You are in: eMedicine Specialties > Ophthalmology > REFRACTIVE DISORDERS Astigmatism, Astigmatic KeratotomyArticle Last Updated: Mar 14, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: James Hays, MD, Consulting Staff, Department of Corneal Transplantation and Refractive Surgery, Atlanta Eye Surgery Center Coauthor(s): Spencer Thornton, MD, Medical Director, Thornton Eye Center 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: arcuate keratotomy, transverse keratotomy, AK, refractive surgery INTRODUCTIONSection 2 of 10
  The ideal refractive surgical procedure is simple to perform, inexpensive, and applicable to a wide range of ametropias. Astigmatic keratotomy (AK) fits these criteria in many ways. It can be useful for numerous refractive problems, including congenital astigmatism, astigmatism with a cataract, posttraumatic astigmatism, and astigmatism after corneal transplantation. Even with excimer laser vision correction (eg, photorefractive keratoplasty [PRK], LASIK), AK can be a useful tool for many eyes. The Prospective Evaluation of Radial Keratotomy (PERK) study addressed only symmetrically placed radial incisions. No astigmatism correction was attempted. In fact, the PERK study demonstrated that radial incisions do not change astigmatism in a reproducible way. It was not until later studies that the effects of transverse or arcuate incisions were investigated. Early investigations of the AK techniques included surgeons Spencer Thornton, Kurt Buzard, Frank Price, Bruce Grene, Lee Nordan, and Dick Lindstrom. History of the ProcedureAK has developed along 2 parallel pathways. Richard Troutman, a classic corneal transplant surgeon, was an early thinker in the reduction of postcorneal transplant astigmatism. Troutman developed the first pathway with his technique of wedge resection for high corneal transplant astigmatism, which helped spawn the field of refractive surgery. The second pathway of AK development was among surgeons who dealt not with corneal transplantation but with congenital astigmatism. This pathway also helped develop the subspecialty of refractive surgery. Three schools of thought initially developed. Nordan proposed a relatively simple method of straight transverse keratotomy, with a target correction of 1-4 diopters. Lindstrom developed a technique and nomogram with a significant age factor. His work later evolved into the Astigmatism Reduction Clinical Trial (ARC-T) study. Thornton's technique involved up to 3 pairs of arcuate incisions, with varying optical zone sizes. More recent techniques involve moving the incisions more peripherally, closer to the limbus. More recently, Nichamin has developed an extensive nomogram for AK at the time of cataract surgery, although this method has been largely replaced by the use of limbal relaxing incision during cataract surgery. INDICATIONSSection 3 of 10
AK is a very useful tool, applicable to a wide range of refractive problems. It can be combined easily with other refractive techniques. Many indications for this procedure exist. AK is useful in cases of corneal transplant astigmatism. The donor recipient interface creates a new functional limbus. Incisions placed just inside the donor-recipient interface are philosophically similar to limbal relaxing incisions. AK has a place in transplant astigmatism whenever there is a less than spherical result. Patients with transplants may be afflicted with irregular astigmatism. Although most congenital astigmatism is regular astigmatism, following transplantation, one quadrant may be especially steep or flat. Nonorthogonal astigmatism may occur where 2 areas of the donor-recipient interface have healed too tightly. AK, when used in conjunction with high-quality corneal topography, can allow an individualized approach to each graft case. Patients with mixed astigmatism may benefit from AK. For the patient who has a refractive error with a spherical equivalent approaching zero (eg, -1.00 + 2.00 X [any axis]), LASIK may not be necessary. The concept of coupling means that when a tangential or arcuate incision is placed 2 events occur. Along the meridian of the incision(s) flattening of the cornea occurs, and the meridian 90° away steepens. The combination of flattening of the steeper axis with steepening of the flatter axis is the total amount of astigmatism correction. If a patient who had LASIK 1-2 years ago desires enhancement of residual mixed astigmatism, then AK may be preferable to recutting a flap or lifting a fairly well-healed flap. For patients presenting for LASIK with high astigmatism, the combined treatment of LASIK with AK may give a more pleasing result than LASIK alone. Since the minor axis of astigmatism treatment with the excimer laser still tends to have a small diameter, reducing a 5 or 6 diopter cylinder with limbal AK prior to LASIK may give a smoother optical zone, with less night glare and ghosting. A 2 or 3 diopter correction at the limbus will have a functional optical zone of greater than 10 mm. It allows a smaller astigmatic correction at the 4- to 5-mm optical zone, which is the largest currently available with broad-beam excimer. AK combined with cataract surgery can improve a patient's chances of excellent uncorrected postoperative vision. For many surgeons, it is already part of their routine surgery. Patients with more than a diopter of topographical astigmatism should be considered for AK at the time of their cataract surgery. Use of AK becomes more important when using multifocal intraocular lenses (IOLs) because good simultaneous uncorrected distance and near vision can be obtained only with a nearly spherical cornea. RELEVANT ANATOMYSection 4 of 10
The thickness of the cornea is measured with pachymetry and generally is 300-600 µm. The setting of the blade is dependent on knowing the thickness of the cornea. CONTRAINDICATIONSSection 5 of 10
The excimer laser is a useful tool for most patients with hyperopia or myopic astigmatism. These patients rarely will be candidates for AK, except as a small "touch-up" procedure. Patients with high astigmatism from Terrien degeneration, Mooren ulcer, or any other peripheral corneal thinning should not have limbal relaxing incisions. Patients with chronic diabetes have more epithelialization problems after corneal surgery than other patients. Take care when operating on patients with diabetes. Exercise caution in patients with connective tissue diseases (eg, rheumatoid arthritis). AK in patients with extreme dry eye, whether or not connected to rheumatoid arthritis, should be performed with great caution and close follow-up care. Similarly, AK in patients with a significant history of chemical burn or other cause of ocular surface failure should be performed with increased caution. Patients who previously had radial keratotomy (RK) may present for late "enhancement." AK is reasonable on these patients but take care in the orientation and the location of newly placed incisions. The crossing of a radial incision with a transverse incision, even years after the initial procedure, may produce excessive and unwanted overcorrection. Even though the initial incisions may have faded (almost invisible), it is still important to avoid them with AK incisions. Map the incisions at the slit lamp by identifying blood vessels or iris pigment landmarks. Operate with a dilated pupil using oblique lighting. Dimpling down the cornea with a smooth instrument may reveal otherwise obscured incisions. Be prepared to perform multiple smaller incisions to obtain the desired effect. Most RK operations carry the incisions to the limbus; therefore, long, uninterrupted limbal relaxing incisions generally are contraindicated. AK is especially difficult on a patient who previously had 16 incisions RK. AK is more useful in the patient who underwent previous 4-, 6-, or 8-cut RK. The potential benefits of astigmatism reduction must be weighed against the risks of the procedure on a case-by-case basis. WORKUPSection 6 of 10
Imaging Studies
TREATMENTSection 7 of 10
Surgical therapyMany individual surgical techniques exist for AK. The basic principles remain the same, but slight differences in variables (eg, type of blade, depth of cut, lengths of cut, location of cuts) exist. The following thorough and detailed technique outlining routine AK is provided for the less experienced surgeon. Individual modifications certainly are expected as the surgeon's experience increases. Developing the surgical plan A surgical plan includes the combination of patient data and appropriate nomogram. When the various nomograms are evaluated critically, very little difference exists between them. When all of the minor variables are equated, the resulting "suggested operation" is basically the same. Most nomograms suggest that age is a variable. Most RK nomograms assumed a nominal patient age to be approximately 30 years. It is appropriate for AK nomograms to have an older nominal age; cataract patients are generally older and many AK patients will be cataract patients. The nominal age of 50 years is appropriate for an AK nomogram. The basic rules are as follows: longer the incision, the more the effect; the smaller the optical zone, the greater the effect; the less uncut tissue under the knife tip, the greater the effect. The following nomogram is intended for limbal incisions (approximately 11-mm optical zones), with smaller 8.0-mm optical zone incisions being reserved for attempted corrections more than 3.0 diopters. The depth of cut for each incision should be reset, and it should be 0.02 mm less than pachymetry in the area of the intended cut. This nomogram is simple and straightforward for less experienced AK surgeons. It is applicable for congenital astigmatism and astigmatism with cataract. Do not use it with astigmatism following corneal transplant because posttransplant astigmatism should be corrected based upon topography. This nomogram is based upon incisions being performed at the limbus with a maximal incision length of 80°. Incisions are intended to be paired, but the surgeon may vary the relative length of each half of the pair based on topography. For example, if the nomogram calls for 100°, one incision may be placed for 60° and one incision may be placed for 40° if topography shows asymmetrical astigmatism. Nominal age is 50 years. Decrease the total amount of cut 1° for each year above 50, and increase it 1° for each year below 50. To correct, use the following:
Nomogram examples Example 1. A 50-year-old patient has 2 diopters of astigmatism. On topography, a symmetrical bow tie appearance is present. A pair of 60° incisions may be placed. See Image 3. Example 2. A 30-year-old patient has 1.5 diopters of astigmatism. On topography, a symmetrical bow tie appearance is present. The 100° in the chart must be increased by 20° based on the patient's age. A pair of 60° incisions may be placed. See Image 4. Example 3. A 75-year-old patient has 2.5 diopters of astigmatism. On topography, a symmetrical bow tie appearance is present. The 140° in the chart must be reduced by 25° based on the patient's age. A pair of 67.5° incisions may be placed. See Image 6. Example 4. A 35-year-old patient has 2.0 diopters of astigmatism. On topography, the bow tie is asymmetrical, with more steepness in 1 hemimeridian. The 2 diopters of astigmatism from the chart yields 120°, which is increased by 15° due to age for a total of 135°. A 60° incision may be paired with a 75° incision with the longer incision placed in the steeper hemimeridian. Higher levels of astigmatism As higher levels of astigmatism need to be treated either the incision needs to be lengthened or the optical zone needs to be made smaller. After reaching the maximal amount of correctable astigmatism using the above nomogram, approximately 1 diopter of additional correction may be achieved by adding a pair of 40° incisions at the 8.0-mm optical zone. Variability increases as the optical zone is decreased, and higher levels of astigmatism may be better approached with LASIK. Combining AK with cataract surgery To use this nomogram for against the rule astigmatism with cataract surgery, it may be adequate to perform a standard cataract clear corneal temporal incision and place all of the necessary AK incisions nasally. If greater correction is required, make the cataract incision in the Langerman style using a 600-µm groove. At the end of the surgery, simply extend the groove to the appropriate length. AK with corneal transplantation The physics of astigmatism after corneal transplant are quite different than with native astigmatism. The donor-recipient interface has created a new limbus, commonly it is approximately 8.0 mm in diameter. Variable wound healing and scar contracture around this circle can induce high levels of cylinder. Mismatch of the donor and recipient tissues can create high levels of permanent astigmatism. The recipient bed of some transplanted eyes may be quite variable, as in the case of progressive keratoconus. Transplants completed for herpes simplex and chemical burns may involve sewing normal elastic tissue into relatively scarred and inflexible beds. For these and other reasons traditional nomograms for AK are inadequate for patients with corneal transplantation. Astigmatism reduction in corneal transplant patients should be based completely on physical inspection of the tissue and topographical appearance of the ocular surface. These cases are approached on an individual basis. The goal in these patients is to reduce the topographical cylinder toward zero. Refraction may be difficult or imprecise in these patients since they often are older and may have co-existent cataract or macular degeneration. The most objective measurement is likely to be topographical astigmatism. Transplant patients often have asymmetrical astigmatism, irregular astigmatism, or high astigmatism. Each astigmatism procedure needs to be specifically designed for each eye. These cases are not routine. The first step in these cases is to try to determine the cause (eg, occult wound dehiscence) for the high cylinder. In these cases, excessive flattening in only 1 hemimeridian may be present. Wedge resection may be the more appropriate procedure when excess flattening occurs. Determine if an area of excessively dense wound contraction exists. In these areas, a small relaxing incision may have a large effect. Determine if an area of scarring in the recipient bed exists. In these areas, a relaxing incision may have almost no effect. If there is no "give" in the cornea (eg, as in a cornea scarred by herpes simplex) a relaxing incision may not budge the cornea. Some form of intraoperative keratotomy is very helpful for cases of transplant astigmatism. Plan the procedure based on the steep areas on topography. Determine the steepest hemimeridian and first incise that area. A Morcher ring or some similar intraoperative keratometer can help to ensure the precise centration of each incision. Look at the topography and mark the exact steepest area using a skin scribe or by making a small corneal abrasion. This technique negates any possible cyclotorsion of the eye or misplacement of the 12-o'clock and 6-o'clock positions. It also negates any possible error in translation from the topography picture to the eye. In a posttransplant case, AK should be placed just central to the donor-recipient interface rather than in the interface. The interface may have variations in thickness and inadvertent perforation may occur. Measure the thickness of the cornea one-half millimeter central to the interface. Since the donor-recipient interface is the weakest spot in the cornea do not plan to cut too deeply. Bias the knife at 90% of the measured pachymetry for each area to be incised. It is better to use a slightly longer cut at slightly less depth because the longer cut will give smoother topography. Use a triple-edged arcuate knife to make the incision parallel to the interface. After the initial incision, observe the keratometry. It is not necessary to completely neutralize the astigmatism; a continuing effect over the next week may be observed. When the cornea is basically spherical, stop cutting even if it is after only one cut. It is better to cut too little than too much. Postoperatively, if the patient has a transplant, treat with more frequent topical antibiotics and steroids than used for routine AK. The eye is neurotrophic, and epithelial healing may be problematic. Use adequate lubrication, especially if there is any tendency to dry eye. If a significant overcorrection is noticed in the postoperative period, return to the operating room and suture the gaping cut. The suture may be removed after 8 weeks. Preoperative detailsFor all AK cases have a surgical plan written down before entering the operating room. The patient's record should have a sheet with the proposed plan written out and a corneal topography for the operative eye. All of your calculations should be completed in advance. In the preoperative area, the patient should have 2 drops of topical anesthetic and 1 drop of topical antibiotic placed in the eye 5 minutes apart. Skin may be prepped with Betadine in the preoperative area or on the table. The corneal topography and the surgical plan are either taped to the operating microscope or placed on a small cart or table next to the microscope. The surgeon must be able to see the plan and the topography at all times to avoid disorientation. Intraoperative detailsAllow the patient to enter the operating room and to sit upright on the table. With the patient fixating at distance, mark the 6-o'clock and 12-o'clock locations at the limbus with a gentian violet skin scribe. The patient may then lie down under the microscope. The tip of the ultrasonic pachymeter is cleaned with an alcohol pad. The end of the tip is wiped dry with a sterile 4 X 4 pad or rinsed with balanced salt solution (BSS). Any alcohol on the pachymeter tip will give an abrasion when touched to the cornea. The thickness of the cornea in the areas of incision are measured and recorded. This measurement can be completed with manual opening of the lids rather than with a lid speculum. Orbscan readings in the area to be incised also may be used to measure the corneal thickness. Set the diamond micrometer knife under the operating microscope. Reset the knife for each incision. Choose the area where the first incision will be made and set the diamond knife for 0.02 mm less than the recorded pachymetry in this area. Set the knife down on a protective block, and put in the lid speculum. One additional drop of anesthetic may be placed on the eye. Remove excess fluid from the cornea using a slightly moist Weck-Cel sponge. The 360° Thornton astigmatic ruler is used to mark the cornea. Line up the flanges with the previously placed 6-o'clock and 12-o'clock marks, and lightly press the ruler on the cornea. The Thornton Ring or the Thornton-Fine ring may be used to fixate the globe. Go to the correct optical zone for the nomogram, and locate the incision starting point using the 10° marks on the cornea. Enter the cornea firmly and perpendicularly. Cut along the desired arc slowly, watching for any inadvertent perforations. Remove the knife, and reset it for the appropriate depth for the next cut. Repeat this procedure until all cuts have been made. Then, remove the lid speculum. Do not irrigate the cuts, especially if a small microperforation is present. Place a drop of topical antibiotic on the eye followed by 1-2 drops of topical Voltaren. If a bilateral procedure is planned, move to the other eye. Postoperative detailsPostoperative medications vary somewhat upon surgeon choice. Ocuflox used 4 times per day is popular. A steroid drop may prevent regression and dexamethasone used 4 times per day for 1 week or more is popular. A topical nonsteroidal anti-inflammatory drug (NSAID), such as Voltaren, used 4 times per day for a few days may reduce patient discomfort. Follow-upAstigmatism is an ever-changing variable. Bruce Grene advises never believing the results of a refractive surgical procedure until the 12-month postoperative gate is reached. The surgeon most likely will follow these patients for many years, and they may present years later for repeat surgery. Surgery completed at the limbus may be repeated as necessary, and no reports have surfaced from difficulties with repeat limbal incisions. For excellent patient education resources, visit eMedicine's Eye and Vision Center. Also, see eMedicine's patient education article Vision Correction Surgery. COMPLICATIONSSection 8 of 10
Relatively few complications of well-performed AK exist. It is one of the safest procedures in ophthalmology. Foreign body sensation may be present for a few days. The use of a thick artificial tear 6 times a day (eg, Celluvisc) will take care of this problem. Overcorrection can be addressed easily by the placement of 1-2 10-0 Vicryl sutures in the excessively flattened hemimeridian. If overcorrection is noted on topography or refraction any time in the month after surgery, close the incision with the Vicryl sutures. The sutures will dissolve in time after their function is complete and do not need to be removed. Undercorrection can be addressed easily by 2 methods. Remeasure the pachymetry in the area of the incisions to see if the incisions may be too shallow. If pachymetry measures significantly thicker after surgery recut the original incisions at the greater depth. If the depth seems to be adequate, recut the incisions and extend the length of each incision by increments of 10° to enhance the effect. Late regression of the result can be observed. Incisions at the limbus are closer to the blood supply than incisions at a smaller optical zone and thus may have a greater tendency to regress. If late regression occurs, recut the same incision. Treat the patient with topical steroids used 4 times per day for 4-6 weeks after the procedure to inhibit the tendency to heal too aggressively. One of the most serious possible complications of AK is a decrease in best-corrected vision caused by using too small of an optical zone. Optical zones as small as 4.00 mm have been advocated in the past. This problem has been eliminated completely by moving the optical zone to the limbus. MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
Astigmatism, Astigmatic Keratotomy excerpt Article Last Updated: Mar 14, 2006 | ||||||||||||||||||||||||||||||||||||||||||
