Astigmatism is important to address during surgical correction of refractive errors. It is important to understand the steep and flat axes of the cornea, the potential for posterior corneal astigmatism, and lenticular astigmatism when analyzing the patient's condition. For patients undergoing laser in-situ keratomileusis (LASIK) or photorefractive keratoplasty (PRK), the concept of irregular astigmatism or higher-order aberrations is also taken into consideration. For patients undergoing cataract surgery, the ability to correct astigmatism with either corneal-relaxing incisions or toric lens implants is important. Many patients desire this type of treatment when they undergo cataract surgery, so it should be discussed with them at the time of the cataract evaluation.
The surgical correction of astigmatism was first performed with astigmatic keratotomy, which is described in this article, along with compression sutures and wedge resection. In addition, excimer laser photoastigmatic refractive keratectomy (PRK), standard LASIK, wavefront-guided LASIK, topographic-guided LASIK, and small-incision lamellar extraction (SMILE) have been used to reduce astigmatism. Astigmatism correcting intraocular lenses (IOLs), such as toric IOLs, can also be used when inserting phakic or pseudophakic IOLs. Although astigmatic keratotomy has been largely replaced by excimer laser techniques, it is still used, especially during cataract surgery, and, as such, the surgical techniques are discussed.[1]
Astigmatic 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.[2] 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.[3] 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.[4] 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.[5, 6] In the 1940s and 1950s, Sato of Tokyo, Japan, investigated both radial and astigmatic keratotomy.[7, 8, 9, 10, 11, 12, 13, 14, 15] 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.[16]
Over the past 15 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 to treat only myopia and low amounts of myopic astigmatism.[17] Since new software has become more available and more reliable, many studies have reported that excellent results can be obtained for correcting low-to-moderate amounts of hyperopia and hyperopic astigmatism.[18, 19]
Today, most astigmatism is corrected with wavefront based laser treatment algorithms.[20, 21, 22, 23, 24, 25, 26, 27] This allows not only regular astigmatism, where the orientation of the flat and steep axes is orthogonal, to be corrected, but also mild degrees of irregular astigmatism and higher order aberrations to be improved.
Astigmatism, like myopia or hyperopia, can decrease visual acuity. However, astigmatism is much more complex because it has both magnitude and orientation. Thus, astigmatism is more difficult to correct in spectacles, contact lenses, or surgery than spherical forms of refractive error. A small misalignment of astigmatic refractive error can have a significant impact on the overall visual acuity.
Naturally 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 general population has more than 0.50 D of astigmatism, 10% has more than 1.00 D, and 8% has 1.50 D or more. Astigmatism can also be found in 22% of patients with Down syndrome.
Visually significant astigmatism is common after various kinds of ophthalmic surgery,[28] including cataract extraction,[29] penetrating or lamellar keratoplasty,[30, 31, 32, 33] retinal repair, 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. Irregular astigmatism after penetrating keratoplasty is even more common.
Astigmatism can be caused by asymmetry of the various structures in the eye, such as the anterior cornea (most common), the posterior cornea, the lens, or the retina. These asymmetric structures then alter the optics of the eye, creating visual distortion. Most of these asymmetries are created by normal variation in ocular tissues, and, generally, these variations translate into regular astigmatism. Astigmatism may also be caused by pathology of the structures or by changes as a result of trauma.[34] A relatively common example of a corneal pathology that induces high amounts of regular and irregular astigmatism is keratoconus. Lenticular irregularity that results from the changes associated with cataract development can also create astigmatism.
The clinical manifestation of astigmatism is blurred vision. Another common symptom is the streak phenomena or rays around point sources of light, most noticeable in dark environments. If the magnitude of astigmatism is high, it may induce shadowing or smearing of letters; in very high amounts, it may cause diplopia.
Correction of astigmatism by means of surgery is indicated when the degree of the astigmatism impacts visual acuity. Typically, visually significant astigmatism is roughly defined as being more than 1.00 D, although many patients may experience symptoms from lower amounts. As surgical techniques for astigmatism correction improve, patients who are symptomatic from lower magnitudes of astigmatism are being treated by means of surgical correction. Excimer laser surgery using integrated wavefront technologies is designed to correct irregular astigmatism. Patients who desire to be free from contact lenses or glasses or patients who have become intolerant to those types of correction may elect to have surgical correction performed. Patients who have developed visually significant astigmatism as a result of other types of ocular surgery have specific problems, as discussed below.
Special considerations for postsurgical astigmatism
Astigmatism caused by wound dehiscence after cataract extraction may be corrected in many instances by revising the wound itself. This may be preferable to astigmatic keratotomy or LASIK, especially if the wound dehiscence is a structural threat to the eye. In cases where the globe is structurally intact and there is a high degree of astigmatic anisometropia between the eyes, refractive rehabilitation using astigmatic keratotomy or photoastigmatic refractive keratectomy may be the preferred approach.
Understanding how these procedures affect the spherical equivalent power is imperative; astigmatic keratectomy incisions, unless extremely short or long, are typically neutral with respect to the spherical equivalent. Photoastigmatic refractive keratectomy typically induces a hyperopic shift in the refractive error, and this should be taken into account with respect to the change in spherical equivalent power. Laser software is now able to correct hyperopic astigmatism by steepening the flat axis.
Postkeratoplasty astigmatism often necessitates surgical correction because of high amounts of irregular astigmatism, rendering spectacles inadequate. Rigid gas permeable contacts can be difficult to fit properly and often result in intolerance in patients with very high amounts of corneal irregularity. Surgical correction itself can be quite challenging because the 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 more comfortably.
In the postkeratoplasty eye with significant astigmatism, an examination of the sutures may reveal the source of the astigmatism; tight, loose, or asymmetric suture placement is often implicated in creating corneal astigmatism in these patients. Sutures should be removed before attempting any type of surgical procedure for the correction of postkeratoplasty astigmatism.
The keratoplasty wound itself should also 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. This may be the preferred technique in some cases of excessive override, with the obvious disadvantage to this being a lengthy recovery process.
Incisional techniques available for the correction of postkeratoplasty astigmatism include relaxing incisions with or without the use of compression sutures.[35, 36, 37, 33, 32, 31, 16, 38] Wedge resections also can be used for high degrees of astigmatism.
Techniques that involve the excimer laser include photoastigmatic refractive keratectomy and LASIK. Through these techniques, the correction can occur either under a corneal flap (now often made with the femtosecond laser) or on the surface of the Bowman layer through surface laser vision correction.
The total amount of astigmatism is entirely dependent upon the anatomy of the eye. All segments of the physical optical media, including the cornea, the lens, and even occasionally the retina, can contribute toward the development of astigmatism, and each should be examined to understand its role in relation to the disorder.
Surgical correction of astigmatism takes place chiefly in the corneal tissues, 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.
Contraindications to the surgical correction of astigmatism include corneal and anterior segment diseases, such as corneal ulcers, keratitis, or conjunctivitis. Ectatic disorders, such as keratoconus or pellucid marginal degeneration, may increase variability of the predicted outcome.
In general, the results of astigmatic correction at the time of LASIK, PRK, or SMILE are favorable. Residual astigmatism is more common than residual spherical refractive error. Toric IOLs are also very useful for correcting astigmatism when cataract surgery is performed.
For any patient undergoing surgical correction of astigmatism, the patient’s general health is important. No routine laboratory workup is typically done, but the status of any concurrent systemic disease should be known.
Diabetes mellitus, in particular, is an example of one systemic disorder that can cause wide fluctuations in the refractive status of the eye. Prior to surgical treatment, diabetes should be well controlled, and relatively stable refractions should be performed over several visits.
The most important imaging studies for astigmatism correction are corneal topography and tomography.[39, 40, 31, 41] These imaging studies are essential in understanding the shape and curvatures of the cornea, and they can be of tremendous help in establishing a diagnosis and in deciding management.
Corneal thickness using ultrasonic pachymetry may also be useful.
In patients who have had prior surgery, such as cataract surgery, phakic intraocular lens implantation, or keratoplasty, the corneal endothelium can be monitored using specular microscopy.
Astigmatic keratotomy Arcuate keratotomies are placed coincident with an optical zone 5-8 mm in diameter (see the image below).
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Typical astigmatic keratectomy incisions.
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 9-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 9 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 the image below).[42, 43, 44, 45]
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Corneal marker indicating optical zone diameter and degrees of arc.
The excimer laser corrects simple myopia by applying a greater amount of laser energy to the central cornea than to the peripheral cornea.[46, 47, 48] 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.[49]
Hyperopic corrections can be achieved by a process in which tissue in the peripheral area receives more laser energy than that in the central area. Myopic astigmatic corrections are achieved by applying the laser energy in an elliptical pattern along the central part of the flat meridian, thereby flattening the steep axis (see the images below).
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LASIK for myopic with-the-rule astigmatism.
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LASIK for myopic against-the-rule astigmatism.
Alternatively, hyperopic astigmatic correction is achieved by applying the laser energy preferentially in the periphery, steepening the flat axis (see the images below).
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LASIK for hyperopic with-the-rule astigmatism.
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LASIK for hyperopic against-the-rule astigmatism.
The laser operates at a wavelength of 193 nm, which ablates approximately 0.25 µm of stromal tissue with each pulse.[50] 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.[51] This technology has also been helpful in the treatment of mixed astigmatism, as reported by Maloney.
Other types of astigmatic correction
Refractive Lenticule Extraction (ReLEx): Femtosecond Lenticule Extraction (FLEx) and Small Incision Lenticule Extraction (SMILE)
ReLEx procedures are surgical techniques using a femtosecond laser that corrects refractive error by sculpting an intrastromal refractive lenticule, which is then removed. The technique can be performed in the following 2 ways:
FLEx: This involves femtosecond lenticule extraction, which is the earlier variant of ReLEx. The technique involves the creation of a flap similar to a LASIK flap.[52, 53] The flap is subsequently lifted, exposing the refractive lenticule, which is then removed.
SMILE: This is small-incision lenticule extraction,[54] which involves the creation of an arcuate incision of approximately 3 mm, through which the refractive lenticule is extracted after dissection. Without the creation of a flap, the SMILE procedure has been proposed to minimize disruption to the subbasal nerve plexus, thereby possibly reducing postoperative dry eye symptoms[55] also eliminating the risk of flap dislocation.
ReLEx techniques can currently treat up to –11D of myopia and up to –5D of astigmatism. Further research is necessary to improve predictability and effectiveness of the treatment of hyperopia.[56] There is minimal difference in refractive outcome documented between the FLEx and SMILE techniques, and visual outcomes are comparable and potentially superior in stability to traditional LASIK and PRK.[57]
This technique is the only theoretically reversible corneal refractive surgery. It may be potentially reversed via cryopreservation of the lenticule and surgical reinsertion later to restore corneal stromal volume. This has the potential to aid in treatment of various forms of corneal irregularities such as ectasia. Additionally, LASIK or PRK can then be performed, such as in the case of presbyopia to restore low myopia to the nondominant eye for monovision.[58]
Compression 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 the image below) 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.
View Image
Wedge resection.
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.[59]
Thermal and radiofrequency-induced keratoplasty
Shrinkage of corneal tissues with the Holmium YAG laser or conductive keratoplasty can induce changes in the corneal curvature, reducing astigmatism and hyperopia.[60, 61, 62, 63]
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 (CK), which uses a radiofrequency probe to alter the stromal collagen in the cornea to a greater depth than what is exerted by the Holmium YAG laser.[64]
Initial results of astigmatic correction using CK have been promising for low-to-moderate amounts of astigmatism. Regression appears to be less of an issue than with Holmium YAG thermal keratoplasty, and predictability has been favorable. In addition, when conductive keratoplasty is combined with Intacs corneal ring segments, it has been shown to effectively decrease the irregular astigmatism found in keratoconus.
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, some 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.[65]
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.[66] 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 alter 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 many 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
There are 2 methods of creating the corneal flap hallmark with the LASIK procedure: a microkeratome, which uses a blade mounted in a movable housing, and, more recently and commonly, a femtosecond laser can be used to perform the process.[67, 68]
The exact steps used to create the flap vary with the method used. Proper care, maintenance, and assembly of the microkeratome are critical to the success of the LASIK procedure. Positioning of the unit on the ocular surface is critical, as the procedure is performed without visual confirmation by the surgeon. The femtosecond laser must also be properly maintained. The laser unit occupies more space in the laser suite than a microkeratome unit. The advantages of the femtosecond laser include performing the procedure with visual monitoring, more control of flap depth, and no shearing force exerted on the corneal epithelium. The visual results between the 2 devices are not statistically significant.
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.
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
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.[69, 70, 71] 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. While the alcohol method is typically faster, is easier for beginning surgeons to learn, and has a quicker recovery period, the visual and anatomical results of both methods are essentially the same after a few months.
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.[70] 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.
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 physiologic 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, which can affect the refractive outcome.
Transepithelial ablation with the excimer laser is sometimes performed, although this practice is not as common because the epithelial thickness varies in different patients.
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.[72, 73]
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.
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 steroids, antibiotics, and nonsteroidal anti-inflammatory drops. If high correction is performed, the patient may also be instructed to begin taking 1 gram of vitamin C daily, which has been shown to inhibit haze formation. Artificial tears should be used copiously to maintain proper hydration of the bandage lens and to aid in wound healing.
The patient should return for follow-up care on postoperative day 1 for an examination of 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 postoperative day 4 or 5 for removal of the bandage lens after the epithelium has healed completely. The authors recommend that the lens be removed with a dull forceps to limit epithelial disruption. After removal of the bandage contact lens, the patient begins using a hyperosmotic ophthalmic ointment (eg, sodium chloride 5%) before bedtime.
Postoperative medications vary, particularly in accordance with the use of steroids. Some physicians prefer to limit topical steroidal use to 2 weeks, while others may elect to keep their patients on steroidal medications much longer.
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. Typically, postoperative medications are used for approximately 2 weeks.
Follow-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 eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Vision Correction Surgery.
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 the image below).[41]
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Corneal topography of a central island.
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. They are now rare with the current excimer laser vision correction software.
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.[74] 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 the images below).
View Image
Corneal topographies of decentered myopic ablations.
View Image
A properly aligned reticle. Note the position over the center of the pupil.
Much of the new software with wavefront corrections can improve the centration by identifying the pupil to limbus relationship at the wavefront aberrometer and then by translating it at the laser for accurate centration.[74, 75]
Visual aberrations
Glare or star burst caused by scattering of light intraocularly occurs commonly for a few months after surgery.[76, 77, 78, 79, 80, 81] 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.[82, 83] 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 the image below).
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Infectious keratitis following LASIK. Note hyperemia, which typically does not occur in cases of diffuse lamellar keratitis.
Epithelial ingrowth
Epithelial ingrowth under the flap can be significant enough to reduce vision, requiring cleaning of the epithelium from the interface (see the images below).
View Image
Epithelial ingrowth following LASIK surgery.
Small areas of peripheral epithelial ingrowth may not require removal.[84] 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 the images below).
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Diffuse lamellar keratitis - Stage 1.
View Image
Diffuse lamellar keratitis - Stage 2.
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 the image below).
View Image
Diffuse lamellar keratitis - Stage 3. Note central clumping of interface cells.
If severe, the infiltrate may cause a breakdown in the collagen of the stroma, resulting in a melting of the LASIK flap (see the image below). 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. As with microbial infection, DLK also may be triggered by an epithelial defect early in the postoperative period.
View Image
Diffuse lamellar keratitis - Stage 4. Note "cracked" appearance in central cornea following stromal melt.
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.[85] 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.[86] 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.[87] 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.[22] 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.[88, 89, 77, 90, 91, 92, 93, 94, 86, 95, 96, 97]
The excimer laser is now frequently used to correct astigmatism. In fact, the excimer laser procedure has almost completely replaced incisional astigmatic keratotomy for correction of astigmatism, except for during IOL surgery. 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.
Long-term monitoring of patients who have undergone LASIK for corrections that involve astigmatism should be considered and discussed with the patient. Although rare, ectasia can occur, and some have advocated using combined LASIK and collagen cross-linking, especially in patients who have high corrections.[98] Over time, the lens or eyelid changes may cause changes in the astigmatism. Eye rubbing can cause ectasia that can create irregular astigmatism.
David R Hardten, MD, Ophthalmologist, 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 Medical School, Minnesota Veterans Affairs Medical Center
Disclosure: Serve(d) as a speaker or a member of a speakers bureau for: Avedro, Johnson & Johnson<br/>Have a 5% or greater equity interest in: ESI, OSD<br/>Received consulting fee from Allergan, OSD, Sightpath, for consulting; Received grant/research funds from Humanoptics; Received honoraria from Johnson & Johnson for speaking and teaching.
Coauthor(s)
Ahmad M Fahmy, OD, Staff Optometrist, Minnesota Eye Consultants, PA; Adjunct Faculty Assistant Professor of Optometry, Illinois College of Optometry; Adjunct Faculty Assistant Professor of Optometry, Southern California College of Optometry
Disclosure: Nothing to disclose.
Kate Montealegre, OD, Staff Optometrist, Minnesota Eye Consultants, PA; Adjunct Faculty Clinical Instructor of Optometry, Illinois College of Optometry
Disclosure: Nothing to disclose.
Scott G Hauswirth, OD, Consulting Staff, Department of Optometry, Minnesota Eye Consultants
Disclosure: Nothing to disclose.
Specialty Editors
Simon K Law, MD, PharmD, Clinical Professor of Health Sciences, Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine
Disclosure: Nothing to disclose.
Louis E Probst, MD, MD, Medical Director, TLC Laser Eye Centers
Disclosure: Nothing to disclose.
Chief Editor
Michael Taravella, MD, Director of Cornea and Refractive Surgery, Rocky Mountain Lions Eye Institute; Professor, Department of Ophthalmology, University of Colorado School of Medicine
Disclosure: Received income in an amount equal to or greater than $250 from: J&J Vision (Consultant)/Proctor<br/> for: Coronet Surgical (Consultant), no income received.
Additional Contributors
Daniel S Durrie, MD, Director, Department of Ophthalmology, Division of Refractive Surgery, University of Kansas Medical Center
Disclosure: Received grant/research funds from Alcon Labs for independent contractor; Received grant/research funds from Abbott Medical Optics for independent contractor; Received ownership interest from Acufocus for consulting; Received ownership interest from WaveTec for consulting; Received grant/research funds from Topcon for independent contractor; Received grant/research funds from Avedro for independent contractor; Received grant/research funds from ReVitalVision for independent contractor.
MacDonald IM, Zabel RW, Chen V. Corneal astigmatism generation and correction using a fiberoptic-delivered holmium laser. Invest Ophthalmol. 1990. 31:S476.
Thompson V. Holmium:YAG laser thermokeratoplasty for correction of astigmatism. J Refract Corneal Surg. 1994. 10:S293.
Suk Kyue Choi, Jin Hyoung Kim, Doh Lee, Sae Hoon Oh, Jong Hyun Lee, Min Soo Ahn. Creation of an extremely thin flap using IntraLase femtosecond laser. Journal of Cataract & Refractive Surgery. May 2008. 34:859-863.
Kanellopoulos AJ; Asimellis G. Combined laser in situ keratomileusis and prophylactic high-fluency corneal collagen crosslinking for high myopia: two-year safety and efficacy. J Cataract Refract Surg. July. 41:1426-33.
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