eMedicine Specialties > Ophthalmology > Refractive Disorders

LASEK

Author: Sanjeev Grewal, MD, FRCS(C), Assistant Professor of Ophthalmology, Residency Program Director, Department of Ophthalmology, Medical Faculty Associates, George Washington University
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
Contributor Information and Disclosures

Updated: Jun 21, 2007

Introduction

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 Procedure

Concepts 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 plano cut whose thickness and diameter established refractive change. The first disc was then repositioned and sutured back onto the cornea. These initial attempts were complex and unpredictable, often leading to keratoconus and other irregular astigmatisms.

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.

In 1993, Slade developed an automated microkeratome to refine the creation of the flap. Slade was one of the first surgeons to perform LASIK in the United States.

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.

Problem

Ocular 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).

Frequency

Astigmatism, 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.

Indications

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:

• Thin corneal pachymetry
• Steep or flat corneas
• Wide scotopic pupil
• LASIK complications in fellow eye
• Predisposition to trauma
• Irregular astigmatism
• Glaucoma suspects
• Recurrent erosion syndrome
• Borderline dry eye syndrome
• Epithelial basement membrane disease

Highly irregular astigmatism, specifically keratoconus, as well as severe dry eye syndrome can serve as contraindications to LASEK as well as to LASIK.

Relevant Anatomy

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).

Contraindications

Contraindications common to LASIK, LASEK, and PRK include the following:

• Unstable refractive error
• Refractive error outside the range of correction (The range varies according to the surgeon's experience and the type of laser being used, but typically it is approximately 9-10 D of myopia, 4-6 D of hyperopia, and 2-3 D of astigmatism.)
• Keratoconus
• Significant dry eye syndrome
• Active inflammation of external eye
• Autoimmune disease
• History of or active herpes simplex keratitis, because of the concern of eliciting reactivation of the virus
• Active collagen vascular disease
• Uncontrolled diabetes
• Uncontrolled glaucoma
• Pregnancy or breastfeeding
• Use of medications that may adversely affect corneal wound healing, such as Accutane (isotretinoin), Cordarone (amiodarone hydrochloride), and Imitrex (sumatriptan)
• Presence of a pacemaker

Contraindications unique to LASEK and PRK include the following:

• Concern regarding postoperative pain
• Requirement of rapid visual recovery

Unlike LASIK, thin, flat, or steep corneas and wide pupils are usually not contraindications to LASEK and PRK.

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