PERIPHERAL LASER IRIDOTOMY

 GUEST AUTHOR

SAMREEN FARHA

AJMAL KHAN TIBBIYA COLLEGE

ALIGARH

INDIA

INTRODUCTION

The term “iridotomy” refers to the creation of a hole in the iris. Through common usage the laser procedure for doing this has become known as “laser iridotomy” or less commonly “laser iridectomy” and the incisional technique as “surgical iridectomy”.

INDICATIONS FOR LASER IRIDOTOMY

• Primary angle closure.
• Pupillary block associated with uveitis.
• Plateau iris configuration.
• Nanophthalmos.
• Ciliary block glaucoma.

PREOPEARTIVE PATIENT PREPARATION

Informed signed consent from the patient should be taken.

Pilocarpine eye-drops maybe instilled twice at 15 minute intervals, if the patient was previously not on these drops. By inducing miosis, pilocarpine unfolds the iris, reducing the thickness of the iris and improving the surgeon’s ability to create a full thickness hole with less amount of energy.

These advantages must be weighed against a possible increase in inflammation and development of posterior synechiae, which are enhanced by the miotic.

To blunt post-laser increase in intraocular pressure (IOP), 1% Apraclonidine can be instilled an hour prior to the procedure and immediately following the laser.

In case Apraclonidine is not available, Tab Acetazolamide 500mg before and after the procedure can be given.

Anesthesia is achieved by topical anesthetic drops.

Patients presenting during an acute attack of pupillary block glaucoma may require special measures to prepare the eye for the laser iridotomy.

In such cases the cornea may be cloudy from acutely elevated IOP and intravenous acetazolamide can be given to reduce the IOP to reduce the corneal edema sufficiently for better visualization of the anterior segment structures and accurate laser application.

Topical hypertonic saline may also be instilled to reduce corneal edema.

In extreme cases peribulbar or sub-tenon anesthesia can be given to reduce the pain.

If argon laser is not possible due to cloudy cornea, Nd:YAG laser alone can be attempted.

In the patient who is unresponsive to medical therapy or a poor surgical candidate, then a laser pupilloplasty or peripheral iridoplasty with argon laser can be used to break the pupillary block and relieve the attack.

LASER TECHNIQUE

The patient is seated at the laser instrument (Slit-lamp delivery system) and the iris viewed through the slit-lamp magnification.

A special contact lens such as the Abraham iridotomy lens is used to stabilize the eye, provide additional magnification and to keep the eyelids open.

The Abraham lens has a +66 diopter plano-convex lens button affixed to its anterior surface. This lens adds increased convergence to the laser beam, reducing its diameter and thus increasing the power density at the iris and decreasing it at the cornea. This facilitates creation of an iridotomy and reduces the risk of producing a corneal burn.

Following the same principles, the Wise lens, which uses a 103 diopter optical button, increases the energy density at the iris surface 2.92 times greater than the Abraham lens and further enhances the efficiency of the laser energy.

SELECTING THE IRIDOTOMY SITE

It is advisable to perform the iridotomy in the superior quadrant of the iris so that it is covered by the upper eyelid (thus avoiding uniocular diplopia in the patient). It is probably preferable to avoid the 12 o’clock area as gas bubbles formed during argon laser application tend to rise up and obscure the surgeon’s view of the iridotomy site.

The iridotomy is easier to achieve where the iris is thinnest. Relatively thin areas are found at the base of the iris crypts.

TREATMENT PARAMETERS

Various combinations of laser parameters have been described in order to perform an iridotomy.

ARGON LASER:

PREPARATORY BURNS	PENETRATING ARGON LASER BURNS
Spot size: 200-500 microns	Spot size: 50 microns.
Duration: 0.1-0.5 seconds.	Duration: 0.2 seconds.
Energy level: 200-600 mW.	Energy: 800-1000 mW.
The thermal energy contracts the underlying iris and increases the tension on adjacent iris tissue. Contraction burns can be placed on either side of the intended site. A single broad laser burn will create an elevated area or “hump” nearby. Placing the iridotomy at the top of the hump may facilitate the penetration of the iris. In the “drumhead technique”, three to six such contraction burns are placed in a ring around the intended iridotomy site. Some surgeons do not use preparatory burns as exposing the iris to additional laser energy releases more pigments which may block the trabecular meshwork.	These settings are usually effective in dark-medium brown eyes. However, in pale irides (with little pigment to absorb) and in dark irides (thick), difficulty may be encountered.

Nd: YAG LASER:

Usually laser settings of 6-8 mJ are sufficient in most cases.

Bursts of 5-6 pulses have been associated with damage to the lens.

Higher energy burns may also cause bleeding into the anterior chamber. Application of pressure by the iridotomy contact lens for sometime may stop the bleeding.

END POINT

The end point of treatment is observation of a gush of aqueous through the patent iridotomy and visualization of the anterior lens capsule through the iridotomy. A size of 150-200 microns for the PI is adequate.

COMPLICATIONS

• Transient and occasionally, chronic uveitis.
• Acute or chronic IOP elevation.
• Late closure of iridotomy.
• Localized corneal and lens damage.
• Hemorrhage from iris vessels.
• Laser burns to the peripheral retina.
• Laser burns to the fovea causing profound visual loss.
• Glare and diplopia through the iridotomy.
• Pupillary distortion and formation of posterior synechiae.

POST-LASER TREATMENT

The patient's anti-glaucoma treatment is not stopped immediately after the laser procedure. The patient is followed up periodically over a month to assess the IOP and the anti-glaucoma medications tailored to the need.
The patient is also put on topical steroid eye-drops (usually 4 times a day for a week). the frequency and duration can be tailored according to the inflammation.