Introduction to photorefractive
Photorefractive keratectomy (PRK) is a procedure
where an excimer laser is used to reshape the cornea
to get the desired vision correction. PRK was the
first laser surgery performed to treat myopia or
short sightedness. PRK can also treat farsightedness
and astigmatism but presently FDA approval is only
available for treating short sightedness. PRK has
proved quite effective in treating short sightedness
with a good risk to benefit ratio. Most people
opt for PRK to reduce dependence on glasses or
The surgery does not involve the creation of an
epithelium flap as in LASIK. It involves the removal
of the epithelium from the center of the cornea.
The surgery is followed by a laser ablation of
the stromal surface to reshape the cornea. The
laser delivery takes less than one minute for most
patients. Once the corneal reshaping is over, a
bandage contact lens is placed on the eye and anti-inflammatory & antibiotic
eye drops administered to provide relief, in case
of any discomfort. The patient can usually leave
within a few minutes after the procedure are over.
Vision improves as the epithelium wound heals.
Benefits of PRK
As in LASIK, in the hands of a competent surgeon,
PRK improves vision and the degree of improvement
is inversely related to degree of correction. Best
results are obtained when low to moderate short
sightedness, not exceeding 6 dioptres, and far
sightedness, less than 2 dioptres, are treated.
However, compared to LASIK, in PRK the incidence
of regression is more. In most cases, after the
operation, the condition remains stable over time
Higher degrees of short sightedness can be corrected,
but this may result in higher degrees of regression
or corneal opacity. Many surgeons have abandoned
PRK treatment for high degree of short sightedness.
Hyperopic corrections have taken more time to develop
but are now routine for low to moderate levels.
Low and moderate degrees of astigmatism can also
be corrected with PRK.
Generally, PRK treatment for short sightedness
is well established having shown acceptable levels
of efficacy and long-term stability. Treatments
have improved with improvements in laser technology.
At present, PRK is preferred for the correction
of short and far sightedness in some eyes. These
include eyes that have thin but otherwise normal
corneas, eyes where, due to the epithelium flap
in LASIK, recovery of vision is delayed and dry
Risks of PRK
Since PRK does not involve the formation of a
flap, which after the laser application in LASIK
is replaced on the cornea, the unprotected superficial
cornea layers can heal only after the operation.
This means the vision correction in PRK is delayed.
Patients after PRK treatment take much longer to
attain their best vision than patients after LASIK
treatment. However, available data is not quite
consistent with these afore mentioned beliefs since
it is difficult to predict the healing outcome
Generally, both PRK an LASIK show similar results
6 months after the treatment but LASIK is considered
safer due to lesser risks of adverse effects, such
as, scarring, and infections which, though, are
rare in both treatments.
Complications can occur with any refractive surgery.
As in LASIK, PRK treatment also results in several
vision related complications, such as, starbursts,
halos, distorted images, and multiple images commonly
seen during the night. The use of excimer laser
increases the likelihood of these complications
Under and overcorrection
can be seen, caused by improper surgery, a malfunctioning
abnormal corneal hydration status, or excessive
or inadequate wound healing response. It is important
to maintain consistent dryness of the cornea,
because excessive fluid can result in an under
If the corneal stroma is too dry, overcorrection
and haze can occur. Delayed wound healing can
lead to regression, resulting in an undercorrection
and possibly scarring. No or minimal tissue healing
can sometimes lead to an overcorrection.
More complications also occur when corrections
are higher than 7 to 8 dioptres in short sightedness
and higher than 2 to 3 dioptres in long sightedness.
Improvements in laser technology and pre-operative
screenings have reduced the incidence of such
complications. Other complications like, infection
also occur, though rarely.
Pre-operative screening for PRK surgery
Pre-operative screening, as rigorous as in LASIK,
is also done on potential PRK patients. The contra
indications for PRK are virtually the same as in
LASIK, regarding the cornea, the pupil and other
systemic or non-systemic medical conditions. The
indications for PRK are correct levels of short
and farsightedness, and astigmatism, relative intolerance
to glasses and contacts and well-motivated and
informed patients with realistic expectations.
PRK is an elective procedure and, like any other
procedures have its own risks and benefits.