You are in: eMedicine Specialties > Ophthalmology > REFRACTIVE DISORDERS LASEKArticle Last Updated: Jun 21, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Sanjeev Grewal, MD, FRCS(C), Assistant Professor of Ophthalmology, Residency Program Director, Department of Ophthalmology, Medical Faculty Associates, George Washington University Sanjeev Grewal is a member of the following medical societies: American Academy of Ophthalmology, Association for Research in Vision and Ophthalmology, College of Physicians and Surgeons of Ontario, and Royal College of Physicians and Surgeons of Canada Coauthor(s): Reecha Sachdeva, BA, George Washington University School of Medicine and Health Sciences; Brad Feldman, MD, Staff Physician, Department of Ophthalmology, George Washington University Medical Center; Ronald R Krueger, MD, Medical Director, Department of Refractive Surgery, Division of Ophthalmology, Cole Eye Institute, Cleveland Clinic Foundation Editors: Daniel S Durrie, MD, Director, Department of Ophthalmology, Division of Refractive Surgery, University of Kansas Medical Center; Simon K Law, MD, PharmD, Assistant Professor of Ophthalmology, Jules Stein Eye Institute; Chief of Section of Ophthalmology Surgical Services, Department of Veterans Affairs Healthcare Center, West Los Angeles; Louis E Probst, MD, Medical Director of Refractive Surgery, Chicago, Madison, Milwaukee, and Windsor Centers, TLC the Laser Eye Centers; Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri; Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences Author and Editor Disclosure Synonyms and related keywords: laser assisted subepithelial keratomileusis, laser-assisted subepithelial keratomileusis, e-LASIK, epi-LASEK, epithelial flap photorefractive keratectomy, excimer laser subepithelial ablation, laser assisted subepithelial keratectomy, laser-assisted subepithelial keratectomy, laser epithelial keratomileusis, subepithelial photorefractive keratectomy, photorefractive keratectomy, PRK, LASIK, laser assisted in situ keratomileusis INTRODUCTIONSection 2 of 12
  Laser assisted subepithelial keratectomy (LASEK) is a laser surgical procedure for the correction of refractive error. LASEK is specifically used to correct astigmatism, hyperopia (farsightedness), and myopia (nearsightedness). It is a "hybrid" technique between laser assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). LASEK attempts to decrease the occurrence of flap-related complications associated with LASIK and is specifically helpful in patients with corneas that are otherwise too thin for LASIK. By retaining a flap of the corneal epithelium, LASEK offers a decreased risk of infection, a decreased incidence of corneal haze, a reduction in recovery time, and a reduction of postoperative discomfort when compared with PRK. History of the ProcedureConcepts of corneal refractive surgery, such as keratectomy, keratotomy, and thermokeratoplasty, were first described in 1898 by Lans who published a set of experiments that focused on treating astigmatism in rabbits. Refractive surgery, as it is known today, was not realized until 1966 when Pureskin first appreciated its potential with the demonstration that refractive changes could be made by removing central tissue underneath a corneal flap. Barraquer later showed that the corneal disc could be resected and frozen so that it could be reshaped using a cryolathe. However, this technique used complex equipment, and the freezing resulted in tissue damage to the disc itself. In the late 1980s, Ruiz and Barraquer performed the first published keratomileusis in situ. They followed principles formulated by Krumeich using a microkeratome to remove a portion of the cornea followed by a second Burratto and Pallikaris then combined the microkeratome technique with the use of the excimer laser to ablate tissue and to induce refractive change. Buratto performed excimer laser ablation on the posterior surface of the resected corneal disc before replacing and resuturing it back to its original position. Pallikaris then used the excimer laser ablation on the corneal stromal bed under a hinged flap in rabbit corneas. Pallikaris attempted this technique on blind human eyes in 1989 and on sighted human eyes in 1991, thereby creating a refractive surgical technique similar to the procedures currently in practice. Since its introduction, LASIK has been associated with various complications, specifically when performed on eyes with thin corneal thickness, wide diameter pupils, irregular astigmatism, dry eye syndrome, recurrent erosion syndrome, or glaucoma, to the point where these entities have become relative contraindications to performing LASIK. For these reasons, LASEK was developed to reduce the chance of complications that occur secondary to LASIK while inducing less discomfort than PRK. Italian doctor Camellin is credited with developing the original LASEK procedure when he described the Camellin technique in ophthalmic literature in 1999. This technique involved the use of alcohol to separate the corneal epithelium from the stroma to create an epithelial sheet that could be repositioned over the ablated stroma. Since then, this method has evolved into multiple techniques, including Butterfly LASEK developed by Vinciguerra and Camesasca in 2002, cruciform LASEK described by Amolis in 2002, and gel-assisted LASEK created by McDonald in 2004.14 Each of these techniques is described in Intraoperative details. ProblemOcular refraction is defined as the ability of the eye to bend light rays to focus them on the retina. The cornea, the lens, and the axial length of the eye are the main contributors to the eye's refraction capability. The total refractive power of an emmetropic (or normal length) eye is approximately 58 diopters (D), of which 43 D come from the cornea and the remaining 15 D from the lens, aqueous, and vitreous. Astigmatism, myopia (nearsightedness), and hyperopia (farsightedness) are common forms of refractive error that cause irregularities of the bending of light rays, thereby leading to blurred or distorted vision. Myopia (nearsightedness) is a condition in which the eye is too long or the refractive power is too great, causing objects to focus at a point before the retina rather than upon the retina itself. This inability to focus appropriately leads to an inability to see distant objects clearly. This problem tends to first appear in school-aged children and may progress through adolescence but usually stabilizes in early adulthood (see Media file 1). In hyperopia (farsightedness), the eye is too short or the refractive error is too weak because the cornea is too flat. This irregularity causes an inability of the eye to bring near objects into clear focus because light entering the eye focuses behind the retina rather than directly on it. Because younger individuals may accommodate (or adjust) to focus near objects, the blurred vision associated with hyperopia is often not appreciated until later years as the eye loses this ability to accommodate (see Media file 2). In astigmatism, the refractive power of the eye is not the same in all meridians. For example, the eye may exhibit more myopia horizontally than vertically. It is usually secondary to an irregular curvature of the cornea that prevents light from properly focusing on the retina (see Media file 3). FrequencyAstigmatism, myopia, and hyperopia are relatively common in the general population. Myopia and hyperopia have an estimated prevalence of 33% and 25%, respectively. The prevalence of astigmatism varies with the definition used as clinically significant astigmatism. Up to 75% of the population has at least minor, clinically insignificant astigmatism present in either one eye or both eyes. Specifically, in the general population, 44% have greater than 0.50 D, 10% have greater than 1 D, and 8% have greater than 1.50 D. A refractive surgery survey conducted in 2004 regarding 2003 practices identified LASIK as the most common refractive surgical procedure, with wavefront-guided ablation as an increasingly popular entity, increasing from 13% to 60% during 2002-2003 alone. Of the more than 1000 ophthalmologists who participated in this retrospective study, 71% were found to perform PRK and 41% were found to perform LASEK. Of those ophthalmologists who performed LASEK, over one half only performed this procedure when LASIK was not an option. Another survey focused on the refractive surgery practices of the United States Army Warfighter Refractive Eye Surgery Program (WRESP) during 2000-2003. Of the more than 16,000 patients over these 4 years, nearly three quarters of cases involved surface ablative procedures, namely PRK or LASEK. PRK was performed on 64.7% of eyes, LASEK was performed on 8.7% of eyes, and LASIK procedures were performed on the remaining 26.6% of eyes. INDICATIONSSection 3 of 12
The major indications for refractive surgery include astigmatism, myopia, and hyperopia, specifically in patients who are intolerant of or who desire to be free from glasses or contact lenses. Typically, up to 9 D of myopia and 4 D of hyperopia are the limits of corneal refractive surgery. Within the realm of refractive surgeries, surface ablative procedures, such as PRK and LASEK, are usually confined to individuals in whom LASIK is not recommended. These characteristics include the following:
Highly irregular astigmatism, specifically keratoconus, as well as severe dry eye syndrome can serve as contraindications to LASEK as well as to LASIK. RELEVANT ANATOMYSection 4 of 12
The cornea is made up of several layers of transparent tissue that serve to protect the light. The cornea accounts for two thirds of the refractive power that acts to focus light rays on the back of the eye. Of this, approximately 80% of the refractive power is created by the air-tear interface. Average cornea diameter is 11 mm vertically and 12 mm horizontally. The cornea consists of 5 layers. From superficial to deep, these layers are the corneal epithelium, the Bowman layer, the stroma, the Descemet membrane, and the endothelium. The corneal epithelium consists of 5-7 layers of stratified squamous epithelium. Defects in this layer may cause severe pain secondary to the rich sensory innervation. Fortunately, damage to the epithelium is quickly repaired in healthy eyes. The Bowman layer, on the other hand, is not replaced after injury, and this tough layer of collagen fibers may become opacified and replaced by scar tissue after trauma. The stroma makes up about 500 µm (90%) of the average 550-µm central corneal thickness. Its 200-250 lamellae (flattened bundles of collagen) give the cornea its clarity, strength, and shape. The lamellae are produced by scattered stromal fibroblasts or keratocytes. Keratocytes are also responsible for wound healing if the cornea becomes damaged. The Descemet membrane serves as the acellular basement membrane of the corneal endothelium. Like the Bowman layer, it is not replaced after injury and may result in scar formation. The deepest layer of the cornea is a monolayer of endothelial cells whose primary function is the maintenance of corneal fluid balance, thereby maintaining clarity across the cornea. Unlike the epithelium, these cells rarely undergo mitosis and instead decrease in number with age (see Media files 4-5). CONTRAINDICATIONSSection 5 of 12
Contraindications common to LASIK, LASEK, and PRK include the following:
Contraindications unique to LASEK and PRK include the following:
Unlike LASIK, thin, flat, or steep corneas and wide pupils are usually not contraindications to LASEK and PRK. WORKUPSection 6 of 12
Other Tests
TREATMENTSection 7 of 12
Preoperative detailsPreoperative testing and workup include the following:
Intraoperative detailsSeveral LASEK techniques exist, and some of these techniques are described below. Each technique focuses on creating an epithelial flap under which an excimer laser is used to sculpt the corneal tissue. Camellin developed the first of these techniques, and it serves as the most widely used form of LASEK. Common to all procedures, topical anesthetic is applied prior to surgery, typically a combination of 0.05% proparacaine and 4% tetracaine. The eye is then prepped and draped in a sterile fashion with a lid speculum placed to maximize exposure. Standard Camellin technique The first step in this procedure involves creating a sharp, partial-thickness incision using a trephine blade to circumscribe the flap area. This blade is a finer tool than the microkeratome used in LASIK so that it enables the surgeon to cut through the outer corneal epithelium without penetrating deeper corneal layers, specifically the Bowman layer, which would promote scar tissue formation. Using the trephine, the surgeon applies constant downward pressure upon the cornea to create a 270-degree incision with a hinge. Next, the surgeon typically uses a holding well to cover the eye with a solution of ethanol in sterile water, balanced salt solution (BSS), or physiologic solution for approximately 20-30 seconds to loosen the epithelial edges. While the concentration of ethanol varies between surgeons, the current standard practice uses approximately 18-25% ethanol solution. This concentration has been shown to allow sharp wound edges and a clean, smooth Bowman layer. Greater concentrations of ethanol, as well as other chemical agents, such as 0.5% proparacaine, iodine, cocaine, and alkali-n-heptanol, have been associated with inflammatory response with a damaging effect on stromal keratocytes. Also, while mechanical epithelial debridement has also been shown to be effective, this technique often causes defects on the Bowman layer, which can result in corneal haze and irregularity. Once the surface of the eye has been immersed in the alcohol appropriately, a sponge is used to dry the area and BSS is used to rinse the area. Usually, the area is also irrigated with an antihistamine in an effort to minimize the amount of histamine induced by the alcohol. An epithelial microhoe starts the flap, followed by use of the short end of an epithelial detaching spatula to detach the epithelium from the Bowman layer. While the initial exposure to alcohol should not exceed 35 seconds, alcohol may be reapplied at this time for up to an additional 15 seconds if the epithelial flap does not loosen easily secondary to adhesions or other epithelial irregularities. Once loosened, the flap is folded at the 12-o'clock position to maintain hydration of the epithelium. The longer side of the spatula is then passed over the stromal surface to remove any debris. The flap typically consists of epithelium with its basement membrane attachment intact, which provides support to the epithelium throughout surgical manipulation. The point of detachment after alcohol submersion appears to be within the epithelial basement membrane or between the basement membrane and the Bowman layer. Ablation with the 193-nm excimer laser is then carried out. The laser is focused and centered onto the pupil, enabling ablation of the tissue at the level of the Bowman layer. This is in contrast to LASIK, in which the ablative energy is transmitted to the midstromal region. During this treatment, the patient must maintain fixation. Modern lasers are typically equipped with a tracking mechanism that allows the laser to follow most small eye movements and to increase the accuracy of the ablations. Within this technique, Camellin proposed a 10% reduction in the attempted correction when treating myopia up to 10 D and a 20% reduction when treating 10-20 D relative to PRK in an effort to prevent overcorrection. Once laser ablation is complete, another spatula is used to return the epithelial flap to its original position. Intact hemidesmosomal structures in the basal epithelium allow adhesion of the epithelial cells to ablated stroma after repositioning of the flap, a feature necessary to promote proper healing that may be disrupted by ethanol toxicity. Lastly, a soft bandage contact lens is applied, usually for 3 days (see Media files 7-9). Azar flap technique The Azar flap technique is similar to the standard Camellin technique, as described above, with the soaking of the corneal surface in ethanol, except this technique uses either one arm of a jeweler's forcep or one arm of a modified Vannas scissors to delineate the wound edge rather than a trephine blade. This difference allows customized variations for different corneal types. The epithelial flap is pushed aside using a dry, nonfragmenting cellulose sponge, after which the excimer laser ablates the tissue appropriately. In this procedure, an anterior chamber cannula is used to hydrate the stroma and to float the epithelial flap back to its original position, after which the area is allowed to dry for 2-5 minutes. Vinciguerra Butterfly technique This technique maintains the limbal connection of the epithelial stem cells and the limbal vascular connections in an effort to increase epithelial viability, thereby improving visual recovery time and reducing discomfort. Using a special spatula, a thin paracentral epithelial incision is made from the 8-o'clock position to the 11-o'clock position. Then, 20% alcohol in BSS is placed on the cornea for 5-30 seconds, allowing the epithelium to be separated from the Bowman layer. The spatula is used to further separate the epithelium from its underlying layer, from the center to the periphery in both directions, thereby creating 2 flaps from the original single paracentral line. The surgeon then retracts the sheets of epithelium toward the limbus. While holding these sheets in place using the retractor, the surgeon uses the excimer laser to ablate the tissue. The surface is smoothed with a hyaluronic acid masking solution, and the stretched epithelium is repositioned with overlapping margins. McDonald gel-assisted technique This alcohol-free technique uses viscous hydroxylpropyl cellulose 0.3% (GenTeal Gel, Novartis Ophthalmics) to allow the separation of the epithelial flap and to prevent dehydration. After applying this gel below the epithelium with a cannula, with fine holes along the side, the cells are stripped using microkeratome suction. Within this procedure, 5% sodium chloride may be used to stiffen the epithelial cells before their manipulation, as the gel does not offer this property. Once the gel is in position, the cells may be manipulated as the surgeon uses Vannas scissors to cut down the middle of the cornea. Within the gel cushion, the epithelium is pushed to the periphery without compromising cellular viability. After the flap has been created and folded, the gel is removed from the Bowman layer using a wet Weck-cel sponge, after which ablation may be performed. Once laser ablation is complete, the gel is again applied so that the epithelial sheet may be repositioned and a bandage contact lens may be placed. Amolis cruciform technique The Amolis cruciform technique is very similar to the standard Camellin technique, except a rotating microbrush is used to cut a cross into the epithelium to allow creation of the flap. Like the Butterfly technique, this method is aimed to protect the epithelial limbal stem cells and vascular connections in an effort to increase epithelial viability. Epi-LASIK technique Epi-LASIK (epipolis laser in situ keratomileusis or epikeratome laser-assisted keratomileusis) was developed by Pallikaris as a revision of the traditional LASEK procedure. Within this technique, the use of alcohol to float the flap is replaced with an epikeratome tool, which mechanically cuts and lifts the flap of epithelium. As discussed above, alcohol may cause potentially toxic responses in the cornea. This technique attempts to use the principles of LASEK without the use of alcohol, thereby promoting faster healing and less pain for patients. Additionally, the epikeratome leads to a precise, reproducible separation within the epithelium, thereby further eliminating many of the flap complications associated with LASIK. Postoperative detailsIn all of the procedures discussed above, a soft bandage contact lens is placed on the cornea at the closing of surgery and remains on the eye for several days to allow complete re-epithelialization. Healing of the corneal surface defect normally takes 3-10 days. This healing time is dependent on numerous factors, including the size of the area treated, the baseline health of the cornea, the patient's immune response, the concentration and the duration of medications applied intraoperatively, and the presence of coexisting medical problems, specifically diabetes. Approximately 78% of patients show complete closure of the defect by day 3 and 98.8% by day 7. If the contact lens is removed before closure is complete, the flap may peel away with the lens. If re-epithelialization remains incomplete at 3 days, the original lens may be replaced by a new lens for 3 additional days. Topical steroids and antibiotics should be used until the defect is healed to prevent inflammation and infection. Steroids may be continued for 3 weeks, up to several months. Typically, approximately 50% of patients experience mild to moderate postoperative pain, lasting 1-2 days postoperatively. This percentage is lower compared to PRK but higher compared to LASIK. Functional vision recovery follows a pattern similar to re-epithelialization, also taking 3-10 days. This time period is similar to PRK but exceeds the less than 24-hour recovery associated with LASIK. In addition, the most commonly encountered adverse effect is light sensitivity with halo effect. This occurrence is similar to that seen in LASIK and PRK. COMPLICATIONSSection 8 of 12
Although LASEK avoids many of the flap-associated complications of LASIK, including free caps, incomplete pass of the microkeratome, flap wrinkles, epithelial ingrowth, flap melt, interface debris, corneal ectasia, and diffuse lamellar keratitis, LASEK has its own disadvantages. Complications associated with LASEK include the following:
OUTCOME AND PROGNOSISSection 9 of 12
The United States Army WRESP Survey (2000-2003) concluded that PRK, LASIK, and LASEK achieved comparable postoperative outcomes. LASEK, specifically, has been found to be an effective procedure, with 76% efficacy in attaining 20/20 UCVA and 99% efficacy in attaining 20/40 UCVA. In a study of 421 eyes, an efficacy index comparing preoperative BSCVA to postoperative UCVA found improvement in 94.7% of patients after LASEK. LASEK is also relatively predictable, with 83% achievement within 0.5 D of the target refraction and 98.4% within 1 D at the 6-month follow-up visit in 152 patients. At 4 years out, approximately 7% of patients need secondary surgical correction, predominantly because of the initial undercorrection in those with a high preoperative refractive error (see Media file 11 for a more detailed table comparing the refractive techniques). FUTURE AND CONTROVERSIESSection 10 of 12
Many refractive surgeons view LASEK as the answer for patients who desire laser correction but who are not ideal candidates for LASIK, most commonly secondary to corneal thinning or irregularities. However, these patients must be educated that many of the risk factors associated with LASIK apply to LASEK as well. Although LASEK avoids many of the flap-associated complications of LASIK, such as free caps, incomplete pass of the microkeratome, flap wrinkles, epithelial ingrowth, and flap melt, it continues to have its own disadvantages, specifically postoperative discomfort and prolonged visual recovery as the patient awaits complete epithelial closure. Additionally, while lower than PRK, the risk of corneal haze continues and is markedly increased if the procedure is converted to PRK intraoperatively secondary to loss of the epithelial flap. Furthermore, like LASIK and PRK, LASEK is a relatively new procedure, developed within the past decade. While the use of the excimer laser is FDA approved for LASIK, it is accepted as only an off-label use for LASEK. No studies have been conducted on the long-term effects of these procedures on the cornea, so their final effects, stability, and prognosis may only be theorized. Of the choices, no one procedure has been established as ideal for all patients. Therefore, each surgeon must determine which refractive technique is appropriate on an individual basis. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
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