LASER EYE SURGERY IRIDOTOMY

Laser Eye Surgery - iridotomy : Laser iridotomy is a surgical procedure that is performed on the eye to treat angle closure glaucoma, a condition of increased pressure in the front chamber (anterior chamber) that is caused by sudden (acute) or slowly progressive (chronic) blockage of the normal circulation of fluid within the eye. The block occurs at the angle of the anterior chamber that is formed by the junction of the cornea with the iris. All one needs to do to see this angle is to look at a persons eye from the side. Angle closure of the eye occurs when the trabecular meshwork, the drainage site for ocular fluid, is blocked by the iris. Laser iridotomy was first used to treat angle closures in 1956.

During this procedure, a hole is made in the iris of the eye, changing its configuration. When this occurs, the iris moves away from the trabecular meshwork, and proper drainage of the intraocular fluid is enabled. The angle of the eye refers to a channel in which the trabecular meshwork is located. To maintain the integrity of the eye, fluid must always be present in the anterior (front) and posterior (back) chambers of the eye. The fluid, known as aqueous fluid, is made in the ciliary processes, which are located behind the iris.

Released continuously into the posterior chamber of the eye, aqueous fluid circulates throughout the eye. Eventually the fluid returns to the general circulation of the body, first passing through a space between the iris and the lens, then flowing into the anterior chamber of the eye and down the angle, where the trabecular meshwork is located. Finally, the fluid leaves the eye. An angle closure occurs when drainage of the aqueous fluid through the trabecular meshwork is blocked and the intraocular pressure builds up as a result.

For most types of angle closure, or narrow angle glaucoma, laser iridotomy is the procedure of choice. Changes in intraocular pressure (IOP) can alter the name of the condition when the IOP in the eye becomes elevated above 22 mm/Hg as a result of an angle closure. Then, angle closure becomes angle closure glaucoma. Lowering of the IOP is important because extreme elevations in IOP can damage the retina and the optic nerve permanently.

The lasers used to perform this surgery are either the Nd:Yag laser or, if a patient has a bleeding disorder, the argon laser. The majority of patients with glaucoma do not have angle closure glaucoma, but rather have an open angle glaucoma, a type of glaucoma in which the angle of the eye is open.

An angle closure occurs when ocular anomalies (abnormalities) temporarily or permanently block the trabecular meshwork, restricting drainage of the ocular fluid. The anatomical anomalies that make an individual susceptible to an angle closure are, for example, an iris that is bent forward in the anterior chamber (front) of the eye, a small anterior chamber of the eye, and a narrow entrance to the angle of the eye. Some conditions that cause an angle closure are a pupillary block, a plateau iris, phacolytic glaucoma, and malignant glaucoma.

The end result of all of these situations is an elevation of the IOP due to a build-up of aqueous fluid in the back part of the eye. The IOP rises quickly when an acute angle attack occurs and within an hour the pressure can be dangerously elevated. The sclera or white of the affected eye becomes red or injected. The patient will usually experience decreased vision and ocular pain with an acute angle closure. In severe cases of acute angle glaucoma, the patient may experience nausea and vomiting. Individuals with neurovascular glaucoma caused by uncontrolled diabetes or hypertension may have similar symptoms, but treatment for this type of glaucoma is very different.

Within a normal eye, the iris is in partial contact with the lens of the eye behind it. Individuals with narrow angles are at greater risk of angle closure by pupillary block because their anterior chamber is shallow; thus, the iris is closer to the lens and more likely to adhere completely to the lens, creating a pupillary block. Patients who experience a pupillary block may have had occasionally temporary blocks prior to a complete angle closure. Pupillary block can be started by prolonged exposure to dim light. Therefore, it not uncommon for an acute angle closure to occur as an individual with a narrow angle emerges from a dark environment such as a theater into bright light. It can also be brought on by neurotransmitter release during emotional stress or by medications taken for other medical conditions. Pupil dilation may be a side effect of one or more of those medications. However, pupillary block is the most common cause of angle closure, and laser iridotomy effectively treats this condition.

The irises of individuals with plateau iris is bunched up in the anterior chamber, and it is malpositioned along the trabecular meshwork. Plateau iris develops into glaucoma when the iris bunches up further; this occurs on dilation of the iris, which temporarily closes off the angle of the eye. Laser iridotomy is often performed as a preventive measure in these patients, but is not a guarantee against future angle closure. This is because changes within the eye, such as narrowing of the angle and increase in lens size can lead to iris plateau syndrome, where the iris closes the angle of the eye even if a laser iridotomy has already been performed. Peripheral laser iridoplasty and other surgical techniques can be performed if the angle still closes after iridotomy.

Other causes of narrow angle glaucoma are not as common. Phacolytic glaucoma results when a cataract becomes hypermature and the proteins of the lens with the cataract leak out to block the angle and the trabecular meshwork. Laser iridotomy is not effective for this type of angle closure. Malignant glaucoma exists secondary to prior ocular surgery, and is the result of the movement of anatomical structures within the eye such that the meshwork is blocked. Patients who have no intraocular lens (aphakic) are at increased risk for angle closure, as well.

Laser iridotomy is also performed prophylactically (preventively) on asymptomatic individuals with narrow angles and those with pigment dispersion. Individuals with a narrow angle are at higher risk of an acute angle closure, especially upon dilation of the eye. Pigment dispersion is a condition in which the iris pigment is shed and is dispersed throughout the anterior part of the eye. If the dispersion occurs because of bowing of the iris (the case in 60% of patients with pigment dispersion) a laser iridotomy will decrease the bowing or concavity of the iris and subsequent pigment dispersion. This decreases the risk of these individuals to develop pigmentary glaucoma, a condition in which the dispersed pigment may clog the trabecular meshwork. Laser iridotomy is also done on the fellow eye of a patient who has had an angle closure of one eye, as the probability of an angle closure in the second eye is 50%.

There are other indications for laser iridotomy. It is performed on patients with nanophthalmos, or small eyes. Laser iridotomy may be also be indicated for patients with malignant glaucoma to help identify the etiology of elevated IOP. Because laser iridotomy changes the configuration of the iris, it is sometimes used to open the angle of the eye prior to performing a laser argon laser trabeculoplasty, if the angle is narrow. Laser trabeculoplasty is another laser procedure used to treat pigmentary and pseudoexfoliation glaucoma.

Laser iridotomy cannot be performed if the cornea is edematous or opacified, nor if the angle is completely closed. If an inflammation (such as uveitis or neovascular glaucoma) has caused the angle to close, laser iridotomy cannot be performed.

Purpose of Laser Eye Surgery - Iridotomy
The purpose of a laser iridotomy is to allow an equalization of pressure between the anterior (front) and posterior (back) chambers of the eye by making a hole in the superior peripheral iris. Once the laser iridotomy is completed, the intraocular fluid flows freely from the posterior to the anterior part of the eye, where it is drained via the trabecular meshwork. The result of this surgery is a decrease in IOP.

When laser iridotomy is performed on patients with chronic angle closure, or on patients with narrow angles with no history of angle closure, the chances of future pupillary blocks are decreased.

Demographics of Laser Eye Surgery - Iridotomy
Acute angle glaucoma occurs in one in 1,000 individuals. Angle-closure glaucoma generally expresses itself in populations born with a narrow angle. Individuals of Asian and Eskimo ancestry appear to be at greater risk of developing it. Family history, as well as age, are risk factors. Older women are more often affected than are others. Laser iridotomy is performed on the same groups of individuals as those likely to experience angle closures due to pupillary block or plateau iris. They are performed more often on females (whose eyes are smaller than those of males), and more often performed on the smaller eyes of farsighted people than on those of the nearsighted because angle closures occur more frequently in those who are farsighted. Most laser iridotomies are performed on those over age 40 with a family history of plateau iris or narrow angles. However, preventative plateau iris laser iridotomies are performed on patients in their 30s. Individuals who are aphakic (have no intraocular lens) are at greater risk of angle closure and undergo laser iridotomy more frequently than phakic patients. Phakic patients are those who either have an intact lens or who are psue-dophakic (have had a lens implant after the removal of a cataract removal).

Description of Laser Eye Surgery - Iridotomy
After the cornea swelling has subsided and the IOP has been lowered, which is usually 48 hours after an acute angle closure, laser iridotomy can be performed. Pilocarpine is applied topically to the eye to constrict the pupil prior to surgery. When the pupil is constricted, the iris is thinner and it is easier for the surgeon to form a penetrating hole. If the eye is still edematous (swollen) - often the situation when the IOP is extremely high - glycerin is applied to the eye to enable the surgeon to visualize the iris. Apraclonidine, an IOP-lowering drop, is applied one hour before surgery. Immediately prior to surgery, an anesthetic is applied to the eye.

Next, an iridotomy contact lens, to which methylcellulose is added for patient comfort, is placed on the upper part of the front of the eye. This lens increases magnification and helps the surgeon to project the laser beam accurately. The patient is asked to look downwards as the surgeon applies laser pulses to the iris, until a hole is formed. Once the hole has penetrated the iris, iris material bursts through the opening, followed by aqueous fluid. At this point, the surgeon can also see the anterior part of the lens capsule through the opening. The hole, or iridotomy, is formed on the upper section of the iris at an 11:00 or 1:00 position, so that the hole is covered by the eyelid. In an aphakic eye, the hole may be made on the inferior iris.

After performing the laser iridotomy, the surgeon may place a gonioscopy lens on the eye if the angle has been opened. There is no pain associated with this surgery, although heat may be felt at the site of the lasering. If a patient has a tendency to bleed, the argon laser will be used to pre-treat the patient prior to completing the procedure with an Nd:Yag laser, or the argon laser alone may be used. The argon laser is capable of photocoagulation, and, thus, minimizes any bleeding that occurs as the iris is penetrated. Formation of a hole is more difficult with the argon laser because it operates with a decreased power density and the tissue response to the argon laser has greater variability. The argon laser can be used with more patients who have medium-brown irises, however, since the energy of this laser is readily absorbed by irises of this color.