2
Laser Subepithelial Keratomileusis (LASEK): Theoretical Advantages Over LASIK
Paolo Vinciguerra, MD
Istituto Clinico Humanitas
Milan, Italy
Daniel Epstein, MD, PhD
University Hospital
Zurich, Switzerland
INTRODUCTION
Although laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) are now accepted refractive procedures that have been performed on millions of eyes, both surgical modalities are associated with problems and complications that can make them less than optimal in specific cases.
Aside from microkeratome and flap complications, LASIK is limited in scope by the thickness of the preoperative cornea and the associated inability to treat high refractive errors with a reasonably-sized optical zone. In addition, there may be a risk for ectasia if too much stromal tissue is removed during the procedure (1).
PRK is burdened by the triple onus of postoperative pain, slow rehabilitation of best spectacle corrected visual acuity (BSCVA), and the often unpredictable development of subepithelial haze (2). In many countries, LASIK has overtaken PRK as the primary refractive procedure for those very reasons.
Laser subepithelial keratomileusis (LASEK) expands the armamentarium of the refractive surgeon and specifically addresses some of the shortcomings of LASIK and PRK.
THE PROCEDURE
LASEK is based on the principle of creating an epithelial flap, which is used to cover the stroma after the laser ablation.The procedure is performed as follows:
A specially-designed 9.0-mm trephine is centered on the entrance pupil and pressed down, making a 70 µm-deep circular cut through the epithelium, except at the 12 o’clock position, where an 80 degree hinge is left. A 9.5-mm marker is then centered on the circular trephination, and two to three drops of a 20% ethanol-distilled water solution are instilled inside the marker. The alcohol is carefully dried after 30 to 40 seconds with a
LASEK, PRK, and excimer laser stromal surface ablation 16
microsponge. Special care must be taken that no alcohol leaks onto the peripheral cornea or the conjunctiva.
After the marker is removed, a spatula is used to lift the edge of the circular epithelial incision. Once the edge has been lifted along the 280 degrees, which had been cut, a specially designed hoe-like spatula is used to carefully free the epithelium and roll it up like a scroll toward the 12 o’clock position. The laser ablation is then performed in the standard manner, after which the epithelial flap is rolled back to cover the ablated stroma. A therapeutic contact lens is then applied for 3 to 4 days. The eye is treated with antibiotics/ steroid drops for approximately 1 week.
PATIENTS STUDIED
Five hundred twenty-six eyes of 268 patients (mean age 35±12 years) were treated with LASEK. A Nidek EC−5000 excimer laser was used. The mean (±standard deviation [SD]) preoperative spherical equivalent (SE) was −5.51 diopter (D) ±3.48 D (range, −15.88 D to +5.50 D), the mean sphere −4.98 D ±3.43 D (range, −15.00 D to +4.25 D), and the mean cylinder −1.08 D ±1.44 D (range, −8.00 D to +6.00 D).
Mean preoperative uncorrected visual acuity (UCVA) was 20/60. Minimum follow-up was 12 months. At 12 months postoperatively, the mean SE was −0.10 D ±0.70 D, the mean sphere +0.20 D ±0.60 D, and the mean cylinder −0.42 D ±0.90 D. Eighty-six percent of the eyes were within 0.50 D of aim (Fig. 1), and 53% had an UCVA of 20/20 or better (Fig. 2). Eighty-one percent of the eyes has a postoperative BSCVA of 20/20 or better at 1 year.
On the first postoperative day, 74% of the patients reported that they had experienced only mild discomfort after the procedure. Twenty percent had noted marked discomfort, but only 6% had perceived pain.
Figure 1 Scattergram of attempted refractive change vs. achieved refractive change 1 year postoperatively.
Laser subepithelial keratomileusis (LASEK) |
17 |
Figure 2 Histogram illustrating the distribution of UCVA 1 year after surgery.
Three days postoperatively, 38% of the eyes had regained their preoperative BSCVA. By 2 weeks after LASEK, that figure had increased to 44%. At 1 month postoperatively, all eyes had recovered their preoperative BSCVA (Fig. 3). Gains of one line or more of BSCVA were noted up to the fourth month after LASEK.
Mean postoperative refraction showed stability over time, with a value of +0.04 D at 1 month, −0.27 D at 6 months, and −0.07 D at 1 year (Fig. 4).
At 1 year, no eye had lost two or more lines of BSCVA. In fact, none had lost one line, whereas 32% had gained one line and 12% gained two lines of BSCVA (Fig. 5). Subepithelial haze never exceeded trace, and in 80% of the eyes no haze was detectable at all 6 months postoperatively.
Figure 3 Stability of BSCVA over time, with 1 standard deviation.
LASEK, PRK, and excimer laser stromal surface ablation 18
Figure 4 Stability of refraction over time depicted with the spherical equivalent refraction and 1 standard deviation.
Figure 5 Histogram of gained/lost BSCVA lines at 1 year postoperatively.
Laser subepithelial keratomileusis (LASEK) |
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Flap management (the preablation rolling and postablation unrolling of the epithelium) was deemed easy in 62% of the cases, required some extra effort in 33%, and was difficult in 5%. Only seven flaps were “lost,” and in those cases a straightforward PRK was performed.
Vector analysis of the cylinder component showed significant angle deviation in only 2.5% of the cases and significant undercorrection of the astigmatism in only 1.9%.
DISCUSSION
LASIK and PRK have revolutionized corneal refractive surgery, but despite impressive achievements, neither can be regarded as ideal for every case. Accordingly, there is room for an alternate procedure that can deliver its own set of advantages.
LASIK is limited by the thickness of the preoperative cornea. A thin cornea imposes bounds on the correction range with LASIK if optical zone size is not sacrificed. Although the minimum required residual stroma under the flap is not known, most surgeons assume that a bed of at least 250 µm is needed to prevent the development of ectasia. If a 180-µm flap is planned, and if the preoperative corneal thickness is less than 500 µm, less than 70 µm would be available for ablation. In fact, even that may be an overoptimistic estimate, because the calculation does not take into account the observation that flap thickness may actually vary by as much as ±30 µm, a factor that should be considered because the majority of surgeons do not perform intraoperative pachymetry.
In the presence of a relatively thin cornea, and if PRK is not acceptable because of the postoperative pain and the slow visual recovery, we suggest that it is now possible for a refractive surgeon to consider LASEK as a reasonable alternative. Using LASEK on a thin cornea widens the range of correction (compared to LASIK) and increases the potential for retreatment, an option often not available in LASIK when the procedure is used in a borderline-thickness cornea. Because there is more stroma to treat, LASEK may in some eyes make possible the use of a larger optical zone than would be feasible if LASIK were performed. The larger the optical zone, the lower the risk for the visual problems in mesopic illumination, which trouble many patients after excimer refractive surgery (3–5).
LASEK may also be appropriate in patients who do not want to be treated with a microkeratome. By definition, LASEK eliminates all potential keratome complications.
Monocularly treated LASIK patients who have had a difficult bout of diffuse lamellar keratitis or who have scars after such an episode may feel more comfortable if the second eye undergoes a LASEK procedure. The same applies to LASIK patients with persistent epithelial ingrowth who may have had their flaps lifted two or three times to remove the epithelial cells (6). Again, by definition, all potential lamellar and interface complications are eliminated by LASEK.
Proposing LASEK to a patient who has had LASIK in the first eye in not unreasonable because, as shown by our results, pain after LASEK is rare, making the procedure much more attractive than PRK from the patient’s point of view. A further argument for LASEK is the impressively rapid recovery of BSCVA, as compared to standard PRK. Admittedly, recovery is not as fast as in LASIK, but considerably faster than after PRK,
LASEK, PRK, and excimer laser stromal surface ablation 20
with almost 40% of the eyes regaining their preoperative BSCVA on the third postoperative day.
In terms of safety, efficacy, and accuracy, our observations suggest that LASEK results are comparable to long-term outcomes after LASIK and PRK. At 1 year postoperatively, no eye had lost any line of BSCVA, 53% of the eyes had an UCVA of 20/20 or better, and 86% of the eyes were within 0.50 D of aim.
It is noteworthy that subepithelial haze is unusually discreet or absent in LASEK eyes, in contrast to what is seen after PRK. Although purely speculative, one explanation for this observation may be that the epithelial flap acts to protect the stroma by decreasing the magnitude of keratocyte apoptosis, thus making for less haze formation. It has been shown that as soon as the corneal epithelium is damaged (whether in PRK or LASIK), a cytokine-mediated cascade is initiated, leading to apoptosis (7). Yet, it is well-established that only rarely is there haze after LASIK, a procedure that causes much less epithelial damage than the pre-PRK epithelial scraping. Creating an epithelial flap (as in LASEK) may also be less of a trauma than the epithelial debridement used in PRK, possibly leading to a more subdued apoptosis response.
LASEK is predicated on the use of ethanol to facilitate the creation of the epithelial flap. In excimer refractive surgery, ethanol has previously been applied to remove the epithelium before PRK. Published reports on the use of ethanol in PRK have not shown any detrimental effect on the refractive outcome or on the postoperative visual acuity (8,9).
We conclude that in eyes in which LASIK is not advisable, in eyes in which retreatment may be necessary, in patients in whom serious post-LASIK sequelae developed in the first eye, and for surgeons or patients who prefer a refractive procedure without a microkeratome, LASEK represents a reasonable alternative. As illustrated by our results, LASEK has fewer complications than PRK (less pain, faster visual rehabilitation, less haze) and, by virtue of not using a keratome and not producing a stromal flap, fewer complications than LASIK.
REFERENCES
1.Gimbel HV, Penno EE, van Westenbrugge JA. Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology; 1998; 105:1839–1848.
2.Seiler T, Holschbach A, Derse M. Complications of myopic photorefractive keratectomy with the excimer laser. Ophthalmology; 1994; 101:153–160.
3.Hersh PS, Stulting RD, Steinert RF. Results of phase III excimer laser photorefractive keratectomy for myopia. Ophthalmology; 1997; 104:1535–1553.
4.Ben-Sira A, Loewenstein A, Lipshiz I. Patient satisfaction after 5.0-mm photorefractive keratectomy for myopia. J Refract Surg; 1997; 13:129–134.
5.Brunette I, Gresset J, Boivin JF. Functional outcome and satisfaction after photorefractive keratectomy. Part 2: survey of 690 patients. Ophthalmology; 2000; 107:1790–1796.
6.Wang MY, Maloney RK. Epithelial ingrowth after laser in situ keratomileusis. Am J Ophthalmol; 2000; 129:746–751.
7.Wilson SE, Kim WJ. Keratocyte apoptosis: implications for corneal wound healing, tissue organization and disease. Invest Ophthalmol Vis Sci; 1998; 39:220–226.
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8.Abad JC, Talamo JH, Vidurrai-Leal J. Dilute ethanol versus mechanical debridement before photorefractive keratectomy. J Cataract Refract Surg; 1996; 22:1427–1433.
9.Carones F, Fiore T, Brancato R. Mechanical vs. alcohol epithelial removal during photorefractive keratectomy. J Refract Surg; 1999; 15:55–62.
3
Indications and Contraindications of
LASEK
Jae Bum Lee, MD, PhD, Puwat Charukamnoetkanok, MD, and
Dimitri T.Azar, MD
Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,
Harvard
Medical School
Boston, MA
The advantages of photorefractive keratectomy (PRK) are its precise predictability and reliable safety as a corrective procedure for low to moderate myopia (1,2). However, its predictability decreases in cases of high myopia, and its effects are limited by postoperative pain, corneal haze, and myopic regression (3). Laser in situ keratomileusis (LASIK) offers fast visual recovery with minimal postoperative discomfort. However, flap-wrinkling, epithelial ingrowth, diffuse lamellar keratitis, and iatrogenic keratoectasia may develop after surgery (4).
Laser epithelial keratomileusis (LASEK) involves creation of an epithelial flap after the application of an alcohol solution and then the repositioning of this flap after laser ablation (5). In theory, LASEK combines the beneficial aspects of both PRK and LASIK. It does not need any flap in the stroma like LASIK, thus eliminating inherent flap complications. This chapter discusses indication and contraindications for LASEK. The goal is to offer patients the most optimal refractive surgical procedures for their ocular conditions.
INDICATIONS FOR LASEK
LASEK-treated eyes were shown to have lower postoperative pain and less postoperative corneal haze than PRK-treated eyes in mild to moderate myopia (Table 1) (Fig. 1) (6). Patients with low to moderate myopia who are at a low risk for subepithelial haze also may benefit from LASEK. The most ideal candidates for LASEK are those with mild to moderate myopia up to −7.00 diopter (D). Gabler et al. (7), however, reported that LASEK has been effective in treating up to −14.50 diopters of myopia. At these pathological ranges, the risk of wound healing abnormalities manifesting as haze and regression might be expected. A high degree of corneal haze was observed in cases with more than −10 diopters of myopia. The excimer laser we used was a broad-beam laser. Scanning spot
Indications and Contraindications of LASEK 23
Table 1. Corneal Haze and Pain Scores After PRK or LASEK.
|
PRK |
LASEK |
P |
Pain score |
2.36±0.67 |
1.63±0.81 |
0.047 |
Haze score |
|
|
|
1 mo |
0.86±0.45 |
0.46±0.24 |
0.02 |
3 mo |
0.45±0.27 |
0.29±0.26 |
0.22 |
PRK, photorefractive keratectomy; LASEK, laser epithelial keratomileusis.
lasers might reduce severe corneal haze because of lower energy used during LASEK. Candidates should be older than age 20 years and have a stable prescription, with less than 0.25 to 0.50 D change within the past 1 year.
Surgeons should consider LASEK for patients whose corneal characteristics render them at greater risk for LASIK or whose professions or lifestyles, such as athletes involved in contact sports and military personnel, predispose them to flap trauma. LASEK might also play a role in refractive surgery when patients are reluctant to undergo incisional surgery, when ophthalmologists have a preference for LASEK, or when small amounts of refractive error may make the risk of flap complication unacceptably high. Patients with redundant conjunctiva that might cause pseudo-suction when the suction ring is applied may avoid this potential complication by undergoing LASEK.
The characteristics that may make LASEK a better choice for the patient include: (1) corneal thinning, in which less than 200 to 250 µm of tissue would be left in the bed should LASIK be performed instead; (2) very small cornea that might cut the limbus when one use the microkeratome; (3) difficulty achieving satisfactory suction for the microkeratome cut; (4) narrow palpebral fissure, in which case the microkeratome cannot be well-applied; (5) epithelial anterior basement membrane dystrophy, because of the risk of epithelial ingrowth; (6) superficial corneal lesion such as anterior stromal dystrophy or granular dystrophy; (7) severe neovascularized cornea caused by long-term contact lens use; (8) presence of tears in the peripheral retina wherein if suction is applied, postoperative complications such as retinal detachment and retinal hemorrhage may occur (9) when the microkeratome is not available.
Additional Advantages of LASEK Over PRK
Kornilovsky (8) reported that the main advantages of LASEK over PRK are the absence of pain and corneal opacities. Other potential advantages of LASEK are as follows. First, if making of epithelial flap is unsuccessful, one can easily convert to PRK. Second, the risk of infection is reduced because the epithelial flap acts as an effective protective barrier. Third, if re-treatment for PRK is needed, LASEK can be used to reduce the risks of corneal haze. Fourth, although the immediate postoperative visual acuity is not as good as LASIK, patients have relatively good visual acuity during the
LASEK, PRK, and excimer laser stromal surface ablation 24
Figure 1 Left (A) shows +2 grade corneal haze in the center of the cornea with 20/30 uncorrected visual acuity 3 months after PRK. Right (B) also shows grade 1 corneal haze with 20/20 uncorrected visual acuity 3 months after LASEK.
early postoperative days, enabling simultaneous bilateral surgery to be performed. However, most patients may prefer to undergo sequential treatment because of postoperative discomfort.
Additional Advantages for LASEK Over LASIK
LASEK avoids the corneal lamellar cut. Therefore, flap-related complications in LASIK such as flap displacement, flap striae, and epithelial ingrowth, are essentially eliminated (Fig. 2). Because LASEK does not require microkeratome, there is no need for wide exposure of the eyeball using a speculum. This minimizes pain during surgery and the risk of ptosis after surgery (9).
LASEK After Other Surgical Procedures
LASEK can be performed to improve the refractive error in patients who underwent previous eye surgeries. Postkeratoplasty corneas tend to be very steep, leading to increased risk of flap complication such as buttonhole. LASEK offers an attractive alternative to treat postkeratoplasty astigmatism. Surface ablation procedures such as LASEK may also be useful after radial keratotomy (RK). By avoiding the flap, the risk of further weakening the corneal biomechanics is minimized. LASEK also circumvents the “pizza effect” occasionally observed when making LASIK flaps after RK.
PREOPERATIVE CONSIDERATIONS
Careful patient selection and review of the risks and benefits of LASEK are important considerations. Patients’ information and education are essential for LASEK. The patient
Indications and Contraindications of LASEK 25
Figure 2 Epithelial ingrowth can be seen 4 weeks after LASIK procedure.
should thoroughly understand the LASEK procedure and nature of postoperative recovery by viewing information sheets and obtaining additional information from patient counselors, the surgeon, and other patients. Patients need to realize that they will experience greater postoperative discomfort after LASEK compared to LASIK. They must also be informed that the visual recovery will be delayed, but the long-term visual results are comparable to those of LASIK.
CONTRAINDICATIONS
Contraindications for LASEK are similar to PRK. Retinal, optic nerve, and hereditary conditions should be inquired and excluded before surgery. Careful slit-lamp examination is essential to rule out any significant corneal abnormalities. Vascularization that involves the optical zone can result in bleeding during the procedure and lead to irregular ablation. Absolute ocular contraindications are clinical keratoconus, monocular patients, exposure keratopathy, and herpes zoster. The relative contraindications are history of herpes simplex keratitis, previous ocular surgery, any active/residual/recurrent ocular disease, unstable/ progressive myopia, irregular astigmatism, corneal scar, and forme fruste keratoconus (topographic changes) (Table 2). Other contraindications for LASEK include uveitis, cataract, retinopathies, and significant lagophthalmos. Because patients are given topical steroids postoperatively, it is important to rule out the presence of glaucoma or a suspected glaucoma that may make the eye vulnerable to raised intraocular pressures (11). Funduscopy is an important examination in myopic patients because of the possibility of a retinal hole or degenerative retina (12). It also rules out any optic disc or macular disease. One should refrain from operating on individuals with large pupils (>7
LASEK, PRK, and excimer laser stromal surface ablation 26
mm) in dim light because of the increased risk of night glare and halos (13). Advanced keratitis sicca with diffuse superficial punctate keratopathy or corneal filaments are a possible contraindication for LASEK. In LASEK, two essential components are the making of an epithelial flap and the use of contact lens. The production of tear film decreases with age. Older patients may find the postoperative contact lens uncomfortable. To maintain good adhesion of epithelial flap to stroma, patients should be instructed to avoid rubbing or blinking excessively, particularly during the first postoperative day.
GENERAL HEALTH
Like all other refractive surgery procedures, the success of LASEK depends on proper wound healing. Any systemic conditions that may potentially be detrimental to the healing process of eyes should be actively uncovered during preoperative examination. Relative general contraindications are diabetes mellitus (type I and II), clinically significant atopy,
Table 2. Absolute Ocular Contraindications for LASEK.
Keratoconus
Monocular patients
Severe dry eye (Sjögren syndrome)
Active infection of cornea and conjunctiva
Herpes zoster ophthalmicus
and pregnancy or lactation. During pregnancy or nursing, there may be hormonal changes that could alter the refractive errors and corneal haze (14).
A recent study (15) has demonstrated a chance of regression 13.5 times higher in women using oral contraceptives. Patients should be warned of this apparent increased risk. Patients with active systemic connective tissue diseases (e.g., systemic lupus, rheumatoid arthritis) are considered poor LASEK candidates because of the potential for poor epithelial healing and the risk of a corneal melt. A history of keloid formation of the skin is no longer considered a contraindication to LASEK. History of keloid does not appear to increase risk of corneal haze.
In our experience (6), contact lens intolerance after surgery occurred in 4% eyes, and most of these had folds in Decemet’s layer. Most patients were older than age 40 years. Although the reason for the contact lens intolerance is not clear, patients older than 40 should be informed of this increased risk.
LASEK offers patients another choice of refractive surgery in mild to moderate myopia. It reduces the incidence of postoperative significant pain and corneal haze and could avoid various flap and interface-related problems after LASIK. However, careful patient selection is essential for successful outcome.
Indications and Contraindications of LASEK 27
REFERENCES
1.Seiler T, Holschbach A, Derse M, Jean B, Genth U. Complications of myopic photorefractive keratectomy with the excimer laser.. Ophthalmology; 1994; 101:153–160.
2.Gartry DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive keratectomy: 18 months follow-up.. Ophthalmology; 1992; 99:1209–1219.
3.Wang Z, Chen J, Yang B. Comparison of laser in situ keratomileusis and photorefractive keratectomy to correct myopia from −1.25 to −6.00 diopters.. J Refract Surg; 1997; 13:528–534.
4.Hersh P, Brint S, Maloney RK. Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia.. Ophthalmology; 1998; 105:1513–1523.
5.Camellin M. LASEK may offer the advantages of both LASIK and PRK.. Ocular surgery news international 1999; March.
6.Lee JB, Seong GJ, Lee JH, Seo KY, Lee YG, Kim EK. Comparison of laser epithelial keratomileusis and photorefractive keratectomy for low to moderate myopia.. J Cataract Refract Surg; 2001; 27:565–570.
7.Gabler B, vonMohrenfels W, Lohmann CP. LASEK: A histological study to investigate the vitality of corneal epithelial cells after alcohol exposure.. Invest Ophthalmol Vis Sci; 2001; 42:S560 [abstract 3222].
8.Kornilovsky IM. Clinical results after subepithelial photorefractive keratectomy (LASEK).. J Refract Surg; 2001; 17(2Suppl):S222–S223.
9.Hori-Komai Y, Toda I, Tsubota K. Laser in situ keratomileusis: association with increased width of palpebral fissure.. Am J Ophthalmol; 2001; 131(2):254–255.
10.Farris RL. Contact lenses and the dry eye.. Int Ophthalmol Clin; 1994; 34(1):129–136.
11.Gimno JA, Munoz LA, Valenzuela LA, Molto FJ, Rahhal MS. Influence of refraction on tonometric readings after photorefractive keratectomy and laser assisted in situ keratomileusis.. Cornea; 2000; 19(4):512–516.
12.Arevalo JF, Ramirez E, Suarez E, Antzoulatos G, Morales-Stopello J, Ramirez G, Torres F, Gonzalez-Vivas R. Rhegmatogenous retinal detachment in myopic eyes after laser in situ keratomileusis. Frequency, characteristics, and mechanism.. J Cataract Refract Surg; 2001; 27(5):674–680.
13.Hersh PS, Steinnert RF, Brint SF. Photorefractive keratectomy versus laser in situ keratomileusis: comparison of optical side effects. Summit PRK-LASIK Study Group.. Ophthalmology; 2000; 107(5):925–933.
14.Sharif K. Regression of myopia induced by pregnancy after photorefractive keratectomy. J Refract Surg; 1997; 13(5Suppl):S445–S446.
15.McCarty CA, Ng I, Waldon B, Garrett SK, Downie JA, Aldred GF, Wolfe RJ, Taylor HR. Relation of hormone and menopausal status to outcomes following excimer laser photorefractive keratectomy in women. Melbourne Excimer Laser Group. Aust N Z J Ophthalmol; 1996; 24(3):215–222.