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Haze, loss of BCVA can result from treating high myopia with PRK
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Photorefractive keratectomy (PRK) produces reasonably good results, and the excimer laser holds great promise for achieving emmetropia. However, these procedures may not always go exactly as planned. Some patients who undergo PRK may develop significant corneal scarring and loss of best-corrected visual acuity (BCVA).
For the success rate of PRK to increase, an increase in knowledge of corneal biological variations in healing is needed, in addition to improvements in laser technology.
---Late onset corneal haze: In some cases, new subepithelial collagen consolidates into a scar, creating a dense layer of irregular fibrotic tissue.
A risk of performing PRK on patients with high levels of myopia is haze formation. The destruction of Bowman's membrane causes healing between the epithelium and stroma, which can precipitate haze formation.
Another risk is that the ablation zone is too small. A small optical zone can result in optical aberrations, such as night glare, flare and halos. The formation of central islands, which occur more frequently using the currently available broad-beam laser, is yet another argument against performing PRK at these high levels of myopia. The higher the degree of myopia, the higher the incidence of central island formation.
Understanding wound healing
Gaining a greater understanding of corneal wound healing in the postoperative PRK eye will help the optometrist manage and alter the final refractive outcome. After PRK, several histological changes occur within the cornea. One of the main areas of corneal rehabilitation is epithelial migration. Immediately after PRK, the epithelium starts its migration over the bare stromal bed.
Several animal studies indicate that fibrinogen and fibronectin form across the ablated stromal surface and provide the extracellular substance so the epithelium can migrate and adhere. The new epithelial cells cover the wound and begin to thicken and become more hyperplastic in nature, especially in the case of broad-beam ablation, where the ablation produces abrupt wound edges. This process occurs generally in 3 to 4 days, but can take as long as several months to occur.
Stromal regeneration
Stromal regeneration is another area of corneal rehabilitation. The stroma responds immediately with an increase in hydration and thickness after PRK. Within the first weeks, active keratocytes in the anterior stroma start to increase fibroblast activity and collagen production. This Type III collagen begins to deposit at about 3 weeks.
Further long-term healing continues with epithelial thickening and reorganization of the stroma. Typically, subepithelial haze reaches maximal intensity between the third and sixth months. The new collagen forms between the epithelium and the ablated stromal surface. From 6 months to 1 year post-PRK, the normal lamellar structure returns to the stroma with a decrease in the magnitude of the subepithelial haze. Aberrant cases of late onset corneal haze have been described. In these cases, new subepithelial collagen consolidates into a scar, creating a dense layer of irregular fibrotic tissue.
---Eight months postop:Over a 3-month period, this patient's healing was unremarkable. After 8 months she returned with a dramatic decrease in visual acuity, regression and late haze
Variations in PRK corneal wound healing can explain the difference in refractive and visual acuity results from patient to patient. Corneal healing is the main determinant of success or failure in the PRK patient.
Daniel Durrie, MD, has described PRK healing as three different healing patterns:
- Type I: normal healer - correct refractive outcome.
- Type II: inadequate healer - leads to overcorrection, hyperopia and no haze.
- Type III: aggressive healer - leads to undercorrection and significant haze.
It may be too simplistic to characterize the healing process with just three classifications, but there are many variations in the corneal healing process. The PRK corneal healing response is variable, and there may be no preoperative indicators that will help identify potential aberrant healers.
Haze is multifactorial
The etiology of corneal haze postoperatively in PRK appears to be multifactorial in origin. Several studies indicate that it could be caused by the beam profile: a broad beam produces more haze than a scanning laser. Several animal studies indicate that the loss of Bowman's membrane and the interaction between the epithelium and the stroma seems to be the main factor in postoperative haze production. Other studies have implicated tear film abnormalities, late-onset ultraviolet B exposure and idiosyncratic healing responses.
Patient with late-onset haze
| Managing Post-PRK/PTK Haze | |
|---|---|
| Month 1 | Dexamethasone four times daily; FML (fluorometholone, Allergan) ointment at bedtime |
| Month 2 | Dexamethasone twice daily; FML-Forte twice daily; FML ointment at bedtime |
| Month 3 | Dexamethasone every day; FML-Forte three times daily; FML ointment at bedtime |
| Month 4 | FML-Forte four times daily; FML ointment at bedtime |
| Month 5 | FML-Forte twice daily; FML ointment at bedtime |
| Month 6 | FML-Forte every day; FML ointment bedtime |
| Note: Do topical steroids really work to manage these patients postoperatively? Double randomized, multicenter trials on animals indicate they do not. What's the answer? This pharmaceutical topical management for late haze PRK retreatment is recommended. Be aware of the possibility of topical steroid-induced complications, such as cataract, glaucoma and ptosis. | |
A 32-year-old woman, presenting with a long-term history of rigid gas-permeable contact lens wear and no significant medical history, underwent a bilateral PRK procedure in April 1997. Preoperatively, the cycloplegic refraction in her right eye was -7.50 -0.75 x 30 correctable to 20/20. The left eye was -6.75 -1.75 x 170 correctable to 20/20. The PRK procedure was unremarkable. The postoperative healing also was unremarkable, with long-term 0.1% FML Forte (fluorometholone, Allergan) tapering over a 3-month period. The patient was asked to return for a follow-up examination in 6 months.
In December l997, 8 months after surgery, the patient returned to the office with a dramatic decrease in visual acuity, regression and late haze development. The patient's manifest prescription in the right eye was -4.50 -1.00 x 005 with a best-corrected visual acuity of 20/30. In the left eye, she had regressed to -4.75D correctable to 20/50. She was a very dissatisfied patient.
The patient then underwent retreatment in both eyes in December 1997, with a transepithelial ablation in phototherapeutic keratectomy mode for epithelial and haze removal and PRK for 60% of the residual myopia and astigmatism. Four months after the retreatment, the patient is -0.25 -0.50 x 75 correctable to 20/20. In the left eye, the patient has a stable refraction at +1.00 -0.50 x 90 correctable to 20/20. The patient is tapering her topical steroids. There is no haze at the present time.
--Stable refraction, successful treatment: The patient underwent retreatment with a transepithelial ablation in phototherapeutic keratectomy mode for epithelial and haze removal and PRK for 60% of the residual myopia and astigmatism.
The Visx Star Laser system is approved for use with up to 12 D of myopia. The question is, if you were a 12-D myope, would you feel safe having PRK? The answer is no. In the 200-patient study that supports this expansion to -12 D, the researcher's postoperative results indicate that 5% of the patients lost two or more lines of best-corrected visual acuity. Is this result good enough for your patients?
To avoid haze, loss of best-corrected visual acuity and regression, avoid recommending PRK for patients with high myopia. As an alternative, laser in situ keratomileusis (LASIK) has numerous clinical advantages. LASIK involves less physiological healing response and is more predictable for high myopia compared to PRK. LASIK is not without risk, but haze does not appear to be one of the complications.
For Your Information:
- Jeffrey M. Augustine, OD, FAAO, is director of Refractive Surgery Optometric Consultative Services at the Cleveland Eye Clinic and is president of the Optometric Refractive Surgery Society. He can be reached at the Cleveland Eye Clinic, 2740 Carnegie Ave., Cleveland, OH 44115. Dr. Augustine has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.