Photorefractive Keratectomy

David J. Tanzer, MD

History

Photorefractive keratectomy (PRK) is a corneal-based refractive surgical procedure designed to reduce reliance on spectacles or contact lenses.^1,2 PRK is approved by the Food and Drug Administration to treat all aspects of refractive error except mixed astigmatism (an off-label indication).

PRK is a corneal surface ablation procedure that does not require the creation of a partial thickness flap as in laser-assisted in situ keratomileusis (LASIK), thereby avoiding a host of potential vision-threatening complications such as buttonholes, free caps, epithelial ingrowth, and microstriae.

Results

A conventional PRK study, which enrolled 785 Naval aviators including 150 pilots in actual control of aircraft, was recently completed. The mean preoperative manifest refractive spherical equivalent (MRSE) was -2.97 D. By 6 months, the MRSE was -0.03 D. At the 6-month postoperative visit, 86% of enrollees were within 0.50 D of intended correction and 96% were within 1 D of intended correction.

Preoperative uncorrected visual acuity (UCVA) averaged 20/160 and by 6 months after PRK, the average UCVA was 20/16 +1. Ninety-five percent of treated eyes were 20/20 or better, 79% were 20/16 or better, and 50% were 20/12.5 or better uncorrected. Regarding low contrast visual acuity (CA), 3% of subjects lost two or more lines of 5% CA; however, 6% gained two or more lines of 5% CA.³

The results of wavefront-guided PRK (currently an off-label application) are showing promise. By 6 months postoperative, the MRSE was -0.10 D, 98% were 20/20 or better uncorrected, and 92% were within 0.50 D of intended correction. No patients lost two or more lines of BCVA, and 6% gained two or more lines of BCVA.

These results were no different from a comparative population of subjects receiving

Slide 1. Apply anesthetic. Slides courtesy of David J. Tanzer, MD

wavefront-guided LASIK. Thus far, studies are demonstrating equal safety and efficacy when comparing wavefront-guided PRK with wavefront-guided LASIK.

Technique

1. Initial anesthesia
Proparacaine is instilled in the eye every 5 minutes for 15 minutes before surgery to provide adequate anesthesia and to loosen the epithelium, thereby facilitating epithelial removal with a rotary brush. (Slide 1)

Slide 2. Mark limbus.

Slide 3. Mark limbus.

2. Marking the cornea
Before surgery, the eye is marked at the slit lamp with a gentian violet surgical marking pen at the 3 o'clock and 9 o'clock meridians, just lateral to the limbus. This is performed before conventional PRK in which the astigmatism exceeds 1.5 D. All eyes are marked before a wavefront-guided procedure. This step is required to allow adjustment for cyclotorsion of the eye when going from an upright to a horizontal position, which can be as much as 16º in up to 25% of patients.⁴ (Slide 2, Slide 3)

3. Patient identity and laser calibration confirmation
It is important for surgeons to verify the identity of patients as they enter the laser suite as well

Slide 4. Confirm patient identity and laser settings.

Slide 5. Align limbal marks with reticle before laser.

as ensure the treatment parameters entered into the laser correspond to the preoperative plan. (Slide 4)

4. Patch non-treated eye
The fellow eye is patched to limit visual confusion by the treated eye when the patient is fixating on the target light during laser ablation.

5. Position patient under laser
The patient is positioned and aligned properly under the laser, using pre-placed limbal marks if previously applied at the slit lamp. (Slide 5)

6. Additional proparacaine to treated eye
Additional drops of proparacaine are instilled in the treated eye before lid speculum insertion. Typically, the eyelids are not draped or covered for PRK as in LASIK.

Slide 6. Place lid speculum.

Slide 7. Place lid speculum.

7. Lid speculum insertion
The lid speculum is gently inserted to retract the eyelids. (Slide 6, Slide 7)

8. Additional proparacaine to treated eye
A final drop of proparacaine is instilled in the eye to uniformly hydrate and lubricate the epithelium.

9. Epithelial removal
An Amoils rotary brush (Innovative Excimer Solutions Inc, Toronto) is used to debride the epithelium. A new, sterile nylon bristle head is used for each eye. The laser reticle (as available) is used to guide

Slide 8. Epithelial removal with Amoils brush.

Slide 9. Epithelial removal with Amoils brush.

the amount of epithelium to be removed, according to the ablation zone to be used. Ensure that the epithelium has been removed to the full extent of the ablation zone and that edges of the epithelium are smooth to promote rapid and uniform healing. (Slide 8, Slide 9, Slide 10, Slide 11, Slide 12)

Because the laser fixation light is temporarily blocked by the brush, the eye has a tendency to rotate during this step. Quick applications of the brush, alternating with temporary removal to allow refixation, is usually required to maintain adequate centration during epithelial removal. A Fine-Thornton fixation ring can be used to stabilize the globe for difficult cases.

Slide 10. Epithelial removal with Amoils brush.

Slide 11. Epithelial removal with Amoils brush.

10. Stromal hydration
Uniformly hydrate the exposed stroma with a surgical sponge moderately moistened with balanced salt solution. The stroma should not be overly hydrated, nor should it be bone dry. The exposed Bowman's layer has a smooth appearance when properly exposed and hydrated. Pools of balanced salt solution should not be anywhere in the ablation field. Uniformity and consistency of stromal hydration is one of the key elements to reproducible results and consistency in technique enables a surgeon to make nomogram adjustments with confidence. (Slide 13, Slide 14)

11. Activate the pupil tracker
Engage the pupil tracker (as available) and ensure that the tracker is centered on the target to be ablated

Slide 12. Epithelial removal with Amoils brush.

Slide 13. Rehydrate stroma.

(usually the center of the pupil) and that the pre-placed limbal marks are aligned properly with the laser.

12. Commence with laser ablation
Activate the laser, keeping the reticle centered on the pupil and the patient's attention focused on the fixation light. Monitor the corneal stroma for any particulate matter (such as fibers or flakes of surgical sponge) which usually fluoresces brightly and for pools of fluid which may accumulate on the stroma. Remove any particulate matter and redistribute the corneal hydration with a surgical sponge as required. For MRSE greater than -6 D, the ablation is stopped at the half-way point and the stroma is uniformly cooled by rehydration using a surgical sponge lightly moistened with balanced salt solution.

Slide 14. Rehydrate stroma.

Slide 15. Irrigate with balanced salt solution.

Before the advent of variable and flying spot lasers, stromal rehydration for highly ametropic eyes was a crucial step to avoid overheating and excessive dehydration of the cornea, with resultant overcorrection. With modern laser platforms, this step is no longer an absolute requirement but is still performed by some surgeons. Again, consistency in technique is required for reproducible outcomes and to enable nomogram adjustments to be made with confidence.

13. Irrigate stroma
Once the laser ablation is completed, irrigate stroma with 2 mL to 3 mL of chilled balanced salt solution to improve pain control and reduce the possibility of postoperative corneal haze. Studies have

Slide 16. Apply bandage contact lens.

Slide 17. Apply bandage contact lens.

shown that cooling the cornea immediately after PRK significantly reduces discomfort and haze after surgery.^5,6 (Slide 15)

14. Place bandage contact lens
Position a bandage soft contact lens on the ablated cornea, ensuring that it is right-side up and well-centered. High oxygen permeability (Dk) soft contact lenses have been shown to speed epithelial healing and improve patient comfort.⁷ (Slide 16, Slide 17, Slide 18)

15. Instill postoperative drops
Instill immediate postoperative eye drops. A fluoroquinolone antibiotic (Ocuflox, Allergan) and a nonsteroidal

Slide 18. Apply bandage contact lens.

Slide 19. Apply postoperative drops.

anti-inflammatory (Voltarin, Novartis) are currently used at our center. (Slide 19)

16. Remove lid speculum
Gently remove the lid speculum, taking care not to dislodge the contact lens. After lid speculum removal and before repositioning the patient, check the position of the bandage lens one final time and adjust as required.

17. Remove patient from under laser
Reposition the patient for treating the fellow eye as indicated, or sit patient up from the laser and ensure there is no dizziness or light headedness before moving the patient to the immediate postoperative area. (Slide 20)

Slide 20. Remove lid speculum and assess final position of contact lens.

Postoperative Medication Regimen

Patients are discharged with medications and the following instruction for the immediate postoperative period:

Oral:

Topical:

Patients are instructed to proceed home immediately after the surgery and to begin their medications. After eating a small meal, patients should begin oral medications for pain and swelling as required.

Patients are seen on the first postoperative day for a comfort check and to ensure the bandage contact lens is in good position. All medications are continued as above. Patients are reminded to use the tetracaine no more frequently than every 2 to 4 hours and to discontinue its use the following day.

Patients are next seen on the fourth postoperative day to evaluate the healing of the epithelium. If the corneal epithelium has completed healing, then the bandage lens is removed. At this time, the oral medications and topical antibiotic are discontinued. After this examination, the patient is continued on nonpreserved artificial tears and topical steroids on a tapering course.

Patients who have an MRSE of -6 D or less are instructed to use the steroid four times daily for 2 weeks after surgery and to reduce the steroid by 1 drop per day every 2 weeks after PRK (2-month total course). Patients with preoperative MRSE above -6 D or patients after a hyperopic treatment, are instructed to use the steroid four times daily for the first 4 weeks after surgery and to reduce it by 1 drop per day every 4 weeks after PRK (4-month total course).

Patients are also encouraged to use ice therapy (ice packs on their eyes for 20 minutes out of each hour) on their closed eyelids to reduce corneal swelling. Because of the induced photophobia secondary to the corneal abrasions, remaining in a darkened room for the first 24 to 48 hours after PRK may aid in reducing overall discomfort. Patient are also instructed to wear 100% UVA and UVB protected sunglasses while exposed to outside sunlight, especially for the first several weeks after PRK surgery, to reduce the chance of developing postoperative corneal haze.

Complications

The Achilles' heel of PRK is the potential for developing corneal haze during the healing phase after surgery, which can last for months after the procedure. A direct relationship exists between the potential for haze after PRK and the preoperative MRSE. There is also a correlation between

Slide 21. Apply mitomycin disc to central cornea.

Slide 22. Apply mitomycin disc to central cornea.

sunlight exposure and corneal haze after PRK (hence the importance of maximal UVB protection). Several studies have documented the efficacy of using low-dose mitomycin C during PRK to reduce the potential for postoperative haze.^8,9

Mitomycin C concentrations between 0.01% and 0.02% (0.1 mg/mL and 0.2 mg/mL, respectively) with contact times between 60 and 120 seconds during surgery have routinely been used. Mitomycin C is typically applied to the cornea via a soaked surgical sponge/disc and is allowed to rest on the cornea for the prescribed amount of time (Slide 21, Slide 22, Slide 23). This application usually occurs immediately after excimer laser ablation. The cornea is then immediately irrigated with a minimum of 15 mL of balanced salt solution and a bandage lens is applied.

Slide 23. Apply mitomycin disc to central cornea.

Slide 24. Hockey stick for further epithelial removal.

The use of mitomycin C has significantly reduced the incidence of haze after PRK and the search is ongoing for the lowest therapeutic dose that will be effective at eliminating corneal haze after PRK.

Mitomycin C can also be applied to the cornea as a therapeutic modality to treat visually significant haze after PRK.^10,11 In these cases, the corneal epithelium is removed and the haze/scar is scraped with a corneal spatula or gently with a knife (e.g., a Took knife). (Slide 24, Slide 25, Slide 26, Slide 27) Mitomycin C is then applied (usually a higher concentration and for a longer duration, such as 0.02% for 2 minutes) with a surgical sponge/disc. The cornea is treated in

Slide 25. Hockey stick for further corneal epithelium removal.

Slide 26. Hockey stick for further corneal epithelium removal.

a similar manner as described above; however, additional excimer laser is not typically applied until the MRSE has stabilized.

Summary

PRK is a safe and effective treatment for all levels of ametropia. PRK compares favorably with LASIK when evaluating clinical and quality of vision outcomes. The visual recovery after PRK is somewhat longer than with LASIK, but this limitation is balanced by the enhanced safety profile of the surface ablation procedure (i.e., no risk of flap-related complications). Surgeons and patients can take comfort in considering PRK for the correction of refractive errors

Slide 27. Hockey stick for further corneal epithelium removal.

knowing that the best possible outcomes are possible with this safe corneal-based refractive surgical procedure.

References

1. Tanzer DJ, McDonnell PJ. Background and principles of photorefractive keratectomy. In: Brightbill FS, ed. Corneal Surgery: Theory, Technique and Tissue. St. Louis, Mo: Mosby, Inc; 1999.
2. Schallhorn SC, McDonnell PJ. History and overview of refractive surgery. In: Kratchmer JH, Mannis MJ, Holland EJ, eds. Cornea: Surgery of the Cornea and Conjunctiva. St. Louis, Mo: Mosby-Year Book, Inc; 1977.
3. Schallhorn SC, McDonnell PJ. History and overview of refractive surgery. In: Kratchmer JH, Mannis MJ, Holland EJ, eds. Cornea: Surgery of the Cornea and Conjunctiva. St. Louis, Mo: Mosby-Year Book, Inc; 1977.
4. Tanzer DJ. Update on photorefractive keratectomy in Naval aviators. Presented at: the American Society of Cataract and Refractive Surgery conference, San Diego, Ca; May, 2004.
5. Smith EM Jr, Talamo JH, Assil KK, Petashnick DE. Comparison of astigmatic axis in the seated and supine positions. J Refract Corneal Surg. 1994; 10:615-620.
6. Stein HA, Salim AG, Stein RM, Cheskes A. Corneal cooling and rehydration during photorefractive keratectomy to reduce postoperative corneal haze. J Refract Surg. 1999; 15(2 Suppl):S232-S233.
7. Kitazawa Y, Maekawa E, Sasaki S, Tokoro T, Mochizuki M, Ito S. Cooling effect on excimer laser photorefractive keratectomy. J Cataract Refract Surg. 1999; 25:1349-1355.
8. Engle AT, Laurent JM, Schallhorn SC, et al. A masked comparison of silicone hydrogel (Lotrafilcon A) versus Etafilcon A extended wear bandage contact lenses after photorefractive keratectomy (Ms. 520-03). J Cataract Refract Surg. (in press).
9. Nagy ZZ, Hiscott P, Seitz B, et al. Ultraviolet-B enhances corneal stromal response to 193-nm excimer laser treatment. Ophthalmology. 1997; 104:375-380.
10. Carones F, Vigo L, Scandola E, Vacchini L. Evaluation of the prophylactic use of mitomycin-C to inhibit haze formation after photorefractive keratectomy. J Cataract Refract Surg. 2002; 28:2088-2095.
11. Raviv T, Majmudar PA, Dennis RF, Epstein RJ. Mytomycin-C for post-PRK corneal haze. J Cataract Refract Surg. 2000; 26:1105-1106.
12. Vigo L, Scandola E, Carones F. Scraping and mitomycin C to treat haze and regression after photorefractive keratectomy for myopia. J Refract Surg. 2003; 19:449-454.