iridocorneal endothelial (ICE) syndrome. Careful gonioscopy is imperative to insure that at least 180 degrees of the trabecular meshwork appears functional.

ALT is generally poorly effective in patients with uveitis. This may be due to permanent angle closure, blockage of the trabecular meshwork by particulate debris in eyes with acute uveitic glaucoma, a poor view of the anterior chamber angle, or growth of a microscopic membrane over the trabecular meshwork. In all of these cases, trabeculoplasty may potentially cause elevated IOP due to a reactivation of the uveitis and further destruction of the remaining functional meshwork.23,28,33

In eyes with chronic angle closure, the exact amount of angle that must be visible and open for ALT to be effective has not been determined. Many have suggested that at least half of the angle needs to be open to treat successfully without creating a permanent IOP elevation. Here it is imperative to weigh the potential adverse effects of permanent angle damage against the chance of improving aqueous outflow. However, if the angle can be adequately visualized, some patients with angle-closure glaucoma can be successfully treated with ALT after iridotomy or iridectomy, even in the presence of peripheral anterior synechiae.10,28

Eyes with angle recession generally do not respond adequately to ALT.28 However, individual cases sometimes do respond well in the short term.10,23 In one series, 63% of eyes were considered successful after 5 months.33 However, most of these eyes eventually required more definitive therapy.

REPEAT ALT

Repeating ALT on 180 or 360 degrees of the angle in an eye that has had a previous 360 degree treatment is usually ineffective.34–36 Successful ALT can occur in 33 to 38%, whereas about 68% will fail.34 Of the eyes considered successful, the effect is short lived and the treatment effect is lost in two thirds of eyes by 1 year.34,36 In addition, 12 to 17% can have an acute rise in IOP of 10 to 37 mm Hg. This can result in rapid progression of visual field loss and optic nerve deterioration, and a few cases will require urgent surgery to control the IOP.35,37 On the other hand, early experience with selective laser trabeculoplasty (SLT), discussed in the following text, suggests that repeated treatment may provide adequate results.

ARGON LASER TRABECULOPLASTY

AS PRIMARY THERAPY

ALT may also be considered as first-line therapy for the patient with newly diagnosed open-angle glaucoma.38 The advantage of this approach is that it avoids the problems of noncompliance and the side effects of long-term medical therapy, as well as the negative effect of medications on the conjunctival epithelium and stroma.39 Several studies have shown that, when performed as initial therapy, ALT can be very effective in reducing IOP.8,9,40,41 The Glaucoma Laser Trial, which compared the efficacy of initial treatment with ALT to that of initial medical therapy with beta-blockers, found that 63% of patients maintained IOP control 1 year after undergoing ALT, and 44% were controlled with ALT alone after 2 years.8 In addition, patients treated initially with ALT were more likely to be controlled on fewer medications. Although differences were small, an additional 3 years of follow-up revealed that eyes that received laser first had overall lower IOP and less visual field loss.9 In spite of these favorable results, may clinicians still tend to use medications to treat glaucoma before trying ALT.

EFFECT OF RACE, SEX, AND AGE ON ARGON

LASER TRABECULOPLASTY EFFICACY

Neither race nor sex has any effect on the response or success of ALT. It is as effective in black as well as white patients.10,11,13,25,42 However, black patients may fare better with respect to visual field and acuity in the long term, as compared with white patients. On the other hand, patients younger than 40 years of age have a dramatic rate of failure with ALT. About 60% develop uncontrolled IOPs and require filtering surgery for control within 1 to 2 years.10,43 Congenital and juvenile-onset glaucomas also respond poorly to ALT.10,20

PRELASER INTRAOCULAR PRESSURE AND

ARGON LASER TRABECULOPLASTY RESPONSE

Some investigators have found that ALT produces a greater lowering of IOP in eyes with higher preoperative

CHAPTER 40 LASER TRABECULOPLASTY • 433

pressures.10,40,44 However, not all studies found such a correlation,25 suggesting that, overall, the percent of IOP reduction may be fairly constant.

SURGICAL TECHNIQUE FOR ARGON

LASER TRABECULOPLASTY

PREOPERATIVE CONSIDERATIONS

Prior to treatment, the surgeon should insure adequate knowledge of the patient, remarkable features of the patient’s angle, and the proper functioning and calibration of the laser. A pretreatment IOP measurement is essential. However, argon laser energy may be absorbed by fluorescein and can cause corneal opacities. Therefore, excess fluorescein should be irrigated from the conjunctival sac prior to treatment. Alternatively, fluorescein can be used in the untreated eye, and proparacaine in the treated eye. By applanating the eye with the fluorescein first, and then carrying this fluorescein on the tonometer tip over to the eye to be treated, epithelial exposure to the dye can be minimized.

Informed consent should include the possibility of a 1 to 3% chance of a transient IOP elevation necessitating further observation or medication, mild inflammation with injection and photophobia, or, rarely, permanent IOP elevation that may require further laser or even surgical management. Most important, the patient should understand that the procedure may not work, and that further medical or surgical therapy may be necessary.

It is advisable to treat the patient with one drop of either brimonidine or apraclonidine one half hour before and/or immediately following trabeculoplasty. These alpha adrenergic agonists have been shown effective in minimizing acute postoperative IOP elevations, as discussed in the following text.

LASER TECHNIQUE

The surgeon and the patient must both be comfortable, with the patient’s head resting on the chin and forehead supports of the slit-lamp. Either a Goldmann gonioscopy lens or a Ritch trabeculoplasty four-mirror lens can be used to perform ALT, although other mirrored gonioprisms have also been used. Following instillation of a topical anesthetic, the surgeon inserts the gonioprism, using the right hand for treating the left eye and the left hand for the right eye, to allow the patient to maintain fixation with the fellow eye. An elbow rest helps keep the prism steady throughout the treatment.

Standard treatment parameters consistently include a spot size of 50 m and 0.1 sec exposure time. The spot should be carefully focused with each application to maximize energy delivery. This is accomplished by keeping the spot size as small and round as possible. Throughout the procedure, the front plane of the prism must be kept

434 • SECTION VI LASER THERAPY OF GLAUCOMA

perpendicular to the laser delivery path. If the aiming spot is no longer round, this is most likely due to the surgeon tipping the lens in an attempt to view the angle over the iris surface. If this occurs, the surgeon should correct the lens orientation while looking around the slit-lamp at the lens position, and then have the patient shift his or her gaze, using the fixation light with the fellow eye, to improve the view of the angle, without tipping the lens. Treating through the middle third of the lens will minimize the effect of spherical aberration.

PEARL… Orienting the lens surface perpendicular to the laser beam will help keep the aiming spot well focused and round.

Laser energy levels should be titrated between 500 and 1000 mW to produce minimal depigmentation or a slight vaporization bubble in the anterior trabecular meshwork.7,45 Treatment spots are typically placed three degrees apart (Fig. 40–2). Because the laser energy is absorbed by pigment, the power may need to be reduced in eyes with a heavily pigmented meshwork, as is frequently seen in pigmentary glaucoma and pseudoexfoliation. Likewise, the power may need to be adjusted throughout the procedure because pigmentation can vary within a single eye. In some lightly pigmented eyes, it may be almost impossible to see the spots. Power levels above 1 W should be avoided, if possible.

Every surgeon should develop a standard routine for treating the meshwork. This minimizes the chance of getting disoriented and helps the surgeon keep track of the clock hour being treated. Many surgeons advocate begin-

FIGURE 40–2 Representation of anterior chamber angle appearance during argon laser trabeculoplasty, illustrating slight blanching of the freshly treated trabecular meshwork regions, spaced approximately 3 degrees apart. Proper placement of the aiming beam, at the junction of the pigmented and nonpigmented meshwork, appears at right.

ning treatment through a superior mirror, into the inferior trabecular meshwork. This is typically the deepest part of the angle, with the best view. In addition, the inferior angle is often more highly pigmented, making it easier to identify the trabecular meshwork. Applying several burns along the meshwork, and then turning the gonio mirror (in a direction opposite to that of the burns) to expose the next stretch of meshwork, is generally more efficient than turning the mirror slightly after each burn application. Noting angle landmarks, changes in pigmentation, and even treatment burns, when visible, helps identify the last area treated when reorienting the lens.

Anterior versus Posterior Placement of Burns

Although initial treatment protocols placed burns either on or just posterior to the most heavily pigmented portion of the meshwork,7 this method was associated with pain, acute rises in postoperative pressure, and the formation of posterior synechiae. Anterior treatment, so that each spot straddles the junction of the pigmented and nonpigmented trabecular meshwork, appears to diminish all of these problems when compared with treatment of the posterior meshwork.45 However, the surgeon should beware of confusing pigment that occasionally appears anterior to Schwalbe’s line with the pigmented trabecular meshwork because this can cause inadvertent treatment to the peripheral cornea. Often, pigmentation might disappear as one moves from the inferior to the superior meshwork. This can be remedied by “back-tracking” to the last area treated.

180versus 360-Degree Treatment

The original protocols for ALT involved placing 100 spots around the entire 360 degree circumference of the angle. Subsequent studies suggested that 180-degree treatments were less likely to produce marked postoperative IOP elevation.45,46 This has led many surgeons to perform ALT in 180-degree increments, assessing the effects of the first treatment after 1 month. If the desired level of IOP has not been achieved, the other half is treated because some patients respond when the entire angle is treated, even when some response was initially attained. If, however, significant IOP reduction does occur, the rest of the angle is not treated and saved for treatment at a later date, if necessary.44

However, as will be discussed, the routine use of apraclonidine and brimonidine with ALT has diminished the incidence of acute postoperative IOP elevations to less than 5%, even following 360-degree treatment. Although there appears to be no striking difference in IOP lowering between the two approaches, eyes that respond well to ALT seem to have a larger absolute reduction when the entire angle is treated.47 In addition, better long-term success with ALT has been correlated with applying 100 or more burns to the entire angle, although excessive treatment can result in a postoperative pressure increase.48

C O N T R O V E R S Y

Many surgeons perform argon laser trabeculoplasty in 180-degree increments, whereas others prefer to treat the entire angle at one sitting.

From these considerations, many surgeons prefer to treat the entire angle at one sitting and save the patient the expense and inconvenience of two treatments. Currently, no prospective, well-controlled studies have validated the superiority of either approach, and the question of 180versus 360-degree treatment comes down to regional preferences.

POSTOPERATIVE MANAGEMENT

The IOP should be measured approximately 30 to 60 minutes following treatment. If the pressure is not elevated or only minimally elevated, the patient can be discharged. If the pressure is elevated, the patient should be monitored or treated, if necessary, with medications until the IOP is back to baseline or clearly returning to a safe level.

Glaucoma medications should not be discontinued immediately after ALT, but continued for at least 4 to 6 weeks, to allow sufficient time for a response. At this time, a decision can be reached whether any medications can be discontinued, but in most cases medications will still be required to achieve an IOP that is lower than the pretreatment pressure.12

Most clinicians prescribe topical corticosteroids, usually 1% prednisolone, four times a day for 1 week to control postoperative inflammation.10,11,23,49 However, no articles have confirmed a definite cost-effective benefit from this treatment. Nonsteroidal anti-inflammatory drugs (NSAIDs) may also be effective in curtailing inflammation,50,51 although other studies found that topical flurbiprofren was no different than placebo in terms of clinically assessed inflammation and comfort.52,53

COMPLICATIONS OF ARGON LASER

TRABECULOPLASTY

INCREASED INTRAOCULAR PRESSURE

The most important and potentially dangerous complication of ALT is an acute, usually self-limited, rise in IOP as early as 1 hour, or sooner, after treatment. Without prophylaxis, about one third of patients can have rises in IOP over 5 mm Hg, and 12 to 50% will have rises over 10 mm Hg 1 hour after treatment, with one documented IOP rise of up to 33 mm Hg.54 As already discussed, the incidence in the rise of IOP is similar after treating either 180 or 360 degrees.55–57 This acute rise in IOP does not

CHAPTER 40 LASER TRABECULOPLASTY • 435

indicate long-term failure of the procedure.55 However, it can cause loss of central fixation in advanced cases of glaucoma10,56 as well as progression of visual-field loss.56 Both brimonidine and apraclonidine effectively prevent a rise in IOP in all but about 3 to 5% of cases.58,59 Both medications are effective instilled 1 hour before and immediately following treatment, or using it only after the

laser treatment.60,61

PERIPERAL ANTERIOR SYNECHIAE

Small, peaked, “tentlike” peripheral anterior synechiae (PAS) can occur in up to 46% of patients by 3 months after treatment.54,57,62 This rate may be higher in eyes with brown irides, or in cases where the laser burns are applied to the posterior meshwork or ciliary body band.54,62,63 These PAS are usually to the posterior trabecular meshwork and do not correlate with IOP rise or average laser power.54

INFLAMMATION

Inflammation after ALT treatment can occur in up to 49% of cases and generally peaks 2 days after treatment. Up to 100% of eyes with pigment dispersion and 69% of eyes with pseudoexfoliation will have inflammation, compared to 23% with POAG.50 Inflammation is not associated with pressure increases54 and, to the contrary, appears to correlate with pressure decrease in the short term50 and can be treated with either corticosteroids or NSAIDs.50,57 Most of the time it is clinically insignificant, and patients are asymptomatic, even when treated with a placebo.

Rarely, inflammatory precipitates can form on the trabecular meshwork, which are thought to be secondary to trabeculitis following ALT. This usually occurs a few weeks after the treatment and is associated with elevated IOPs.64

PROGRESSIVE FIELD AND VISION LOSS

Visual field and central vision loss from a significant rise in IOP can develop very rapidly following ALT treatment.10,56,65 As a result, some patients may require emergency glaucoma surgery for marked and sustained pressure elevation.65 Field loss and progression can also occur despite a decrease in IOP as a result of ALT. This is usually due to inadequate IOP lowering and progression of the disease itself.57

TRABECULOPLASTY WITH OTHER LASERS

Although argon blue or green irradiation (488 through 515 nm) have been widely accepted for performing trabeculoplasty, other lasers can produce a comparable decrease in IOP. These include the krypton red (647 nm),66 continuous wave (CW) Nd:YAG (1064 nm),67 diode (810 nm),68 and frequency-doubled Nd:YAG (fd-YAG) (532 nm).69 Early experience with the latter two shows potential unique advantages for glaucoma treatment.

436 • SECTION VI LASER THERAPY OF GLAUCOMA

DIODE LASER TRABECULOPLASTY

The semiconductor diode laser has been shown to produce a decrease in IOP comparable to ALT.68,70 However, this solid-state laser has the great advantage of being durable, compact and portable, adaptable to the standard biomicroscope, and less expensive, and it uses standard electric current. The same laser can be used for transscleral cyclophotocoagulation (Chapter 42) and photocoagulation of the retina.

For diode laser trabeculoplasty (DLT) the patient sits at the biomicroscope to which the laser has been adapted, and the procedure is similar to that used with ALT. Using a 75m spot size, 750 to 1000 mW of power is delivered with 0.1 to 0.2 sec duration to either 180 or 360 degrees of the trabecular circumference. Compared with ALT, DLT produced less visible reaction in the anterior chamber angle, less postoperative flare, less postoperative pain, and fewer PAS.71 In a study of 25 eyes followed for 24 months, the ocular hypotensive effect of DLT was equivalent to ALT.72

The mechanism by which DLT lowers the IOP is presumed to be the same as with ALT. A comparison of the trabecular pathology after DLT and ALT revealed similar alterations in the trabecular meshwork.73,74

SELECTIVE LASER TRABECULOPLASTY

Selective laser trabeculoplasty (SLT) describes trabeculoplasty using a Q-switched, frequency-doubled Nd:YAG laser.75 In a nonrandomized, prospective 26-week study, trabeculoplasty with this laser significantly reduced the IOP both in open-angle glaucoma eyes undergoing their first laser treatment and in eyes that had had previous ALT.76 In a prospective trial 36 patients were randomized to treatment with either SLT or ALT and followed for 6 months. SLT was equivalent to ALT in lowering IOP during the first 6 months after treatment. Equally important, patients with previous failed ALT had a significantly greater drop in IOP when treated with SLT compared with retreatment with ALT.77

Patients are treated at the biomicroscope with the fdYAG laser (Selecta 7000, Coherent, Inc., Palo Alto, CA) emitting at 532 nm with a pulse duration of 3 nsec and a spot size of 400 m. The helium-neon aiming beam is focused onto the pigmented trabecular meshwork and 180 degrees of meshwork is treated with approximately 50 adjacent spots. Pulses of 0.8 mJ energy are adjusted in 0.1 mJ decreasing increments until there is no cavitation bubble formation.77

The mechanism by which SLT lowers IOP may be similar to that of ALT. Like ALT, SLT increases the number of monocytes and macrophages present in the treated trabecular meshwork tissues and may cause release of factors and chemoattractants that are required to recruit macrophages. These macrophages, in turn, clear the pigment granules from the trabecular meshwork.78 However, unlike ALT, SLT does not produce mass destruction of

cells and trabecular beams or result in scar tissue.79 This is believed to result from selective photothermolysis of trabecular melanin granules, whose absorption spectrum includes the wavelength of the fd-YAG laser. The short, 3 nsec pulse confines the heat to the target tissue without spreading outside the irradiation zone. Because there is no widespread tissue destruction, it has been suggested that SLT might be effectively repeated.76,77

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19a. Parshley DE, Bradley JM, Fish A, et al. Laser trabeculoplasty induces stromelysin expression by trabecular juxtacanalicular cells. Invest Ophthalmol Vis Sci 1996;37:795–804.

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24.Shingleton BJ, Richter CU, Bellows AR, et al. Longterm efficacy of argon laser trabeculoplasty. Ophthalmol 1987;94:1513–1518.

25.Shingleton BJ, Richter CU, Dharma SK, et al. Longterm efficacy of argon laser trabeculectomy: a 10-year follow-up study. Ophthalmol 1993;100:1324–1329.

26.The AGIS Investigators. The Advanced Glaucoma Intervention Study (AGIS), IV: Comparison of treatment outcomes within race: seven-year results. Ophthalmol 1998;105:1146–1164.

27.Ritch R, Podos SM. Laser trabeculoplasty in exfoliation syndrome. Bull N Y Acad Med 1983;59:339.

28.Robin AL, Pollack IP. Argon laser trabeculoplasty in secondary forms of open-angle glaucoma. Arch Ophthalmol 1983;101:382–384.

29.Ritch R, Liebmann J, Robin AL, et al. Argon laser trabeculoplasty in pigmentary glaucoma. Ophthalmol 1993;100:909–913.

30.Lehto I. Long-term follow up of argon laser trabeculoplasty in pigmentary glaucoma. Ophthalmic Surg 1992;23:614–617.

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31.Lunde MW. Argon laser trabeculoplasty in pigmentary dispersion syndrome with glaucoma. Am J Ophthalmol 1983;96:721–725.

32.Fellman RL, Starita RJ, Spaeth GL, Poryzees EM. Argon laser trabeculoplasty following failed trabeculectomy. Ophthalmic Surg 1984;15:195–198.

33.Spaeth GL, Fellman RL, Starita RJ, Poryzees EM. Argon laser trabeculoplasty in the treatment of secondary glaucoma. Trans Am Ophthalmol Soc 1983; 81:325–330.

34.Richter CU, Shingleton BJ, Bellows AR, et al. Retreatment with argon laser trabeculoplasty. Ophthalmol 1987;94:1085–1089.

35.Brown SV, Thomas JV, Simmons RJ. Laser trabeculoplasty re-treatment. Am J Ophthalmol 1985;99:8–10.

36.Messner D, Siegel LI, Kass MA, et al. Repeat argon laser trabeculoplasty. Am J Ophthalmol 1987;103: 113–115.

37.Starita RJ, Fellman RL, Spaeth GL, Poryzees E. The effect of repeating full-circumference argon laser trabeculoplasty. Ophthalmic Surg 1984,15:41–43.

38.American Academy of Ophthalmology. Preferred Practice Pattern: Argon Laser Trabeculoplasty. 1996.

39.Lavin MJ, Wormald RPL, Migdal CS, Hitchings RA. The influence of prior therapy on the success of trabeculectomy. Arch Ophthalmol 1990;108:1543.

40.Thomas JV, El-Mofty A, Hamdy EE, Simmons RJ. Argon laser trabeculoplasty as initial therapy for glaucoma. Arch Ophthalmol 1984;102:702–703.

41.Rosenthal AR, Chaudhuri PR, Chiapella AP. Laser trabeculoplasty primary therapy in open-angle glaucoma: a preliminary report. Arch Ophthalmol 1984;102:699–701.

42.Krupin T, Patkin R, Kurata FK, et al. Argon laser trabeculoplasty in black and white patients with primary open-angle glaucoma. Ophthalmol 1986;93:811–816.

43.Safran MJ, Robin AL, Pollack IP. Argon laser trabeculoplasty in younger patients with primary open-angle glaucoma. Am J Ophthalmol 1984;97: 292–295.

44.Klein HZ, Shields MB, Ernest JT. Two-stage argon laser trabeculoplasty in open-angle glaucoma. Am J Ophthalmol 1985;99:392–395.

45.Schwartz LW, Spaeth GL, Traverso C, Greenidge KC. Variation of techniques on the results of argon laser trabeculoplasty. Ophthalmol 1983;90:781–784.

46.Weinreb RN, Ruderman J, Juster R, Wilensky JT. Influence of the number of laser burns administered on the early results of argon laser trabeculoplasty. Am J Ophthalmol 1983;95:287–292.

47.Lustgarten J, Podos SM, Ritch R, et al. Laser trabeculoplasty, a prospective study of treatment variables. Arch Ophthalmol 1984;102:517–519.

48.Wise JB. Ten-year results of laser trabeculoplasty: does the laser avoid glaucoma surgery or merely defer it? Eye 1987;1:45–50.

438 • SECTION VI LASER THERAPY OF GLAUCOMA

49.Shin DH, Frenkel RE, David R, Cheetham JK. Effect of topical anti-inflammatory treatment on the outcome of laser trabeculoplasty. The Fluorometholone-Laser Trabeculoplasty Study Group. Am J Ophthalmol 1996; 122:349–354.

50.Mermoud A, Pittet N, Herbort CP. Inflammation patterns after laser trabeculoplasty measured with the laser flare meter. Arch Ophthalmol 1992;110:368–370.

51.Kim YY, Glover BK, Shin DH, Lee D, Frenkel RE, Abreu MM. Effect of topical anti-inflammatory treatment on the long-term outcome of laser trabeculoplasty. Fluorometholone-Laser Trabeculoplasty Study Group. Am J Ophthalmol 1998;126:721–723.

52.Hotchkiss ML, Robin AL, Pollack IP, Quigley HA. Nonsteroidal anti-inflammatory agents after argon laser trabeculoplasty: a trial with flurbiprofen and indomethacin. Ophthalmology 1984;91:969–974.

53.Weinreb R, Robin AL, Brevardt G, Drake M, Blumenthal M, Wilensky J. Flurbiprofen pretreatment in argon laser trabeculoplasty for primary open-angle glaucoma. Arch Ophthalmol 1984;102:1629–1632.

54.The Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial, I: acute effects of argon laser trabeculoplasty on intraocular pressure. Arch Ophthalmol 1989;107:1135–1142.

55.Krupin T, Kolker AE, Kass MA, Becker B. Intraocular pressure the day of argon laser trabeculoplasty in primary open-angle glaucoma. Ophthalmol 1984; 91:361–365.

56.Weinreb RN, Ruderman J, Juster R, Zweig K. Immediate intraocular pressure response to argon laser trabeculoplasty. Am J Ophthalmol 1983;93:279–286.

57.Hoskins HD, Hetherington J, Minckler DS, et al. Complications of laser trabeculoplasty. Ophthalmol 1983;90:796–799.

58.Robin AL. Argon laser trabeculoplasty medical therapy to prevent the intraocular pressure rise associated with argon laser trabeculoplasty. Ophthalmic Surg 1991;22:31–37.

59.Brown RH, Stewart RH, Lynch MG, et al. ALO 2145 reduces the intraocular pressure elevation after anterior segment laser surgery. Ophthalmol 1988;95: 378–384.

60.Holmwood PC, Chase RD, Krupin T, et al. Apraclonidine and argon laser trabeculoplasty. Am J Ophthalmol 1992;114:19–22.

61.Barnebay HS, Robin AL, Zimmerman TJ, et al. The efficacy of brimonidine in decreasing elevations in intraocular pressure after laser trabeculoplasty. Ophthalmol 1993;100:1083–1088.

62.Traverso CE, Greenidge KC, Spaeth GL. Formation of peripheral anterior synechiae following argon laser trabeculoplasty. Arch Ophthalmol 1984;102:861–863.

63.Rouhiainen HJ, Terasvirta ME, Tuovinen EJ. Peripheral anterior synechiae formation after trabeculoplasty. Arch Ophthalmol 1988;106:189–191.

64.Fiore PM, Melamed S, Epstein DL. Trabecular precipitates and elevated intraocular pressure following argon laser trabeculoplasty. Ophthalmic Surg 1989;20:697–701.

65.Levene R. Major early complications of laser trabeculoplasty. Ophthalmic Surg 1983;14:947–953.

66.Spurney RC, Lederer CM Jr. Krypton laser trabeculoplasty: a clinical report. Arch Ophthalmol 1984;102: 1626.

67.Kwasniewska S, Fankhauser F, Larsen, et al. The efficacy of CW Nd:YAG laser trabeculoplasty. Ophthalmic Surg 1993;24:304.

68.McHugh D, Marshall J, Ffytche TJ, et al. Diode laser trabeculoplasty (DLT) for primary open-angle glaucoma and ocular hypertension. Br J Ophthalmol 1990; 74:743.

69.Latina MA, Sibayan SA, Shin DH, et al. Q-switched 532-nm Nd:YAG laser trabeculoplasty (selective laser trabeculoplasty): a multicenter, pilot, clinical study. Ophthalmology 1998;105:2082.

70.Brancato R, Carassa R, Trabucchi G. Diode laser compared with argon laser for trabeculoplasty. Am J Ophthalmol 1991;112:50.

71.Moriarty AP, McHugh JDA, Spalton DJ, et al. Comparison of the anterior chamber inflammatory response to diode and argon laser trabeculoplasty using a laser flare meter. Ophthalmology 1993;100:1263.

72.Moriarty A, McHugh J, Ffytche T, et al. Long-term follow-up of diode laser trabeculoplasty for primary open-angle glaucoma and ocular hypertension. Ophthalmology 1993;100:1614.

73.McMillan TA, Stewart WC, Legler UFC, et al. Comparison of diode and argon laser trabeculoplasty in cadaver eyes. Invest Ophthalmol Vis Sci 1994;35:706.

74.McHugh D, Marshall J, Ffytche TJ, et al. Ultrastructural changes of human trabecular meshwork after photocoagulation with a diode laser. Invest Ophthalmol Vis Sci 1992;33:2664.

75.Latina MA, Park C. Selective targeting of trabecular meshwork cells in vitro studies of pulsed and CW laser interactions. Exp Eye Res 1995;60:359.

76.Latina MA, Santiago A, Sibayan MD, et al. Q- switched 532-nm Nd:YAG laser trabeculoplasty (selective laser trabeculoplasty): a multicenter, pilot, clinical study. Ophthalmology 1998;105:2082.

77.Damji KF, Shah KC, Rock WJ, et al. Selective laser trabeculoplasty v argon laser trabeculoplasty: a prospective randomised clinical trial. Br J Ophthalmol 1999;83: 718.

78.Alvarado JA, Murphy CG. Outflow obstruction in pigmentary and primary open-angle glaucoma.

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79.Kramer TR, Noecker RJ. Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes. Ophthalmology 2001;108:773