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.
