Peripheral Iridotomy

Background

Laser peripheral iridotomy (LPI) is the preferred procedure for treating angle-closure glaucoma caused by relative or absolute pupillary block. ^[1] LPI eliminates pupillary block by allowing the aqueous to pass directly from the posterior chamber into the anterior chamber, bypassing the pupil. ^[2] LPI can be performed with an argon laser, with a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser, or, in certain circumstances, with both.

Indications

Indications for LPI include the following:

• Acute angle-closure glaucoma

• Chronic angle-closure glaucoma

• Fellow eye of acute angle-closure glaucoma

• Narrow/occludable angle

• Miscellaneous conditions, including phacomorphic glaucoma, aqueous misdirection, nanophthalmos, pigmentary dispersion syndrome, and plateau iris syndrome

In patients with acute angle-closure glaucoma, LPI should be performed after intraocular pressure (IOP) and intraocular inflammation are controlled. The aim is to prevent another attack of acute angle-closure glaucoma or progression to chronic angle-closure glaucoma. In patients with chronic angle-closure glaucoma, IOP may remain the same or be lowered after LPI, depending on the extent of peripheral anterior synechiae.

The fellow eye in a patient with acute angle-closure glaucoma or chronic angle-closure glaucoma has a 50% chance of developing acute angle-closure glaucoma. ^[2]

Certain patients, especially hyperopic patients, are at increased risk of having narrow angles. Therefore, gonioscopy should be performed. Patients who have primary angle closure suspect may benefit from LPI if they are at risk for poor follow-up, require frequent pupillary dilation, have a positive provocative test result, are monocular, or have a family history of primary angle closure or primary angle closure glaucoma. ^[3]

LPI has been performed in phacomorphic glaucoma, aqueous misdirection, and nanophthalmos to relieve pupillary block. In pigmentary dispersion and plateau iris syndrome, LPI is used to confirm the diagnosis.

Contraindications

Contraindications for LPI include conditions that cause poor visualization of the iris, angle closure due to synechial closure of the anterior chamber angle, and a patient who is unable to cooperate.

Conditions causing poor visualization of the iris include the following:

• Corneal edema

• Corneal opacity

• Flat anterior chamber

Conditions causing synechial closure of the anterior chamber angle include the following:

• Neovascular glaucoma

• Iridocorneal endothelial (ICE) syndrome

Patients who are unable to cooperate include the following:

• Patients who cannot sit comfortably at the laser table

• Patients who cannot keep the head still

Patient Education/Informed Consent

Initially, informed consent for laser peripheral iridotomy (LPI) is obtained. The indications for the procedure (see Introduction), its benefits, and its complications (see Technique) should be discussed in detail with the patient.

Equipment

To perform LPI, a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser (see the first image below), an argon laser (see the second image below), or both are needed.

Additionally, using a contact lens makes the procedure easier. The advantages of doing so include the following ^[1] :

• The laser energy is concentrated at the level of the iris

• The number of corneal epithelial burns is minimized because the lens acts as a heat sink

• The target structure is magnified with less loss of depth of field than occurs if magnification is simply increased with the slit lamp controls

• The lens acts as a speculum; keeping the eye open minimizes fine eye movements

Either an Abraham lens or a Wise lens may be employed. Of the 2, the Abraham lens (see the image below) is more commonly used. It is a modified Goldmann-type fundus lens with a flat glass plate bonded to its anterior surface. ^[4] The glass plate has a +66 diopter planoconvex button bonded into a decentered 8-mm hole. ^[1] The Wise lens has a +102 diopter button, which provides higher magnification.

Patient Preparation

Topical anesthesia with proparacaine 0.5% is usually adequate for performing LPI. The patient must be comfortable at the laser table, as when a patient is examined at the slit lamp.

Monitoring and Follow-up

At 1 hour after completion of LPI, the intraocular pressure (IOP) should be checked to make sure that it did not increase significantly (ie, that IOP has not increased by 8 mm Hg or more and that IOP does not exceed 30 mm Hg). Topical prednisolone acetate 1% is given 4 times a day for 5-7 days. AT 1 week, the patient is seen to monitor IOP, to confirm the patency of the iridotomy site, and to check for any significant intraocular inflammation.

At 1 month, the patient is seen again for a complete examination that includes IOP measurement, slit-lamp examination, gonioscopy, and dilated fundus examination. IOP is also measured after dilation. If IOP rises by more than 8 mm Hg, the anterior chamber angle is still occludable, and the patient must be evaluated for other causes of angle closure (eg, plateau iris).

Approach Considerations

Before laser peripheral iridotomy (LPI) is performed, intraocular pressure (IOP) and anterior segment inflammation must be controlled. Generally, LPI can be performed with the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser alone. In cases of dark irides, a combination of the argon laser and the Nd:YAG laser works well. The objective is to be able to visualize the anterior capsule.

Because postlaser IOP spike is a common complication of LPI, the eye should be pretreated with topical proparacaine, pilocarpine 1%, and either apraclonidine (0.5% or 1%) or brimonidine (0.1%, 0.15%, or 0.2%); the use of apraclonidine or brimonidine significantly reduces the risk of this complication. ^{[5, 6, 7]} Pilocarpine is used to stretch the peripheral iris, making it thinner and easier to penetrate. ^[1] Higher concentrations of pilocarpine are not recommended, because they can cause paradoxical angle closure.

Laser Peripheral Iridotomy

LPI is performed as follows.

Initial measures

Gonioscopy is used to assess the anterior chamber angle. IOP and intraocular inflammation are controlled. A topical anesthetic (proparacaine 0.5%), a topical alpha-agonist (apraclonidine or brimonidine), and pilocarpine 1% are placed on the eye.

Placement of contact lens on eye

The patient is asked to sit comfortably at the laser table. The contact lens is placed on the eye. When the laser is used on the right eye, the contact lens should be held with the left hand, and vice versa. The plane of the contact lens must always be oriented parallel to the iris plane and the laser spot centered within the button. ^[1] The laser beam should always be in sharp focus.

If a corneal opacity develops during LPI, the procedure can be continued by asking the patient to look in a different direction. ^[1] If the view to the iris is still limited after this maneuver, another site must be chosen.

Identification of iridotomy site

The iridotomy site should be in the peripheral third of the iris just anterior to the arcus. A crypt or a thinned area of the iris is recommended. Most ophthalmologists place the iridotomy between 11 o’clock and 1 o’clock, where it is superiorly covered by the lids. The author, however, prefers using the 3-o’clock or 9-o’clock position. With a nasal or temporal iridotomy site, the view is not limited by the arcus; also, optical aberrations are less frequent than they would be with a superior site.

Fleck has recommended that the iridotomy be at least 200 μm in size. ^[8] Lam et al prefer that it be 500 μm in diameter. ^[2]

Application of laser

Different laser settings are employed, depending on the device used, the clinical situation, and the physician's preference. The values given below reflect the author's practice and should not be construed as representing rigid guidelines.

Recommended laser settings can be broadly divided into 2 groups on the basis of iris color: (1) recommendations for blue or green/light brown irides and (2) recommendations for dark brown irides.

In patients with blue or green/light brown irides, LPI can be performed with a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser, using the following settings:

• Power - 4-8 mJ

• Pulses/burst - 1-3 (the author prefers 2)

• Spot size - Fixed

In patients with dark brown irides, the iris can be thick. Because the Nd:YAG laser causes photodisruption, significant pigment dispersion, iris bleeding, and possibly hyphema can occur. To prevent these complications, the iris stroma is initially thinned with the argon laser, and the iris pigment epithelium is penetrated with the Nd:YAG laser.

First, the argon laser is employed to remove the anterior border of the iris, using the following settings:

• Power - 300-400 mW

• Spot size - 50-100 mm

• Duration - 0.05 seconds

If an air bubble develops, the power is reduced. The bubble can be easily dislodged by aiming the next laser shot at the inferior margin of the bubble. Aiming at the center of the bubble is not recommended, because the laser energy may be reflected back toward the cornea and causes a corneal burn. ^[1]

Next, the argon laser is employed to remove the iris stroma, using the following settings:

• Power - 900 mW

• Spot size - 50 mm

• Duration - 0.03-0.04 seconds

Alternatively, a pattern scan laser (Pascal) laser photocoagulator (Optimedica; Santa Clara, CA) may be used in place of the argon laser (see the image below). This device uses a short, 532-nm laser. Typical settings are as follows:

• Power - 600 mW

• Spot size - 100 mm

• Duration - 0.1 seconds

  Pascal laser.

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The Nd:YAG laser is then used to remove the iris pigment epithelium. Recommended laser settings are as follows:

• Power - 1.7-3 mJ

• Pulses per burst - 2

• Spot size - Fixed

The goal is to visualize aqueous following through the iridotomy site with iris pigment release. Additionally, the anterior capsule should be visible. If iris bleeding develops, pressure is applied to the globe with the contact lens (for approximately 10-20 seconds) until the bleeding is stopped.

Complications of Procedure

Complications of laser peripheral iridotomy include postoperative IOP spike, intraocular inflammation, iris bleeding and hyphema, focal cataract, ^[9] posterior synechiae, visual symptoms (eg, blurred vision, haloes, lines, glare, and diplopia), corneal decompensation, and miscellaneous complications.

Postoperative intraocular pressure spike

Elevation of IOP after LPI is common. Typically, the increase is transient, lasting less than 24 hours. When an IOP spike occurs, it is usually in the first hour after LPI (as many as 70% of cases) ^[10] or, less commonly, in the second hour (as many as 40% of cases). ^[11] A rise in IOP greater than 6 mm Hg occurs in as many as 40% of patients, and an IOP higher than 30 mm Hg is noted in as many as 30%. ^{[11, 12]}

To prevent postoperative IOP spike, a drop of topical apraclonidine (0.5% or 1%) or brimonidine (0.1%, 0.15%, or 0.2%) is placed on the eye before LPI. The IOP should be checked within an hour after LPI.

Anterior uveitis

Anterior segment inflammation is another common complication of LPI. It is thought to be due to the release of prostaglandins. ^[13] The inflammation is usually mild and can be successfully treated with topical steroids. Prednisolone acetate 1% 4 times a day for 5-7 days is prescribed.

Iris bleeding and hyphema

Because the Nd:YAG laser is a photodisruptive device, bleeding is common with its use, occurring in as many as 50% of patients. ^{[11, 14]} However, iris bleeding is uncommon with the use of the argon laser, which causes photocoagulation. Usually, iris bleeding can be controlled by applying pressure on the globe with the contact lens. In severe cases, the iris bleeding can lead to hyphema. ^[15]

Focal cataract

Lens opacities can develop if the iridotomy site is too close to the pupil. Cataract formation is attributable to heat buildup during argon laser use and direct tissue disruption during Nd:YAG laser use. ^{[1, 9]}

Posterior synechiae

Synechiae may occur between the iris and the lens at the pupillary border or at the iridotomy site. ^[16] Formation of posterior synechiae can be reduced by using postoperative topical steroids; any synechiae that form can be broken up by means of early postlaser dilation. ^[1]

Visual symptoms

Different visual symptoms can present after LPI. Transient blurred vision may occur in the immediate postlaser period. Possible causes include pigment dispersion, inflammation, and retained methylcellulose from contact lens placement. ^[13]

One study found that optical aberrations (eg, shadows, ghost images, lines, haloes, spots, glare, diplopia, and spots) were reported in 9% of eyes with completely covered iridotomies, 26% of those with partially covered iridotomies, and 17.5% of those with fully exposed iridotomies. ^[17] The authors concluded that a fully covered iridotomy is less prone to visual disturbances than a fully exposed or partially covered iridotomy. ^[17] One randomized study found that the location of the LPI (superior vs nasal/temporal) does not affect the likelihood of postoperative visual symptoms. ^[18] Six-month follow-up of these patients revealed that these visual symptoms frequently resolve over time. ^[19]

Corneal decompensation

After argon LPI, corneal decompensation can occur. Corneal edema may be focal ^[20] or generalized. ^[21]

Miscellaneous complications

The following complications are rare but have been reported in the literature:

• Aqueous misdirection ^[22]

• Recurrent herpetic keratouveitis ^[2]

• Retinal and subhyaloid hemorrhage ^[23]

• Choroidal and retinal detachment after argon LPI ^[24]

• Stage I macular hole ^[25]

• Lens subluxation and dislocation ^[3]

Closure of the iridotomy site is rare, especially when the Nd:YAG laser is used; however, it is common in patients with uveitis that requires LPI. ^[13]