CHAPTER 44 Laser peripheral iridotomy and iridoplasty

Introduction

Angle closure is an anatomic disorder. It is not a single entity, but a number of entities related by iris apposition to the trabecular meshwork. They are characterized by abnormalities of size or position of structures. The resulting physical barrier to aqueous outflow acutely or gradually leads to elevated intraocular pressure (IOP). Angle closure can be primary or secondary to other pathological processes, such as ‘malignant’ glaucoma after pan-retinal photocoagulation or intraocular tumor. Angle closure glaucoma is now defined as glaucomatous optic neuropathy secondary to elevated IOP caused by closure of the anterior chamber angle.

Pupillary block is the most common cause of primary angle closure (PAC) and can be eliminated by laser peripheral iridotomy (LPI), which allows aqueous to pass directly from posterior to the anterior chambers, bypassing the pupil. Once pupillary block has been eliminated, any residual appositional angle closure is due to other contributing factors, such as plateau iris, lens-related angle closure, or pressure originating posterior to the lens (‘malignant’ glaucoma, ciliary block, posterior aqueous misdirection). Appositional closure in some, if not all, of these entities can be eliminated effectively and safely by argon laser peripheral iridoplasty (ALPI).

Laser peripheral iridotomy

The continuous wave argon laser revolutionized the treatment of glaucoma. In 1973, Beckman and Sugar reported successful argon laser iridotomies in humans ¹. Others soon reported success in human eyes with angle closure². The ease and convenience of the procedure for both patient and surgeon and the paucity of severe complications led to its rapid acceptance. ALPI was first described in 1982³.

Indications

LPI is the procedure of choice for all forms of angle closure in which there is a component of pupillary block.

Acute primary angle closure

LPI should be performed after the acute attack has been terminated with medical therapy or ALPI and the eye is no longer actively inflamed, usually about 2 days later. Attacks of acute angle closure which do not respond to a brief period of medical treatment to lower IOP and to open the angle may be treated successfully with ALPI, although in some cases the media may be clear enough to perform an LPI. If LPI is to be attempted during an acute attack, the argon laser is preferred because of the greater incidence of iris bleeding in these inflamed eyes.

A recent randomized controlled trial reported that LPI may not be as effective as early phacoemulsification in reversing angle closure and preventing the subsequent IOP increase in acute angle closure eyes ⁴. It was suggested that if LPI could be performed within 7 days after the onset of symptoms in acute angle closure the chance of successful post-iridotomy IOP control would be higher⁵. After LPI in acute angle closure eyes, up to 55.6% may still have appositionally closed angles⁶. It is therefore important to monitor both the drainage angle and IOP in acute angle closure patients following LPI.

Other situations

Secondary angle closure glaucoma

In situations of angle closure secondary to other eye diseases, e.g. anterior uveitis or Behcet’s disease ⁸, LPI may help relieve the pupillary block and reverse, partially or totally, the appositionally closed portions of the angle. This may improve IOP control.

In eyes with an intumescent lens presenting with acute angle closure, if the cornea is clear enough, LPI may be performed prior to medical treatment. This approach, when successful, would avoid the risk of a paradoxical reaction to pilocarpine.

Before argon laser trabeculoplasty in eyes with narrow angles

If a narrow angle makes visualization or treatment of the angle structures difficult, LPI may be performed if this is on the basis of relative pupillary block, often suggested by a peripheral bombé configuration. If the angle is narrow because of a plateau iris configuration, ALPI is a better alternative to improve visualization of angle structures.

Nanophthalmos

Eyes with nanophthalmos are at considerable risk of develop angle closure because of anterior chamber crowding. Both prophylactic iridotomy and ALPI can be useful.

Pigment dispersion syndrome

Peripheral iridotomy reduces the iris concavity often found in pigment dispersion syndrome, by eliminating the pressure differential across the iris from reverse pupillary block.

Contraindications

Contraindications to LPI include corneal edema or opacities, flat anterior chamber, a completely sealed angle, and angle closure from primary synechial closure of the angle, such as in uveitis, neovascular glaucoma, or iridocorneal–endothelial (ICE) syndrome (where pupillary block is not contributory).

Surgical techniques

The various techniques for LPI as developed in the late 1970s and early 1980s, with multiple variations, are detailed elsewhere ⁹. We describe our current most commonly used techniques and settings.

Contact lenses for laser procedures

Firm control of the contact lens reduces saccades and extraneous eye movements that interfere with accurate superimposition of burns. The lens helps to separate the lids, to focus the laser beam, and to minimize loss of laser power from reflection. By absorbing heat delivered through the cornea, the gonioscopy solution decreases the incidence of corneal burns.

The Abraham lens (Fig. 44.1) consists of a fundus lens with an anterior +66 diopter planoconvex button. The button magnifies without loss of depth of focus. The effective size of a 50 µm spot is reduced to approximately 30 µm; this provides higher energy per unit area and permits the procedure to require a lower total energy. Posterior to the site of focus, the beam is more rapidly defocused, decreasing potential injury to structures behind.