Photorefractive keratectomy

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Photorefractive keratectomy (PRK) and Laser-Assisted Sub-Epithelial Keratectomy (LASEK) are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. The first LASEK procedure was performed at Massachusetts Eye and Ear Infirmary in 1996 by ophthalmologist, refractive surgeon, Dimitri Azar. The procedure was later popularized by Camellin, who coined the term LASEK for laser epithelial keratomileusis. LASEK and PRK permanently change the shape of the anterior central cornea using an excimer laser to ablate (remove by vapourization) a small amount of tissue from the corneal stroma at the front of the eye, just under the corneal epithelium. The outer layer of the cornea is removed prior to the ablation. A computer system tracks the patient's eye position 60 to 4,000 times per second, depending on the brand of laser used, redirecting laser pulses for precise placement. Most modern lasers will automatically center on the patient's visual axis and will pause if the eye moves out of range and then resume ablating at that point after the patient's eye is re-centered.

The outer layer of the cornea, or epithelium, is a soft, rapidly regrowing layer in contact with the tear film that can completely replace itself from limbal stem cells within a few days with no loss of clarity. The deeper layers of the cornea, as opposed to the outer epithelium, are laid down early in life and have very limited regenerative capacity. The deeper layers, if reshaped by a laser or cut by a microkeratome, will remain that way permanently with only limited healing or remodelling. In LASEK, the corneal epithelium is preserved with a chemical solution, peeled off, and replaced after the laser ablation is complete. In PRK the epithelium removed is discarded and allowed to regenerate. Both procedures are distinct from LASIK (Laser-Assisted in-SItu Keratomileusis), a form of laser eye surgery where a permanent flap is created in the deeper layers of the cornea.

PRK versus LASIK

Because PRK does not create a permanent flap in the deeper corneal layers (the LASIK procedure involves a mechanical microkeratome using a metal blade or a femtosecond laser microkeratome to create a 'flap' out of the outer cornea), the cornea's structural integrity is less altered by PRK.

The LASIK process covers the laser treated area with the flap of tissue which is from 100 to 180 micrometres thick. This flap can mute the nuances of the laser ablation, whereas PRK performs the laser ablation at the outer surface of the cornea. The use of the anti-metabolite mitomycin can minimize the risk of post-operative haze in persons requiring larger PRK corrections.

PRK does not involve a knife, microkeratome, or cutting laser as used in LASIK, but there may be more pain and slower visual recovery. Unlike LASIK, PRK does not create the risk of dislocated corneal flaps which may occur (especially with trauma), at any time after LASIK.

An evolved form of PRK is called No Touch laser vision correction. It also treats the surface of the cornea but unlike other techniques, requires no assistance from manual surgical instruments. It is the only technique to use exclusively an excimer laser from start to finish.

PRK eligibility

It is estimated that up to 80% of the myopic population may physically qualify as potential PRK candidates. There are a number of basic criteria which a potential candidate should satisfy:

• Normal ocular health
• Age 20 years or older
• Stable refraction error (no noticeable change in the last year) correctable to 20/40 or better
• Between -1.50 to -7.00 diopters of Myopia
• No gender restriction, with the exception of pregnancy
• Realistic expectations of the final results (with a complete understanding of the benefits, as well as the possible risks)
• Pupil size 6 mm in room light

There are also some pre-existing conditions that may complicate or preclude the treatment.

• Collagen vascular disease (e.g., corneal ulceration or melting)
• Ocular disease (e.g., dry eye, keratoconus, glaucoma)
• Systemic disorders (e.g., diabetes, rheumatoid arthritis)
• History of side effects from steroids

Possible complications

Some complications of PRK include:

• Long healing period
• Pain
• Glare, halos, or starburst Aberrations
• Under- or over-correction
• Recurrence of myopia
• Corneal haze
• Scarring
• Reduced best corrected visual acuity
• Reduced acuity in low light

PRK may be performed on one eye at a time to assess the results of the procedure and ensure adequate vision during the healing process. Activities requiring good binocular vision may have to be suspended between surgeries and during the sometimes extended healing periods.

A few post-PRK patients have complained of glare, halos, and starburst aberrations, which may be the result of postoperative corneal haze that may develop during the healing process. Using modern lasers as of the year 2005, this is quite rare after 6 months but reportedly, symptoms have occasionally lingered longer than a year in some cases.

Predictability of the resulting refractive correction after healing is not totally exact, particularly for those with more severe myopia. This can lead to under/over-correction of the refractive error. In the case of the over-correction, premature presbyopia is a possibility.

In 1 to 3% of cases, loss of best corrected visual acuity (BCVA) can result, due to decentered ablative zones or other surgical complications. PRK results in improved BCVA about twice as often as it causes loss. Decentration is becoming less and less of a problem with more modern lasers using sophisticated eye centering and tracking methods.

Aviator usage

Operation of an aircraft is a visually demanding activity performed in an environment that is not always user friendly. Currently, over 50% of the civil airman population uses some form of visual correction. Aviators considering PRK should know that clinical trials claiming success rates of 90% or higher are based on criteria of 20/40 or better, not 20/20 or better, uncorrected visual acuity.

Some PRK patients have reported dissatisfaction with their vision under low ambient lighting (dusk/nighttime) conditions. Pilots who experience postoperative vision problems could be further compromised by the variations in lighting common to the aviation environment. In addition, exposure to intense UV radiation has been associated with late-onset corneal haze and recurrence of myopia.

The US Federal Aviation Administration will consider applicants with PRK once they are fully healed and stabilized, provided there are no complications and all other visual standards are met. Pilots should be aware, however, that potential employers, such as commercial airlines and private companies, may have policies that consider refractive surgery a disqualifying condition. Also, civilians who wish to fly military aircraft should know that there are restrictions on those who have had corrective surgery. The Army now permits flight applicants who have undergone PRK or LASIK, though it still requires a standard waiver. The Navy and Marines will routinely grant a waiver for pilots or student naval aviators to fly after PRK, assuming no complications and acceptable vision. LASIK is currently disqualifying for the Navy. In one study 967 of 968 naval aviators having PRK returned to duty involving flying after the procedure. In fact, the U.S. Navy now offers free PRK surgery to Naval Academy Midshipmen who intend to pursue career paths requiring perfect uncorrected vision, including flight school and special forces training. The U.S. Air Force approves the use of PRK and recently approved LASIK (pilots must have LASIK performed by Air Force Ophthalmologists at Wilford Hall Medical Center)..

In the majority of patients, PRK has proven to be a safe and effective procedure for the correction of myopia. PRK is still evolving with other countries currently using refined techniques and alternative procedures. Many of these procedures are under investigation in the U.S. Given that PRK is not reversible, a patient considering PRK is recommended to contact an eye-care practitioner for assistance in making an informed decision concerning the potential benefits and liabilities that may be specific to him or her.

See also

• Phototherapeutic keratectomy
• FLIVC, femtosecond laser intrastromal vision correction

External links

• LASIK, Epi-LASIK & LASEK: Eye Digest Reviews(University of Illinois)