Ординатура / Офтальмология / Английские материалы / Step by Step Minimally Invasive Glaucoma Surgery_Garg, Melamed, Bovet, Pajic, Carassa, Dada_2006

52 Step by Step Minimally Invasive Glaucoma Surgery

not significantly increase the endothelial cells lost.15,17,23 Nevertheless, caution should be exercised not to focus the laser beam onto the Descemet’s membrane. The iris stroma is strongly vascularized by arteries from the major iris circle, and injury to arterioles cause instant bleeding in the anterior chamber. When this complication occurs, a gentle pressure with the contact glass will temporarily increase the IOP and promote hemostasis. Dramatic hyphema have been reported after such laser procedures.29 Argon laser, or Nd:YAG laser set on multi-mode iridotomy, generally prevent such complications, as the thermal effect of the laser burns coagulates the vessels and avoids bleeding when hitting a vascular branch.13 The stroma and iris pigment epithelium dispersion phenomena observed during completion of laser iridotomies increase the amount of particles in the anterior chamber. Inflammation and anterior uveitis are generally present after the procedure. To prevent extension and persistence of this inflammation, topical steroids are given and tapered in a few days. Posterior synechiae may develop as a result of prolonged inflammation.10 As mentioned before, the IOP rises in the next few hours after laser therapy. Close monitoring of the IOP is mandatory and adequate medication given in case of persistent elevated values. Some case of damages to the lens have been reported as the result of excessive laser application to the capsule behind the iris plane.32,33 Care should therefore be exercised when applying laser energy on an patent iridotomy.

LASER TRABECULOPLASTY

When laser burns are applied on the trabecular meshwork, persistent IOP reduction is followed.30,31 The pathophysiology of the laser burns on the trabecular

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meshwork was hypothesized as follows: Thermal effect of the burns induces collagen shrinkage and scarring of the meshwork. This leads to mechanical traction on the adjacent intertrabecular space that becomes open, thus increasing the outflow facility.5,27 Argon blue-green laser (488 nm) was the laser of choice for this procedure. This has been named argon laser trabeculoplasty (ALT). It can be used in several forms of open-angle glaucoma. A Nd:YAG laser trabeculoplasty (YLT) has also been used as an alternative to the ALT. In case of hypopigmented trabeculum meshwork, YLT is a safe and effective alternative technique to perform laser trabeculoplasty, which is especially useful in poorly pigmented angles where ALT is known to be less effective.14

A non-contact laser delivery system coupled with a slitlamp, is used for the ALT procedure. To gain access to the entire angle structure, a contact lens with a gonioscopic mirror (CGI from LASAG, Bern, Switzerland) is placed on the cornea of the patient after topical anesthesia. In some cases, when the angle is too narrow to allow a good visibility of the trabecular meshwork, use of pilocarpine may enlarge the angle to enable the treatment. Laser spot parameters are set commonly with 50 μm spot size, 0.1 sec. of duration and 800 mW of power. The correct effect of the laser burns should be a blanching of the trabecular meshwork. When too much power is applied, a bubble will form and intense blanching and scarring will result. The power should then be reduced just enough to get blanching with minimal bubble creation. The amount of power required to get this result may vary with the degree of pigmentation of the trabecular meshwork.20 It is important to readjust the power setting throughout the laser session to get homogeneous treatment all over the treated quadrants. The laser beam spot must be kept on focus over

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endothelial pathology such as Fuch’s endothelial dystrophy or cornea guttata have greater risks to induce complications of the corneal endothelium.28

Care should be given not to treat too many areas of the trabecular meshwork. An experimental glaucoma model in rhesus monkey has been created by giving a large number of long duration laser burns.8 Postoperative IOP rise peak may also depend on the number of burns.25 Generally, 50 to 70 burns over 180° are enough to create a sustained consistent IOP drop overtime.13,26 It should be kept in mind that the long-term overall outcome of ALT is about 50 percent after 5 years.18,19 Most of the patients will probably require other types of treatment, for instance a filtration surgery. In order not to compromise the result of further filtrating surgery, it is advisable not to treat the upper quadrant, and to leave the trabecular meshwork untouched.

It is not uncommon to encounter some pressure spike after argon ALT. To prevent or avoid the extension of such pressure rise, α-2 agonist may be given before and/or just immediately after the laser treatment.17 Apraclonidine or brimonidine are the therapy of choice.1,2,4 Even with an adequate postoperative therapy, the IOP sometimes remains elevated several weeks after the ALT. It should be emphasized that the ALT produces or enhance inflammation in the anterior chamber and over the trabecular meshwork.15 Inflammatory cells and inflammation products collect into the trabecular meshwork and dramatically reduce the outflow facility, resulting in persistent IOP elevation. This inflammatory reaction is thought to be the result of transient breakdown of the bloodaqueous barrier and is correlated with the type of glaucoma. Pigmentary and pseudoexfoliative glaucoma show the greatest inflammatory reaction after ALT compared with chronic open-angle glaucoma.

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Beside the inflammatory reaction, the ALT can also induce other reactions or changes in the anterior chamber. Laser burns placed too posteriorly result in creation of peripheral anterior synechiae.19 This complication does not happen when burns are placed in the anterior trabecular meshwork. It is still unclear whether peripheral anterior synechiae play a role in the long-term outcome of ALT or not.12

The long-term results of the ALT is relatively modest. A mean success rate of 45 percent 5 years after ALT has been reported in several studies.3,6,18,19 Despite this rate, the ALT might be indicated in patients where classical filtrating surgeries cannot be performed for general health state reasons, for ophthalmological reasons, or because of nonmotivated patients.7,9,11,29 Primary ALT gives a longlasting and favorable effect in chronic open-angle glaucoma where 2/3 of the eyes were still managed without additional medication for 8 years. The success in pseudoexfoliation glaucoma was even higher the first 3 years, and stayed above 50 percent for 10 years.3,16,22 In young patients below 50 years, the overall results without additional medication is even higher.10 Some variations in the efficiency of the ALT may be seen between individuals having different intensity in the pigmentation of the trabecular meshwork. The caucasian patients have generally less pigmented meshwork in comparison to the black African patients, and the former may respond poorly to the ALT.21,24

LASER PERIPHERAL IRIDOPLASTY

Not every angle-closure glaucoma may be relieved by laser iridotomy. Structural modifications of the anterior chamber angle, like appositional angle closure in the plateau-iris syndrome, peripheral anterior synechiae or attacks of

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severe acute angle-closure glaucoma with corneal edema, anterior chamber flattening and inflammation, will prevent laser iridotomy to be effective.4,5 In case of narrow anterior chamber angle induced by lens intumescence or anterior chamber crowding as the result of short anteroposterior axis eyes (hypermetropia or nanophthalmos), the trabecular meshwork might be also difficult to see through the gonioscopic mirror. In such situations, alternative laser procedures on the iris could be performed to open the angle.5,7,10

Localized heat application onto biological tissues induces protein coagulation and shrinkage around the burn spot. This produces contractions of the tissue fibers and can be used as mechanical retractors. The argon laser peripheral iridoplasty consists of using the thermal effect of argon laser burns of large spot size, long duration and low power applied at the iris periphery to promote contractions of the iris stroma that will retract the iris root and open the angle.8,6,10,11

The treatment is performed with a contact lens under topical anesthesia. In order to have maximum efficiency, the iris surface should be as smooth as possible. To stretch the iris stroma, pilocarpine 4 percent is given one hour before initiating the procedure. Alpha-agonist is also recommended to avoid postoperative rise of the IOP. The laser is set at 500 μm, 0.5 sec and 400 mW of power, depending on the irides coloration. Excess power results in bubble formation and pigment dispersion. In that case, power should be reduced. Enough power should be applied to produce a noticeable stromal contraction. Lighter irides require more power than darker, as pigments are less dense in the former and absorb less power at a time. Location of the burns must be at the most peripheral part of the iris to produce better results. Should the burns

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be placed less peripherally, the contraction would not be acting directly against the iris root and the resulting effect would be much less effective. For instance, placement of burns in the mid-periphery would lead to failure of this procedure. The effect of laser burns on tissues is almost immediate and iris stroma shrinkage is followed by local opening of the anterior angle and local deepening of the anterior chamber. The spots are placed around the circumference of the iris over 360 degrees with two to three spot diameters inbetween.

Postoperative treatment consists of an alpha-agonist like apraclonidine given once just after the laser session and topical steroids 3 times a day for a week. Caution should be given to the IOP rise in the early hours and adequate medication be administered accordingly. Complications are relatively seldom and consist in mild iritis lasting no longer than a few days. Corneal burns and endothelial decompensation might occur as the laser beam strikes the iris plane with a narrow angle in the case of plateau iris and shallow anterior chamber. Contrary to laser iridotomy, the iris is not cut through the entire stromal portion and the lens or retina will not suffer from damages linked to laser application.2,3,9 A case of malignant glaucoma has been reported.1

CYCLOCOAGULATION WITH YAG AND DIODE LASER

The intraocular pressure can be lowered not only by enhancing the outflow through the trabeculum meshwork, but also by reducing the production of aqueous humor from the ciliary body.1,18,21 During the era when laser techniques were not currently used, cyclodestruction was made by applying either an intense and focal heat onto the ciliary body, a procedure called cyclodiathermy, or by freezing

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the ciliary processes with the help of a cryoprobe applied close to the limbus.3,19

The laser technique used to destroy the ciliary body belongs to the group of cyclophotocoagulation, a way of reducing the activity of the ciliary body by the means of light as a vector of energy. Nd:YAG laser and diode laser are both used to achieve this goal. The laser delivery system can be of contact or non-contact type, if the laser beam has to be directed through a fiberoptic end probe or applied through the air to the globe via the optic devices of a slit lamp. The latter has the advantage that the end part of the delivery system does not carry the risk of transmitting infectious diseases and does not need to be sterilized, while the former can be more compact and easier to use on a patient in a bed. The energy delivery mode can be set on a pulse mode, whereas during short time intervals a predetermined burst of energy is emitted, or the beam is continuously emitted from the laser source during a preset time.

The Nd:YAG laser produces a laser beam of 1064 nm, that is well below the lowest visible wavelengths. The pulse-wave mode creates a mechanical photodisruption that is concentrated in the pigment epithelium of the ciliary body. The continuous wave mode gives a very high level of energy that is mostly absorbed thermally over the ciliary body and partially absorbed in the sclera.16 The effects of Nd:YAG laser application onto the ciliary body result in destruction of tissue, inflammation fibrosis and coagulation necrosis. At the end stage, the production of aqueous humor is markedly reduced, thus lowering the IOP.12,20

The patient is given a retrobulbar injection of anesthetic to reduce the pain induced by the laser treatment and to avoid unexpected eye movements during the procedure. With a non-contact delivery system, the patient is seated

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diode treatment is set between 1000 and 2000 mW, the duration is 2 sec, and the spot size varies between 150 and 500 μm.9 In the contact mode, the duration is slightly longer, giving a power level of 2000 mW.17 The laser beam or the diode probe are placed 1 mm behind the limbus and 20 to 40 burns are made over the entire 360 degrees of the limbus (Fig. 5.5).7,10,13

A new technique for controlling refractory glaucoma has been developed that acts directly from inside the eye instead of acting through the sclera. One reason for failure of trans-scleral cyclophotocoagulation, particularly in congenital glaucoma, may be the displacement of the ciliary processes. This displacement does not permit the indirect treatment to reach the appropriate area. Because endoscopic laser cyclophotocoagulation allows direct visualization of the processes, treatment can be accurately

Fig. 5.5: Inflammation is noticeable after Nd:YAG cyclophotocoagulation, with mixed perilimbal conjunctival injection. The laser burns are clearly visible around the limbus as white scars evenly spaced