Photorefractive keratectomy at U.S. Naval Medical Center San Diego.
Photorefractive keratectomy (PRK) and Laser-Assisted Sub-Epithelial Keratectomy (or Laser Epithelial Keratomileusis) (LASEK) are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. LASEK and PRK permanently change the shape of the anterior central cornea using an excimer laser to ablate (remove by vaporization) 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 microtome, will remain that way permanently with only limited healing or remodelling. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate after the surgery. The procedure is 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.
The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University Medical Center in Berlin, Germany. The first procedure similar to LASEK was performed at Massachusetts Eye and Ear Infirmary in 1996 by ophthalmologist, refractive surgeon, Dimitri Azar. Dr. Massimo Camellin, an Italian surgeon, was the first to write a scientific publication about the new surgical technique in 1998, coining the term LASEK for laser epithelial keratomileusis.
PRK versus LASIK
There have been a number of studies comparing LASIK to PRK. LASIK is associated with decreased inflammation and quicker recovery, but at the cost of decreased expression of Nerve Growth Factor (NGF) at the surgical bed. PRK, on the other hand, has longer recovery time with more postoperative discomfort and irritation, but rates of ocular dryness are less, as only the epithelium is denuded in this approach. LASIK, on the other hand, involves creation of a corneal flap. The medical literature is not uniform, however, regarding comparative effects on ocular dryness, although the majority of research supports PRK as causing less reduction in tear film. Surgical technique has improved with time, and a more recent study has shown no difference in ocular dryness in LASIK or PRK at 12 months, as compared to pre-operative baseline, although interval assessments at months 1, 3 and 6 did show dryness in both groups. Quantitative changes occurring at the eye surface are more pronounced with LASIK, but more irritation, pain and eyelid sticking are felt with PRK, which could be related to increased denervation with LASIK. A systematic review that compared PRK and LASIK concluded that LASIK may probably offer benefits in terms of recovery time and pain, however the two interventions probably perform similarly after a period of one year.
PRK does not create a permanent flap in the deeper corneal layers, while LASIK involves a mechanical microtome using a metal blade or a femtosecond laser microtome to create a 'flap' out of the outer cornea. As such, 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, which is referred as M-LASEK, can minimize the risk of post-operative haze in persons requiring larger PRK corrections, although the medication can worsen the dry eye that occurs after surgery in some patients. Unlike LASIK, PRK does not involve a knife, microtome, or cutting laser, but there may be more pain and slower visual recovery. Unlike LASIK, PRK does not have an increased risk of dislocated corneal flap, which may occur with trauma after LASIK.
PRK versus LASEK
Although PRK and LASEK use basically the same technique, there are minor differences between them. In PRK, epithelium is removed and the outermost layer below the epithelium is treated with laser. In LASEK, epithelium is not removed, but an alcoholic solution is used to cause the epithelial cells to weaken; the surgeon will fold the epithelial layer out of the laser treatment field, and fold it back in its original place after the cornea has been reshaped by the laser. If the epithelial flap is not strong enough to be laid back in its original place, it will be removed, and the LASEK procedure becomes a PRK procedure. Recent studies show that removing the epithelial flap results in less pain and faster epithelial recovery after LASEK. As a result, although alcohol is used to loosen the epithelium, surgeons are routinely discarding the epithelium, thus converting LASEK into alcohol assisted PRK surgery.
Types of PRK
- Traditional PRK
Uses Amoils Brush or a 'Hockey Stick' to remove the corneal epithelium before excimer laser ablation
- Alcohol assisted PRK
Uses alcohol to loosen the corneal epithelium before its removal. This allows gentler surgery as it avoids the need for scrubbing or scraping to remove the corneal epithelium. This originated from LASEK, and essentially is LASEK but with removal of the corneal epithelial layer at the end of surgery.
Uses an excimer laser to remove the corneal epithelium. A one step, no touch technique is used where the excimer laser performs both epithelial removal and corneal reshaping sequentially. This procedure has been shown to result in less pain and faster healing of the corneal surface than alcohol assisted PRK.
Types of LASEK
- ASA (Advanced Surface Ablation) LASEK
Uses Amoils Brush and gas cooling to reduce the pain
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 18 years or older
- Stable refraction error (no noticeable change in the last year) correctable to 20/40 or better
- Between −1.00 to −12.00 diopters of Myopia
- Not pregnant at the time of surgery
- Realistic expectations of the final results (with a complete understanding of the benefits, as well as the possible risks)
- Pupil size 6 mm or less in a dark room is ideal (but some newer lasers may be acceptable for larger pupils)
- Assessment of allergies, (e.g., pollen) where allergy may complicate the eyelid margins following surgery leading to dry eye.
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
- Granular corneal dystrophy type II
Some complications that can be temporary or permanent include:
- Dry eyes
- Recurrent erosions during sleep
- Long healing period
- Glare, halos, or starburst aberrations
- Increased ocular straylight
- Under- or over-correction
- Recurrence of myopia
- Corneal haze
- Reduced best corrected visual acuity
- Reduced acuity in low light
- Increased sensitivity
As with other forms of refractive surgery, keratoconjunctivitis sicca, colloquially referred to as 'dry eye,' is the most common complication of PRK, and can be permanent. In more advanced cases, recurrent erosions occur during sleeping from adherence of the corneal epithelium to the upper eyelid with rapid eye movement. Adjuvant polyunsaturated fatty acids (PUFAs) with high Omega-3 content before and after surgery improves sicca, possibly due to their anti-inflammatory effects. Foods containing PUFAs include flax and fish oil. Brush PRK to denude the epithelium, instead of alcohol based techniques, also result in quantitatively lower ocular dryness after surgery. The amount of corneal hazing after surgery is also decreased with brush technique. The platelet activating factor LAU-0901 has shown effect in mitigating dry eye in mouse models. Rabbit models have also shown improvement with topical nerve growth factor (NGF) in combination with docosahexaenoic acid (DHA). Mitomycin C worsens post-surgical dry eye.
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.
Halos, starbusts and refractive errors
PRK can be associated with glare, halos, and starburst aberrations, which can occur with postoperative corneal haze during the healing process. Night halos are seen more often in revisions with small ablation zone size. With more recent developments in laser technology, this is less common after 6 months though symptoms can persist beyond a year in some cases. A dilute concentration of the chemotherapeutic agent, Mitomycin-C, can be applied briefly at the completion of surgery to reduce risk of hazing, although with increased risk of sicca.
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. Experienced surgeons employ a custom-profile algorithm to further enhance predictability in their results.
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.
Operation of an aircraft is a visually demanding activity performed in an environment that is not always user friendly. Currently, over 50% of the civilian pilot 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 patients' post-operative refractive errors 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 U.S. 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. Uncomplicated, successful corneal refractive surgery does not require a waiver and is noted as information only.  The Navy and Marines will routinely grant a waiver for pilots or student naval aviators to fly after PRK, assuming preoperative standards are met, no complications in the healing process were encountered and passing their standard vision tests. LASIK is currently under study for the Navy, however only current Naval Aviators can be admitted into the study. 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 at the National Naval Medical Center to Naval Academy Midshipmen who intend to pursue career paths requiring good uncorrected vision, including flight school and special operations training. The U.S. Air Force approves the use of PRK and LASIK. Since 2000 the USAF has conducted PRK for aviators at the Wilford Hall Medical Center. More airmen were allowed over the years and in 2004 the USAF approved Lasik for aviators, with limits on the type of aircraft they could fly. Then in 2007 those limits were lifted. Most recently in 2011 the USAF expanded the program, making it easier for more airmen to qualify for the surgery. Current airmen (Active Duty and Air Reserve Components who are eligible) are authorized surgery at any DOD Refractive Surgery Center. Those airmen not eligible, are still able to get the surgery done at their own expense by a civilian surgeon, but must first be approved (Approval is based on the same USAF-RS program). Others that do not fall into those categories (i.e. applicants who are seeking a pilot slot) can still elect to have the surgery done, but must follow the criteria in accordance with the USAF Waiver Guide. Those applicants will be evaluated at the ACS during their Medical Flight Screening appointment to determine if they meet waiver criteria. Applicants are strongly encouraged to read the references contained within this article as information obtained from sources other than those listed could lead to disqualification for a pilot candidate.
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.
U.S. Army Special Operations
In the U.S.A. candidates who have had PRK can get a blanket waiver for the Special Forces Qualification, Combat Diving Qualification and Military Free Fall courses. PRK and LASIK are both waived for Airborne, Air Assault and Ranger schools. However, those who have had LASIK must enroll in an observational study, if a slot is available, to undergo training in Special Forces qualification. LASIK is disqualifying/non-waiverable for several United States Army Special Operations Command (USASOC) schools (HALO, SCUBA, SERE) per Army Regulation 40-501.
- Automated lamellar keratoplasty
- Phototherapeutic keratectomy
- Femtosecond laser intrastromal vision correction
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