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

Figure 35.1 Schematic of laser system. Laser material is placed in a tube between two mirrors. When an energy source is pumped into the tube, atoms in the laser material (1) are excited to a higher energy level

(2). In the excited state, atoms have an enhanced probability of being stimulated by photons to decay back to the lower energy level (3) by emitting photons (4). The emitted photons bounce between the mirrors, stimulating other excited atoms, until sufficient light amplification is achieved, at which time the light is allowed to leave the cavity as a laser beam.

Collimation (Directionality)

Because light amplification occurs only for photons that are aligned with the mirrors, a nearly parallel beam, in which all the waves travel in the same direction, is produced, as opposed to the diverging beam of an incandescent lamp. Although limited divergence occurs with all laser beams, it is minimal enough that a small focal spot can be created when the light is delivered through an optical system. Monochromacy

Because the photons are emitted through the release of energy between two defined levels of the atom, the resulting light has only one discrete wavelength. In contrast, ordinary white light is a combination of many different wavelengths.

High Intensity

The light amplification of a laser can produce a beam with significantly more intensity than that of the sun.

LASER-INDUCED TISSUE INTERACTIONS

The tissue effects produced by laser surgery are of three types: thermal, ionizing, and photochemical (7). Thermal Effects

In this situation, the absorption of laser energy by the target tissue produces temperatures high enough to induce chemical changes that can cause local inflammation and scarring (photocoagulation) or to vaporize intracellular and extracellular fluids, creating an incision in the tissue (photovaporization). Factors influencing the laser thermal effect include (a) wavelength of the incident light, (b) duration of exposure, and (c) amount of light energy per area of exposure. Melanin, the pigment of most target tissues in glaucoma laser surgery, has a peak absorption in the blue-green portion of the visible spectrum. Therefore, lasers with wavelengths between 400 and 600 nm are most useful for these procedures, and the argon laser is the prototype photocoagulator.

The heat generated by the absorption of laser energy is dissipated by the surrounding tissue. A short exposure time and a high-energy level and area reduce heat conduction, which causes tissue temperatures to reach the critical boiling point, producing gas bubbles with tissue disruption and photovaporization through a microexplosion. This reaction can be used to create holes in ocular tissues, as with laser iridotomy. At lower energy levels, photocoagulation may produce contraction of collagen, which is the mechanism of pupilloplasty and iridoplasty, and possibly of laser trabeculoplasty. Ionizing Effects

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If intense laser energy is focused into a very small area for a very short period of time, a reaction occurs that is independent of pigment absorption and is referred to as photodisruption. An instantaneous electric field is generated, which strips electrons from target atoms, producing a gaseous state called plasma (6). As ionized atoms of plasma recombine with free electrons, photons with a wide range of energies are emitted, producing a spark of incoherent white light. Associated shock and pressure waves create additional mechanical damage to target tissues, resulting in a reaction that can disrupt both pigmented and nonpigmented structures. Thermal effects are also involved in the mechanism of photodisruption

(8).

The Nd:YAG (neodymium:yttrium-aluminum-garnet) laser is the most commonly used photodisruptor. The pulse

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may be Q-switched or mode-locked, both of which have been shown to produce the same size of rupture in polyethylene membranes (8). The main clinical use has been for the disruption, or cutting, of relatively transparent anterior segment structures, most notably the posterior lens capsule. For glaucoma surgery, the primary application of the Q-switched Nd:YAG laser is the creation of iridotomies. Nd:YAG lasers can also be used in a pulsed thermal or continuous-wave mode for transscleral cyclophotocoagulation.

Photochemical Effects

The target tissue in this laser-induced effect is volatilized (vaporized) by short-pulsed ultraviolet radiation (photoablation). Some tissues, such as tumors, can be photosensitized with hematoporphyrin or other photosensitizing agent and selectively destroyed with laser energy of a specific wavelength (photodynamic therapy or photoradiation).

LASER DELIVERY SYSTEMS

Most laser units use a slitlamp biomicroscope, in which a system of fiber optics or mirrors in an articulated arm direct the laser beam from the laser tube, through the slitlamp, and into the patient's eye. Various types of contact lenses are normally used during laser surgery with slitlamp delivery. Some contain mirrors to direct the laser beam into the anterior chamber angle, and others incorporate convex lenses to concentrate the light energy on the iris. Other laser delivery systems use contact probes attached to the fiber optics, which allows application of laser energy to the ocular tissues by external placement of the probe on the eye or by aiming directly at internal ocular structures with the probe tip in the eye. By using a fiber-optic camera and fiber-optic delivery, it is also possible to deliver laser energy (diode) endoscopically.

For lasers in the visual spectrum, an aiming beam of attenuated laser energy can be used to allow positioning and focusing of the laser beam on the target tissue. For lasers with wavelengths outside the visual spectrum, an additional laser such as a helium-neon, or semiconductor diode, with wavelengths of 633 and 640 nm, respectively, is used as the aiming beam. A foot pedal or finger trigger is used to release the full laser energy, producing the tissue alteration. The variables on the control units of most laser systems include spot size (usually expressed in microns), exposure duration (expressed in tenths of seconds, milliseconds, microseconds, or nanoseconds), and energy (joules or millijoules) or power (watts or milliwatts). Energy in joules equals power in watts times duration in seconds.

SPECIFIC LASERS FOR GLAUCOMA SURGERY

Lasers differ primarily according to the medium in which the atoms exist that produce the stimulated emission of photons. The lasers used most commonly for glaucoma surgery are argon, Nd:YAG, and semiconductor diode, although experience with many other lasers has also been reported.

Argon Lasers

The medium in these instruments is argon gas, which is pumped by an electrical discharge. The wavelengths are in the blue (488-nm) and green (514-nm) portions of the visible spectrum, which are optimum for absorption by melanin. Most argon lasers operate in the continuous-wave mode and have maximum power levels of 2 to 6 watts. Units are also available, however, that produce pulses of approximately 100 microseconds with powers of 20 to 50 watts. The latter instruments achieve full

power only as needed, which reduces heat buildup and improves energy efficiency. Nd:YAG Lasers

In these instruments, the neodymium atoms are embedded in a crystal of yttrium-aluminum-garnet (YAG) and are pumped by a xenon flash lamp. The laser wavelength is in the near-infrared (1064-nm) range, although it can be made a visiblelight emitter by frequency doubling or an ultraviolet emitter by frequency tripling (7). Nd:YAG lasers can be operated in the continuous-wave mode to provide a photocoagulation effect but are more commonly used with pulsed delivery, by using Qswitching or mode-locking, to allow photodisruption. The selective laser uses spectrum of the wavelength that is selectively absorbed by a pigment in the tissue. The laser destroys melanin in the tissue (e.g., trabecular meshwork), while minimizing thermal injury to surrounding structures. The mechanism is based on the principle of selective photothermolysis (9), developed in the Wellman Laboratory by Parrish and Anderson in the early 1980s. This principle is used in selective laser trabeculoplasty (10). Semiconductor Diode Lasers

Two light-emitting diodes are used in this system to produce a wavelength in the near-infrared spectrum (800 to 820 nm). Solid-state construction allows compact size, durability, and low maintenance. The wavelength, between that of the argon and Nd:YAG lasers, provides better scleral penetration than the argon and better absorption by melanin than the Nd:YAG lasers, making it useful for transscleral cyclophotocoagulation (11). Diode lasers can also be operated in the red range of the visible spectrum (640 nm), in which case they are used as an aiming beam.

Other Lasers

Other lasers are being developed and evaluated for ocular surgery. Among these are the dye lasers, which use a solution of complex organic dyes, such as rhodamine, and can produce monochromatic wavelengths at relatively high-output powers through a large range of the visible spectrum. This allows the selection of a wavelength that would be most highly absorbed by the target tissue, thereby minimizing the transmittal of laser energy through the ocular media (12). Carbon dioxide lasers in the infrared spectrum (10,600 nm) have been used in the continuous-wave mode to cut tissue by vaporization with very

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little coagulation necrosis, whereas excimer lasers in the ultraviolet range (193 to 248 nm) are being evaluated in the pulsed mode to cut tissue with no visible necrosis (13). The ruby laser in the visible spectrum (694 nm) can produce photoablation with high-energy pulses, and the krypton laser in the yellow-red wavelength can be used for photocoagulation. The heliumneon laser in the red wavelength is, as previously noted, used as an aiming beam in many laser systems that operate at nonvisible wavelengths (7).

LASER SAFETY

Although the properties of laser energy make lasers ideal tools for the surgical manipulation of tissues, they also pose serious hazards, including electric shock, direct laser burns, explosions, and fires. Probably, the most common and serious health hazard, however, is accidental exposure of the retina, either directly or from reflected laser light.

The following classification of lasers is generally accepted regarding hazards (14). Class I: Do not emit hazardous levels. Class II: Visible-light lasers that are safe for momentary viewing but should not be stared into continuously; an example is the aiming beam of ophthalmic lasers, or laser pointers. Class III: Unsafe for even momentary viewing, requiring procedural controls and safety equipment. Class IV: Also pose a significant fire and skin hazard; most therapeutic laser beams used in ocular surgery are in this class.

During glaucoma laser surgery, the patient has the greatest risk of injury from accidental exposure of the retina or lens. The risk to the corneal endothelium has been evaluated with specular microscopy 1 year after laser trabeculoplasty or iridotomy; some investigators have found a significant increase in cell size and endothelial cell loss (15, 16), but others have found no significant changes (17, 18).

The surgeon is theoretically protected during each exposure of therapeutic laser energy in most slitlamp

delivery systems by a built-in filter. There is some evidence, however, that subtle, but definite, alterations in color vision can be seen in ophthalmic laser surgeons who are exposed chronically to argon blue light (19, 20). Because there is no apparent clinical advantage to blue-green wavelengths in ophthalmic surgery, it is advisable to use green-only whenever possible.

Aside from the patient and the surgeon, the individuals at greatest risk for retinal burns are other personnel in the laser room during the treatment whose eyes may be exposed to reflected laser light. One study with argon lasers and various contact lenses indicated that a hazard can exist for a bystander at the side of the slitlamp who is exposed to unattenuated back reflections of the treatment beam within 1 m of the contact lens (21). To minimize this hazard, only antireflective-coated contact lenses should be used, ancillary personnel should wear protective goggles or look away from the laser when it is in use, and access to the laser room should be limited to necessary individuals during the treatment.

KEY POINTS

Lasers operate on the principle that excited atoms can be stimulated to emit photons, resulting in a markedly amplified light that possesses the unique properties of coherence, collimation, monochromacy, and high intensity.

The nature of this light allows precise alteration of tissues by thermal effects (photocoagulation and photovaporization), ionizing effects (photo disruption), and photochemical effects (photoablation and photodynamic therapy or photoradiation).

The tissue interactions, especially the photocoagulation and photodisruption, are used in a wide variety of glaucoma surgical procedures.

REFERENCES

1.Meyer-Schwickerath G. Light Coagulation. [Translated by Drance SM]. St. Louis, MO: CV Mosby; 1960.

2.Maiman TH. Stimulated optical radiation in ruby. Nature. 1960;187: 493-494.

3.Peyman GA, Raichand M, Zeimer RC. Ocular effects of various laser wavelengths [review]. Surv Ophthalmol. 1984;28:391-404.

4.Belcher CD III. Photocoagulation for Glaucoma and Anterior Segment Disease. Baltimore, MD: Williams & Wilkins; 1984.

5.Schwartz L, Spaeth G. Laser Therapy of the Anterior Segment: A Practical Approach. Thorofare, NJ: Slack; 1984.

6.Mainster MA, Sliney DH, Belcher CD III, et al. Laser photodisruptors: damage mechanisms, instrument design and safety. Ophthalmology. 1983;90:973-991.

7.Lasers in medicine and surgery [review]. Council on Scientific Affairs. JAMA. 1986;256:900-907.

8.Vogel A, Hentschel W, Holzfuss J, et al. Cavitation bubble dynamics and acoustic transient generation in ocular surgery with pulsed neodymium: YAG lasers. Ophthalmology. 1986;93:1259-1269.

9.Anderson RR, Parrish JA. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science. 1983;220:524-527.

10.Latina MA, Park C. Selective targeting of trabecular meshwork cells: in vitro studies of pulsed and CW laser interactions. Exp Eye Res. 1995;60:359-371.

11.Schuman JS, Jacobson JJ, Puliafito CA, et al. Experimental use of semiconductor diode laser in contact transscleral cyclophotocoagulation in rabbits. Arch Ophthalmol. 1990;108:1152-1157.

12.L'Esperance FA Jr. Clinical photocoagulation with the organic dye laser: a preliminary

communication. Arch Ophthalmol. 1985;103:1312-1316.

13. Gibson KF, Kernohan WG. Lasers in medicine—a re view. J Med Eng Technol. 1993;17:51-57.

14.Sliney DH, Wolbarsht ML. Safety with Lasers and Other Optical Sources: A Comprehensive Handbook. New York, NY: Plenum Press; 1980.

15.Hong C, Kitazawa Y, Tanishima T. Influence of argon laser treatment of glaucoma on corneal endothelium. Jpn J Ophthalmol. 1983;27:567-574.

16.Wu SC, Jeng S, Huang SC, et al. Corneal endothelial damage after neodymium:YAG laser

iridotomy. Ophthalmic Surg Lasers. 2000;31: 411-416.

17.Thoming C, Van Buskirk EM, Samples JR. The corneal endothelium after laser therapy for glaucoma. Am J Ophthalmol. 1987;103:518-522.

18.Schwenn O, Sell F, Pfeiffer N, et al. Prophylactic Nd:YAG-laser iridotomy versus surgical iridectomy: a randomized, prospective study. Ger J Ophthalmol. 1995;4:374-379.

19.Arden GB, Berninger T, Hogg CR, et al. A survey of color discrimination in German ophthalmologists: changes associated with the use of lasers and operating microscopes. Ophthalmology. 1991;98:567-575.

20.Berninger TA, Canning CR, Gunduz K, et al. Using argon laser blue light reduces ophthalmologists' color contrast sensitivity: argon blue and surgeons' vision. Arch Ophthalmol. 1989;107:1453-1458.

21.Sliney DH, Mainster MA. Potential laser hazards to the clinician during photocoagulation. Am J Ophthalmol. 1987;103:758-760.

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Shields > SECTION III - Management of Glaucoma >

36 - Surgery of the Anterior Chamber Angle and Iris

Authors: Allingham, R. Rand

Title: Shields Textbook of Glaucoma, 6th Edition Copyright ©2011 Lippincott Williams & Wilkins

> Table of Contents > SECTION III - Management of Glaucoma > 36 - Surgery of the Anterior Chamber Angle and Iris

36

Surgery of the Anterior Chamber Angle and Iris

In this chapter, we consider the laser and incisional operations that are designed to reduce the intraocular pressure (IOP) through increased aqueous outflow by treating specific structures of the anterior chamber angle and the iris. (Filtration procedures and glaucoma drainage-device surgery, which involve not only the anterior chamber angle but also limbal and external ocular tissues, are considered in Chapters 38 and 39, respectively; procedures for children are discussed in Chapter 40.)

LASER TRABECULOPLASTY Historical Background

In 1961, Zweng and Flocks (1) introduced the concept of applying light energy to the anterior chamber angle for the treatment of glaucoma. Using the xenon-arc photocoagulator of Meyer-Schwickerath (discussed later in this chapter), they selectively coagulated the filtration angles of cats, dogs, and monkeys and reported subsequent lowering of the IOP. Histopathologic examination of the treated tissue revealed fragmentation of the trabecular lamellae, atrophy of ciliary muscle, and destruction of ciliary processes. Little more was said about this technique, however, until more than a decade later, when several investigators revived the concept by using the light energy of the laser. Yet another decade of investigative work would elapse before the operation would achieve widespread clinical popularity.

In the early 1970s, reports began to appear from several parts of the world, most notably from Krasnov

(2) in Russia, Hager (3) in Germany, Demailly and associates (4) in France, and Worthen and Wickham

(5) in the United States, regarding attempts to improve aqueous outflow by creating holes in the trabecular meshwork with laser energy. Although trabecular perforations were achieved, they eventually closed in most cases due to fibrosis, and IOP reduction was usually temporary. The value of laser treatment to the trabecular meshwork came under further question when, in 1975, Gaasterland and Kupfer (6) reported that experimental glaucoma could be produced by applying argon laser energy to the meshwork of rhesus monkeys. The following year, however, Ticho and Zauberman (7) noted that longterm reduction in IOP occurred in some patients despite the lack of permanent trabecular openings. This led to a new concept in laser trabecular therapy in which lower energy levels were used to

photocoagulate, rather than to penetrate, portions of the meshwork. In 1979, Wise and Witter (8) described the first successful protocol of what has become known as laser trabeculoplasty. Their preliminary work was corroborated in 1981 (9, 10 and 11).

In the subsequent years, many different energy sources producing different wavelengths of laser light, such as krypton (red [647.1 nm] or yellow [568.2 nm] wavelengths), Nd:YAG (neodymium:yttrium- aluminum-garnet) (continuous-wave [1064 nm] and frequency-double Q-switched [532 nm]), and diode (840 nm), have been studied for laser trabeculoplasty (12, 13, 14, 15, 16, 17, 18, 19 and 20). At the time of publication, the only other laser that has attained popularity is the frequency-doubled Nd:YAG laser, otherwise known as selective laser trabeculoplasty (SLT).

Theories of Mechanism Argon Laser Trabeculoplasty

Tonographic studies indicate that argon laser trabeculoplasty (ALT) reduces IOP by improving the facility of outflow (12, 21, 22, 23 and 24), while showing no significant influence on aqueous production on fluorophotometric investigations (23, 25, 26). Although fluorescein leakage into the anterior chamber is seen during the first week after trabeculoplasty, suggesting a breakdown in the blood-aqueous barrier, it is gone within 1 month and does not seem to be a factor in the long-term effect of this procedure (27).

The mechanism of improved aqueous outflow facility by ALT is uncertain. Wise and Witter (8) originally postulated that the thermal energy produced by pigment absorption of laser light caused shrinkage of collagen in the trabecular lamellae. They believed that the subsequent shortening of the treated meshwork might enlarge existing spaces between two treatment sites or expand the Schlemm canal by pulling the meshwork centrally. Laboratory studies have provided partial support for this theory but have also suggested alternative or additional mechanisms of action.

Light and electron microscopic and immunohistochemical evaluations of trabecular meshwork from normal and glaucomatous human eyes, obtained hours to weeks after ALT, revealed disruption of trabecular beams, fibrinous material, and necrosis of occasional cells, followed by shrinkage of the collagenous components of the meshwork and accumulation of fibronectin in the aqueous drainage channels (28, 29, 30, 31, 32 and 33). Surviving endothelial cells near the laser lesions showed phagocytic and migratory activity (29, 30). Specimens obtained several months after therapy had partial or total occlusion of intertrabecular spaces by a monocellular layer (28, 30, 31). These observations were thought to support the theories of heat-induced shrinkage of collagen in the trabecular lamellae with possible

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stretching of the meshwork between two treatment sites and fibronectin-mediated attachment of trabecular beams supporting an adhesive tightening of the trabecular components (31).

Studies with monkeys have provided similar observations to those noted in humans, with some additional insight into the mechanism of ALT. Within the first few hours, there is disruption of the trabecular beams and coagulative necrosis with accumulation of debris in the juxtacanalicular region (34). As with human eyes, surviving trabecular endothelial cells are noted to have increased phagocytic activity with removal of tissue debris and increased cell division (34, 35). By 1 month, the treated regions are flat with collapsed beams and are covered with an endothelial layer (36). The latter is more likely to occur when the laser energy is applied to the anterior portion of the trabecular meshwork (37). Perfusion with ferritin shows lack of flow through the treated meshwork, with diversion of flow through the adjacent nonlasered meshwork, which becomes structurally altered to compensate for the overload of flow (38). It has also been suggested that concomitant collagen degeneration and loss of trabecular cells may widen the intertrabecular spaces with improved outflow (39). However, light and electron microscopic studies of the trabecular meshwork and the inner wall of the Schlemm canal 3 to 17 months after 360-degree trabeculoplasty in monkeys revealed no significant difference from untreated eyes (40). Whether the human eye has similar reparative capacity is unclear, but this and other studies suggest that alternative or additional mechanisms to the mechanical theory must account for the long-term benefit of

laser trabeculoplasty.

Studies of human autopsy eyes treated with ALT revealed a significant reduction in the trabecular cell density and an increase in radioactive sulfate incorporation into the extracellular matrix of laser-treated eyes (41). The latter findings have also been reported with human trabecular tissue treated with ALT before trabeculectomy and subsequently studied with radioactive leucine (31), and in cat eyes that were studied in vivo with radioactive thymidine following trabeculoplasty (42, 43). Studies with a human corneoscleral organ culture system indicate that ALT causes an early trabecular endothelial cell division in the anterior meshwork, with migration of the new cells to repopulate the burn sites over the next few weeks (44, 45).

It has been postulated that ALT eliminates some trabecular cells, which may stimulate the remaining cells to produce a different composition of extracellular matrix with improved outflow properties (41, 42 and 43). This hypothesis is further supported by demonstration of induction of matrix metalloproteinases in response to laser trabeculoplasty (46, 47 and 48). The matrix metalloproteinases are the enzymes that normally break down the extracellular matrix to maintain normal turnover of the trabecular meshwork (49). Manipulation of activity of these enzymes has been demonstrated in perfused human anterior segment organ culture to increase outflow facility with increasing matrix metalloproteinases (50). Evaluation of two members of the matrix metalloproteinases family, stromelysin and gelatinase B, after ALT of anterior segment organ cultures also supports the hypothesis that extracellular matrix turnover is important in the regulation of aqueous humor outflow. An increase of stromelysin expression has been demonstrated in the juxtacanalicular region of the meshwork in response to laser trabeculoplasty (47). This would be expected to degrade trabecular proteoglycans, a presumed source of outflow resistance in the juxtacanalicular meshwork. If reduced juxtacanalicular extracellular matrix turnover is responsible for the reduction in aqueous humor outflow, an increase in stromelysin in this specific area of the meshwork should increase the outflow (47).

Additional studies have been designed to identify factors that mediate the matrix metalloproteinases response to ALT. Matrix metalloproteinases expression was increased by adding recombinant interleukin-1a in human anterior segment organ cultures and tumor necrosis factor-a in porcine trabecular meshwork (50, 51). Expression of stromelysin was partially blocked by either interleukin-1 receptor antagonist or tumor necrosis factor-a-blocking antibodies (48).

Although the precise mechanism of ALT still remains only partially understood, an initial mechanical injury appears to trigger activation of unique signaling pathways resulting in cellular response and tissue remodeling, leading to an improved outflow (52).

Selective Laser Trabeculoplasty

In 1995, Latina and Park reported that the energy of a Qswitched, frequency-doubled Nd:YAG laser would preferentially be absorbed by pigmented trabecular meshwork cells, in culture (53), called an SLT (54, 55). The laser selectively targets pigmented trabecular meshwork cells without causing structural damage to nonpigmented cells. Experimental study on the trabecular meshwork from human autopsy eyes after SLT revealed no coagulative damage or disruption of the corneoscleral or uveal trabecular beams (32). The only evidence of laser tissue interaction with SLT was cracking of intracytoplasmic pigment granules and disruption of trabecular endothelial cells, suggesting that it may potentially be a repeatable procedure (32). Evaluation of the trabecular meshwork after ALT revealed crater formation in the uveal meshwork at the junction of the pigmented and nonpigmented trabecular meshwork, with coagulative damage at the base and along the edge of craters, disruption of the collagen beams, fibrinous exudate, lysis of endothelial cells, and nuclear and cytoplasmic debris (32). However, in another study, the mechanical damage observed after low-power ALT and SLT was similar, with both lasers producing disruption of trabecular beams, cellular debris, and fragmentation of endothelium (33). The similarity of changes in the trabecular meshwork produced by both lasers may explain their similar IOP-lowering responses (33). The impact of 360-degree SLT on free oxygen radicals and antioxidant enzymes of the aqueous humor has been evaluated in rabbits. Concentrations of lipid peroxide in the aqueous humor of the treated eyes were significantly higher than those in the untreated eyes until the 7th day (56). Glutathione Stransferase levels were significantly decreased between 12 hours and 7 days after the

trabeculoplasty, suggesting that free oxygen radicals are formed in the pigmented trabecular meshwork during SLT and may be responsible for the inflammatory complications of this procedure (56).

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Basic Techniques Instruments

The original laser unit for trabeculoplasty is the continuouswave argon laser. It has traditionally been operated in the bluegreen, biochromatic wavelength spectrum (454.5 to 528.7 nm). No differences were noted in the postoperative IOP course or incidence of complications when compared with the use of green, monochromatic laser light (514.5 nm) (57). As noted in the previous chapter, however, greenonly argon light may be safer for the surgeon with regard to an influence on color vision. The Q- switched Nd:YAG laser has only one wavelength setting, at 532 nm.

A contact lens with a mirror for visualization of the anterior chamber angle (gonioprism) is used in trabeculoplasty. As with all contact lenses for laser application, it should have an antireflection coating on the front surface. A standard Goldmanntype three-mirror lens, in which one mirror is inclined at 59 degrees for gonioscopy, or a single-mirror gonioscopy lens can be used (Fig. 36.1). Both, however, have the slight disadvantage of requiring rotation of the lens to view all quadrants of the anterior chamber angle. This disadvantage can be eliminated by using the Thorpe four-mirror gonioscopy lens, in which all mirrors are inclined at 62 degrees, or the Ritch trabeculoplasty laser lens, in which two mirrors are inclined at 59 degrees for viewing the inferior quadrants and two at 64 degrees for viewing the superior angle (58, 59). In the latter lens, a 17-diopter (D) planoconvex button lens over two mirrors provides 1.4× magnification, reducing a 50-µm laser spot to 35 µm, which may be particularly useful, because a 50-µm spot size with most argon lasers produces a b urn in excess of 70 µm (60). A double-mirror gonioscopic lens has also been developed to facilitate the visualization of the anterior chamber angle (61). The Latina lens was specifically designed for SLT and has a single mirror at a 63-degree angle; it has a 1.0× magnification to maintain the 400-µm spo t size.

Figure 36.1 The Goldmann-type three-mirror lens, modified with antireflection coating, is a commonly used gonioprism for visualizing the anterior chamber angle and for use in performing laser trabeculoplasty.

Gonioscopic Considerations

Successful laser trabeculoplasty requires accurate identification and treatment of the trabecular meshwork. The surgeon must, therefore, have a detailed knowledge of the anterior chamber angle anatomy and its many variations. The basic aspects of this subject are discussed in Chapters 2 and 34; additional features that are pertinent to laser trabeculoplasty are considered here.

Two variations of the anterior chamber angle that may interfere with accurate laser application to the trabecular meshwork are (a) the degree of pigmentation and (b) the width of the chamber angle. With regard to pigmentation, some angles are so diffusely pigmented from the ciliary body band to the Schwalbe line that the exact location of the meshwork is obscured. This is usually most marked in the inferior quadrants, and a careful inspection of all quadrants before starting treatment usually discloses the functional position of the meshwork in some areas, which can then be used as a guide in locating the meshwork in the remainder of the angle. At the opposite extreme, the trabecular meshwork in some angles is so lightly pigmented that it is hard to see. In some cases, iris processes, which normally extend to the meshwork, may be a useful indicator. Identification of the ciliary body band or Schwalbe line may also help determine the relative position of the meshwork.

A narrow anterior chamber angle can lead to improper placement of the laser burns or may prohibit

performing trabeculoplasty. If the peripheral iris obscures the visualization of the meshwork, a heavily pigmented Schwalbe line may be mistaken for the meshwork. Rotating the contact lens in relation to the eye, by asking the patient to look in the direction of the mirror being used, often provides a deeper view into the angle, enhancing visualization of the meshwork. Care must be taken with this maneuver, however, not to distort the size and shape of the aiming beam. If positioning of the contact lens is not sufficient to expose the meshwork, the chamber angle may be deepened by applying low-energy laser burns to the peripheral iris, a technique called iridoplasty or gonioplasty (discussed later in this chapter). If the angle is still too narrow, a laser iridotomy (also discussed later in this chapter) should be performed, and the trabeculoplasty should be done at a later date.

Original Protocol

The original protocol of Wise and Witter (8) has remained the standard approach to ALT against which variations in technique have been evaluated. A 25× magnification in the slitlamp delivery system usually provides an optimum balance between detail and field of view. Argon laser settings of 0.1-second duration exposure and 50-µm beam diameter have rema ined constant through most variations in protocol. One study compared durations of 0.2 to 0.1 second and found no advantage to the former (62). The most commonly used power levels range between 700 and 1500 mW, with an average of 1000 mW. A survey by the American Society of Cataract and Refractive Surgery in 1999 indicated that most general ophthalmologists use a duration of 0.1 second and a spot size of 50 µm, and that 39% of the respondents use initial power between 501 and

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799 mW and 41% use 800 to 1000 mW (63). One study evaluated powers ranging from 100 to 1000 mW and found that power of more than 500 mW gave the maximum success rates (64). The power should be adjusted to produce a depigmentation spot or a small gas bubble at the treatment site (Fig. 36.2). This response is influenced by the amount of pigment in the trabecular meshwork. With a heavily pigmented meshwork, a lower power level may be sufficient, whereas lightly pigmented meshworks require higher levels. In a retrospective study, the decrease of IOP was greater in the eyes in which ALT was a primary therapy and was not influenced by the power level (65). The initial IOP response to ALT in patients with glaucoma associated with exfoliation syndrome was greater than in patients with chronic open-angle glaucoma (COAG) (66, 67), although the long-term outcome was similar (66). A preoperative IOP higher than 31 mm Hg and visual field defect and light pigmentation of the trabecular meshwork were found to be predictive of ALT failure (67).

Figure 36.2 Placement of laser burns (A) along anterior portion of trabecular meshwork (TM). Desired visual result is depigmentation of the treatment site (B,C) or a small gas bubble (B). SL, Schwalbe line; SS, scleral spur; CBB, ciliary body band; I, iris.

Originally, ALT laser burns were applied onto or immediately posterior to the pigmented band of the