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

44 Step by Step Minimally Invasive Glaucoma Surgery

LASER PRINCIPLE AND INTRODUCTION

The word laser was first used in the 60s when light was generated by stimulated emission of radiation. The word LASER is an abbreviation for Light Amplification by Stimulated Emission of Radiation.9 The physical and wavelength characteristics of the laser light were so specific that they gained rapid interest in the field of medical treatment. Laser light is monochromatic and spatially and temporally coherent. This results in a laser beam that is highly directional with little divergence, thus enabling focusing of the beam into a very small spot of high power per unit area. Monochromaticity means that the laser beam is composed of almost a single wavelength, which has great consequences in the surgical eye treatment. This allows light to be effectively absorbed by the pigments contained in the ocular tissues, e.g. the melanin, hemoglobin or the xanthophyll pigments.4

The molecules in biologic tissues are not transparent to wavelength shorter than 300 nm or greater than 100 nm. Between this range, the molecules absorb selectively the light depending on the spectral absorption characteristic of the pigments in the tissues. Laser effects in tissues can be summarized into three groups: photochemical, thermal and ionization effects. The photochemical effect results in photon absorbtion by molecules inducing chemical reactions. A thermal effect is present when photon absorbtion by electrons increases the molecular vibrations to a temperature peak that results in denaturation of proteins in biologic tissues. When the temperature reaches 60°C, the weak van der Waals forces that maintain the molecular bonds are broken and denaturation of the several intracellular proteins occurs.1 If laser energy is strong enough to tear electrons from the outer orbit, the atoms or molecules become ionized and relax this metastable state

Laser Treatment in Glaucomas 45

by producing mechanical (pressure) waves (photodisruption effect).7

The efficiency of laser beam onto biological tissues depend on the absorption spectra of cellular pigments.5 Melanin best absorbs wavelengths between 400 and 100 nm. Hemoglobin, naturally presents a red appearance that strongly reduces its absorption characteristics in the long wavelengths above 650 nm. Xanthophyll absorbs very well in the short wavelengths below 500 nm.

The laser delivery systems used in laser glaucoma treatments are divided into two groups: non-contact and contact laser source.8 The non-contact delivery system produces a beam that is focused onto the area to be treated via a slit-lamp biomicroscope. It may be useful to place a contact lens to stabilize the eye and the lids, to reduce the optical aberrations of the anterior surface of the cornea and to gain access to structures that are otherwise invisible without a contact lens.

The contact delivery system consists of laser source and optical guidance device such as fiberoptic and delivery tip that directly apply onto the tissue to be treated.10,11 The energy distribution of the laser beam can be set on two different modes.7 The fundamental mode gives a sharp focus spot with maximum energy at the focus point and minimum energy anterior or posterior to that point. The mode locking produces a series of brief spikes of photon, with a peak of energy that is much higher than the average energy of equivalent photon bursts that are delivered in a continuous flow. It is also possible to modulate the production of the laser light before delivery by placing an electronic shutter that keeps the excited molecules to a high energy level before relaxation. When the shutter opens, an extremely brief pulse (20 ns) of high energy is produced (Q-switch mode).7

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Several wavelengths are used in ophthalmic laser photocoagulators. Krypton lasers produce red light (647 and 676 nm). Argon lasers produce green light (514.5 nm) and green and blue light (514.5 and 488 nm). Both are used in photocoagulation,14 whereas Nd:YAG lasers deliver infrared light (1064 nm) which is used with Q-switch mode in photodisruption. Diode lasers deliver red (640 nm) and infrared light (800-820 nm) that can be guided through a fiberoptic device for contact photocoagulation. A new type of laser has recently been developed to enhance the photodissociative effect of the YAG laser in intraocular surgery. It is made by combination of a classic YAG laser on which an erbium component has been added to produce fragmentation effects. The wavelength of 2940 nm is longer than Nd:YAG, thus water absorbs most of the energy with an important water absorption photothermal effect.6 The transfer of heat to the adjacent tissue is therefore reduced. The beam is directed through a fiberglass tip with a ceramic end. The first attempts have been made in cataract surgery to replace the ultrasonic emulsification currently used.13 But technical problems regarding the safety of this method in respect to the posterior capsule have not yet been completely solved. New attempts have been made in glaucoma therapy to increase the aqueous outflow by disrupting the trabecular meshwork and inner wall of Schlemm’s canal by an ab-interno approach. A quartz fiber contact endoprobe (320 micron core-diameter, 385 micron coating-diameter) applying single neighboring laser pulses (5-7 mJ) to the trabecular meshwork. The procedure was gonioscopically visualized. Although IOP lowering effect of erbium:YAG laser trabecular ablation did not prove as effective as in filtering procedures. LTA might be a valuable alternative in glaucoma surgery especially in order to avoid conjunctival scarring and postoperative hypotony.3-5 It is

Laser Treatment in Glaucomas 47

not known whether this new technique can be widely used in the field of glaucoma laser surgery to add new prospectives or replace existing and efficient laser technique and procedure that will be explained in more details in the subsequent chapters.

The physical properties of the laser beam are such that safety aspects should always be considered when using or being in close proximity to laser devices. Every laser delivery system protects the operator from direct damages when using the laser by shutting off the beam with a filter. But this is not the case for personnel being nearby and staring at the laser system, for the filter will not prevent hazardous direct or reflected beam from reaching the retina of the observers.12

LASER IRIDOTOMY

Laser iridotomy is a laser surgical procedure by which a small aperture through the iris is created to treat several forms of angle-closure glaucomas and more recently pigmentary glaucoma.25,31 It was introduced in the 80’s with the development of laser surgeries.21 The indication for laser iridotomy is given in the case of acute angle-closure glaucoma when the acute attack has been medically controlled, and the cornea is clear enough to allow sufficient penetration of the laser beam into the anterior chamber without excessive scattering.5,26 In the case of chronic angleclosure glaucoma, or eyes with peripheral anterior synechia, laser iridotomy may reduce the intraocular pressure (IOP) and allow the angle structures to resume a wider space, by deepening the anterior chamber.24 Pupillary block may be relief thus allowing a better control of the IOP. In aphakic or pseudophakic eyes, pupillary block from pockets of aqueous behind the iris plane could be relieved by adequate

48 Step by Step Minimally Invasive Glaucoma Surgery

laser iridotomy.1,6,30 In eyes with intumescent cataracts, or in the fellow eye of an acute angle-closure glaucoma eye, prophylactic laser iridotomy can be performed in order to avoid an attack of angle-closure glaucoma.30 In eyes with narrow angle, good visualization of the angle structures may be rendered difficult by the geometric disposition of the iris over the angle. This situation could prevent the performance of laser trabeculoplasty in such eyes. To avoid this complication, laser iridotomy can be performed to open a too narrow angle. In the plateau iris syndrome, anteriorly located ciliary processes support the peripheral iris. Variation in the angle values between dark and light are solely related to changes in iris thickness. Pilocarpine produces iris thinning and is an effective method of opening the angle.18 General disturbance of the geometry of the eye, like in the case of nanophthalmos, have greater risks for developing angle-closure glaucoma. A prophylactic laser iridotomy could reduce the incidence of such complications.

In pigment dispersion syndrome, the iris often presents a marked concavity, forming an inverted pupillary block. Accommodation increases iris concavity in some patients with pigment dispersion syndrome. The most likely explanation is an accommodation-induced relative increase in anterior chamber pressure, secondary to anterior movement of the lens surface. Iridotomy prevents change in the iris profile with accommodation.19 In this case too, performance of laser iridotomy would reduce the pressure differential between the anterior and posterior plane of the iris, by creating a channel through the iris.2,4,7,12,22 In the field of refractive surgery, the placement of an intraocular contact lenses (ICL) in the posterior chamber in front of the anterior capsule of the lens reduces the angle and increases the risk for angle-closure glaucoma. Laser iridotomy performed before refractive surgery prevents the likelihood

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of such complications.22 Conversely, in the presence of strong hypermetropic eyes, the current IOL may not have sufficient refractive power for the intended emmetropization. Two intraocular lenses (piggyback) have been placed simultaneously in the capsular bag, thus also reducing the depth of the chamber. In that case too, a laser iridotomy will prevent an attack of angle-closure glaucoma.14 Laser iridotomy is contraindicated in any situation where the angle-closure is not mechanicaly reversible by deepening the anterior chamber, for instance, a flat anterior chamber, synechial closure of the angle by uveitis, neovascular glaucoma or by iridocorneal endothelial syndrome (ICE). In this situation they are absolute contraindications for laser iridotomy. Any corneal edema or opacities will strongly reduce the quality of the transmission of the laser, and are relative contraindications for the performance of the treatment.

A gonioscopic contact lens, like the Goldmann is placed on the cornea of the patient after topical anesthesia. It sometimes happens that the iris is not contracted enough to present a smooth surface and thin stroma. Only a few shallow crypts are visible. The use of pilocarpine will contract the sphincter iridae and enlarge the angle of view before treatment. In some cases, the IOP rises a few hours after laser iridotomy.3,27 To prevent the peak of pressure, an alpha agonist such as apraclonidine can be given preand perioperatively.28 The location of the iridotomy should be either the 11 or 1 O’clock positions, so that the upper lid will cover the aperture made by the laser and prevent monocular diplopia (Fig. 5.1).11,16 A crypt should be selected if possible to facilitate the creation of the crater. As laser light is absorbed by ocular pigments, very light iridies are more difficult to perforate than darker. Conversely, very dark pigmented iris have generally thicker stroma and are

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Fig. 5.1: Iris aspect after Nd:YAG laser iridotomy located at 12 O’clock position. Note the depigmentation of the anterior surface of the stroma around the aperture

also difficult to penetrate.13 Laser energy used will then depend on color and density of irides pigments. Current power level for argon type laser ranges from 500 upto 1500 mW for a duration of 0.1 sec. Spot size is set at 50 μm. Nd:YAG laser energy level is set between 2.0 and 8.0 mJ with single burst mode. Small bubbles form around the spot site and, in case of dark brown pigmented iris, some pigments will be released into the anterior chamber. This can severely reduce visibility of the treated area and preclude the completion of the treatment. In such case, a break between laser applications should be made. When passing through the posterior plane of the iris, clouds of pigments burst in the iridotomy crater may appear. As soon as the iris stroma has been totally penetrated, a flow of aqueous humor “leaks” through the aperture and the iris

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stroma moves slightly backwards. Retroillumination through the pupil will reveal a red reflex via the iridotomy. Care should be exercised not to be overconfident in the patency of the iridotomy, as red reflex is not enough to ensure complete burn of iris stroma. Some very thin transparent membranes could still be present, preventing aqueous from freely flowing through the iridotomy. Gonioscopy should always be performed in doubtful cases. Ultrasound biomicroscopy will show an increase in the iridocorneal angle after completion of a successful iridotomy (Fig. 5.2).

Corneal edema and epithelial erosion can occur when performing laser iridotomies. Such complications generally resolve in a few days. Endothelial cell can be affected by Nd:YAG laser, caused by the shock waves produced during the treatment. Endothelial cells count have shown that proper application of laser burn with Nd:YAG laser does

Fig. 5.2: Ultrasound biomicroscopy of the anterior chamber angle after patent iridotomy through the iris stroma, the angle is wider