Introduction to photorefractive keratectomy (PRK)

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 contacts.

PRK procedure

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 eyes.

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 in PRK.

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 occurring.

Under and overcorrection can be seen, caused by improper surgery, a malfunctioning excimer laser, 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 correction. 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 and ectasia 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.