Larissa Camejo, MD
Introduction
Peripheral iridectomy (PI) was introduced in 1956 but later gained popularity with the advent of argon laser and, more recently, with the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser. Peripheral iridectomy involves creating a full-thickness defect in the peripheral iris and is most commonly performed to treat pupillary block angle closure. In pupillary block, a functional block develops between the lens and the iris, which causes pressure to build up posterior to the iris and makes the iris bow forward. In this manner, the peripheral iris closes the trabecular meshwork by apposition.
Laser peripheral iridectomy (LPI) enables free passage of aqueous humor between the posterior and anterior chambers of the eye, which equalizes the pressures in between and lets the peripheral iris rest in a natural position. (i.e., away from the angle). This laser procedure may be called iridotomy or iridectomy depending on whether a surgeon is opening a hole or destroying and removing iris tissue. Laser peripheral iridectomy has replaced surgical iridectomy.
Today, surgical iridectomy is reserved for patients who cannot cooperate at the slit lamp or for those with severe inflammatory eye conditions that cause repetitive closure of the PI site with fibrin. Laser peripheral iridectomy is the treatment of choice for angle-closure glaucoma when the mechanism involved is pupillary block.
LPI may be performed using an argon laser or an Nd:YAG laser. Diode laser has also been used. Considerable differences are found among the laser-tissue interactions with the various technologies, and the Nd:YAG has been proven to be the laser of choice for LPI.
Nd:YAG is the most commonly used type of laser for LPI. The PI closure rate is lower when Nd:YAG is used given to disruption of the iris pigment epithelium with this type of laser. Pigment migration and plugging accounts for most cases of PI closures, except in uveitis, where fibrin may close the PI (Slide 1).1-3
Indications for LPI
Indications for LPI include:
- Angle closure with pupillary block
- Narrow, occludable angles with a pupillary block mechanism
- Chronic angle-closure glaucoma with pupillary block
- Fellow eye in acute pupillary block angle closure
- Combined mechanism glaucoma including pupillary block-mediated angle-closure mechanism
- Plateau iris: LPI can treat the pupillary block component of this pathology and therefore help distinguish between plateau iris configuration in which case LPI would suffice to open and deepen the angle, and plateau iris syndrome in which case the angle remains narrow or closed regardless of LPI.
- The use of LPI in reverse pupillary block in pigmentary dispersion syndrome or pigmentary glaucoma is controversial. Laser peripheral iridectomy reverses the posterior bowing of the iris that is typically seen in this pathology and that causes the iris chafing against the lens zonules with prominent release of pigment into the angle. The effect of LPI on IOP and the disease process, however, is uncertain.
- Nanophthalmos with angle closure
Contraindications for LPI
Aside from lack of an indication for LPI, perhaps the absolute contraindication is inability to adequately view the iris to perform LPI. Caution is recommended if corneal pathology is present or if the anterior chamber is flat. Visibility is key to this procedure.
In cases of acute angle-closure glaucoma in which the IOP is high and significant corneal edema is present, visibility may be compromised. IOP-lowering and application of topical glycerin to the corneal surface may be pursued in an attempt to clear the cornea before performing the LPI.
Finally, it is important to remember that a PI is not helpful if the angle closure is caused by a non-pupillary block mechanism. Laser peripheral iridectomy is not effective for synechial angle closure like that seen in neovascularization or iridocorneal endothelial syndrome.
The Technique for LPI
Laser peripheral iridectomy is performed most commonly with an argon or Nd:YAG laser. The latter is used by most ophthalmologists. The argon laser may be useful in combination with the Nd:YAG laser in some occasions (e.g., in patients with dark brown irides that may be thick and difficult to perforate). In those situations, it may be convenient to pre-treat the intended iridectomy area with argon laser to thin it before full iris perforation with the Nd:YAG laser is attempted. The laser settings are shown in Table 1.
After full explanation to a patient of what the procedure involves, what to expect (e.g., seeing a flash, hearing and feeling a "pop," electric shock sensation), and after informed consent is obtained from a patient, pilocarpine 2% drops are instilled every 5 minutes times three in the procedure eye. Adequate constriction and akinesia of the pupil must be ensured. The goal is to stretch the iris to facilitate its perforation with the laser. A drop of apraclonidine or brimonidine is given. Proparacaine is instilled for anesthesia.
A patient and operator are adequately positioned for the laser treatment. A patient's head may be secured with a head strap if necessary. Laser energy is selected to be between 4 mJ and 6 mJ and single pulse mode is preferable. Generally, one to 10 single pulses are sufficient. The less energy used to perforate the iris, the better. A corneal contact lens with a plano-convex button (Abraham or Wise lens) is placed on the eye after filling it with a coupling solution (Slide 2).
The laser beam is focused on the iris along the superior peripheral iris, usually at 11 o'clock or 1 o'clock. The peripheral iridectomy should be placed in a location where it can be covered by the upper eyelid. Furthermore, location is influenced by the presence of iris crypts along the superior peripheral iris, in which the iris is already thinned and penetration may be easier.
Once location for laser delivery is chosen and the beam is focused on the iris, the slit lamp is pushed forward toward the patient to defocus the beam slightly posteriorly (single spot separates into two or four spots, depending on the laser manufacturer), placing it deeper in the iris, in midstroma (Slide 3).
Laser energy is delivered until full thickness perforation is achieved. A "gush" of fluid and pigment may be seen once this is achieved. This represents aqueous passage from a higher pressure posterior chamber to a lower pressure anterior chamber, thus breaking the pupillary block. The iris should fall back away from the angle to a flat position because the pressures in the anterior and posterior chambers are now equalized.
Once a full thickness hole is created in the iris, it can be enlarged if necessary by directing the beam to the edges of the hole (avoid applying laser to underlying structures such as the lens). Before the contact lens is removed, patency of the PI is confirmed by disappearance of the aiming beam in the iridectomy and being able to see and focus on the lens capsule in the area of the PI. The contact lens is then removed, the patient is reassured and asked to wait for 1 hour for an IOP check.
In the absence of complications, the patient is instructed to use prednisolone acetate 1% drops four times a day for 4 days in the operated eye. Pre-laser medications are continued except for miotics (to avoid synechiae formation). Follow-up is usually in 1 to 2 weeks, at which time the PI patency is evaluated, IOP is rechecked and gonioscopy is performed to evaluate the angle for openness and occludability. Gonioscopy and dilation may be reserved for the patient's 6-week visit. In a study on acute primary angle closure in 44 Asian subjects, deepening of the angle after LPI was observed in the first 2 weeks and did not vary further in 1 year.4
Contact Lens Use and Options
A corneal contact lens with a plano convex button is used. Two main types are available: Abraham and Wise lenses. The Abraham iridectomy lens has a 66 D plano-convex button that is larger and easier to focus through than the Wise lens, especially when used with the Nd:YAG laser, which has a larger cone angle than the argon. On the other hand, the Wise lens has a plano-convex button of 103 D that although smaller, concentrates the laser energy more.
The corneal contact lens:
- Concentrates the energy by minifying the beam spot
- Magnifies the target site
- Acts as a heat sink, decreasing corneal burns (with photocoagulators)
- Helps in keeping the eye open
Complications
Complications from LPI are rare but can occur.
Cornea
Corneal whitening at the LPI overlying site can occur secondary to unexpected energy absorption by the endothelium when using argon or diode laser or due to corneal edema secondary to disruption of the endothelium in case of using the Nd:YAG laser. Prompt discontinuation of laser application through this area is recommended. The corneal whitening will generally disappear with time.
Uveitis
Variable amounts of inflammation can be expected after LPI. It is usually a mild transient iritis that resolves in a few days. Prednisolone acetate 1% is routinely prescribed and seems to control the inflammation with low doses, namely 1 drop four times a day for 4 days.
IOP Spike
IOP is routinely checked 1 hour after the procedure. It is not rare to find an elevated IOP. All patients should be pre-treated with apraclonidine 0.5% or 1% or brimonidine 0.15% or 0.2% for prophylaxis. If the IOP spike is significant compared to the patient's baseline or appears hazardous given the patient's degree of glaucoma damage, adequate IOP lowering should follow.
Topical medications are tried first and control the spike in the majority of cases. Oral medications such as acetazolamide or glycerin may be needed in certain patients. IOP elevation may persist for 1 month and this should be appropriately managed. Laser peripheral iridectomy is commonly performed in individuals who already have more susceptible optic nerves than healthy patients, hence, it is important to avoid any further damage from an untreated pressure spike.
Lens Injury
Lens injury is a potential complication of this procedure but to date no evidence of long-term consequences of LPI to the lens exists.
Pigment Dispersion
It is common to observe additional pigment in the angle after LPI. Consequences are not known.
Retinal Burn
This risk can be practically eliminated with the use of the contact lens and adequate caution.
Corectopia
The pupil can change its shape, elongating towards the quadrant lasered. This may happen with photocoagulation and usually does not persist.
Monocular Diplopia
Rarely patients may complain of diplopia or "ghost image" after LPI. This will be seen if the laser spot is small and in the right position for light refracted by the upper lid tear meniscus to enter and become focused on the retina.
Iris Atrophy
This was a complication when low-power photocoagulation was used, but is now rarely seen.
Malignant Glaucoma
Laser peripheral iridectomy can precipitate malignant glaucoma, also known as aqueous misdirection syndrome. Laser peripheral iridectomy is the treatment for angle closure caused by pupillary block and malignant glaucoma is mostly seen in patients with angle closure after an incisional or laser procedure. Shallowing of the anterior chamber is seen in pupillary block and malignant glaucoma but the pattern is different. In pupillary block, iris convexity may be observed, with the chamber remaining deep centrally and shallow peripherally. On the other hand, in malignant glaucoma, the chamber shallows centrally and peripherally.
Failure
Patency should always be confirmed after the procedure is done. Positive transillumination may not be enough proof of patency, given that partial thickness PIs may transilluminate as well. Visualization of structures beyond the iris, such as the lens capsule, may be more accurate in indicating patency. If full thickness penetration is not achieved, then more laser applications are needed until this is accomplished.
Late Closure
Once the LPI is patent, it rarely closes and if it does, it happens during the first few weeks or during a bout of uveitis. Pigment migration is the most common cause of peripheral iridectomy closure. Nd:YAG laser has been proven to be more efficient than argon or diode in removing the iris pigment epithelium, presumably due to its shock wave. Peripheral iridectomy closure after Nd:YAG laser peripheral iridectomy is rare.
Posterior Synechiae
Avoid the use of miotics after LPI because they can contribute to synechiae formation. Posterior synechiae may form after anterior segment inflammation. If the inflammation persists, it may be wise to employ cycloplegia for the eye.
Hemorrhage
Blood vessels can be ruptured when using the Nd:YAG laser because it does not photocoagulate the tissue (as argon laser does). Bleeding during LPI is generally self-limited and can be treated by gentle pressure on the eye through the contact lens.
References
- Latina MA, Puliafito CA, Steinert RF, et al. Experimental iridotomy with the q-switched neodymium-YAG laser. Arch Ophthalmol. 1984;102:1211-1213.
- Robin AL, Pollack IP. A comparison of neodymium: YAG and argon laser iridotomies. Ophthalmology. 1984;91:1011-1016.
- Pollack IP, Robin AL, Dragon DM, et al. Use of the neodymium: YAG laser to create iridotomies in monkeys and humans. Trans Am Ophthalmol Soc. 1984;82:307-328.
- Lim LS, Aung T et al. Acute primary angle closure: configuration of the drainage angle in the first year after laser peripheral iridotomy. Ophthalmology. 2004;111:1470-1474.
- Epstein D, Allingham R, Schuman J. Laser Peripheral Iridectomy. In: Chandler's and Grant's Glaucoma. 4th ed. Baltimore, Md: Williams & Wilkins; 1996:472-483.
- Alward, WL. Laser surgical treatment. In: Krachmer JH ed. Glaucoma: The requisites in Ophthalmology. St. Louis, MO: Mosby Ed. 2000;206-213.
- American Academy of Ophthalmology. Angle-Closure Glaucoma. In: BCSC Glaucoma. AAO: 2003;100-106.