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 person's 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,
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 mesh-work 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.
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.
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 psuedophakic (have had a lens implant after the removal of a cataract removal).
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 photo-coagulation, 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.
To determine if laser iridotomy is indicated, the surgeon must first determine if and how the angle is occluded. The eye is anesthetized and the aonioscopic lens, which enables the surgeon to see the interior of the eye, is placed on the front of the eye. This is done at the slit lamp biomicroscope in a dark room. In cases of prophylactic surgery, an image of the eye is taken with a ultra-sound biomicroscope in both dim and bright light; this shows the doctor how the patient's iris moves with dilation and constriction, and how this movement can close an angle if the patient has ocular features that predispose the eye to an angle closure.
When an angle is completely occluded (blocked), the elevated IOP usually causes corneal edema (swelling). Because this swelling can obscure the surgeon's view of the iris, prior to performing a laser iridotomy, the IOP must be lowered. One technique to lower the IOP is corneal indentation, in which the gentle pressure is applied several times to the cornea with a lens or hook to open the angle. This pressure on the cornea causes a shift in the internal structures of the eye, enhances aqueous drainage, and lowers the IOP.
The doctor can attempt to lower the IOP medically, as well. One drug that lowers the pressure is acetazolamide, which is given either orally or by intravenous(IV) to decrease aqueous production in the eye. This may be administered up to four times a day, until the adhesion is broken. Another method of lowering the IOP, if acetazolamide is not effective, is with the use of hyperosmotic agents, which through osmosis causes drainage of the aqueous fluid from the eye into the rest of the body. Hyperosmotic agents are given orally; an example of such an agent is glycerine. Given by IV (intravenous administration), mannitol can be used. As the fluid drains from the eye, the vitreous—the jelly-like substance behind the lens in the posterior chamber—shrinks. As it shrinks, the lens in the eye pulls away from the vitreous, creating an opening to the anterior chamber such that aqueous fluid can flow to the anterior chamber. The success of this procedure is increased, due to gravity, if the patient is laying supine.
Once the IOP has begun to decrease, the pressure is further decreased using topical glaucoma medications, such as pilocarpine, or beta blockers. Any inflammation that occurs because of the iridotomy must be controlled with steroid eye drops.
If glaucomatous-like visual field is present prior to surgical intervention, the prognosis for the patient is not as good as if the visual field were completely intact. Thus, a visual field test may be done prior to surgery.
Immediately after the procedure, another drop of aproclonidine is applied to the eye. The IOP is checked every hour for a several hours postsurgery. If the IOP increases dramatically, then the increased IOP is treated until lowered. Because of inflammation is inherent in this procedure, corticosteroids are applied to the eye every five minutes for 30 minutes, then hourly for six hours. This therapy is then continued four times a day for a week. Thereafter, the patient is seen by the surgeon at one week post-surgery and again at two to six weeks post-surgery. If there are complications, the patient is seen more frequently.
After the pressure has been stabilized, a visual field test to determine the extent of damage to the optic nerve may be performed again.
The greatest risk of laser iridotomy is an increase in intraocular pressure. Usually, the IOP spike is transient and of concern to the surgeon only during the first 24 hours after surgery. However, if there is damage to the trabecular meshwork during laser surgery , the intraocular pressure may not be lowered enough and extended medical intervention or filtration surgery is required. Patients who undergo preventative laser iridotomy do not experience as great an elevation in IOP.
The second greatest risk of this procedure is anterior uvetis, or inflammation within the eye. Usually the inflammation subsides within several days, but can persist for up to 30 days. Thus, the follow-up care for laser iridotomy includes the application of topical corticosteroids. A posterior synechia, in which the iris may again adhere to the lens, may occur if intraocular inflammation is not properly managed.
Other risks of this procedure include the following: swelling of, abrasions to, or opacification of the cornea; and damage to the corneal endothelium (the part of the cornea that pumps oxygen and nutrients into the iris); bleeding of the iris during surgery, which is controlled during surgery by using the iridotomy lens to increase pressure on the eye; and macular edema, which can be avoided by careful aim of the laser during surgery to avoid the macula. The macula is the part of the eye where the highest concentration of photoreceptors is found. Perforations of the retina are rare. Distortion of the pupil and rupture of the lens capsule are other possible complications. Opacification of the anterior part of the lens is common, but this does not increase the risk of cataract formation when compared with the general population.
When the iridotomy hole is large, or if the eyelid does not completely cover the opening, some patients report such side effects as glare and double vision. The argon laser produces larger holes. Patients may also complain of an intermittent horizontal line in their vision. This may occur when the eyelid is raised just enough such that a small section of the inferior part of the hole is exposed, and disappears when the eyelid is lowered. Blurred vision may occur as well, but usually disappears 30 minutes after surgery.
In successful laser iridotomy, the IOP differential between the anterior and posterior chambers is relieved and IOP is decreased, and the pupil is able to constrict normally. These are the results of the flatter configuration of the iris after laser iridotomy. If an angle closure is treated promptly, the patient will have minimal or no loss of vision. This procedure is successful in up to 44% of patients treated.
For up to 64% of patients, one to three years after laser iridotomy, the IOP will rise above 21 mmHg, and long-term medical treatment is required. One-third of argon laser iridotomies will close within six to 12 weeks after surgery and will require a repeat laser iridotomy. Approximately 9% of Nd:Yag laser iridotomies must be redone for this reason. Closure of the iridotomy site is more likely if a uveitis presented after surgery. Up to 45% of patients will have anterior lens opacities after laser iridotomy, but these opacifications do not put the patient at an increased risk of cataracts.
An alternative to laser iridotomy is surgical iridectomy , a procedure in which part of the iris is removed surgically. This was the procedure of choice prior to the development of laser iridotomy. The risks for iridectomy are greater than for the laser iridotomy, because it involves an incision through the sclera, the white tunic covering of the eye that surrounds the cornea. The most common complication of an iridectomy is cataract formation, occurring in more than 50% of patients who have had a surgical iridectomy. Since an incision in the eye is required for surgical iridectomy, other procedures, such as filtration surgery—if needed in the future—will be more difficult to perform. Studies comparing the visual outcomes and IOP control of laser iridotomy with surgical iridectomy show equivalent results.
In the case of acute angle closures that occur because of reasons other than, or in addition to pupillary block, argon laser peripheral iridoplasty is performed. During this procedure, several long burns of low power are placed in the periphery of the iris. The iris contracts and pulls away from the angle, opening it up and relieving the IOP.
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Martha Reilly, OD
A laser iridotomy is performed in an office setting by an ophthalmologist, a doctor or osteopahic doctor with residency training in the treatment of eye diseases. The doctor who performs a laser iridotomy may have advanced fellowship training in the treatment of glaucoma, after completing his or her three-year residency.