You are in: eMedicine Specialties > Ophthalmology > REFRACTIVE DISORDERS Astigmatism, LASIKArticle Last Updated: Jan 2, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: David R Hardten, MD, Phillips Eye Institute; Adjunct Clinical Associate Professor, Director of Refractive Surgery at Minnesota Eye Consultants, Pennsylvania College of Optometry; Clinical Associate Professor, Department of Ophthalmology, University of Minnesota, Minnesota VAMC David R Hardten is a member of the following medical societies: Alpha Omega Alpha, American Academy of Ophthalmology, American College of Surgeons, American Medical Association, American Society of Cataract and Refractive Surgery, and International Society of Refractive Surgery Coauthor(s): Ahmad M Fahmy, OD, Fellow, Department of Ophthalmology, Minnesota Eye Consultants; Scott G Hauswirth, OD, Consulting Staff, Department of Optometry, Minnesota Eye Consultants 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: LASIK, astigmatism, PRK, PARK, refractive error, refraction, photoastigmatic refractive keratectomy, conventional laser in-situ keratomileusis, wavefront-guided laser in-situ keratomileusis, wavefront-guided LASIK, photorefractive keratoplasty, laser epithelial keratomileusis, LASEK, astigmatic keratotomy INTRODUCTIONSection 2 of 11
  The surgical correction of astigmatism was first attempted with astigmatic keratotomy, which is described in this article, along with compression sutures and wedge resection. More recently, excimer laser photoastigmatic refractive keratectomy (PARK), conventional laser in-situ keratomileusis (LASIK), and wavefront-guided LASIK have been used to reduce astigmatism. Although astigmatic keratotomy is gradually being replaced by excimer laser techniques, it is still used by many surgeons, and, as such, a discussion of the surgical techniques is warranted. History of the ProcedureAstigmatic keratotomy was first performed in 1885, when Schiötz, a Norwegian ophthalmologist, treated a patient with 19.50 diopters (D) of astigmatism after cataract surgery with a 3.5-mm penetrating incision at the limbus in the steep meridian, which reduced the astigmatism to 7.00 D. Faber, a Dutch ophthalmologist, performed perforating anterior transverse incisions in a 19-year-old patient with 1.50 D of idiopathic astigmatism, which reduced the astigmatism to 0.75 D and allowed him to pursue a career in the Royal Military Academy. Lucciola of Terrin, Italy was the first surgeon to use nonperforating corneal incisions to correct astigmatism. In 1894, Bates of New York City, described 6 patients who developed flattening of the cornea in the meridian that intersected a surgical or traumatic scar. He postulated that incisions of the cornea made at right angles to the steeper meridian might be used to correct astigmatism. Lans showed that flattening in the meridian perpendicular to a transverse incision was associated with steepening in the orthogonal meridian, as well as demonstrating that deeper and longer incisions have a greater effect. In the 1940s and 1950s, Sato of Tokyo, Japan, investigated both radial and astigmatic keratotomy. He used tangential posterior corneal incisions to decrease astigmatism an average of 2.50 D in 15 eyes and also reduced astigmatism an average of 4.20 D in 18 eyes with perforating tangential incisions near the limbus. Fyodorov later described the correction of myopic astigmatism using several nonperforating anterior keratotomy patterns. Over the last few years, the use of photorefractive techniques to treat various types of refractive error has increased in popularity among surgeons. Initially, the excimer laser was used only to treat myopia and low amounts of myopic astigmatism. Since new software has become more available and reliable, it has been shown in many studies that excellent results also can be obtained for correcting low-to-moderate amounts of hyperopia and hyperopic astigmatism. ProblemAstigmatism, like myopia or hyperopia, can decrease visual acuity. However, astigmatism is much more complex because it has both magnitude and orientation. Thus, it is more difficult to correct in spectacles, contact lenses, or surgery than are spherical forms of refractive error. FrequencyNaturally occurring (idiopathic) astigmatism is common. Clinically detectable refractive astigmatism is present in as many as 95% of eyes. Incidence of clinically significant astigmatism has been reported to be 7.5-75%, depending on the specific study and the definition of what degree of astigmatism is determined to be clinically significant. Approximately 44% of the population has more than 0.50 D of astigmatism, 10% has more than 1.00 D, and 8% has 1.50 D or more. EtiologyVisually significant astigmatism is common after various kinds of ophthalmic surgery, including cataract extraction, penetrating or lamellar keratoplasty, and trabeculectomy. Astigmatism greater than 1.00 D often occurs after extracapsular cataract extraction, and astigmatism greater than 3.00 D is present in as many as 20% of cases with 10-mm incisions. High astigmatism after penetrating keratoplasty is even more common. INDICATIONSSection 3 of 11
Correction of astigmatism by means of surgery is indicated when the amount of the astigmatism affects the vision. Usually visually significant astigmatism is defined as being more than 1 D. Surgical intervention is especially important in those patients who have had previous forms of ocular surgery. Patients who are intolerant or desire to be free from contact lenses or glasses also can become candidates for surgical correction. Special considerations for postsurgical astigmatism If astigmatism is caused by wound dehiscence after cataract extraction, in many cases revising the wound itself may be preferred over astigmatic keratotomy or LASIK. If the wound dehiscence is a structural threat to the eye, wound revision is recommended. If the globe is structurally intact, refractive rehabilitation with the reduction of astigmatic anisometropia becomes most important, and astigmatic keratotomy or photoastigmatic refractive keratectomy may be appropriate. The spherical equivalent should be taken into account when considering these procedures. Astigmatic keratectomy incisions, unless extremely short or long, typically are neutral with respect to the spherical equivalent. Photoastigmatic refractive keratectomy typically induces a hyperopic shift in the refractive error. Newer laser software is now able to correct hyperopic astigmatism by steepening the flat axis. Postkeratoplasty astigmatism often necessitates surgical correction because of intolerance of glasses or contact lenses. The surgical correction can be difficult because astigmatism can be irregular and nonorthogonal, with a significant amount of higher order aberrations. Although an ideal goal is to correct all of the astigmatism, a more realistic goal is to reduce the amount of astigmatism, so that spectacles or contact lenses can be worn comfortably. Before any surgical procedures for the correction of postkeratoplasty astigmatism are considered, all sutures should be removed because they can be the cause of the astigmatism. Also, the keratoplasty wound should be inspected for focal abnormalities. Wound dehiscence and graft override cause flattening of the central cornea in that meridian and may be best corrected by opening and resuturing the wound, despite the lengthy recovery. Incisional techniques available for the correction of postkeratoplasty astigmatism include relaxing incisions with or without the use of compression sutures. Wedge resections also can be used for high degrees of astigmatism. Techniques that involve the excimer laser include photoastigmatic refractive keratectomy and LASIK. RELEVANT ANATOMYSection 4 of 11
The total amount of astigmatism is entirely dependent upon the anatomy of the eye. Therefore, the physical optical media including the cornea, the lens, and occasionally the retina can contribute toward development of astigmatism. Surgical correction of astigmatism takes place chiefly in the tissues of the cornea, so it is most desirable to have an intact, healthy cornea. Postsurgical patients, such as those who have undergone penetrating keratoplasty, will benefit from surgical correction of astigmatism, but careful consideration should be given to the strength and health of the corneal tissues and the method of correction used. CONTRAINDICATIONSSection 5 of 11
Contraindications to the surgical correction of astigmatism include forms of corneal or anterior segment diseases such as corneal ulcers, keratitis, or conjunctivitis. Ectatic disorders, such as keratoconus, may increase variability of the predicted outcome and may not be predictable in their outcome. TREATMENTSection 6 of 11
Surgical TherapyAstigmatic KeratotomyArcuate keratotomies are placed coincident with an optical zone 5-8 mm in diameter (see Image 1). Incisions placed coincident with a 5-mm optical zone achieve the greatest effect, yet they have the highest chance of induction of irregular astigmatism. Incisions should not be placed more centrally because of the risk of encroaching on the visual axis and producing glare. Typically, incisions are placed at the 7-mm optical zone. Younger patients achieve less effect from a given incision than older patients do; therefore, the incision length to achieve a desired correction should be adjusted according to the patient's age. The expected result is decreased by 2% per year for patients younger than 30 years and increased by 2% per year for patients older than 30 years. Arcuate incisions flatten the steeper meridian the same amount as they steepen the flatter meridian; therefore, the net effect is no change in the spherical equivalent. Arcuate incisions greater than 90° are not recommended because of the risk of late wound dehiscence. The authors prefer arcuate incisions with an optical zone diameter of 7 mm for almost all cases. Nomograms are surgeon dependent and should be adjusted based on individual experience and outcome analysis. Special arcuate keratotomy markers are available that imprint both the desired optical zone and a graduated scale for the degrees of arc (see Image 3). Photorefractive Excimer Laser Astigmatic CorrectionThe excimer laser corrects simple myopia by applying a greater amount of laser energy to the central cornea than to the peripheral cornea. This technique can be accomplished by opening and closing a circular aperture through which the laser light passes or by using a scanning laser to direct pulses primarily to the central cornea. This results in the central stromal tissue receiving more ablation than the peripheral cornea, thereby creating a convex surface. The laser operates at a wavelength of 193 nm, which ablates approximately 0.25 µm of stromal tissue with each pulse. Greater amounts of tissue removal correspond with greater amounts of correction induced. While the laser calculations and ablation methods are essentially the same for photorefractive keratoplasty (PRK) and laser epithelial keratomileusis (LASEK) as they are for LASIK, the chief difference between the 2 surgical techniques is the depth within the cornea where the laser energy is applied. In LASIK, the energy is transmitted into the midstromal layers after first creating the LASIK flap and exposing the stromal tissue. In PRK, ablation of tissue occurs at the level of the Bowman layer and anterior stroma rather than in the midstromal region. In LASEK, the epithelium is saved as a sheet and repositioned over the ablated area after the treatment. Wavefront-guided laser vision correction Recent advances in techniques used to gather refractive data allow for correction of not only myopia, hyperopia, and astigmatism but also higher order aberrations. This wavescan digital technology was originally developed for astrophysics to reduce atmospheric distortions when viewing distant objects in space through high-powered telescopes. By applying wavescan analysis, coma, trefoil, quadrafoil, higher order spherical aberration, and astigmatism can be corrected. These higher order aberrations are visually significant for many patients. The LASIK or PRK or LASEK procedure does not change but rather the method of mapping the visual imperfections (optical aberrations) is different. Zernike polynomials are used to provide a convenient mathematical expression of the aberration content in the optical wavefront, resulting in more precise measurements than with standard methods. The wavefront aberrations are then transferred into an ablation profile that is applied using variable beam or scanning spot technology. Variable spot sizes are used to remove corneal tissue with an excimer laser. This translates to decreased subjective perception of halos and glare, especially in mesopic conditions, along with increased visual acuity in low-contrast conditions. Clinical results of wavefront-guided LASIK to correct myopic astigmatism by Mrochen et al showed that, in a group of 35 eyes tested, 93.5% of eyes were at an uncorrected visual acuity level of 20/20 or better at 3 months. This technology has also been helpful in the treatment of mixed astigmatism, as reported by Maloney. Other Types of Astigmatic CorrectionCompression sutures Compression sutures used simultaneously with astigmatic keratectomy can markedly increase the effect of the incisions. Compression sutures are placed 90° from the incisions. Suture depth should be approximately 80% of the corneal thickness. The sutures are tied with a slipknot, and tension is adjusted under intraoperative keratometric or keratoscopic control until an overcorrection of 25-50% is achieved. Then, the knots are tied, the ends are trimmed, and the knots are buried. The recommended suture to use for compression sutures is 11-0 polyester. Nylon is easier to work with but biodegrades with time. Polypropylene is too elastic to maintain strength. Polyester suture is permanent and can be left without the possibility of loss of effect from biodegradation. Sutures of any material may attract blood vessels to a corneal graft, and they also tend to loosen with time. Wedge resection A wedge resection technique (see Image 2) is used for the correction of extreme amounts of astigmatism, usually following penetrating keratoplasty. This procedure is reserved for patients with more than 10 D of astigmatism and is capable of correcting up to 20 D of astigmatism. A wedge resection requires a prolonged postoperative rehabilitation because of the need to place multiple sutures, which may induce irregular astigmatism. In general, a minimum of 6 months must be allowed for adequate wound healing before selective suture removal. The overall effect of wedge resection is to steepen the flat meridian approximately twice as much as it flattens the steeper meridian. The net effect is an increase in myopia or a decrease in hyperopia. The surgical technique involves removing a wedge of tissue along the flat meridian of the cornea. Thermal keratoplasty Shrinkage of corneal tissues with the Holmium YAG laser or conductive keratoplasty can induce changes in the corneal curvature, reducing astigmatism and hyperopia. Regression has been a problem with corrections obtained with the Holmium laser for hyperopia, and this regression also may prevent long-lasting results for astigmatism. Regression is less of an issue with conductive keratoplasty. Further studies need to be carried out to determine if a role exists for the Holmium YAG laser or conductive keratoplasty in treating hyperopia and astigmatism. Preoperative DetailsPatient evaluation Age is an important criterion in screening patients for surgical astigmatism correction. Progression of myopia is greatest during the first and second decades of life; therefore, patients younger than 21 years may not have a stable refraction. The surgical correction of idiopathic astigmatism is considerably more predictable than the surgical correction of postkeratoplasty or postcataract astigmatism, in which the wound healing is more variable. Careful manifest refraction, keratometry, videokeratography, and wavefront analysis are performed preoperatively. Patients considering surgical correction of astigmatism must have a stable refraction confirmed by comparison of the preoperative refraction with previous refractions. In most cases, the refractive, keratometric and topographic cylinder power and axis are similar. In cases of significant disparity, remeasurement and reevaluation are recommended. Refractive astigmatism may consist of several components, including corneal astigmatism and lenticular astigmatism. Keratometry and videokeratography are better measures of true corneal astigmatism than refraction. Wavefront analysis, especially with cyclotorsional tracking capabilities, may be an even better method of analyzing true total eye astigmatic and higher order aberrations than a manifest refraction. Surgery for naturally occurring and postcataract astigmatism typically is based on the refractive cylinder and axis. For postkeratoplasty astigmatism, keratometric and videokeratographic analysis generally are used in conjunction with the refractive cylinder and axis, because nonorthogonal astigmatism is common in these patients. Wavefront analysis has the potential to even further refine these results. Videokeratography helps identify patients who would be poor surgical candidates for astigmatic keratotomy because of irregular astigmatism, keratoconus, or other corneal abnormalities. Wavefront analysis of the total refractive power of the eye may be useful in the future to further direct surgical management of patients with disparities between refraction, keratometry, and topography. Astigmatic keratectomy Preoperatively, a mild sedative, such as diazepam, is given if needed. An oral nonsteroidal anti-inflammatory drug, such as ibuprofen or naproxen, is useful to aid in any early postoperative discomfort. Topical pilocarpine is not recommended because it can skew the position of the pupil and make localization of the center of the true entrance pupil inaccurate. Some surgeons prefer to identify the steep meridian preoperatively by placing a drop of topical anesthetic on the eye, having the patient fixate on a distance object, and using a surgical marking pen to mark the limbus in the steep meridian. Care should be taken to avoid the use of excess anesthetic, as it may cause epithelial toxicity. Cyclotorsional tracking and registration are now available with some laser systems to define the same registration with the wavefront analysis and the laser treating system to ensure that alignment is the same for treatment as for the diagnostic system, thus giving astigmatic keratotomy an alignment disadvantage. Peribulbar or retrobulbar anesthesia usually is not necessary and prevents patient fixation, which is useful in proper placement of the incisions. The eyelids are prepped with 5% povidone-iodine solution. A drape may be used to keep the eyelashes out of the way. An eyelid speculum is inserted. A closed-bladed speculum is particularly useful in isolating the eyelashes. The eye is centered under the operating microscope and is positioned such that the iris plane is perpendicular to the microscope. The corneal thickness at the optical zone in the steep meridian is measured on both sides of the cornea with an intraoperative ultrasonic pachymeter. The blade is set to 100% of the thinnest measurement. Photorefractive keratectomy As with other types of astigmatic correction, perhaps the most critical measurement a surgeon can make is to be sure the axis of astigmatism is properly aligned to correspond with the desired treatment effect. A small amount of axis misalignment can lead to a substantial decrease in treatment effect. The limbus may be marked with gentian violet dye in the 12-o'clock and 6-o'clock positions to aid in the identification and alignment with the proper meridians. This mark can be completed with a Sinskey hook at the slit lamp prior to the procedure. Videokeratography also is a useful tool for accurate identification of the steep and flat meridians. Cyclotorsional tracking and registration is now available with some laser systems to define the same registration with the wavefront analysis and the laser treating system to ensure that alignment is the same for treatment as for the diagnostic system. Laser in situ keratomileusis The cornea first is marked with a 4-mm optical zone marker, so that in the event of a free cap, proper realignment can be obtained. The exact steps used to locate the flap vary with the microkeratome that is used. Proper care, maintenance, and assembly of the microkeratome are critical to the success of the LASIK procedure. Various microkeratomes are now used for LASIK, and the exact protocol for use varies from one machine to another machine. The desired ablation parameters are entered into the computer prior to generation of the LASIK flap, keeping in mind the surgeon's individualized adjustment parameters based on prior results. Both PRK and LASIK are performed under topical anesthetic. One drop of 0.5% proparacaine is instilled, and the surrounding tissues are cleansed with a povidone-iodine solution. Next, the lid speculum is inserted and the patient is positioned under the laser. Intraoperative DetailsAstigmatic keratectomy The patient is asked to fixate on the operating microscope light or the fixation light. The blade is inserted into the cornea and, after a pause of 1 or 2 seconds, the blade is slowly and steadily drawn through the cornea. A blade with a vertically oriented cutting edge allows the surgeon good visibility while the incision is being made. A square blade may track a more stable course through corneal tissue than an angled blade. Photorefractive keratectomy Once the epithelium is completely removed, the cornea is moistened again with a sponge to smooth out any surface irregularities and return the cornea to a physiological hydration status. The patient is asked to maintain fixation on the fixation target, and the laser energy is centered on the pupil. Laser treatment should proceed at this point, ensuring the patient maintains proper fixation. The corneal hydration status should be monitored carefully to prevent excessive dehydration or extra fluid collection in the stromal tissue. The epithelium is removed in PRK to reveal the Bowman membrane and usually is accomplished either by mechanical epithelial removal or by loosening the epithelium with alcohol. In either case, care should be taken to remove all the epithelium within the ablation zone, because any residual epithelial cells will create small areas of irregularity and may affect the smoothness of the ablation surface. It has been shown that while the alcohol method is typically faster, easier for beginning surgeons to learn, and has a quicker recovery period, the visual and anatomical results of both methods after a few months are essentially the same. Transepithelial ablation with the excimer laser sometimes is performed, although this practice is not as common because the epithelial thickness varies in different patients. If one is to use the alcohol method, a solution of 20-25% ethanol is prepared. A 7-mm optical zone marker is centered on the pupil, and 2 drops of the alcohol solution are placed in the well. After 20 seconds, a dry cellulose sponge is used to absorb the alcohol solution. The loosened epithelium is removed using another dry sponge or forceps if preferred. The alcohol interferes with the bonding of the basal epithelial cells to the underlying basement membrane, leaving the Bowman layer exposed for ablation. A study by Agrawal et al showed that using alcohol may lead to greater loss of keratocytes in the stroma as well as a greater inflammatory response. However, a solution of 70% ethanol was used for this study, and more recent studies have used considerably more dilute concentrations. The other method of removing the epithelium is by direct mechanical removal. This method can be completed with a PRK spatula, starting in the periphery and completing the removal centrally. A rotating brush also may be used. Mechanical abrasion may lead to dehydration of the cornea if not completed in a timely fashion. Laser in situ keratomileusis The suction ring of the microkeratome is positioned over the limbus and is centered on the pupil or slightly decentered toward the flap hinge. After proper positioning is obtained, suction is turned on. Tonometry is used to check that the intraocular pressure is raised to a minimum of 65 mm Hg. Next, the microkeratome is brought into place. The blade is passed across the cornea, keeping a small hinge intact. The microkeratome and suction ring are removed, and the flap is lifted to reveal the corneal stromal bed. A Merocel sponge is wiped across the stromal bed to ensure even hydration. Ablation should be centered on the pupil, and the patient is instructed to concentrate on the fixation light. Care should be taken not to ablate the posterior face of the flap. Treatment of astigmatism requires treating a wider zone in one meridian than in the other meridian, and this should be considered when planning flap size and hinge location. The hydration status of the cornea should be monitored carefully. If the cornea dries excessively, overcorrection may occur. If the cornea is too moist, undercorrection or irregular astigmatism may occur. After treatment, any debris should be removed from the stromal surface and usually is completed with a Merocel sponge. A drop of balanced salt solution (BSS) is placed on the ablation bed, and the cap is floated back into position. Then, the flap is smoothed out with a moist sponge. The eye is allowed to dry under the laser for approximately 5 minutes to aid in flap re-adhesion. A drop of lubricant is placed onto the cornea prior to removing the lid speculum. The patient should be allowed to blink to ensure a secure flap. Postoperative DetailsAstigmatic keratectomy Postoperatively, a drop each of topical antibiotic solution, corticosteroids, and a nonsteroidal anti-inflammatory drug are applied to the eye. The eye is not routinely patched, and cycloplegia is not necessary. The patient is seen 1 day, 1 week, and 1 month postoperatively. Topical antibiotics and corticosteroids are used 4 times a day for 1-2 weeks until the incisions have reepithelialized. Photorefractive keratectomy Following laser treatment, a bandage contact lens is placed over the eye. The patient is given topical steroid, antibiotic, and nonsteroidal anti-inflammatory drops. Artificial tears also may be used copiously to maintain proper hydration of the bandage lens. The patient should return for follow-up care the next day to examine the newly forming epithelial surface underneath the bandage lens. This examination can be accomplished without removal of the lens, so as not to interrupt the leading edge of the ingrowing epithelium. The patient is seen next around day 4 or 5 postoperatively for removal of the bandage lens after the epithelium has healed completely. Laser in situ keratomileusis Antibiotic and steroidal drops are placed in the eye 4 times daily. Frequent artificial tears also are used postoperatively. If an epithelial defect occurs as the flap is being smoothed out, a bandage contact lens may be used to promote proper healing. However, in most cases, this bandage is not necessary. Follow-upFollow-up visits for astigmatic keratotomy, PRK, and LASIK usually are at the first postoperative day, first postoperative month, and then at 6-12 months following surgery. At each postoperative visit, uncorrected visual acuity is taken, along with a manifest refraction, slit lamp examination of the cornea and anterior segment structures, and videokeratography to monitor changes in the corneal topography. For excellent patient education resources, visit eMedicine's Eye and Vision Center. Also, see eMedicine's patient education article Vision Correction Surgery. COMPLICATIONSSection 7 of 11
Central islands Central islands are small central elevations in the corneal topography, which may occur for a variety of reasons and can occur in myopic and myopic astigmatic treatments (see Image 8). Central islands have been reported after excimer laser treatment using broad-beam lasers. Beam profile abnormalities or increased hydration of the central corneal stroma have been implicated as potential causes. A flat ablation beam may direct stromal fluid into the central area of ablation, and the hydrated tissue is ablated at a slower rate. This can lead to less tissue removal in the central 1-3 mm of the cornea. Laser software can add extra pulses in the central cornea to compensate for this, yet careful monitoring of the hydration of the central cornea is important. If excessive hydration of the central cornea is noted, the procedure should be paused to remove excess fluid from the center of the cornea. Central islands tend to resolve with time after photorefractive keratectomy, probably due to differential wound healing or epithelial hyperplasia. Central islands are associated with loss of best-corrected visual acuity because of the irregular astigmatism that they create. Undercorrection and overcorrection Undercorrection and overcorrection can be seen as a result of improper surgical programming, decentration or malfunctioning of the excimer laser, abnormal corneal hydration status, or excessive or inadequate wound healing response. Maintaining consistent hydration of the cornea is crucial, because if excessive fluid is on the cornea, then an undercorrection will result. If desiccation of the corneal stroma is present, then overcorrection and haze may occur. An enhanced wound healing response can lead to corneal scarring with an undercorrection. Minimal tissue healing may sometimes lead to an overcorrection. Cap dislocation A complication unique to LASIK is displacement of the flap. If displacement should occur, the flap should be lifted and repositioned after cleaning the interface of debris and epithelial cells. Epithelial ingrowth under the flap can be significant enough to reduce vision, requiring cleaning of the epithelium from the interface. A protective shield worn for the first 24 hours after LASIK can reduce the chances of inadvertent trauma. Decentration Decentration of the refractive ablation can result in glare, irregular astigmatism, and a decrease in best-corrected visual acuity. Some controversy still exists about proper centration landmarks, but centration on the pupil, with the patient looking directly at the fixation light of the laser, is suggested most frequently (see Images 9-10). Visual aberrations Glare or star burst caused by scattering of light intraocularly occurs commonly for a few months after surgery. Glare or star burst may persist for a year or more in radial keratotomy, and, in 9% of patients, it may cause diminished vision, particularly at night. With astigmatic keratotomy alone these complaints are quite rare. Glare and halos at night are uncommon after photorefractive keratectomy, but if persistent, can result in patient dissatisfaction in occasional cases. Low-contrast visual acuity is a more sensitive measurement of visual function after photorefractive keratectomy and may better objectively measure the degree of visual aberration seen. Wavefront analysis also may better define visual aberrations seen in postsurgical corneas. Infectious keratitis Infectious keratitis seen following excimer photorefractive keratectomy or LASIK should be treated with appropriate culture and frequent antibiotics. Infectious keratitis is rare after LASIK, yet it may occur especially if an epithelial defect is present in the first 1 or 2 days postoperatively (see Image 11). Epithelial ingrowth Epithelial ingrowth under the flap can be significant enough to reduce vision, requiring cleaning of the epithelium from the interface (see Image 12). Small areas of peripheral epithelial ingrowth may not require removal. However, if the ingrowth is progressing toward a 7-mm optical zone, then it should be removed to reduce the likelihood of visual aberrations. Diffuse lamellar keratitis Another potential complication unique to LASIK is diffuse lamellar keratitis (DLK). A severe form of this condition is known popularly as sands of the Sahara. This condition usually is seen beginning on the first day postoperatively and consists of white blood cells infiltrating the flap interface. It usually presents as a light, striated infiltrate in the peripheral zone of the flap and may increase in area and intensity in the days following surgery (see Images 13-14). DLK usually can be treated with an increase in strength and frequency of topical steroids, and by discontinuing nonsteroidal anti-inflammatory drugs in favor of topical steroids. However, in severe cases a need may exist to lift the flap and irrigate the interface to remove the white cells (see Image 15). If severe, the infiltrate may cause a breakdown in the collagen of the stroma, resulting in a melting of the LASIK flap (see Image 16). It usually is best to continue the use of frequent topical steroids following irrigation to ensure no residual damage develops from the remaining infiltrative cells. OUTCOME AND PROGNOSISSection 8 of 11
Astigmatic correction results using the excimer laser steadily have improved as surgical techniques have improved. Several years ago, Nordan et al reported a series of 5 eyes with postkeratoplasty anisometropia treated with the excimer laser. The mean preoperative myopia was -7.40±4.37 D with a mean of +3.05±1.47 D of astigmatism. Postoperatively, the myopia was reduced to a mean of -3.35±3.04 D, and the astigmatism was reduced to +1.30±0.60 D. The improvement in refractive error also was associated with an improvement in uncorrected visual acuities. Thirteen eyes with high astigmatism were treated with a rotating mask system using the Aesculap Meditec MEL 193-nm excimer laser. In this study, the astigmatism was reduced an average of 2.80 D. Inadequate correction was seen in some patients due to decentration of the ablation. Haze was seen in most patients but was graded as severe in only 2 patients and eventually cleared with topical steroids. The ability of the excimer laser to treat high levels of astigmatism in all degrees of myopia also was examined by Lee et al. One hundred ten eyes were included in the study with myopic spherical component ranging from -3.00 D to -13.00 D, and astigmatic component ranging from -2.00 D to -5.50 D. All ablations were completed with an elliptical and multizone treatment pattern. Results were examined using vector analysis and showed that the low-to-moderate levels of myopia had a high level of predictability. However, high levels of myopia, greater than -10.25 D, showed little improvement in astigmatic correction in this study. The authors' group reported a series of 19 eyes undergoing LASIK for hyperopic astigmatism. Preoperative average hyperopia was +2.20 D±0.99, and cylinder corrections ranged from 0 D to 4.50 D. Postoperatively, no eyes lost more than 2 lines of best-corrected visual acuity, and 63% of eyes were within +1.00 D of emmetropia. In the eyes with the longest follow-up care (6 mo), refractive stability was good, although it was still unclear if the slight tendency for the primary hyperopes to regress would continue over the long term. In both groups, however, predictability was good, and patients gained excellent uncorrected visual acuity quickly after surgery. This suggests that LASIK may help hyperopic astigmatic patients resume a normal, active lifestyle after only a brief recovery period. Another study by this group looked at the results of LASIK for myopic astigmatism and compared unilateral and bilateral treatments. The procedure rapidly reduced spherical equivalent and astigmatism in both groups. No statistical difference in spherical equivalent or astigmatism was noted postoperatively between groups. The effect of LASIK on the astigmatism was rapid and remained stable without regression between the visits at 1 day and 1 month and between visits at 1 month and 1 year in both the unilateral (p=1, p=0.94) and bilateral groups (p=0.14, p=0.45). The difference in cylinder between the unilateral and bilateral groups was not statistically significant at any of the postoperative visits. Another study by Febbraro et al examined the efficacy of excimer laser photoastigmatic refractive keratectomy in patients treated with a Nidek EC 5000 excimer laser. Preoperative myopia averaged -4.50 D, and average astigmatism preoperatively was -1.64 D. Results at 1 year showed residual subjective astigmatism to be -0.40 D. Uncorrected visual acuity of 20/40 or better occurred in 22 out of 27 eyes and 21 of the eyes were within ± -1.00 D of emmetropia. In a study by Vinciguerra et al, 25 eyes underwent correction for hyperopic astigmatism using the excimer laser with an algorithm designed to remove as little tissue as possible. Mean preoperative hyperopia was +3.76±1.70 D, and mean preoperative cylinder was 2.20±0.80 D. Six month follow-up results showed that mean sphere was corrected by 3.08 D and mean cylinder by 1.60 D, while best-corrected visual acuity actually improved. As is clearly demonstrated, PRK and LASIK are predictable and effective means of correcting various types and degrees of ametropia, including astigmatism. FUTURE AND CONTROVERSIESSection 9 of 11
The excimer laser is now frequently used to correct astigmatism. Although early results appear promising, whether the excimer laser procedure will completely replace incisional astigmatic keratotomy awaits to be seen. Complications are unusual and if managed appropriately, a minimal incidence of losing best-corrected visual acuity exists. Thorough understanding of the corneal response to both incisional astigmatic surgery as well as excimer laser correction of astigmatism is essential to refractive surgeons. As these responses are better understood and as software packages are refined, more success using excimer laser techniques is expected. Continued advancements in many areas of refractive surgery are improving results obtained in correction of astigmatism. The excimer laser used to resculpt the cornea under a corneal flap in the LASIK procedure is currently the most reliable method of correcting astigmatism. Proper attention to surgical details as well as preoperative and postoperative management will continue to provide safe and reliable astigmatic corrections. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
Article Last Updated: Jan 2, 2007 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
