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

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58.Neufeld AH, Das S, Vora S, et al. A prodrug of a selective inhibitor of inducible nitric oxide synthase is neuroprotective in the rat model of glaucoma. J Glaucoma. 2002;11(3):221-225.

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71.Wax MB, Tezel G, Saito I, et al. Anti-Ro/SS-A positivity and heat shock protein antibodies in patients with normal-pressure glaucoma [see comments]. Am J Ophthalmol. 1998;125(2):145-157.

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75.Ben Simon GJ, Bakalash S, Aloni E, et al. A rat model for acute rise in intraocular pressure: immune modulation as a therapeutic strategy. Am J Ophthalmol. 2006;141(6):1105-1111.

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Shields > SECTION III - Management of Glaucoma >

34 - Anatomic Principles of Glaucoma Surgery

Authors: Allingham, R. Rand

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

> Table of Contents > SECTION III - Management of Glaucoma > 34 - Anatomic Principles of Glaucoma Surgery

34

Anatomic Principles of Glaucoma Surgery

All laser and incisional surgical procedures for glaucoma are designed to reduce the intraocular pressure (IOP) by increasing the rate of aqueous humor outflow or reducing aqueous production. The involved anatomy, therefore, is the anterior ocular structures related to aqueous outflow and the portions of the ciliary body associated with aqueous inflow. To properly perform any of the operations that make up the armamentarium of glaucoma surgery, the surgeon must be familiar with both the internal and external aspects of these structures. In this chapter, we consider these portions of the ocular anatomy as they

interrelationship between these structures is considered in Chapter 1 with a stepwise construction of a schematic model that may be summarized as follows.

At the junction between the cornea and the sclera is the transitional zone of connective tissue known as the limbus. On the inner surface of the limbus, extending for 360 degrees, is a depression, referred to as the scleral sulcus. The anterior margin of this sulcus slopes gradually into the peripheral cornea, while the posterior margin contains a lip of connective tissue called the scleral spur. This spur might be

thought of as the dividing point between the structures of aqueous outflow anteriorly and those of aqueous production posteriorly. The trabecular meshwork attaches in part to the anterior side of the scleral spur and extends forward to blend into the sloping anterior wall of the scleral sulcus, which converts the sulcus into the Schlemm canal. The bulk of aqueous humor in the anterior chamber flows through the trabecular meshwork to the Schlemm canal, from where it leaves the eye via intrascleral channels and episcleral veins.

The ciliary body inserts into the posterior portion of the scleral spur. This is actually the only firm attachment of the ciliary body, with the remaining surfaces between the sclera and the ciliary body creating a potential space, referred to as the supraciliary space. The ciliary processes, the actual site of aqueous production, occupy the innermost and anteriormost portion of the ciliary body. The iris inserts into the ciliary body just anterior to the ciliary processes. Consequently, a peripheral iridectomy, as performed during glaucoma filtering surgery, often allows visualization of two to four ciliary processes. The insertion of the iris is usually such that a portion of the anterior ciliary body remains gonioscopically visible between the iris root and scleral spur. This is referred to as the ciliary body band, the physical entrance to the uveoscleral outflow pathway. The remainder of the trabecular meshwork— that is, the portion not inserted to the scleral spur—attaches to this band and to the peripheral iris . INTERNAL ANATOMY

Ciliary Body

Most of the ciliary body is located posterior to the iris (Fig. 34.1) and cannot be directly visualized except in unusual circumstances, such as with marked iris retraction or absence of portions of the iris. The anterior 2 to 3 mm of the ciliary body, the pars plicata, is thicker than the posterior portion and contains the radial ridges of the ciliary processes. The latter are the site of aqueous production and the target of cyclodestructive procedures. In those unusual circumstances in which they can be visualized directly (by using cycloscopy), direct treatment with laser transpupillary cyclophotocoagulation or endoscopic visualization may be possible. When direct visualization is not

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possible, an indirect, transscleral route can be used for cyclodestruction, requiring the use of external landmarks (discussed later in this chapter). The posterior 4 mm of the ciliary body is the thinner pars plana, which must also be approached by using external landmarks.

Figure 34.1 Internal anatomy. The ciliary body is located just posterior to the iris and is divided into the pars plicata (A) and the pars plana (B). The remaining internal structures can be seen by gonioscopy and include 1, iris; 2, ciliary body band; 3, scleral spur; 4, trabecular meshwork; 5, Schwalbe line.

Structures Visualized by Gonioscopy

The following structures in the anterior chamber can be visualized by gonioscopic examination and are involved in several laser and incisional glaucoma surgical procedures.

Iris

The iris is the posteriormost structure of the anterior chamber angle. It is helpful to remember that the peripheral portion of the iris is thinner than the more central iris, which makes it, among other reasons, the preferred site for a laser iridotomy. Other anatomic considerations related to optimum laser iridotomy sites are iris crypts, or thinner areas of stroma that may be easier to penetrate. In addition, areas of increased pigmentation, such as iris freckles, may improve the absorption of laser energy in lightly pigmented eyes when using argon laser. It is generally preferred to place the iridotomy so that it is fully covered by the upper lid, to minimize the side effect of intermittent glare (1). However, peripheral iridotomies can result in symptomatic glare in any position.

Ciliary Body Band

The ciliary body band is located just anterior to the root of the iris; it typically has a dark gray or brown appearance on gonioscopic examination. The width of this band varies considerably from one patient to the next. Eyes with myopia often have a wide band, and those with hyperopia a narrow band. Surgeons should avoid confusing the pigmented ciliary body band with the trabecular meshwork in patients with lightly pigmented meshwork, especially when interpreting the depth of the anterior chamber angle. This is particularly relevant when performing laser trabeculoplasty. The patient usually lets the surgeon know when the latter mistake is made, because the ciliary body contains many nerve endings and is sensitive

to the application of laser energy. Scleral Spur

The scleral spur is seen gonioscopically as a white line just anterior to the ciliary body band. In some patients, visualization of the spur may be obscured because of variable degrees of high iris process insertion (but discontinuous; continuous areas of high iris insertion are peripheral anterior synechiae) or heavy pigment dispersion. This was the principal site of surgery with a cyclodialysis procedure, an operation of historical interest, in which an aqueous outflow pathway in the suprachoroidal space was constructed by separating ciliary body from the scleral spur. In the early stages of neovascular glaucoma, new vessels may be seen extending across the scleral spur from the iris and ciliary body to the trabecular meshwork. The vessels can be obliterated at this site with laser applications in a procedure called goniophotocoagulation, which is also rarely used today.

Trabecular Meshwork

Just anterior to the scleral spur is the functional portion of the trabecular meshwork, the portion adjacent to the Schlemm canal through which the aqueous humor drains. This portion of the meshwork is demarcated gonioscopically by the presence of variable amounts of pigment. Because this pigment is presumably carried to the meshwork from uveal tissue by the aqueous humor flow, it is typically light in young individuals and varies considerably among individuals later in life according to the amount of intraocular pigment release. In some patients, especially with pathologic states such as the pigment dispersion syndrome and exfoliation syndrome, the meshwork is heavily pigmented. In other individuals, the meshwork may be so lightly pigmented that it is hard to see, which can lead to the incorrect diagnosis of a narrow, or even closed, anterior chamber angle. In some of these cases, blood reflux into the Schlemm canal or iris processes, which typically extends to the meshwork, may help identify this structure.

It is this pigmented portion of the trabecular meshwork to which the laser energy should be applied during argon laser trabeculoplasty, which delivers its energy within a 50-µm spot. However, there is another, less pigmented portion of the meshwork just anterior to the functional, pigmented portion.

When performing argon laser trabeculoplasty, overlapping the laser beam between the pigmented and nonpigmented portions of the meshwork—that is, alon g the anterior border of the pigmented portion— may help reduce the complications of transient postoperative IOP rise and peripheral anterior synechia formation. Selective laser trabeculoplasty delivers its energy in a 400-µm spot; with this procedure, centering the spot over the entire trabecular meshwork is preferable.

When performing trabeculectomy ab interno, the scleral spur and trabecular meshwork must be clearly identified to initially penetrate through the trabecular meshwork into the Schlemm canal. If one penetrates posterior to the scleral spur, the probe will enter the suprachoroidal space, resulting in a substantially increased risk of complications.

Schwalbe Line

The Schwalbe line is the anteriormost structure in the anterior chamber angle and represents the junction between the nonpigmented portion of the trabecular meshwork and the peripheral cornea. In most individuals, a portion of this junction is represented by a small ridge. This is an important landmark when performing a goniotomy, in that the internal incision in that operation is made just posterior to the Schwalbe line. The structure may be difficult to visualize gonioscopically, unless there has been a moderate degree of pigment dispersion, in which case there may be a buildup of pigment along the anterior side of the ridge, especially inferiorly. Care must be taken to avoid confusing this pigmented line with the trabecular meshwork when performing laser trabeculoplasty. In other

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cases in which pigmentation is minimal, the location of the Schwalbe line can be established gonioscopically to help determine the depth of the peripheral anterior chamber. A fine beam of light from the slitlamp can be seen reflecting from both the anterior and posterior surfaces of the peripheral cornea. As the clear portion of the peripheral cornea approaches the Schwalbe line, it is replaced externally by opaque limbal tissue, which causes the two beams to converge at the Schwalbe line,

providing a useful way for determining the location of this structure. EXTERNAL ANATOMY

Anterior Limbus

On the external surface of the eye, the anterior boundary of the limbus is defined as the termination of the Bowman membrane, which is approximately 0.5 mm anterior to the insertion of the conjunctiva and Tenon capsule (Fig. 34.2). This has been referred to as the corneolimbal junction, or the apparent or anterior limbus. It is important to note that the conjunctiva inserts more anteriorly in the superior and inferior quadrants. Consequently, the limbus is wider in these quadrants, ranging between 1 and 1.5 mm, and gradually tapers to the narrowest width in the nasal and temporal quadrants, where the range is between 0.3 and 0.5 mm (2). In performing glaucoma filtering surgery, some surgeons choose to take advantage of the wider areas of the limbus by placing the surgical site at the 12-o'clock position. When performing surgery that involves the ciliary body, such as a cyclodestructive procedure or a pars plana incision, the surgeon should remember that these structures are slightly more posterior in relation to the apparent limbus in the superior and inferior quadrants.

Figure 34.2 External anatomy. On the external surface, the limbus is bounded posteriorly by the sclerolimbal junction (SLJ) and anteriorly by the corneolimbal junction (CLJ). The width of the limbus varies from a maximum superiorly to a minimum on the sides (inset) due to the relative insertion of the conjunctiva (C). The Tenon capsule (TC) is firmly attached to limbal connective tissue approximately

0.5 mm behind the conjunctival insertion, creating a potential space (PS). Cyclodestructive procedures should be placed over the pars plicata, usually 1.0 to 1.5 mm posterior to the corneolimbal junction, whereas a posterior sclerotomy should be made through the pars plana, approximately 3 to 4 mm posterior to the corneolimbal junction.

Conjunctiva and Tenon Capsule

The conjunctiva and Tenon capsule cover the limbus. The Tenon capsule is firmly attached to the connective tissue of the limbus approximately 0.5 to 1.0 mm posterior to the insertion of the conjunctiva, which creates a potential space between the anterior conjunctiva and Tenon capsule tissue and the limbal connective tissue. If the surgeon wishes to obtain maximum exposure of the limbus when preparing a limbus-based conjunctival flap, it is necessary to dissect this adherence between the Tenon capsule and the limbal tissue. Such a technique is not recommended for filtering surgery with use of adjunctive antimetabolites, because the resulting filtering bleb may be too thin. The adhesions between conjunctiva and the Tenon capsule are moderately firm, so that sharp dissection is required to dissect between these two structures when preparing the conjunctival flap. The adhesions between the Tenon capsule and the underlying limbus and sclera posteriorly are less firm, and these structures can often be separated with blunt dissection. With the current trabeculectomy techniques, adequate anterior dissection is possible under the partial-thickness scleral flap without dissecting the insertion of the Tenon capsule. It is also preferable, especially when performing filtering surgery with

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an adjunctive antimetabolite, to leave this adhesion intact to avoid creating a filtering bleb that is too thin at the limbus.

Posterior Limbus

When the conjunctiva and Tenon capsule have been reflected, the posterior boundary of the limbus can be seen. This has been referred to as the sclerolimbal junction, or the surgical or posterior limbus. It is identified as the junction of the opaque white sclera posteriorly and the translucent bluish-gray limbus anteriorly. This boundary of the limbus is more useful than the anterior limbus in glaucoma surgery because it helps identify the location of the deeper structures of the anterior chamber angle. The scleral spur, for example, is located just posterior to the sclerolimbal junction and the Schlemm canal and therefore would be found just anterior to this landmark. In performing a trabeculotomy ab externo, a radial scratch incision across the sclerolimbal junction should reveal the Schlemm canal in the posterior portion of the gray zone. When performing a trabeculectomy, a circumferential incision beneath the partialthickness scleral flap at the corneolimbal junction enters the anterior chamber just in front of the trabecular meshwork. By extending the dissection posteriorly with radial incisions to the sclerolimbal junction, a flap of deep limbal tissue is created that can be reflected to expose the anterior chamber angle structures and can then be excised along the scleral spur. If the latter incision is mistakenly made more posteriorly, the ciliary body may be damaged, resulting in brisk bleeding. A fistula that is too posterior is also at risk for obstruction by uveal tissue, hence the importance of correctly identifying the external landmarks during glaucoma filtering surgery.

The vasculature of the limbus originates primarily from the anterior ciliary arteries (3). The anterior ciliary arteries enter the ciliary body behind the scleral spur in locations corresponding to the positions of the rectus muscle tendons. These vessels should be avoided when possible during surgery to minimize excessive bleeding. Because the ciliary body cannot usually be visualized internally, external landmarks must be used when performing surgical procedures associated with these structures. In performing cyclodestructive procedures, which involve the pars plicata, it was once suggested that the destructive element, for example, the cryoprobe, should be placed 2 to 3 mm behind the corneolimbal junction, allowing for the previously discussed variation in this landmark (2). In most eyes, however, using this location would result in entry into the eye that is posterior to the pars plicata. This may not have been significant with the earlier cyclodestructive procedures in which the area of tissue destruction was so broad. With transscleral laser cyclophotocoagulation, however, the zone of tissue destruction is more precise, and a placement of the laser beam 1.5 mm behind the corneolimbal junction superiorly and

inferiorly and 1.0 mm temporally and nasally is most likely to reach the pars plicata. Some probes, such as the G probe (Iridex Corporation, Mountain View, CA), are designed with a footplate that is placed at the limbus, and the energy is delivered posteriorly, approximating the correct location. When making a pars plana incision, as during a posterior sclerotomy for malignant glaucoma or when draining a suprachoroidal detachment or hemorrhage, the incision should be made 3 mm (in aphakic or pseudophakic eyes) to 4 mm (in phakic eyes) behind the corneolimbal junction.

KEY POINTS

Laser and incisional surgical procedures for glaucoma are directed at the anatomic structures associated with aqueous inflow, that is, the ciliary body, and aqueous outflow, which includes the iris and the trabecular meshwork and related outflow pathways.

For successful glaucoma surgery, it is necessary to be familiar with these structures by direct internal visualization through slitlamp and gonioscopic examination, and by their relationship to the external aspects of the limbal connective tissue and the overlying conjunctiva and Tenon capsule.

REFERENCES

1.Spaeth GL, Idowu O, Seligsohn A, et al. The effects of iridotomy size and position on symptoms following laser peripheral iridotomy. J Glaucoma. 2005;14(5):364-367.

2.Sugar HS. Surgical anatomy of glaucoma. Surv Ophthalmol. 1968;13:143.

3.Van Buskirk EM. The anatomy of the limbus. Eye (Lond). 1989; 3(pt 2):101.

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Shields > SECTION III - Management of Glaucoma >

35 - Principles of Laser Surgery for Glaucoma

Authors: Allingham, R. Rand

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

> Table of Contents > SECTION III - Management of Glaucoma > 35 - Principles of Laser Surgery for Glaucoma

35

Principles of Laser Surgery for Glaucoma

The introduction of laser (i.e., light amplification by stimulated emission of radiation) therapy was a significant advance in the surgical treatment of glaucoma during the second half of the 20th century. The concept of using light energy to alter the structure of intraocular tissues, however, actually preceded the development of laser technology. Meyer-Schwickerath (1), beginning in the late 1940s, pioneered this field of ocular surgery, first using focused sunlight and later the xenon-arc photocoagulator. Although the latter technique was useful for certain retinal disorders, xenon-arc photocoagulation for the treatment of glaucoma never gained clinical acceptance.

In 1960, Maiman (2) described the first laser that used a ruby crystal stimulated by a flash lamp to emit red laser light at a wavelength of 694 nm. It was the development of the continuous-wave argon laser, near the end of that decade, that brought on a virtual explosion of laser applications for ocular diseases. Since the first report of argon laser use for ocular disease in the late 1960s, numerous wavelengths arising from different energy-emitting sources have been tried. Lasers are now used to treat various forms of glaucoma, and today it is the most commonly used mode of glaucoma surgery (3, 4, 5 and 6). This chapter briefly reviews the physical and biologic aspects of laser therapy. The application of these principles to the treatment of specific forms of glaucoma is considered in subsequent chapters.

BASIC PRINCIPLES OF LASERS

When light is shined on a metal surface in a vacuum, it may free electrons from that surface. These electrons can be detected as a current flowing in the vacuum to an electrode. Only certain wavelengths can cause photoemission of electrons. In 1917, Albert Einstein wrote “Zur Quantum Theorie der Strahlung” (the quantum theory of radiation), in wh ich he speculated that light consists of photons, each with discrete quantum of energy proportional to its wavelength. For an electron to be freed from the metal surface, it would need a photon with enough energy to overcome the energy that bound it to the atom. His theory formed the basis of laser technology.

When atoms absorb energy, called “pumping,” they ar e “excited” from a lower to a higher energy level. When a substance (e.g., gas, liquid, or a semiconducting material) is excited by energy, it emits light in all directions. The sources of energy used to excite the lasing medium typically include electricity from a power supply or flash lamps, or the energy from another laser. If more atoms are in the excited state than in the unexcited state, population inversion is said to exist. Under such circumstances, photons with energy equal to the difference between the two levels of excitation have an enhanced probability of stimulating the atoms to decay back to their lower energy level by emitting photons, a process called stimulated emission. The emitted photons stimulate the emission of more photons, leading to a chain reaction.

If this system is enclosed between two mirrors, the photons bounce back and forth, creating multiple stimulated emissions of light, or light amplification. The mirrors form an optical cavity, which, in addition to amplifying the light, creates a parallel beam and acts as a resonator to limit the number of wavelengths. When the light amplification is sufficient, some photons are allowed to leave the cavity in the form of a laser beam through a partially permeable mirror (Fig. 35.1).

The laser beam can be delivered as a continuous wave or in a pulsed mode. In the latter situation, the energy is concentrated and delivered in a very short period of time, which can be accomplished in one of two ways. With one technique, called Q-switching, light is not allowed to travel back and forth in the cavity until maximum population inversion is reached. This is accomplished with an electronic shutter or misalignment of the mirrors. When the shutter is opened or the mirrors are aligned, stimulated emission and light amplification occur suddenly, and the energy is released in a pulse of a few to tens of nanoseconds. In the other form of pulsed delivery, called modelocking, the energy is also released after achieving maximum population inversion, but different modes of light are synchronized, creating peaks of energy, which are emitted in tens of nanoseconds as a chain of pulses, each of which lasts a few tens of picoseconds. To provide some appreciation for the brevity of these exposures, it has been noted that the ratio between the duration of a Q-switched laser pulse and a conventional continuous-wave argon laser exposure is roughly the same as the ratio between the argon exposure and a human lifetime (6). PROPERTIES OF LASER ENERGY

Light emitted by a laser differs from normal “white ” light in several ways. Coherence

Unlike the photons in a light bulb, which are emitted randomly, the resonator effect of the laser cavity causes the photons to be synchronized or coherent—t hat is, in phase with each other in time and space. P.449