53

Laser Trabeculoplasty

Joel S. Schuman, MD, FACS and Jeremy B. Wingard, MD

Since the tissue between the anterior chamber and Schlemm’s canal is the site of both normal resistance to aqueous humor outflow and abnormal resistance,¹ it was natural to attempt to penetrate this tissue with the laser, and thus bypass the abnormality. However, similar to the results of Barkan’s surgical goniotomy in adult open-angle glaucoma, only temporary lowering of intraocular pressure (IOP) was achieved with laser applications that attempted to penetrate to Schlemm’s canal. Gaasterland and Kupfer then demonstrated that heavy applications of argon laser energy to the trabecular meshwork (TM) can actually produce glaucoma in animals.² After this demonstration, enthusiasm for laser therapy to the TM in glaucomatous eyes diminished.

We owe a great debt to Wise and Witter³ for reintroducing laser treatment to the TM. Instead of attempting to penetrate the TM, they popularized circumferential, nonpenetrating applications to the tissue to produce small superficial scars. They hypothesized that this corrected a supposed mechanical “sag” of the tissue, which they felt to be the basic defect in open-angle glaucoma. One can debate whether these authors were the first to apply such a laser method, and whether this hypothesis is correct. It is clear, however, that they deserve much credit for advocating and promulgating this technique in the face of adverse laboratory data, and for introducing the modern laser era of open-angle glaucoma therapy.

The results of Wise and Witter have been confirmed by many authors.^4–12 It is remarkable how consistent the results have been. Depending on the pretreatment IOP and type of glaucoma, an average 6 to 10 mm Hg, or approximately 20%, pressure reduction can be expected. The best response is obtained in pseudoexfoliation glaucoma, whereas the poorest results tend to be in young patients with glaucoma.¹⁰ Certain secondary glaucomas such as uveitic glaucoma also respond poorly, although occasional successes are reported. It is often worthwhile now to attempt laser trabeculoplasty (LTP) in almost all forms of open-angle glaucoma before embarking on filtration surgery, and the availability of selective laser trabeculoplasty (SLT) obviates the need to treat less than 360 degrees of the angle, even in cases with poor prognosis, because the risk of worsening IOP with the laser is much lower than with argon laser trabeculoplasty (ALT).

SLT was introduced in this text in the fifth edition,¹³ and this technology has remained important in most glaucoma practices, gradually replacing ALT in many centers. As opposed to ALT, SLT employs short pulse durations (3 to 10 ns compared with 0.1 sec for ALT) and delivers significantly less total energy than ALT. The SLT unit is a Q-switched, frequency-doubled (532 nm) neodymium:yttrium-aluminum-garnet laser with a spot size of 400 μm that affixes to a slit lamp. Comparative studies performed by Damji and colleagues showed equivalence between the 2 laser treatments in IOP reduction for open-angle glaucoma patients at both 6 months¹⁴ and 12 months¹⁵ of follow-up. This equivalence was verified over up to 5 years of retrospective follow-up by Juzych and colleagues.¹⁶ Other lasers have been used for trabeculoplasty as well, with clinical IOP-lowering success demonstrated for diode LTP (0.1-s duration, 50-μm spot size, power 700 to 1200 mW),¹⁷ micropulse diode LTP (15% duty cycle with 100 pulses over a 0.2-s application, 200-μm spot size, power 2 W),¹⁸ and recently titanium-sapphire LTP.¹⁹ The use of micropulse diode LTP deserves some mention, as clinical units are available, and this technology is employed in some centers. The longest duration study of diode LTP (0.5 s, 100 μm, 570 to 850 mW) showed equivalent IOP lowering compared with ALT at 5 years of follow-up.20

Depending on the indications for LTP, most patients who are uncontrolled before laser treatment need to maintain medical therapy after laser treatment. For less severe glaucoma or in eyes with dramatic responses, some medications can be discontinued. In addition, SLT particularly can be offered as the initial treatment for newly diagnosed open-angle glaucoma, before embarking on a regimen of medications, and in these cases SLT alone may suffice as the initial treatment, in some cases for a number of years. The efficacy of LTP should be assessed 4 to 6 weeks after treatment. Shorter-term IOP measurements may be misleading. In some cases, even longer follow-up may be required to find the final effect of the laser.

The major risk of the procedure is acute IOP elevation, and although this risk seems lower with SLT than with ALT, it can be severe and can compromise vision. This should be monitored and treated appropriately. The magnitude of IOP elevation after ALT has been reduced by the tendency to divide the ALT sessions into 2 or 3, using 35 to 50 spots per session, with lower power settings than originally described. In addition, both SLT and ALT are routinely performed with the perioperative use of alpha-2 agonists, such as apraclonidine^21,22 or brimonidine. Although less common, the other notable risks of treatment are inflammatory in nature, including both anterior chamber cellular reaction and corneal edema.

Mechanistically, it is unclear how LTP works. It is possible that Wise’s hypothesis is correct, but there are certain inconsistencies with this mechanical hypothesis. Treating smaller portions of the angle than 360 degrees often produces a disproportionally greater effect than would be anticipated mechanically. Also the type of glaucoma that responds best to LTP, pseudoexfoliation glaucoma, is the one type of glaucoma in which mechanical trabecular collapse is not believed to be involved, but rather it seems that accumulation of extracellular pseudoexfoliation material in the juxtacanalicular tissue is responsible for the increase in outflow resistance.²³ Histopathologic study in human autopsy eyes showed that ALT caused coagulative damage and disruption of the trabecular beam structure, whereas SLT did not cause these changes, suggesting that the 2 lasers may work differently.²⁴ More likely, it may be that ALT and SLT create their IOP-lowering effects similarly, but SLT does so without changing the structure of the TM, creating a margin of safety and also preserving the TM for the possibility of surgical treatment of Schlemm’s canal in the future. This last consideration has gained importance with the recent availability of numerous ab interno Schlemm’s canal–based surgical options.

The effects of LTP on IOP seem to persist for years in most patients, although some patients demonstrate drift of IOP upward much sooner. This seems especially common in pseudoexfoliation glaucoma. (Is more material being deposited?) As a general rule, ALT maintains IOP control in 80% of eyes for 1 year, 50% for 5 years, and 30% for 10 years.²⁵ The largest long-term SLT study showed slightly lower success over 5 years, but the results are identical to ALT in that study, suggesting that SLT results probably follow the same trend as ALT.¹⁶ In addition to static IOP lowering, diurnal IOP fluctuation was demonstrated to improve at 6 months’ follow-up after SLT,²⁶ and at 1 month of follow-up, tonographic outflow facility improved after either 180 or 360 degrees SLT.²⁷

Roughly 30% of eyes respond to retreatment with ALT,²⁸ and we have observed some success with retreatment in patients who initially demonstrated a good but not long-lasting response. (In contrast, no success has been observed with retreatment in those who failed to respond initially.) More importantly, no dramatic worsening of the glaucoma has been demonstrated in those who failed to respond to ALT retreatment on half the angle (with less than 50 applications), and subsequently, filtration surgery has been performed uneventfully. It should be noted, however, that there is potential for scarring in the angle with primary and repeat ALT, and this may preclude potential surgical access to Schlemm’s canal for ab interno approaches to glaucoma surgery and therefore is to be avoided in certain patients who might benefit from these approaches before filtration surgery is considered. In addition, patients previously treated with ALT appear to respond better to subsequent SLT treatment than to repeat ALT,^14,29 and so we recommend this approach in most situations. As for repeat SLT, there is good evidence that patients with prior successful SLT whose effect fails after 6 months may be treated successfully with repeat SLT.³⁰ We have had success with this strategy in some patients, and there does not appear to be an increased risk of treatment complications. In fact, there is no a priori limit to the number of times SLT may be repeated with respect to risks of treatment, although we have observed a diminishing chance of treatment success with each retreatment after a prior effective treatment’s effect has failed, such that the risk of failure eventually approaches 100% on a population basis. Nonetheless, there are rare patients who respond well even to a fourth or fifth retreatment.

There are many unanswered questions about LTP. In open-angle glaucoma, it is not certain what the basic abnormality may be (nor even is it known precisely how aqueous humor normally makes its way through the aqueous outflow system). Yet it is clear that LTP works, at least for several years. It is also clear that ALT damages the angle in ways that SLT does not, resulting in destruction of TM tissue that has been demonstrated in a cynomolgus monkey (Figure 53-1) and has been shown in contradistinction to SLT-related damage in human autopsy eyes (Figure 53-2). There is still the potential for long-term adverse effects (eg, downgrowth of endothelial cells from the cornea and hyalinization of the angle). (Some have hypothesized that TM cells may normally act by contact inhibition to prevent this endothelial downgrowth.) It is likely that SLT provides a safety margin compared to ALT in this regard. It is obvious that because of the initial good results of LTP, it is now being applied to earlier stages of open-angle glaucoma. With the favorable safety profile of SLT, some have in fact raised the question whether LTP should be a substitute for medical glaucoma therapy (see the discussion of the Glaucoma Laser Trial), either at initial diagnosis or as a later substitution for medical therapy, especially if there are adherence issues or significant side effects of medical therapy. In a study designed to assess the ability of SLT to provide a reduction in IOP-lowering medication use, a predefined IOP target was maintained while discontinuing an average of 2 medications at 6 months and 1.5 medications at 1 year,31 supporting the idea that SLT could replace medications, at least early in the disease. Our current practice is to offer SLT as an initial option for most patients newly diagnosed with open-angle glaucoma, and we frequently discuss SLT as an option for limiting a patient’s reliance of long-term medical therapy.

It is interesting to compare the dose of laser energy utilized in ALT with the dose required to produce glaucoma in monkeys. Quigley has demonstrated that these differ by an approximate factor of 6.³² Given the much lower total energy delivered, this concern is diminished with SLT.

Wise has stressed the importance of precise focusing of the laser and proper alignment of the instrument, both of which are quite appropriate.³³ Although the SLT beam is much wider (400 μm compared to 50 to 100 μm for traditional ALT), it remains possible to inappropriately aim or focus the beam, thereby compromising the effectiveness of the laser, and also risking iris or endothelial damage, depending on the direction of dislocation. It is likewise important that the actual power output of the laser be checked and calibrated periodically. Care and caution are necessary to avoid errors in LTP (Table 53-1).

HOW DO WE DO LASER TRABECULOPLASTY?

We are frequently questioned regarding our technique for this commonly performed procedure. In our center, SLT has replaced ALT entirely, although we continue to see patients on referral who were previously treated with ALT, sometimes relatively recently. Therefore, we will describe our prior technique for ALT as well as our current protocol for SLT.

Following informed consent, the patient receives a drop of brimonidine (0.1% to 0.2%) or apraclonidine 0.5% in the eye to be treated. Proparacaine 0.5% is placed in both eyes to anesthetize the treated eye, but also to decrease blinking in the contralateral eye, which can disrupt the treatment. A Goldmann 3-mirror lens is applied for ALT, or the Latina SLT lens is used for SLT, using a hypromellose 0.3% gel as a coupling solution. Although this is our procedure, any goniolens with the user’s choice of viscous coupling solution could be employed.

The decision of initial treatment zone is nuanced in ALT, but we usually chose the temporal angle. The rationale is that if the ALT is ineffective, and the patient requires filtering surgery, then the nasal angle is not directly involved by the ALT, and the filtering surgery may be less likely to suffer bleb encapsulation from the prior ALT.34 Alternately, the nasal angle may be treated first, for the same reason, as the completion of the ALT (the second 180 degrees) then involves the temporal 180 degrees. If then the completion of the ALT is not successful, filtering surgery can ensue shortly after, without the nasal angle having recent exposure to laser energy. Still others suggest treatment of the inferior angle as the initial site, as it is generally the widest area of the angle. This is acceptable, but was not our practice. The single most important factor in deciding where to treat is consistency: if one treats the temporal angle first, one should always treat this area first, so as to avoid later errors.

We routinely carry out treatment for a full 360 degrees with SLT, although in select patients at high risk for an acute IOP spike, the nasal 180 degrees may be treated first. Given the relative safety of SLT, it would be extremely rare to move on to filtering surgery before the angle had adequately healed, and so the choice of where to begin is less important. It remains important, however, to maintain a consistent pattern, so that if a treatment is interrupted for any reason, it will be clear where to begin at the next session.

The area to receive LTP is visualized in the goniolens, and the 6 o’clock position is treated first. Additional burns are then made moving temporally in the mirror, treating more temporally inferiorly. It is crucial to remember that the mirror does not cross the image of the angle, and that the more temporal portion in the mirror is indeed the more temporal part of the angle (see Figure 53-1). The goniolens is rotated clockwise in the right eye, or counterclockwise in the left eye, in order to keep the beam in the center of the goniomirror, to keep the spot round, and to deliver the most effective treatment with the least possible energy.

Approximately 50 spots are applied over 180 degrees, 8 to 9 spots per clock hour. The laser settings for ALT are a 50-μm spot, 0.1-s duration, and the starting power is generally 600 mW. The surgical endpoint is a blanch or small bubble. If the bubble bursts, then the power setting is too high; if there is no blanch, the power is too low. Burns are placed at the junction of the anterior third and posterior two-thirds of the TM, in an effort to avoid injury to the filtering portion of the TM (see Figure 53-2). Also, the more posterior the burn, the more likely inflammation and peripheral anterior synechiae (PAS) are to occur postoperatively.

SLT is continued for the second 180 degrees in almost all cases, as described above. The laser settings are 400-μm spot, 3-ns duration, and a starting power generally 0.7 mJ, which may be titrated in 0.1-mJ increments. The desired tissue effect is either a champagne bubble or just below the threshold that produces this effect. Large cavitation bubbles are not desired, and the energy is titrated lower if these occur. The power setting may be varied during the procedure if necessary, which may occur if pigmentation is variable as one proceeds around the angle. Burns are placed across the entire TM, over which the aiming beam should be centered. Approximately 100 spots are required to provide a full, nonoverlapping treatment.

Following ALT, the patient is given prednisolone acetate 1% every hour for the first day, then 4 times a day for 4 days, and then this drug is discontinued. Our SLT protocol is less aggressive, and we do not routinely use postoperative steroid drops. Anti-inflammatory treatment after SLT has been evaluated in several recent studies. A comparison of prednisolone acetate 1% drops, ketorolac tromethamine 0.5% drops, and placebo drops 4 times daily for 5 days following SLT demonstrated no difference in IOP lowering at 1 month or treatment failure at 1 year.³⁵ A study evaluating indomethacin 0.1% or dexamethasone 0.1% drops 3 times daily for 1 week after SLT vs an SLT-treated fellow eye without anti-inflammatory treatment found no treatment effect from the postoperative regimens, including no difference in IOP at 6 months.³⁶

Following the laser session, the patient is examined after 30 minutes to 1 hour to ensure that an acute IOP elevation has not occurred. The follow-up intervals for ALT are then 1 day, 1 week, and 4 to 6 weeks; the efficacy is judged at the 4- to 6-week follow-up examination. The regimen for SLT is less rigid, and, excepting patients in whom IOP spikes at 30 to 60 minutes, follow-up in clinic need not occur 1 week, or even 2 to 4 weeks in certain patients, depending on the patient’s specific history and clinician preference.

IMMEDIATE POST-LASER TREATMENT

Immediately after LTP the patient may experience minor photophobia and irritation, and examination may show conjunctival injection, ciliary flush, and anterior chamber cellular reaction. Topical steroids are prescribed over a 5-day period after ALT, but this is not necessary after SLT (see Table 52-2). The patient is free to ambulate without restriction. As mentioned previously, the principal short-term risk is acute IOP elevation, which should be monitored. Additional antiglaucoma therapy with carbonic anhydrase inhibitors or osmotics may be prescribed as dictated by an early IOP spike. Data show that neither pretreatment with corticosteroids nor antiprostaglandin drugs reduces the magnitude of the acute post-ALT IOP elevation.³⁷ However, a single drop of preoperative brimonidine 0.5% greatly reduced the likelihood of postoperative acute IOP elevations.³⁸ Commercially available brimonidine 0.15% and apraclonidine 0.5% exhibit equal abilities to prevent acute IOP elevation after anterior segment laser surgery.³⁹

POST-LASER VISITS

When we began using SLT in our clinics, it became apparent that the incidence of postoperative day 1 problems was exceedingly low. We have therefore changed our pattern of follow-up visits, and unless there is an acute IOP elevation at 1 hour after SLT, the patient is not asked to return to clinic for 1 week, or even longer, as determined by the treating physician and the patient’s specific history. In cases where the IOP is acutely elevated immediately following the laser treatment, it is wise to see patients back sooner. Depending on when the patient is first seen after the laser treatment (1 week vs 3 or 4 weeks), the visit evolves from a check for postoperative inflammation or residual IOP spike into an initial measure of longer-term treatment effect.

The ALT protocol is somewhat different given the higher energy delivered and correspondent increased risks of IOP elevation and inflammation in the near term. Following the patient’s first ALT treatment in his or her first eye, it is wise to have the patient return 24 hours after treatment for evaluation. At all post-ALT visits, in addition to routine ophthalmological examination in which applanation tonometry and examination for anterior chamber inflammation are performed, the angle should be carefully examined with gonioscopy for signs of inflammatory exudates and beginning PAS formation. Such synechiae are not always benign, and when observed to be forming, steroid dosage should be increased. On the first day after ALT occasionally we have seen true inflammatory precipitates, keratic precipitates, on the TM.40 At these sites subsequent PAS to the TM have evolved, similar to organization of keratic precipitates in the angle in certain inflammatory glaucomas (see Chapter 44). (These must be differentiated, of course, from mere blanched areas, which are often still visible 24 hours after ALT, but soon disappear.) After all, one is actually “burning” the TM and initiating an inflammatory reaction, which may be part of the mechanism for its efficacy. It is important to detect these inflammatory changes in the meshwork at an early stage in order to increase steroid therapy and attempt to prevent PAS, which, when extending up to the meshwork in a substantial portion of the circumference, can result in impairment of outflow and in poorer glaucoma control.

Depending on the findings at the first post-ALT visit, the interval for the next visit is determined. For example, if IOP is elevated or if there are inflammatory signs in the angle, the patient should be seen in a few days. Otherwise, if this is the first laser treatment, the patient is seen within a week, at which time the above examination is repeated, including gonioscopy. SLT patients are routinely seen for their first post-laser visit at 1 week or later. Often there is some IOP elevation at 1 week, which might relate to the laser “healing process” or just slow resolution of an initial IOP spike. If nothing adverse is detected on early post-laser visits, the patient is usually next seen at 4 to 6 weeks postoperatively, at which time the efficacy of LTP (SLT or ALT) for the first time is truly assessed.

At this 4- to 6-week post-LTP visit, some IOP lowering is to be expected, usually more than half the ultimate total effect on “outflow pressure” (ie, IOP minus episcleral venous pressure). It is to be expected that the lower the starting IOP before the treatment, the less the absolute reduction in mm Hg for an equivalent effect. For example, with an episcleral venous pressure of 10 mm, an IOP reduction from 20 to 15 mm is equivalent to that from 30 to 20 mm. Both represent a 50% decrease in outflow pressure. If no IOP reduction is manifest, it is unlikely that completing the remaining half of the angle, when applicable, will result in substantial IOP reduction, but there are notable exceptions to this. Further, if a complete 360-degree treatment has not produced any effect by this time, repeat treatment with either LTP modality is likely to fail, as discussed above.

Unless there has been actual worsening of IOP from the first half of ALT, or extensive PAS formation, the second half of the angle is treated. For the second half treatment, one is tempted in nonresponsive cases to alter the laser parameters somewhat, for example, in a pale meshwork to increase the power to higher levels to try to achieve bubble formation, or if fewer than 50 applications were made for the first half, to apply a full 50 applications to the second half. It is not certain that these maneuvers result in any improvement in efficacy, however. If the first half of LTP results in an adequate IOP decrease, it is justifiable not to perform the second half of treatment until a time when IOP drifts higher. In some patients this has not yet happened and only one-half of the angle has been treated. Especially if some PAS formation has occurred, even if not to meshwork but only to scleral spur, one should pause in patients with good responses before completing LTP therapy. Gonioscopy should, thus, always be performed at this 1-month post-ALT visit.

It is curious that there is little correlation between the acute effects of LTP on IOP and the long-term efficacy. Patients with acute IOP elevation seem, at first glance, just as likely to show a long-term IOP improvement as those without such acute IOP elevation. This question has not been extensively studied, but it will be interesting to see whether some correlation between short-term and long-term IOP effects does, however, emerge.

It is important to state that when some IOP decrease occurs with full LTP treatment, it is still necessary for the ophthalmologist to judge whether there has been sufficient lowering of IOP to obviate the need for further surgery. There seems to be a tendency, which to us is not at all justified, not to proceed with filtration surgery in patients with advanced glaucoma who have had LTP with minimal IOP lowering. For example, in a patient with a central island of vision due to glaucoma and an IOP of 30 mm Hg, if LTP lowers the IOP to only 25 mm Hg 1 month after full treatment, filtration surgery should typically be performed. A pressure of 25 mm Hg is clearly too high for such a patient (an IOP in the teens or even low teens is required), and if such an IOP were encountered on maximum medical therapy, filtration surgery would have been proposed.

INDICATIONS

Treatment paradigms for all forms of open-angle glaucoma have changed over the past decade. There are a variety of reasons for this, but the notable success and minimal risk of SLT have catapulted this procedure to an earlier place in open-angle glaucoma management. Even if one considers the general class of LTP options, including SLT, micropulse diode LTP, and ALT, there is little reason why any patient would proceed to filtration surgery without first, at some point, undergoing LTP. There are of course exceptions, commonly based on disease severity and type at presentation. For example, a recent retrospective study showed that in a group of patients who underwent SLT after maximally tolerated medical therapy had been instituted, only 14.2% of patients demonstrated a 20% IOP reduction from treated baseline after 1 year.⁴¹ This corroborates what is commonly understood in clinical practice, that trabecular function cannot be restored in some patients, and likely those with more severe or longer duration disease present a higher failure risk.

There has been unsettled debate regarding the effect lens status may have on LTP success. In aphakic and many pseudophakic eyes, the response to LTP, although often significant, seems less than in the phakic eye. (The reason for this is obscure; there does not seem to be any “mechanical” basis for this.) Pseudophakic eyes may behave no differently with respect to LTP response than do phakic eyes, although this point has been controversial.42 Recent studies do not adequately address this question, and there remains a lack of concrete data regarding relative effectiveness of either ALT or SLT in phakic, aphakic, or pseudophakic patients. Dreyer and Gorla⁴² studied the effectiveness of ALT in pseudophakic eyes in a retrospective fashion using historical controls. A subgroup analysis⁴³ based on the data set by Damji and colleagues¹⁵ has suggested that there is no difference between phakic and pseudophakic eyes in response to ALT or SLT. While these data suggest no difference in the effectiveness of LTP in pseudophakic and phakic eyes with primary open-angle glaucoma, this problem deserves further investigation in a prospective study.

In patients with chronic angle-closure glaucoma with synechiae in the angle, we will generally treat the open portions of the angle (if 3 or more hours are open) with LTP prior to embarking on filtration surgery. This is most often unsuccessful, but there are occasional gratifying exceptions.

All these discussions have concerned patients on maximum-tolerated medical therapy who are facing filtration surgery. What is the role of LTP in replacing medical therapy for glaucoma? The Glaucoma Laser Trial, a study of 271 patients with primary open-angle glaucoma, attempted to answer these questions for ALT.^44,45 In this experiment, one eye received 360-degree ALT divided into 2 sessions separated by 1 month, while the other eye was treated with timolol. There was a series of medical steps that the medication-first eye could undergo for IOP control, from timolol to dipivefrin, to low-dose pilocarpine, to high-dose pilocarpine, to timolol and high-dose pilocarpine, to dipivefrin and high-dose pilocarpine, then to the ophthalmologist’s discretion. The laser-first eye initially received no medications but was medically treated if necessary for IOP control.

Of the eyes treated with ALT, 44% were controlled on no medications, 70% were controlled with the addition of timolol, and 89% were controlled with ALT and medications but no surgery. The medication-first eyes showed 30% controlled on timolol alone, 51% controlled on a single medication, and 66% controlled on multiple medications. These short-term, 2-year results indicate that ALT is efficacious in lowering IOP, but that it is not a panacea.^44,45 Indeed, more than half the eyes treated with ALT first later required some sort of medical or surgical therapy. A further follow-up of 3.5 years examining visual field progression found little difference between the laser-first and medicine-first groups, although the laser-first eyes performed slightly better on some measures of visual field function.⁴⁶

ALT is a destructive procedure,⁴⁷ eliminating a portion of an already marginally functional TM. It may make future filtering surgery less likely to succeed, and may predispose to bleb encapsulation following trabeculectomy.³⁴ In addition, if synechiae are formed within the angle as a result of ALT, it may become difficult to perform a Schlemm’s canal–based surgery in the future. For these reasons, we would place ALT in our armamentarium after conventional antihypertensive eye drops but before systemic carbonic anhydrase inhibitors and trabeculectomy.

The situation is different for our current LTP modality of choice, the SLT. We are much more liberal in its application because of its reduced risk of side effects and lack of observable intrinsic angle damage on histopathologic study and clinical follow-up exams. In most patients, we think it is reasonable to offer SLT at diagnosis of any open-angle glaucoma, especially if there is a high risk of medication nonadherence or if failure to follow up is expected over the long term. As previously noted, SLT has been shown to reduce the need for medication use while maintaining a predefined goal IOP,³¹ suggesting that SLT could be used at any point during the treatment of open-angle glaucoma prior to incisional surgery.

It must be remembered that approximately 3% of patients with open-angle glaucoma actually get worse following ALT,6 although this rate has not been reported for SLT. This, of course, does not specifically refer to or include patients with acute IOP elevation shortly after the procedure. The mechanism for this sustained worsening is not known, but it possibly may relate to the destruction of meshwork tissue with the laser, with possible scar formation.⁴⁷

Acute Pressure Elevation

Permanent visual loss is possible with extreme IOP elevation, even if relatively short lived. The IOP shortly after LTP needs to be monitored and treated.

Uveitis

Occasionally, uveitis is severe and must be vigorously treated to prevent synechiae formation. Occasional cases of corneal edema may present along with uveitis.

Peripheral Anterior Synechiae

PAS relate more strongly to ALT and are rare if present at all following SLT. They are often described as being benign and extending only up to scleral spur. However, cases of extensive functional angle closure due to PAS have been documented following ALT. The appearance of the angle post-ALT needs to be carefully monitored and steroid dosage appropriately adjusted.

Corneal Burns

Occasionally, corneal epithelial opacities are encountered, but these heal within a few days without sequelae. Corneal endothelial burns are rare.

Hyphema

Hyphema is an uncommon complication, but it has been observed. As with most potential complications, this risk is much less of a concern with SLT than ALT. There seem to be 2 causes of hyphema after ALT. Occasionally, there are meandering normal vessels in the angle that are inadvertently disrupted by the laser energy. Treatment immediately consists of pressing the goniolens against the cornea to raise IOP and thus achieve hemostasis, or if the vessel is visible through the goniolens, to treat it directly with further ALT laser applications in order to coagulate it as in goniophotocoagulation. (This approach would be unlikely to succeed using the SLT laser, which does not have coagulative ability.) The other cause of hyphema is encountered when there is blood in Schlemm’s canal and the laser energy penetrates deeply into the meshwork to the canal. Most patients with open-angle glaucoma do not have blood in Schlemm’s canal (it is important to contemplate why this is so), but occasionally it is found, especially in patients with abnormally elevated episcleral venous pressure (eg, in association with a dural shunt). Treatment consists of raising IOP with goniolens pressure and/or tilting the lens to relieve compression at the limbus in the involved portion of the angle. It is this initial compression of limbal episcleral vessels with the goniolens that promotes blood reflux into the canal. One should avoid the temptation to increase the laser power further and treat the bleeding portion of the angle further (unless a blood vessel can be identified), for this simply may allow further penetration into the blood-filled canal. If hemostasis is obtained by these means, one can proceed to complete the LTP, but lower power settings should be chosen for the remaining portions of the angle. Usually only a small amount of bleeding is encountered in such cases, but rarely significant hyphema has been observed with associated elevation of IOP.

The Unknown

In any procedure whose optimal parameters and long-term biological effects are not clearly known, we must all be diligent in looking for and documenting unexpected long-term consequences of our treatment.

A discussion of the mechanisms of action of LTP and the treatment of acute IOP elevation are covered in the following chapters.

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