PRK Refractive Program at Vance Thompson Vision
PRK Top Line
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.
What is the Procedure?
PRK, or photorefractive keratectomy, was the first refractive excimer laser surgery to be approved in the United States. Its longevity as a viable refractive procedure is attributable to its relatively low surgical risk, excellent visual outcomes, and long term stability.
Although the recovery and return of vision is slower than other popular procedures like LASIK, it is hard to argue against PRK for low levels of myopia, hyperopia, and astigmatism. We were involved in the initial US FDA clinical trials as one of the first 20 study centers investigating PRK. After six years of well-controlled and exhaustive clinical trials, PRK was approved for the correction of myopia in October 1995. We have over 20 years of clinical data on PRK, and it continues to be a common procedure performed in our center.
When and Why Do It?
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.
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.
How is the Procedure Done?
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.
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. The patient is placed on:
- Pred/Gati (Prednisolone acetate 1% / Gatifloxacin): 1 drop QID for one week, then 1 drop BID for 1 week
- Acuvail: as needed for pain with a maximum of BID for 3 days
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.
Side Effects & Post-Op Complaints
Post-operative complications can include, infection, slowed or non-healing epithelial defects, persistent epithelial ridge formation, steroid response, under and over-correction, anterior reticular haze, and recurrent erosion.
Infection rates with PRK are extremely low, but, with the large epithelial defects that we are dealing with, patients must be monitored closely and treated aggressively to prevent loss of vision or corneal scar formation from a virulent infection. Any infiltrate noted during healing should be brought to our attention and referred in for evaluation.
As we talked about in the previous section, if slowed or non-progressive epithelial healing is determined, we increase lubrication and decrease anti-inflammatory medication to promote epithelial healing. Using this medication protocol along with continued bandage contact lenses will usually promote epithelial healing and the defect should seal within several days. If the epithelium is still not sealed after 6 days, the patient should be scheduled back with us at the refractive surgery center for evaluation and treatment.
Commonly, a central epithelial ridge will form where the epithelial edges seal together. This ridge usually smoothens itself out over the first 2 to 4 weeks post operatively. If the ridge persists and is blurring vision at 4 weeks post-op, we discuss with the patient that we may need to mechanically remove the aberrant epithelium to grow smoother and more viable epithelium. A persistent epithelial ridge could lead to an elevated central island of tissue and cause significant visual problems.
Some believe steroid use following PRK is important to help reduce the potential for aggressive healing and corneal haze formation. The smoother ablation patterns of advanced lasers and intra-operative treatment with anti-metabolites have helped reduce the risk of haze. Therefore, our current impression is that patients do not need extended steroid treatment in order to reduce the chance of haze. Since we recommend discontinuing steroid use after 2 weeks, steroid responders with elevated IOP should be very rare. We recommend waiting at least 2 weeks before performing applanation tonometry (unless you are noting signs of elevated IOP), to help reduce the risk of damaging the healing epithelium.
As with all refractive procedures, over and under-corrections can occur. All patients are counseled on the importance of waiting at least six months for healing and refractive stability before considering an enhancement. Remodeling of the epithelium can be very powerful in improving uncorrected visual acuity and decreasing residual refractive error, so being patient with PRK healing is important.
As mentioned earlier, Central corneal haze with PRK is not as prevalent as in the past, due to the smoother ablation patterns of advanced lasers, better patient selection, use of anti-metabolites during surgery, and controlled steroid use. Even so, anterior reticular haze can occur after PRK. Small amounts (Trace haze, Figure 1) of anterior haze are very normal with PRK, and usually do not effect vision or BCVA. This haze typically develops at around 1 month post-operatively and peaks at 3 months. After 3 months, the haze begins to diminish and returns to pre-operative levels within 6 to 18 months. This early haze, which does not affect acuity or image quality, used to be concerning to clinicians. It is now recognized as a normal healing reaction following PRK.
More aggressive healing reactions producing 1+ (Figure 2: 1+ Haze) to 2+ haze (Figure 3: 2+Haze) are rare, but require treatment. If more than trace haze is noted, we recommend referring the patient back to Vance Thompson Vision for evaluation and potential surgical treatment to reduce haze. We know there are several factors that can contribute to the formation of haze. Significant UV exposure is one factor that some believe increases the risk of visually significant haze. We recommend UV protection (sunglasses) for PRK patients for at least 6 months following the procedure.
Recurrent erosion is rare with PRK as it has been demonstrated that epithelial adherence to the laser ablated corneal stroma is stronger than it was pre-laser. This is why PTK has such a strong therapeutic effect on patients with anterior basement membrane dystrophy and recurrent erosion. Rarely, recurrent erosions can occur at the edge of the PRK treatment zone and can be treated with additional therapeutic laser treatment.
Maximizing Patient Outcomes
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.