Photorefractive keratectomy
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PRK, an acronym for PhotoRefractive Keratectomy, is an eye surgery procedure intended to reduce a person's dependency on glasses or contact lenses.
PRK is a procedure that permanently changes the shape of the central cornea using an excimer laser to ablate a small amount of tissue from the front of the eye, just under the corneal epithelium. The epithelium is removed prior to the ablation and must grow back. A computer system tracks the patient's eye position 4,000 times per second, redirecting laser pulses for precise placement.
PRK is also known as LASEK; Laser-Assisted Sub-Epithelial Keratectomy.
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LASEK versus LASIK
Because it does not involve a flap (as in LASIK where a microkeratome is used), the cornea's integrity is less altered, but there is more pain and visual recovery is slower. The use of the antimetabolite mitomycin can minimize the risk of post-operative haze.
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 must satisfy:
- Normal Ocular Health
- Age 18 Years or Older
- Stable Refractive 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 6mm 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
LASEK versus LASIK
LASEK, an acronym for Laser-Assisted Sub-Epithelial Keratectomy, is an eye surgery procedure intended to reduce a person's dependency on glasses or contact lenses.
LASEK is a procedure that permanently changes the shape of the cornea using an excimer laser to ablate a small amount of tissue from the front of the eye, just under the eye's outer layer or epithelium. Unlike LASIK, the epithelium is kept during the procedure and replaced to act as a natural bandage.
Because it does not involve a knife/microkeratome as used in LASIK, the cornea's stability is left virtually unaffected, but there is more pain and visual recovery is slower than with the latter procedure; it is, however, faster than its older relative PRK.
Issues with patients requiring high visual acuity
Operation of an aircraft is a visually demanding activity performed in an environment that is not always user friendly. This becomes particularly evident if your choice of vision correction is ill-suited for the task. 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 complications of PRK that could affect civilian pilots:
- Long Healing Period
- Pain
- Glare/Halos/Starburst Aberrations
- Under/Over-correction
- Recurrence of Myopia
- Increased Intraocular Pressure
- Corneal Haze
- Scarring
- Cataracts
- 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. (Note: The FDA recommends a 3-month period before a second PRK surgery.) Activities requiring good binocular vision may have to be suspended between surgeries and during the, sometimes extended, healing periods.
Post-PRK patients have complained of glare, halos, and starburst aberrations, which may be the result of postoperative corneal haze that develops during the healing process. Reportedly, symptoms have occasionally lingered longer than a year.
Predictability of the resulting refractive correction is less than 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. (Note: Loss of BCVA is reportedly 5 to 15x more likely with PRK than that resulting from the use of extended wear contacts.)
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, civilian pilots who wish to fly military aircraft (Army, Air Force, or Naval Reserves) should know that the military does not allow its pilots to have refractive surgery.
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. If you are considering PRK, it is recommended that you contact your eyecare practitioner, who will assist you in making an informed decision concerning the potential benefits and liabilities that may be specific to your situation. Knowing how refractive surgery could affect your visual performance is essential since PRK is NOT reversible!