PRK

Evaluation

PRK continues to be a safe and predictable procedure to correct low and high levels of refractive error and is a great alternative for those patients who it’s not a good idea to make a flap on their cornea. With advanced laser technology, the risk of aggressive healing reactions has been reduced and predictability has been improved. Wavefront guided custom laser treatments work extremely well with PRK, and because of the excellent visual quality obtained with PRK, we will continue to see it as an increasingly common selection for refractive surgery patients.

• Patient Education is critical with the extended recovery and return of vision with PRK
• Agressive and consistent lubrication is critical to epithelial healing and improved outcomes
• The risks of PRK will continue to be extremely low with quality post-operative care and attention to healing variables with the procedure.
• As with all refractive procedures, patient expectations and realistic goal setting will remain the hallmark of providing our patients with quality refractive care.

Indications

Initially, PRK was approved for myopia up to -6.00 D of myopia. Later astigmatic and hyperopic correction were approved and the upper limits of PRK were extended to -14.00 D. As we evaluated PRK in the early clinical trials, it became evident that higher levels of laser correction could cause aggressive healing reactions in the cornea, showing up as central corneal reticular haze. At this same time, LASIK was becoming a more common procedure and was not exhibiting this reticular haze with higher diopter corrections. PRK was then relegated to low levels of correction, generally below 3.00 D. We now know that the reticular haze was due not only in part to larger laser ablations, but also due to the relative roughness of the early excimer laser ablation patterns. With advanced small spot scanning excimer lasers and with improved optical blends zones, ablation patterns are smoother. These smoother and more accurate ablations, along with the use of the anti-metabolite Mitomycin C have allowed us to increase the levels of correction with PRK and enjoy an extremely low incidence of haze. We are very comfortable going up to 6.00 D of hyperopia or astigmatism and we are performing myopic PRKs up to 9.00 diopters very successfully.

PRK has the advantage of not thinning corneal thickness deep into the stroma and is the procedure of choice for those patients with moderate refractive error and thinner corneas. For patients with 500 micron or less corneas, we prefer PRK. We also find that patients with mild corneal irregularity, asymmetric astigmatism, and displaced corneal apexes tend to have more predictable results with PRK than with LASIK. Patients who excessively rub their eyes are at increased risk of post-operative ectasia, particularly with LASIK and may be better candidates for PRK. We also use Corneal Resistance Factor (CRF) as measured with the ORA Corneal Hysteresis diagnostic device to determine the resiliency of patient’s corneas. Patients with low CRF may also be better candidates for PRK. Patients with mild amounts of epithelial basement membrane dystrophy are also good candidates for PRK. Just as PTK is used therapeutically to treat EBMD with recurrent erosion, PRK can have a positive effect on epithelial smoothness and adherence. Custom wave-front guided laser treatments also do exceptionally well with PRK.

Candidacy

As with all refractive procedures, a comprehensive ocular examination is performed both to rule out any contraindications and to provide an accurate assessment of refractive error. During the slit lamp examination of the anterior segment, we pay particular attention to anything that could slow or hamper epithelial healing following the procedure. Any blepharitis or meibomian gland dysfunction should be treated prior to surgery. Even asymptomatic lid disease can cause significant symptoms post-operatively. Tear deficient and evaporative dry eye should be tested for with tear osmolarity (TearLab), Schirmer’s and TBUT and treated appropriately and aggressively prior to surgery.

Corneal thickness must be determined. Even though there is more room (without a flap) to work with in the cornea with PRK, extremely thin corneas could be a sign of forme fruste or pre-clinical keratoconus. Patients who cannot be refracted to 20/20 or better need further evaluation. If the cornea, lenses, maculae, and optic nerves appear fine, a rigid contact lens over-refraction can be performed to rule out irregular astigmatism. If this yields better results, a search for keratoconus or contact lens-induced corneal warpage is undertaken. We use the Oculus Pentacam to evaluate any irregularity or variations in thickness or curvature of the cornea before PRK to determine if there are any signs of pre-clinical keratoconus, irregular astigmatism, or corneal warpage.

During the pre-operative evaluation, we have a very frank discussion with the patient about the healing time and symptoms associated with the large corneal abrasions after PRK. They must understand that it takes 3 to 5 days to adequately heal the epithelium. They must also understand that they may experience symptoms of burning, tearing, light sensitivity, foreign body sensation, and blurred vision. By using bandage contact lenses, NSAIDs and oral analgesics most patients get through the initial epithelial healing quite well. We reassure them that most patients are functioning visually by the fourth or fifth day (usually around 20/25 to 20/50), and that vision tends to improve daily after the epithelium is intact. Daily improvement in acuity occurs for 2 to 4 weeks and often this is the most frustrating time for PRK patients as it is slow. Another way we instruct patients on the slow visual recovery is; 90% of the vision is recovered at 1 month, 95% at 3 months and 100% by 6 months. If the vision is not quite sharp enough for the patient 6 months post-op and best corrected vision is sharp, it is a good time to consider a fine tune.

Surgery

PRK surgery is great option for those patients who may not be a candidate for a flap on the cornea. Topical anesthetic is applied to the eye and a lid speculum is placed to control lid movement and provide adequate corneal exposure. There are many methods to remove the epithelium: mechanical scraping, power brushing, excimer laser removal of the epithelium, and dilute alcohol removal. We prefer a 15 to 30 second soak in 18% ethanol diluted to 20% with sterile water, using an 8 mm optical zone marker with a deep well to ensure the alcohol bath only bathes the central epithelium. The dilute alcohol solution loosens the tight junctions and adherence of the epithelium to allow quick and thorough removal of the epithelium. Once the epithelium is removed, the basement membrane is cleaned of any remaining epithelial cells. The patient is instructed to focus on the fixation light, the laser tracker is engaged and the laser procedure is performed. Following the laser treatment, an antibiotic drop (fluoroquinolone) and a NSAID drop are instilled. A bandage contact lens is then placed on the cornea. We typically use a Bausch and Lomb Ultra lens.

Epithelial healing occurs at the same rate with or without a bandage contact lens, but patients are much more comfortable with the lens. We do respect that bandage contact lenses can increase the chance of infection; thus, we monitor the patient closely for any infiltrates. The bandage contact lens should show very little movement, but not be too tight on slit lamp exam.

Post Op

The immediate post-operative care of PRK is concentrated on healing the epithelial defect. We monitor the bandage contact lens fit and typically don’t remove it until we are sure there is no epithelial defect. A bandage contact lens that is too tight can cause more discomfort to the patient, while a loose contact lens can make it more difficult for the epithelium to heal.

As with all corneal procedures, adequate lubrication becomes paramount to speedy re-epithelialization and patient comfort. We recommend liberal and unrestricted use of preservative free artificial tears and night time gel or ointment. We will typically place inferior extended duration dissolvable collagen plugs and for those patients who have drier eyes prior to surgery we will place inferior silicone and superior collagen plugs prior to surgery.

The patient is given a prescription for a narcotic pain reliever (to use for pain) and cold compresses are encouraged especially on the temporal region. These can help for those patients with more discomfort. The patient is evaluated at one day to assess vision and monitor epithelial healing. Surprisingly, many patients have fairly good vision at one day as the epithelium hasn’t moved centrally and the BCL is creating a fairly smooth surface. Patients are warned that vision may take a few steps backward over the next several days as the epithelium moves centrally and crosses over the pupil. A smooth epithelial-leading edge should be noted under the contact lens, and epithelial growth of up to 20% can be seen at one day. The patient is re-instructed on the drop regimen and is scheduled to be seen 3 days later. At the four day appointment, the epithelium should be intact or at least very close. If a fairly large, greater than 2 mm epithelial defect is present at 4 days, suspect a slowing of epithelial growth.

If a delay in epithelial growth is suspected we recommend reducing the steroid and discontinuing NSAID use, continue the bandage contact lens, increase lubrication and follow daily. Refrain from discontinuing the steroid completely in order to minimize the chance of a sterile infiltrate. The combination drop (Pred/Moxi) can be reduced to BID to decrease steroid use, but our preference would be to switch the patient to two drops, a fluoroquinolone (Vigamox) and Prednisolone Acetate, so steroid use can be controlled but not sacrificing antibiotic use.

If not re-epithelialized by 6 days call to schedule to be evaluated in our center. Typically, the epithelium will seal with a small centrally located epithelial ridge. It can be star shaped or linear, will usually pick up stain and depending on where the ridge is located, it can have varied effects on vision. Once re-epithelialization has occurred, the bandage contact lens is removed. The patient is instructed to continue the Pred/Moxi QID for one week and BID for the next week. The use of consistent daily and night-time lubrication will help promote continued epithelial healing and adherence, as well as aiding in the epithelial remodeling that occurs following PRK. It is this epithelial remodeling that will continue to improve the patient’s vision with time

Follow-up Regimen: 1 Day, 1 Week, 1 Month, 3 Months, 6 Months, 1 Year.

1 Day Post-op:

Tests to Perform: visual acuity, refraction, slit lamp exam

The day following surgery is when most patients feel their vision has improved a lot. Most patients are not only comfortable, but drive to their 1 day post-op exam and go back to work this day. Your patient can have some dryness on this visit. If so, artificial tears and possibly night time ointment can be added at this stage. Of course, punctal plugs (temporary or permanent) are also an option. Diffuse lamellar keratitis (DLK) will sometimes be seen at this time. If so, notify us and then we can determine whether to increase steroids or consider a potential lift flap and interface rinse. If there are any flap striae, we should also be notified for a potential lift and smooth. Subconjunctival hemorrhages can be noted at this visit. We are not too concerned with this, as these will usually resolve within the first few weeks.

1 Week Post-op:

Tests to Perform: Visual acuity, refraction, slit lamp exam

This is when your patient will typically start tapering their drops. If your patient is not correctable to the preoperative visual acuity, and you can see some striae, it would be appropriate to have our surgeon smooth the flap. Evaluate the flap edge for epithelial ingrowth. This is another time DLK can be observed, and we should be notified.

1 Month Post-op:

Tests to Perform: Visual acuity, refraction, slit lamp exam, IOP (if applicable)

Patients sometimes complain of photophobia at this visit due to a rebound inflammation. This can usually be remedied with a steroid qid for 1 week.

3 Month Post-op:

Tests to Perform: Visual acuity, refraction (if considering enhancement), slit lamp exam, IOP (if applicable).

This is typically the earliest we will want to consider an enhancement. Although we would prefer to wait until the six month post-op exam. We also want to see a stable refraction before considering an enhancement. If we are considering an enhancement, then a cycloplegic exam, corneal thickness and topography must also be performed. Before considering an enhancement it’s also important to assess the tear film. In many situations, if a person has not achieved their target following LASIK, it is secondary to an underlying dry eye. If we are going to consider an enhancement, we want to ensure that there’s a minimum of 400 microns of total corneal tissue and at least 300 microns of untouched corneal tissue under the flap before we will consider re-lifting the flap, as well as the duration of time since their primary procedure. If we are uncomfortable with lifting the flap, we will perform a PRK enhancement instead of a LASIK enhancement.

6 Month Post-op:

Tests to Perform: visual acuity, refraction, slit lamp exam, IOP (if applicable)

If we are considering an enhancement, then a cycloplegic exam, corneal thickness and topography must also be performed. We also want to a stable refraction at this point before considering an enhancement.

1 Year Post-op:

Tests to Perform: visual acuity, refraction, slit lamp exam, IOP

If we are considering an enhancement, then a cycloplegic exam, corneal thickness and topography must also be performed. A stable refraction is also important for a successful enhancement.