Is PRK considered safe for individuals with severe myopia (greater than -10)?
Photorefractive keratectomy (PRK) has shown satisfactory outcomes and holds the potential for achieving emmetropia by utilizing the excimer laser. However, it is important to acknowledge that these procedures may not always proceed as intended. Certain patients who undergo PRK might experience significant corneal scarring and a decline in best-corrected visual acuity (BCVA).
To enhance the success rate of PRK, it is crucial to deepen our understanding of the biological variations in corneal healing and make advancements in laser technology. Late-onset corneal haze is a potential risk, where the formation of dense, irregular fibrotic tissue occurs due to the consolidation of new subepithelial collagen into a scar.
Another risk factor is the possibility of a small ablation zone. A reduced optical zone can lead to optical aberrations like night glare, flare, and halos. Central islands, which are more prevalent with the current broad-beam laser technology, are another concern associated with performing PRK on individuals with high myopia. The incidence of central island formation increases with the degree of myopia.
Understanding the process of wound healing is crucial. Expanding our knowledge of corneal wound healing in postoperative PRK eyes will aid optometrists in managing and modifying the final refractive outcome. Following PRK, various histological changes occur within the cornea. Notably, epithelial migration plays a significant role. Immediately after PRK, the epithelium initiates its migration over the exposed stromal bed.
Animal studies indicate that fibrinogen and fibronectin form a substance across the ablated stromal surface, facilitating epithelial migration and adhesion. The new epithelial cells cover the wound and gradually thicken, becoming more hyperplastic, particularly in cases where broad-beam ablation is employed, leading to abrupt wound edges. Typically, this process occurs within 3 to 4 days, but it can extend to several months.
What is Photorefractive Keratectomy (PRK)?
While LASIK is the most well-known and popular form of laser eye surgery for correcting refractive errors, there are alternative options available that can be suitable for individuals who may not qualify for LASIK. One such alternative is photorefractive keratectomy, commonly known as PRK.
Similar to LASIK, PRK utilizes a guided laser to reshape the cornea and correct myopia, hyperopia, or astigmatism. However, PRK is a preferable choice for individuals with thin corneas or chronic dry eye.
PRK was initially developed in 1983 and further improved in 1987. It received approval from the U.S. Food and Drug Administration (FDA) in 1996, a few years prior to the introduction of LASIK to the market.
Despite being a more effective and faster procedure compared to previous refractive surgeries, PRK was quickly overshadowed by the popularity of LASIK. Nonetheless, PRK remains a highly sought-after option for improving visual acuity.
Who Can Benefit from PRK?
If you are eligible for LASIK, you are also a suitable candidate for PRK. However, due to cost and name recognition, more people tend to choose LASIK.
Nevertheless, PRK is an excellent choice for individuals who may not qualify for LASIK. Certain individuals may experience long-lasting side effects from LASIK, particularly those with naturally thin corneas, individuals who have already undergone LASIK once, individuals with high levels of myopia (nearsightedness), or those with chronic and severe dry eye (which would worsen with LASIK).
The following individuals are considered good candidates for PRK:
- Adults aged 18 or older.
- Those with a stable refractive error that has not changed within the past year.
- Individuals with refractive errors are suitable for laser eye surgery.
- Individuals with healthy corneas.
- Those without serious or untreated underlying health conditions.
While PRK is suitable for individuals who may not benefit from LASIK, there are still certain individuals who are not ideal candidates for PRK. This group includes:
- Individuals younger than 18 or older than 60.
- Those with an unstable refractive error consistently change.
- Individuals with underlying health conditions that impede the healing process.
- Older adults with presbyopia, or age-related farsightedness.
- Individuals with a history of scarring, particularly on the eyes.
- Those with specific eye conditions such as corneal abrasions, cataracts, or glaucoma.
- Pregnant or nursing individuals.
|1||Dexamethasone (four times daily), FML (fluorometholone, Allergan) ointment at bedtime|
|2||Dexamethasone (twice daily), FML-Forte (twice daily), FML ointment at bedtime|
|3||Dexamethasone (every day), FML-Forte (three times daily), FML ointment at bedtime|
|4||FML-Forte (four times daily), FML ointment at bedtime|
|5||FML-Forte (twice daily), FML ointment at bedtime|
|6||FML-Forte (every day), FML ointment at bedtime|
A 32-year-old woman, who had been wearing rigid gas-permeable contact lenses for an extended period and had no significant medical history, underwent bilateral PRK surgery in April 1997. Prior to the procedure, her right eye had a cycloplegic refraction of -7.50 -0.75 x 30, correctable to 20/20, while her left eye had a refraction of -6.75 -1.75 x 170, correctable to 20/20. The PRK surgery proceeded without complications, and the postoperative healing process was uneventful. The patient received long-term treatment with 0.1% FML Forte (fluorometholone, Allergan) over a 3-month period. A follow-up examination was scheduled for 6 months later.
In December 1997, 8 months after the initial surgery, the patient returned to the office reporting a significant decline in visual acuity, regression, and the development of late haze. The manifest prescription for her right eye was -4.50 -1.00 x 005, with a best-corrected visual acuity of 20/30. In the left eye, she experienced regression to -4.75D, correctable to 20/50. The patient expressed dissatisfaction with these outcomes.
Consequently, in December 1997, retreatment was performed on both eyes using a transepithelial ablation in phototherapeutic keratectomy mode to remove epithelial and haze formation, and PRK was performed to address 60% of remaining myopia and astigmatism. Four months after the retreatment, the patient’s refraction in the right eye improved to -0.25 -0.50 x 75, correctable to 20/20. In the left eye, she achieved stable refraction of +1.00 -0.50 x 90, correctable to 20/20. The patient is currently in the process of tapering her topical steroids, and there is no presence of haze at present.
Regeneration of the Corneal Stroma
Stromal regeneration is a vital aspect of corneal rehabilitation following PRK. Immediately after PRK, the stroma undergoes rapid changes, including increased hydration and thickness. Within the initial weeks, the active keratocytes in the anterior stroma stimulate fibroblast activity and collagen production. This process results in the deposition of Type III collagen, which typically begins around the 3-week mark.
As the healing process progresses, there is ongoing epithelial thickening and reorganization of the stroma. The subepithelial haze, a common occurrence, usually reaches its peak intensity between the third and sixth months. During this time, new collagen forms between the epithelium and the ablated stromal surface. Between 6 months and 1 year after PRK, the stroma gradually restores its normal lamellar structure, and the magnitude of the subepithelial haze decreases.
However, there are rare cases of abnormal late-onset corneal haze, where newly formed subepithelial collagen consolidates into a scar, resulting in the formation of a dense layer of irregular fibrotic tissue.
Recovery after PRK (Photorefractive Keratectomy) involves a slightly longer healing period compared to LASIK. While LASIK patients can typically resume work or school the following day, individuals who undergo PRK should allow themselves three to five days of rest. It is important to avoid strenuous activities, including exercise, for up to one week after the procedure to prevent complications such as corneal flap displacement, scar tissue formation, infections, or delayed healing.
PR Recovery: Healing Process After Photorefractive Keratectomy
After PRK surgery, you can expect the following:
- Some temporary eye pain or discomfort that should subside within two or three days.
- Haloes or glares around lights, which may persist for a few days.
- Initial blurry vision as your eyes gradually heal.
- The use of prescription eye drops, if prescribed, typically for approximately one month.
- A follow-up examination the day after PRK to monitor the healing progress.
- Wearing sunglasses outdoors for several weeks, regardless of the season or cloud cover, until advised otherwise by your doctor.
- Avoiding water exposure in pools, oceans, lakes, and hot tubs until you receive clearance.
- Taking precautions to protect your eyes during showers or baths for up to one week.
- If you experience persistent side effects such as dry eye, poor or blurry vision, itching, eye discomfort, or difficulty seeing at night, it is important to inform your doctor. While these side effects can last for a few weeks, similar to LASIK, they should resolve on their own. If they persist, additional adjustments or treatments may be necessary.
PRK boasts a reported success rate of approximately 95 percent, comparable to the outcomes of LASIK. Both procedures have high patient satisfaction rates.