Photorefractive Keratectomy (PRK)

Photorefractive keratectomy (PRK) is a procedure in which the surface of the cornea is reshaped by an ophthalmologist using an excimer laser.  It is actually the first excimer laser procedure approved by the FDA, which occurred in 1995.  PRK may be used to treat myopia (nearsightedness), hyperopia (farsightedness), or astigmatism.  PRK does not involve creation of a corneal flap utilizing a microkeratome or the femtosecond laser (Intralase), and therefore, the protective superficial layers (epithelium) of the cornea must heal post-operatively. This generally means that patients who undergo PRK will require significantly more time than LASIK patients to achieve their best vision.

The American Academy of Ophthalmology considers PRK safe and effective for mild to moderate myopia[i].  Most people with nearsightedness fall in this mild-to-moderate range of myopia, which is approximately –1.0 to –6.0 diopters.  The American Academy of Ophthalmology also considers PRK safe and effective for the treatment of hyperopia.  In fact, the surgery has been approved for use in the U.S. in people age 21 and older that have hyperopia of +1.0 to +6.0 diopters.  However, I would submit that the risks are too great to go much beyond +3.0 diopters.  As with myopic PRK, the greater the degree of treatment, the greater the risk of corneal haze as the eye heals.  I’ll discuss this issue in further detail below.

Studies have shown that PRK is safe and effective in reducing mild to moderate nearsightedness[ii].  Results show that after surgery:

· 67% to 98% of eyes have 20/40 vision or better uncorrected acuity

· 48% to 86% of eyes have 20/20 or better uncorrected acuity

In the hyperopic (farsighted) eye, PRK was first given FDA approval for treatment in 1998.  It has been shown to be relatively safe and effective in correcting mild to moderate hyperopia.  One clinical trial did show that one year after surgery, 92% of eyes had 20/40 or better uncorrected vision and 95% were within 1.0 diopter of the intended correction[iii].

Although the outcomes of PRK and LASIK are quite comparable at the six- month post-operative visit, it has been argued that LASIK may be safer than PRK due to the greater risk of scarring (haze) and unpredictable healing of the cornea with PRK.  In general, the greater the degree of treatment required, the greater the risk of significant corneal haze developing as the eye heals.  Some corneal haze is normal and expected, however, as the eye heals.  This healing process must be modulated in the post-PRK eye with the use of topical (eyedrop) steroids that are generally used for about three months post-operatively.  The haze nearly always resolves.  If significant corneal haze persists beyond eight months to a year, the epithelium is sometimes removed again and the eye is re-treated with a second course of topical steroids.  Only a small percentage of eyes will have persistent corneal haze, assuming that appropriate parameters of treatment are adhered to in the first place.  That is, PRK should only be used in patients who are myopic up to about –6.0 diopters or hyperopic up to about +3.0 diopters, in my opinion. 

The risk of infection is also slightly higher with PRK than with LASIK, although infections following either procedure are rare.

Interestingly, Marguerite McDonald, M.D. clinical professor at Tulane University in New Orleans and the first physician to perform an excimer laser procedure on a sighted human eye, continued to be a proponent of PRK when the greater majority of ophthalmologists had already largely abandoned the procedure in favor of LASIK.  Because PRK falls into the realm of what are sometimes referred to as excimer laser “surface procedures”, such as Epi-LASIK and LASEK, there are theoretical and perhaps actual benefits to the procedure in terms of maintaining the intended shape of the cornea following the laser ablation. 

On LASIK page, I reviewed the fact that creating a LASIK flap at all induces unpredictable biomechanical changes in the shape of the cornea and therefore, causes unpredictable postoperative higher order aberrations (HOA’s).  Again, and this is a review, LASIK entails the creation of a protective corneal flap that is relatively thick.  It must include surface corneal epithelium and underlying stroma.  The procedures Epi-LASIK and LASEK involve creation of a thin flap made only of epithelium, which theoretically, should not alter the shape of the laser-ablated cornea once it is laid back into position, thus preserving the intended shape of the cornea at the completion of the laser procedure.  PRK, on the other hand, involves complete removal of the epithelium followed by excimer laser ablation of the underlying stroma.  At the completion of this procedure, there is no protective flap of any kind and, therefore, the surgeon uses a bandage contact lens placed on the cornea until the underlying epithelium heals back over the cornea, the latter of which usually occurs in about three to four days.

PRK is certainly a good choice, in my opinion, for patients who have thin corneas and are not candidates for LASIK.  However, the greater the degree of myopia, the greater the risk of developing troublesome corneal haze during the healing phase in the post-PRK eye as I mentioned above.  Most ophthalmologists will probably not use PRK in patients who have more than –6.0 diopters of myopia.  Here again, Epi-LASIK, now used extensively by Marguerite McDonald, M.D. and others on the cutting edge, seems the better choice.  It can be used in most patients with thin corneas, it preserves a protective flap of epithelium, and it has (theoretically) the least induction of higher order aberrations.

Overall, PRK has an excellent track record though.  One study from the Department of Academic Ophthalmology, St. Thomas’ Hospital, London, showed PRK patients treated for myopia (nearsightedness) had refractive stability, that is, little or no change in their glasses prescription, between one year and twelve years following their procedure[iv].  Corneal haze decreased with time with only four percent having any long-term residual haze.  There was complete recovery of best-spectacle corrected visual acuity.  Twelve percent of these patients had persistent nighttime halos but this was because of the small excimer laser ablation zone that was abandoned in the mid 90’s.  Again, the machine makes a difference.  A big difference!  The results are really very good - even in these patients whose procedures were completed prior to 1992!  Let’s take a look at the procedure. 

Indications for the PRK Procedure

· Appropriate levels of nearsightedness, farsightedness, or astigmatism

· Relative intolerance of glasses or contact lenses

· Properly motivated and realistic patient    

PRK may be used to treat nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. However, not all Excimer lasers are presently FDA approved to treat all three types of refractive errors. For example, some excimer lasers are not yet FDA approved to treat farsightedness. Your surgeon can provide this information to you at your request.

PRK is an elective procedure and, like any procedure, poses risks as well as potential benefits. In general, indications for surgery must include an appropriate level of nearsightedness, farsightedness, or astigmatism, as well as an educated and properly motivated patient with realistic expectations. The best candidate for PRK is an individual who desires to be less dependent on glasses or contact lenses, is willing to accept the risks of the procedure, and understands that an enhancement procedure may sometimes be required. The primary potential risks include postoperative glare, halos or starburst around lights at night, and infection in the cornea with loss of best-corrected visual acuity. Fortunately, risks that threaten vision in the eye, such as infection in the cornea or within the deeper contents of the eye, are very rare.

The PRK Procedure

Patients who wear soft contacts or rigid gas-permeable contacts should discontinue their contact lens wear at least 3 days or 3 weeks prior to the evaluation, respectively. Prior to the PRK procedure, one or more careful refractions (determinations of eyeglass correction needed) will be completed.  Contact lens wearing patients who are believed to have an unstable refraction will be asked to discontinue contact lens wear and return for a repeat refraction in one to three weeks. When back-to-back refractions are stable (equivalent), the procedure may be scheduled. Corneal topography, or a detailed surface map of the cornea, will also be completed to rule out keratoconus and irregular astigmatism of the cornea.  If all looks stable and keratoconus has been ruled-out, then the procedure may be scheduled.

A few minutes prior to the PRK procedure, anesthetic drops will be applied to the eye to prevent pain during the procedure. A speculum will be placed to hold the lids apart for the procedure, eliminating any concern that blinking during the procedure would present a problem. The eye surgeon will then gently remove the surface corneal cells (epithelium) and proceed with the laser aspect of the procedure. The laser, being computer driven for accuracy and precision, is programmed based on the patient’s refractive error (nearsightedness, farsightedness, astigmatism).  Wavefront-guided (WFG) excimer laser may certainly be used (see Chapter Nine for details). 

Post-operative Regimen

Post-operatively, most patients will need to be re-evaluated one day, three days, one week, one month, two to three months, and four to six months following the PRK procedure. The bandage contact lens can usually be removed on the second or third post-operative day when the epithelium is healed. Most patients will require eye drops to control and modulate healing during the first 6 to 12 weeks, but other patients may require topically applied medications for up to 6 months or more following the procedure. In general, the greater the refractive error, the more likely the patient is to require eye drop medications for a prolonged period following the procedure.

What to Expect After PRK Surgery

Immediately after the procedure, a bandage contact lens will usually be applied to the eye. This contact lens is worn for the first 2 to 3 days until the surface epithelium is healed.  During this period of time there may be mild to moderately severe (unusual) discomfort, although this can be minimized substantially with pre-operative use of topical non-steroidal anti-inflammatory medications such as Acular. Expect at least a few visits to your EyeMD during the first 6 months following surgery, with the first visit being one to three days following surgery. When the surface epithelium is healed, the eye will be comfortable, and the bandage contact lens is removed. Eye drops are required to prevent infection and control inflammation following surgery. Your vision will gradually improve. Generally, vision will be good enough to drive a car within two to three weeks following surgery, but your best vision may not be obtained for up to 6 weeks or even 6 months following surgery.

Conclusion

PRK is a very good procedure, although I believe that even in the patient who has thin corneas and is not a candidate for LASIK, Epi-LASIK is probably the better choice.  However, most surgeons today are just beginning to use Epi-LASIK, which makes it less available, whereas PRK is a relatively simple procedure for nearly any refractive surgeon to perform.  The results tend to be very good when the procedure is completed within the range of refractive error that I have previously given.  This procedure does tend to result in some degree of post-operative discomfort or even pain, whereas LASIK type procedures do not.  Visual recovery is relatively slow.  Many post-operative visits are required and eye drop medications will typically be needed for six to twelve weeks, and occasionally even longer.