Photorefractive keratectomy (PRK) for near and farsight correction
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Photorefractive keratectomy (PRK) is a refractive surgical procedure that applies an excimer laser to modify the cornea. It is an ideal choice for patients who seek an alternative to glasses or contact lenses for correcting their refractive errors.

The discovery of the excimer laser triggered the rapid advancement of refractive surgery in the last 15 years, especially for correction of myopia. Excimer lasers were first implemented in 1975 at Kansas State University and since then it has added a new dimension to various refractive surgeries. PRK, armed with the excimer laser, is now considered a highly effective treatment of mild-to-moderate myopia (-1.5 to -8.0D). Higher myopia can also be corrected, but it may involve greater regression or corneal haze.

Some unique aspects of treatment exist for the correction of moderate and high myopia. However, PRK for hyperopic corrections and astigmatism came about much later and even now it is applicable more for mild-to-moderate degrees only. Eximer laser scar removal can also replace penetrating keratoplasty in patients with anterior corneal scars who have not gained from the use of contact lenses or glasses. PRK is also a viable mode of treatment for residual myopia after radial keratotomy.

Excimer lasers operate through a process known as ablative photodecomposition, wherein ultraviolet radiation at the 193 nm wavelength can remove exact amounts of tissue from the anterior cornea. The 193 nm laser is the most commonly used laser for corneal refractive surgery.

Laser Ablation Protocol

The Food and Drug Administration lays down certain guidelines for ablation profiles in the US. These are strictly based on those profiles used in research for the approval of the apparatus used in the ablation process.

Most of these ablation systems incorporate a computer control unit with a user-friendly menu. This helps in the formulation of an ablation protocol according to specific cases.

Since accuracy of laser application is vital in improving the ablation quality, the excimer laser systems should also provide a microscope to ensure proper alignment of the eye along all axes. Several laser makers also market laser-tracking systems to improve the precision of laser shots.

Other important additions could be a vacuum apparatus to remove residues from the ablation plume, a foot pedal and fingertip regulation for surgeons to maneuver the eye position and the ablating ray. So far as the laser is concerned, various 193 nm excimer lasers are available in the market.

Pre-Operative Measures

  • Patients must be counseled about the risks and benefits according to their specific cases. This is especially vital in presbyopic patients.
  • Pre-surgical screening should include tests for visual perception such as for manifest and cycloplegic refraction, ocular dominance and far and near vision with and without correction.
  • Anterior segment and posterior segment tests should also be conducted to eliminate the possibility of other syndromes affecting surgical outcome.
  • Pachymetry measurements are required to ascertain whether the cornea is of normal thickness.
  • Computerized topographical analysis is necessary to detect subclinical keratoconus or other corneal diseases.

Contraindications

Relative contraindications to laser treatment are:

  • Advanced diabetes.
  • Collagen vascular disease.
  • History of herpes (simplex or zoster) infection.
  • Acute dry eye.
  • Uncorrected blepharitis.
  • Neurotrophic cornea.
  • Peripheral ulcerative keratitis.
  • Use of medication like isotretinoin (accutane), amiodarone (cordarone), or sumatriptan (imitrex).

Surgical technique

There are various PRK techniques, but whatever it may be the most important point is that surgeons should keep their practice consistent and monitor the outcome. The surgical setting is also vital with careful control of temperature and humidity. This is because the degree of corneal hydration during the surgery has a great impact on the refractive results. A customized nomogram can be handy to monitor variations in particular technique, laser options and changes in environment.

Safety measures in the operation theatre are also important. These include:

  • Use of safety glasses that can shield from the wavelength of the 193 nm excimer laser.
  • The OT should have sufficient ventilation to ensure the quick escape of the highly toxic fluorine gas (used in eximer lasers) in case of a leak.
  • To minimize harmful effects of the laser, there should be a constant check on exposure time, laser pulse duration, assimilation by the body and the laser wavelength.

At this backdrop, two most prevalent surgical techniques are:

Laser Radial Keratectomy

Laser radial keratectomy applies the excimer laser to make incisions in the cornea quite like as in radial keratotomy. It is considered more precise than the latter.

Excimer Laser Keratomileusis

Laser keratomileusis enables ablation over a larger surface and the option of resculpting as well. It is also highly accurate and does not involve hyperplasia or a noteworthy wound rehabilitation reaction.

Postoperative Restoration

The standard procedure for post surgical care is:

  • Bandage soft contact lens used till the epithelium heals. Pressure patching is often considered an alternative to bandage soft contacts, since some researches have reported its better epithelial healing ability. However, these patches do not ensure binocular vision.
  • Antibiotic, steroid and non-steroidal medications prescribed four times a day.
  • Pain management with non-steroidal drops.

Complications

Complications can occur with any refractive surgical procedure. They are:

  • Undercorrection.
  • Overcorrection.
  • Corneal scar formation.
  • Central islands.
  • Infectious keratitis.
  • Pain, decentration.
  • Visual aberrations.
  • Endothelial cell loss.
  • Recurrent erosion syndrome.
 
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