Photorefractive Keratectomy2019-03-13T23:33:23+00:00

Photorefractive Keratectomy

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The procedure known as Photorefractive keratectomy (PRK) is a form of laser eye surgery that is used for correcting the vision of a person who otherwise would have to wear contact lenses or eye glasses to correct their vision. Similar to LASEK eye surgery, PRK will change the anterior central cornea’s shape permanently through the use of an excimer laser which vaporizes to remove (ablate) smaller tissues from that are present on the corneal stroma located within the front of the eye underneath the corneal epithelium. Before the ablation occurs, the cornea’s outer layer must be removed.

The procedure is conducted using a computer system that tracks the eye position of the patient between 60 and 4,000 times per second based on the laser being used. Laser pulses are redirected by the computer to precisely position the laser, which are centered onto the visual axis automatically by most modern lasers. In the event the eye moves, it will pause then continue the ablation process once re-centered.

The Epithelium

This is the term referring to the cornea’s outer layer, which is a layer that is soft and regrows quickly, and can fully replace itself with limbal stem cells in just a couple days without losing clarity. The cornea’s deep layers in contrast with the outer layers of the epithelium are developed earlier during life while having limitations when it comes to regenerating abilities. These deep layers can be reshaped with microtome to cut or laser the layers, and remains permanent with limited regrowth.

The PRK procedure removes and discards the epithelium which allows cell regeneration post-surgery. Unlike Lasik surgery, it is a distinct procedure while Lasik is a type of laser surgery that creates a permanent flap within the cornea’s deeper layers.

History

In 1987, Dr. Theo Seiler conducted the first ever PRK procedure during his time at the Free University Medical Center (Berlin, Germany). LASEK was initially conducted in 1996 by Dimitri Azar, a refractive surgeon and ophthalmologist. In 1998, Dr. Massimo Camellin would become the first person to write and publish work regarding the latest surgical techniques, which he had coined the term LASEK as a acronym for Laser Epithelial Keratomileusis.

Eligibility

To be eligible for photorefractive keratectomy a person must satisfy a number of basic criteria, including being in normal ocular health other than the corrective issue being treated. Additionally, patients must be 18 or older to be considered for operation, without noticeable changes of refraction error within the last 12 months, indicating it is stable and can be corrected to 20/40 or more.

Myopia diopters much range between -1.00 and -12.00 while expecting realistic results. Therefore, patients must be aware and understand all potential benefits and risk factors associated with the treatment.

Pupil size commonly needs to be 6mm or under in dark environments, but some of the latest devices are able to treat larger sizes.

Additionally, women cannot be pregnant at the time of surgery, and an allergy assessment is required to determine any potential complications that could interfere with eyelid margins, cause dry eye, or other issues.

Pre-existing conditions that can preclude or complicate surgery:

  • Ocular disease, such as keratoconus, dry eye, or glaucoma
  • Collagen vascular disease, such as corneal melting or ulcerations
  • History of steroid reactions or side effects
  • Systemic disorders, such as rheumatoid arthritis or diabetes
  • Granular corneal dystrophy Type-II

Potential complications

Below is a list of complications that could potentially occur from treatment, and may be permanent or temporary, including:

  • Pain
  • Corneal haze
  • Increased ocular straylight
  • Recurrence of myopia
  • Under or over correction
  • Scarring
  • Increased sensitivity
  • Lowered acuity within lower light environments
  • Lower corrected visual acuity

Recurrent erosion when sleeping

Dry eye is the common term for keratoconjunctivitis sicca, and is a common complication with PRK and other types of refractive surgery. Although many experiences temporary dry eye, it can become permanent and extreme cases experience recurring erosion while asleep because the corneal epithelium adheres to the upper eyelid during REM sleep.

Glared, starburst or halo aberrations

A possible side effect that occurs after surgery where the patient gets a corneal haze, often reporting a halo, glare or starburst effect. Smaller ablations tend to have halo’s at night time when healing, but this complication is commonly temporary lasting up to 6 months.

Compared to LASIK surgery

When comparing these two treatment methods, LASIK frequently has a much shorter time to recover than PRK. In addition, there tends to be less pain experienced too. However, after one year the results are similar based on a systematic review from 2016. This review indicated the differences were unclear for the accuracy, efficacy, and adverse reactions from either method for those with moderate or low myopia.

At the time of the review, it said no trails had been done to compare the differences between LASIK and PRK on those with high myopia. Meanwhile, in 2017 uncertainty was shown in the visual acuity, but one study discovered patients that received PRK was often less at risk of refractive error and over-corrections in comparison with LASIK.

Military and Pilots

For those in the military who’ve had PRK are able to receive a blanket waiver clearing them for courses in the Special Forces Qualifications, Combat Diving and Military Free Fall classes. Whereas Airborne, Ranger and Air Assault schools will wave both LASIK and PRK.

Pilots who undergo PRK surgery can also be considered for U.S. Federal Aviation Administration, but must be fully healed and without any complications that impair visual standards, which must still be met. Although, private and commercial airlines can still consider refractive surgery to be a means of disqualifying conditions depending on their policy.

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