Fig. 30-8  This scanning electron micrograph of Nd:YAG laser iridotomy performed in a monkey eye demonstrates the dispersion of power that occurs at the site of laser impact. This is a full-thickness iridotomy through which a ciliary process can be seen (lower half of photo). Note the damage to the adjacent trabecular meshwork and the corneal endothelium caused by the shock-wave (upper half of photo).

(Photo courtesy of Thomas M Richardson, MD, Harvard Medical School, Boston.)

Corneal stroma

A poorly focused Nd:YAG laser treatment or backward movement by the patient in the instant the treatment is delivered can result in the energy being delivered to the mid stroma.This results in a localized but rather spectacular effect called corneal emphysema, in which a series of small bubbles gives the affected cornea the appearance of a shattered windshield. This effect usually dissipates within an hour or so without clinically significant sequelae.

Increased trabecular pigmentation may occur as a result of iris pigment released by laser iridotomy.This pigment cleared with no microscopic sequelae in healthy monkey eyes,55 but it could likely reduce outflow in an impaired trabecular meshwork.

Failure to perforate

Failure to perforate is rare. Occasionally patients will require a second treatment in 1–3 days. Waiting an hour often allows anterior chamber debris to clear adequately to complete the procedure. In patients with acute angle-closure glaucoma, it is important to obtain a patent iridotomy at the first attempt. An incomplete attempt will generate inflammation that can cause permanent peripheral anterior synechiae if the angle is not opened. If laser iridotomy cannot be accomplished, urgent surgical iridotomy is advisable unless the angle is opened medically. Medical therapy for angle-closure glaucoma usually clears the cornea adequately for successful laser iridotomy by Nd:YAG laser when argon laser iridotomy is not possible.

Late closure

It is estimated that a 15-  m hole is adequate for aqueous flow through the eye,56 but an iridotomy may become narrow as the pupil dilates.55 Also, pigment clumps may be released into the posterior chamber, which can subsequently occlude small iridotomies.57 Deposition and proliferation of pigment may narrow a patent

laser iridotomy during the first few weeks after treatment. To prevent subsequent closure, it is best to have an opening of at least 100  m. Although rare 2 months after the procedure, closure can even occur years later. If the opening does occlude, retreatment in the same place with either Nd:YAG or photothermal laser at relatively low doses easily re-opens the site. When removing pigment with argon or diode laser, it is important that gaze be directed such that any potential for the laser beam reaching the macula is prevented.

Retinal burn

Retinal burns, including burns of the fovea, have been reported with argon lasers.46 Rarely are peripheral retinal burns of any consequence. They can be prevented by taking care not to fire the argon or solid-state laser directly through a patent iridotomy.

Aphakia and pseudophakia with pupillary block

Pupillary block is more common in pseudophakic eyes with anterior chamber lenses than in eyes with posterior chamber lenses. It can occur with any type of lens, however, especially in diabetic eyes and eyes without an iridotomy. Anterior chamber lenses often present with the iris bulging forward around the implant. A first iridotomy should be performed through a portion of the iris that is not touching the cornea. This breaks the block and allows the iris to fall back after a few minutes. Additional iridotomies should be placed in other quadrants of the iris because, especially with anterior chamber intraocular lenses, the posterior chamber may act as though it were divided into separate pockets of sequestered fluid. The vitreous body is often pressed against the back of the iris and can occlude an iridotomy. Because the vitreous seems to encourage closure of these iridotomies with pigment, iridotomies must

be observed carefully for the first 6 months and subsequently three to four times a year.58–62 The Nd:YAG laser may be more effective

in treating this problem because it creates less local iris inflammation and can disrupt the anterior hyaloid at the iridotomy site, if necessary.

Laser iridoplasty (gonioplasty)

Plateau iris

Plateau iris (Fig. 30-9) may not be resolved by iridotomy alone. The peripheral iris may continue to crowd and obstruct the angle. Laser iridoplasty can be used to contract the peripheral iris, pulling it away from the angle.Argon or solid-state lasers are used with the lowest power setting that creates contraction of the iris. The surgeon should avoid explosive or vaporizing effects. Typical settings are a 100–200-  m spot size and 100–30 mW at 0.1 second. Lighter irides will require slightly higher energy levels than darker. Ten to twenty spots evenly distributed over 360º of the iris are usually sufficient, but more or fewer may be placed as the condition warrants.63,64 In some cases, the effect is not permanent. Higher energy levels and longer exposures cause greater contraction of collagen but also cause more trauma and iritis. A safe approach is to use lower energy levels initially and, if the condition recurs after 1–6 months, repeat the treatment with higher power. Peaking of the pupil can be accomplished by a similar technique used in only one section of the iris. Laser iridoplasty may assist in breaking an acute attack of angle-closure glaucoma but is less satisfactory than is a completed iridotomy.