Photorefractive Keratectomy (PRK) for Myopia Correction Treatment & Management

Following photorefractive keratectomy (PRK), topical antibiotics should be used with a therapeutic contact lens until reepithelialization is complete. Furthermore, a weak corticosteroid, such as fluorometholone 0.1%, is frequently used to avoid excessive collagen deposition.

Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK. ^{[6, 7, 8]}

Preoperative evaluation before PRK includes the following:

• Refractive history (including history of contact lens wear)
• Keratometry
• Uncorrected and best-corrected visual acuity
• Manifest and cycloplegic refraction
• Videokeratography and keratometry
• Slit lamp examination
• Tonometry
• Dilated funduscopy
• Assessment of a patient's ability to tolerate the procedure under topical anesthesia, to fixate steadily, to lie flat without difficulty, to understand the nature of the procedure, and to give an informed consent

After placement of antibiotic and anesthetic drops in the eye, the eyelid is held open using a speculum. When using the mechanical epithelial removal technique, a 6.0- or 6.5-mm marker can be used, centered over the entrance pupil to mark the area where the epithelium is to be removed. The epithelium can be removed using a blunt spatula or a small blade, such as a Beaver 64.

After epithelial removal, the surface of the cornea must be wiped with a nonfragmenting sponge that has been soaked with balanced sterile saline and then squeezed so it is moist but not saturated. During the procedure, focusing properly on the stromal surface and avoiding excessive illumination to allow patient fixation are important. A Thornton ring or similar instrument can be used to gently help the patient maintain fixation without distorting the corneal surface.

The excimer laser beam is centered on the entrance pupil and focused on the anterior stromal surface, and the laser treatment is applied. Excimer lasers frequently have tracking devices to help properly center treatment.

Placing a therapeutic contact lens for an average of 3 days (and not uncommonly for up to 5 days) is helpful, and application of a combination of antibiotic/steroid drops is routine. Alternatively, a pressure patch following application of an antibiotic/steroid ointment is well tolerated. Analgesic and antianxiety oral medications can be used during the first 24 hours postoperatively.

Patients should be monitored daily after PRK until the reepithelialization is complete. At this time, the therapeutic contact lens can be removed, and the patient can be monitored 1 week later. The remaining follow-up interval times can double until the vision stabilizes.

For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Vision Correction Surgery.

Complications following PRK include the following:

• Undercorrections, overcorrections, and induced astigmatism: As the most common complications that occur following PRK, they may be caused by inadequate centration or focusing at the time of surgery or an inadequate surgical plan. Management of these complications often includes a retreatment using either a second PRK procedure or LASIK after the initial result has stabilized.
• Regression: This complication may occur postoperatively, particularly in young patients or those with high corrections for up to 6 months following PRK. Initial treatment with topical steroids may halt regression. If a regression results in visually significant changes, a PRK or LASIK retreatment can be performed once refractive stabilization has been documented if residual tissue is adequate.
• Reticular stromal haze: Why some patients develop this pattern on the anterior stroma is unclear. It has been associated with the use of postoperative nonsteroidal anti-inflammatory drugs (NSAIDs). In many cases, it does not affect a patient's visual acuity. Initial management of corneal haze usually consists of waiting until the opacity fades spontaneously. Treatment of persistent moderate-to-severe haze includes the use of topical steroids and/or a second laser application (eg, PRK, phototherapeutic keratectomy [PTK]). Only haze that is causing a reduction in visual acuity should be considered for retreatment. An association exists between significant regression and the formation of severe reticular haze.
• Increase in intraocular pressure (IOP) associated with the use of postoperative steroids: In all cases, IOP reverts to normal values on discontinuation of topical steroids. No reports of persistent increased IOP exist in the literature.
• Steroid-induced herpes simplex keratitis: This complication may be associated with the use of postoperative steroids or possibly from the stress of the excimer laser. In patients with a known history of herpes simplex viral keratitis, the use of preoperative and postoperative topical/oral acyclovir or trifluorothymidine drops is warranted, and some feel this is a relative contraindication.
• Posterior subcapsular cataracts: These have been reported infrequently in patients who used dexamethasone phosphate postoperatively.
• Superficial keratopathy: This has been reported in association with topical NSAIDs or the preservatives used in other drops.
• Complications related to the use of therapeutic contact lenses, including infectious keratitis: Treatment of this complication requires corneal scrapings for culture and sensitivity, as well as the use of fortified topical antibiotics.
• Infectious and sterile paracentral corneal infiltrates: These infiltrates can be treated with fortified topical antibiotics and topical steroids, respectively. Atypical organisms have been reported to cause infectious cases in addition to usual organisms.
• Delayed epithelial healing: This can occur in some patients, requiring a longer period of time with therapeutic contact lens to allow epithelium regeneration.
• Recurrent corneal erosions: This complication may occur in the presence of basement dystrophy unrecognized during the preoperative evaluation. The treatment may include PTK and the use of hypertonic saline solution.
• Decentration of the ablation zone: This may occur following inadequate centration on the entrance pupil at the time of laser application. The treatment may include a retreatment guided by corneal topography and/or 3D corneal topography (elevation map).
• Central islands: Excimer lasers with a flattop energy beam profile have the potential of producing central islands. Four main theories exist as to the cause of central islands, as follows: focal central epithelial hyperplasia, the vortex plume theory, degradation of laser optics, and the acoustic shock wave theory. However, central island formation is most likely multifactorial. Modern excimer lasers have built-in software to neutralize the possible appearance of central islands. This software acts by centrally applying additional pulses.
• Corneal melting and perforation: This has been reported in patients with connective tissue disease. A penetrating keratoplasty may be necessary in these cases to restore vision and ocular integrity.

For predictability of outcome, investigators collected the following data from patients who were operated on with a VISX excimer laser in a prospective, nonrandomized, unmasked, multicenter PRK clinical study and followed them for at least 2 years:

• In myopic PRK, refractive stability achieved at 1 year was maintained up to 12 years with no evidence of hyperopic shift, diurnal fluctuation, or late regression in the long term. Corneal haze decreased with time, with complete recovery of BSCVA. Night halos remained a significant problem in a subset of patients due to the small ablation zone size.
• After hyperopic PRK, refractive stability achieved at 1 year was maintained up to 7.5 years with no evidence of hyperopic shift, diurnal fluctuation, or late regression. Peripheral corneal haze decreased with time but was still evident in a number of eyes at the last follow-up visit.
• PRK for severe anisometropic amblyopia in children resulted in long-term stable reduction in refractive error and improvement in visual acuity and stereopsis, with negligible persistent corneal haze.
• The safety and efficacy of PRK after LASIK show good reduction of refractive error and improvement of UCVA and BSCVA. A significant undercorrection of astigmatism was attributed to surgically induced astigmatism. Further studies are necessary to determine the long-term safety and stability of outcomes.

Even though several reports demonstrate that long-term visual outcome of patients treated with PRK versus LASIK is equivalent for mild-to-moderate myopia, PRK has become a second choice procedure for most refractive surgeons.

When using the excimer laser, LASIK has become the preferred technique because of the lack of a significant amount of discomfort, the faster rate of postoperative visual rehabilitation, and the greater amount of stability.

However, PRK remains useful when treating patients whose corneas are too thin to perform LASIK and leaves a 250-µm stromal bed after the ablation is performed. PRK also is the procedure of choice when treating a refractive error associated with an uneven corneal surface or a superficial leukoma.

New tools for mapping the cornea, including wavefront technology, will provide more accurate information that can be linked to the excimer laser and allow a customized ablation. ^[9] However, one study found that wavefront-guided PRK offered no advantage over non – wavefront-guided PRK in predictability or safety. ^[10] Similarly, technical improvements in the design of excimer machines, such as eye-tracking devices, have minimized potential complications, such as decentration, following PRK.