Photorefractive Keratectomy (PRK)
Definition
Photorefractive keratectomy (PRK), as well as its variants LASEK and Epi-LASIK, is a refractive surgery procedure that applies the excimer laser to the surface of the cornea to correct refractive error and decrease or eliminate the need for glasses or contact lenses. These procedures differ only in how the surface cells of the cornea, the epithelium, are removed before the laser is applied to the corneal tissue.
How Does PRK Work?
PRK surgery works by sculpting the cornea in a precise manner to correct refractive error. By changing the corneal shape, light becomes focused on the retina, allowing clear vision. During PRK surgery, the surface cells of the corneal, called the epithelium are loosened with dilute alcohol then brushed away with a spatula or rotating brush. The excimer laser is then applied to the underlying corneal tissue in a precise manner that can correct nearsightedness, farsightedness and/or astigmatism. A contact lens is then placed on the eye to act as a bandage while the surface cells heal back over the cornea; this process usually takes 5 to 7 days.
How does LASEK Work?
LASEK is very similar to PRK. The surface cells are first loosened with dilute alcohol. However, instead of removing the surface cells, they are pulled back in a sheet away from the central cornea. The excimer laser is then applied to the underlying cornea to correct the vision. The surface cells are then replaced over the corneal surface. A bandage contact lens is then placed in the eye while the surface cells heal. Some doctors believe that the surface cells heal faster and more comfortably with LASEK, though clinical studies have not clearly demonstrated this to be true.
How does Epi-LASIK Work?
Epi-LASIK is also very similar to PRK. However, rather than using dilute alcohol and a spatula to brush away the surface cells of the cornea, the cells are removed in a sheet with an instrument called an epikeratome. The laser is then applied to the cornea to correct the vision, and a bandage contact lens placed after the laser is done. Some doctors feel that this technique, which avoids the use of alcohol to loosen the surface cells, may promote faster and more comfortable healing of the cornea. As this is a relatively new procedure, very few clinical studies have been done to support this opinion one way or another.
What are the main differences between PRK, LASEK and Epi-LASIK versus LASIK surgery?
In LASIK surgery, a thin flap of cornea is made, lifted back then the excimer laser applied underneath to correct the vision. The flap is then replaced. Healing is very rapid, with the eyes feeling mostly normal within a day or two. Most surgeons will perform LASIK on both eyes on the same day, due to the rapid healing time of LASIK. LASIK flaps heal back into place, but are never as strong as the unaltered cornea. For this reason, LASIK may not be used in people with abnormally shaped or thin corneas.
In PRK, LASEK, and Epi-LASIK, however, no flap of cornea is made. The surface cells are removed, and then the laser applied directly to the top of the cornea. A bandage contact lens is placed in the eye. The surface cells typically heal over the cornea in 5-7 days. During this time, the eye can be uncomfortable and the vision a little blurry. For most people, this is only mildly irritating, but for a few, the discomfort is significant, requiring oral pain medications in addition to the usual postoperative eye drops. For this reason, only one eye is typically done at a time. Though the recovery time of PRK and surface ablation surgeries is slower, it is somewhat safer in that no complications can occur with the corneal flap making process, as there is no flap made. Lastly, PRK and surface ablation surgeries may be less traumatic to eyes that are predisposed to dryness.
Who are candidates for PRK, LASEK and Epi-LASIK?
Surface ablation refractive surgeries help correct the vision of people with myopia, hyperopia, and/or astigmatism. In fact, a large majority of people who wear glasses or contacts and suffer from such refractive errors as these could be candidates for PRK and other surface ablation surgeries.
PRK and surface ablation refractive surgeries can be used on almost all patients who would otherwise qualify for LASIK surgery. Additionally, they can also provide laser vision correction options for some individuals who are not candidates for LASIK, due to certain eye or corneal conditions.
In general, candidates for PRK, LASEK or Epi-LASIK should be:
- 21 years of age or older: younger people may still have eyes that are growing. In certain cases, individuals younger than 21 may have LASIK surgery.
- There is no upper age limit for LASIK.
- Dissatisfied with wearing glasses or contact lenses.
- Have had no change in glasses or contact lens prescription for at least a year.
- Have otherwise healthy eyes.
- Be willing to accept a small amount of risk associated with surgery.
- Understand that glasses and/or contacts are occasionally still needed for some activities after surgery.
- Not have excessively thin corneas or extremely high levels of refractive error. Your doctor will test for these conditions on your evaluation exam.
These conditions may prevent you from undergoing PRK or other surface ablation refractive surgeries. You should alert your eye surgeon if you have one or more of these conditions so that he or she can help you make the best choice about undergoing refractive surgery:
Condition:
Reason for caution:
Examination prior to PRK, LASEK or Epi-LASIK
Before you arrive at the doctor's office
If you wear contact lenses you must stop wearing them prior to surgery–at least two weeks for soft contacts and one month for hard contacts. This is because contact lenses can cause mild warping of the corneal shape, which may interfere with the preoperative measurements of the eye and calculations for refractive surgery.
Tests you may have at the doctor's office
The evaluation for PRK, LASIK or Epi-LASIK surgery typically includes a complete eye exam of the front and back of the eye, plus several additional tests including:
- Your vision with and without glasses will be tested, as well as a refraction to determine if your current vision differs markedly from the vision corrected in your current glasses. If they do differ markedly, you may need to return for another visit several weeks later for a repeat refraction to insure that your prescription is not changing.
- The thickness of your corneas will also be tested. Since these surgeries remove some corneal tissue during the reshaping process of vision correction, a minimum amount of corneal thickness is required.
- Your pupil size will also be examined. People with large pupils may be at increased risk for night vision symptoms, such as glare and halos, after refractive surgery.
- Lastly, several machines may be used to assess the shape of your cornea, including a topographer and/or a tomographer, and possibly a wavescan abberometer. Your doctor will review the information from these machines in order to determine if your corneas are regularly shaped. Individuals with abnormally shaped corneas may not be ideal candidates for refractive surgery due to a possible increased risk of irregular corneal warpage after the surgery. PRK and surface ablation surgeries may reduce, but not eliminate, this risk. Your doctor will have a detailed discussion about any abnormal corneal findings with you and help you choose the best option for your vision correction.
Custom (Wavefront) vs. Standard LASIK Surgery
The terms "Custom" (also known as "Wavefront") and "Standard" laser refractive surgery are descriptions of the computer program used by the excimer laser to change the shape of the cornea. Though approved by the FDA for LASIK surgery excimer laser technology is often applied off label for PRK and surface ablation surgeries, and with excellent results.
Standard laser surgery is the term now used for the older computer programs for the excimer laser. With these programs the system used the patient's glasses prescription to change the shape of the cornea. As such, two different patients who happened to wear the same glasses prescription would receive the same corneal shape change by the excimer laser.
In contrast, Custom laser surgery, available since 2003, uses a much more sophisticated way of measuring the eye before surgery. A specialized map, called a wavescan, is obtained using an instrument called an aberrometer. This measures not only the glasses prescription, but also many other optical properties of the eye that cannot be measured by a simple glasses fitting. The result is an optical "fingerprint" of the patient's eye; the data is used to correct the patient's optical aberrations during the refractive surgery procedure. No two treatments, like fingerprints, are exactly alike between patients or even between the two eyes of one patient! Results from the clinical trials show that more patients achieve 20/20 or better vision with Custom laser treatment.
While Custom laser surgery is the optimal way to reshape the cornea for vision correction, not all patients are candidates for Custom. Uncommonly, a patient with very small pupil size, high degrees of astigmatism or myopia, or people interested in monovision treatments may fall outside the treatment ranges for Custom laser surgery. Standard laser surgery, however, is still a remarkably effective surgery, and offers a very good option for laser vision correction. The procedure for PRK, LASEK and Epi-LASIK, that is, what your surgeon does and you experience during the operation, is essentially the same whether Standard or Custom excimer ablation is used.
Is 20/20 vision guaranteed with PRK, LASEK and Epi-LASIK?
Excimer laser surgery is an extremely advanced technology for surgical vision correction. The excimer laser is one of the most precise instruments available for use in modern medicine, and it can be programmed to match the exact refractive correction your eye needs to see well. As such, the vast majority of people undergoing excimer laser surgery are very happy with their post-procedure vision. In fact, a person's happiness with their vision after the procedure is a far more important measure of success than the somewhat arbitrary "20/20" measurement done in a dark room of a doctor's office.
However, factors that may lead to a person requiring glasses or contact lenses after surgery include:
- Individuality–The excimer laser is precise enough to inscribe letters on a grain of sand and could correct an inert object exactly as predicted; however, in LASEK surgery it is being applied to a living tissue, the cornea. Because of this and the individual healing response of each patient, there is some variability of response to the treatment.
- Increased Level of Refractive Error–Higher levels of refractive error make it more difficult to hit an exact refractive outcome target.
What are the risks of PRK, LASEK and Epi-LASIK surgery?
Excimer laser surgery is an extremely advanced technology for surgical vision correction. The excimer laser is one of the most precise instruments available for use in modern medicine, and it can be programmed to match the exact refractive correction your eye needs to see well. As such, the vast majority of people undergoing excimer laser surgery are very happy with their post-procedure vision. In fact, a person's happiness with their vision after the procedure is a far more important measure of success than the somewhat arbitrary "20/20" measurement done in a dark room of a doctor's office.
However, factors that may lead to a person requiring glasses or contact lenses after surgery include:
- Individuality– The excimer laser is precise enough to inscribe letters on a grain of sand and could correct an inert object exactly as predicted; however, in LASEK surgery it is being applied to a living tissue, the cornea. Because of this and the individual healing response of each patient, there is some variability of response to the treatment.
- Increased Level of Refractive Error– Higher levels of refractive error make it more difficult to hit an exact refractive outcome target.
What are the risks of LASEK surgery?
The risks of these surgeries fall into two main categories: Vision Loss Risks, and Nuisance Risks.
Vision Loss Risks
It is very rare to lose vision after laser refractive surgery. When vision is lost in an otherwise uncomplicated procedure, it typically is 1 or less lines on the eye chart of best corrected vision even with extra help from glasses. As such, these surgeries have a very good safety profile. Two other ways may exist in which vision could be lost to a more severe degree.
- Infection: Since cuts are made on the eye, it is possible that bacteria could gain access to the corneal tissue and start an infection. Scarring from such an infection could lead to vision loss. This is very uncommon as powerful antibiotics are used after surgery to prevent infection. The risk of severe infection is less than 1 in 500.
- Progressive Corneal Warpage (Ectasia): In this condition, the cornea begins to warp in odd directions, leading to loss of vision. Occasionally, a corneal transplant is required to fix this condition. Ectasia, however, typically is seen only in patients with abnormal corneal shapes, or corneal dystrophies, such as keratoconus, even before the surgery is done. Your surgeon will screen your corneas very closely to help identify any preexisting corneal shape irregularity. The risk of Ectasia is less than 1 in 3500.
Most of the other risks associated with laser refractive surgery will not cause significant loss of vision. Rather, they may cause nuisance problems with the eyes that may not have been present before the surgery.
- Epithelial Healing: In most cases, the scratches made on the surface of the cornea for laser refractive surgery heal within 5-7 days. A bandage contact lens is left in the eye during this time to promote healing. Eye drops are also given to help healing. Occasionally, though, it can take up to several weeks for the surface cells to heal, during which time the vision can be blurry.
- Dry Eye: Almost everyone undergoing laser refractive surgery has some mild dryness, in which the eyes feel. Typically, these symptoms resolve after a few months; however a small percentage of people have chronically worse dry eyes after laser refractive surgery requiring continuous treatment with drops or dry eye medications.
- Night Vision Symptoms: Some patients notice their night vision after laser refractive surgery is different than before. Usually, this occurs in the form of halos around streetlights, added glare from oncoming traffic, or increased difficulty seeing dimly light shapes in the dark. Typically, these symptoms improve with time.
Data from several studies has shown that if 100 patients had such symptoms at 1 month after surgery, only 20 to 30 will still have symptoms by 1 year. Some surgeons believe that a larger pupil size may put a patient at increased risk for night vision problems. However, such complaints can still happen in people with small pupil sizes. Most people with night vision complaints find them only mildly annoying. However, a few individuals may feel that they are unable to function well in low light settings. Medications can be used to change the size of the pupil in low light or nighttime settings, which can help reduce night vision symptoms if they occur.
- Corneal Haze: Occasionally after surface ablation laser surgery, such as laser refractive surgery a mild haze will form in the front of the cornea during the healing process. This haze can sometimes cause mildly decreased vision. Haze is uncommon with current surgical techniques, which use a medication called mitomycin-c during the procedure to help reduce the risk of haze formation. If it does occur, it typically improves over several months with steroid treatment.
- Enhancements: Even though the excimer laser is extremely precise, the fact that the eye is a living tissue and that individual healing response can vary means that sometimes laser refractive surgery over or under corrects the refractive error of the eye. If this happens, a "touch up" procedure, or enhancement, can be done several months later to correct the remaining refractive error. Enhancement procedures carry a small risk of all of the above complications, just like the original procedure. In general, about 5-10% of patients undergoing laser refractive surgery will need an enhancement procedure to obtain their vision goals.
What will I experience during the PRK, LASEK or Epi-LASIK procedure?
On the morning of your procedure, your surgeon will ask you not to wear any makeup (which may stain the cornea) or perfume or cologne (which may damage the laser). At the laser surgery center, you will usually be given a Valium pill to help you feel calm during the procedure. You lie on a special bed under the laser, and the procedure itself usually takes less than 10 minutes an eye. A blinking red light serves as your target to focus on during the procedure. A lid holder is used to hold your eyelids open, and numbing drops are placed on the eye.
The doctor then removes the surface cells from the eye. In LASEK and PRK, this typically involves some cool fluids and brushing on the cornea, during which you feel nothing but see blurry shapes. In Epi-LASIK, the epikeratome is placed on the eye with a suction ring, during which you will feel some squeezing and pressure lasting approximately 10 to 20 seconds. Once the surface cells are removed your vision will be blurry.
The laser is then activated, and it typically takes less than a minute to change the shape of the cornea. The laser tracks the eye, so if small eye movements occur it will follow and still apply the laser to the right position. Large eye movements will cause the laser to stop treatment, ensuring that only the correct treatment is performed. A bandage contact lens and eye drops are placed on the eye.
After the procedure is done, your surgeon may examine your eye, or simply have you go home and take a long nap. Your eyes will start to burn and feel irritated about half an hour after the surgery as the numbing medicine wears off. The nap, plus the eye drops your surgeon will give you helps make your eyes feel more comfortable.
Your doctor will see you the next day, at which point the eyes feel more comfortable. You typically see your doctor again in about a week to assess the healing of the surface cells and remove the contact lens. You will continue using eye drops for several weeks after the surgery, and then see your doctor again in about a month for a vision check. If all is well, as it typically is, your doctor will usually see you again in 6 months to a year for another vision check. During the few months after the surgery, artificial tears should be used regularly to help limit dryness of the eyes while they heal.
