eMedicine Specialties > Ophthalmology > Refractive Disorders

Astigmatism, Astigmatic Keratotomy

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
Contributor Information and Disclosures

Updated: Mar 14, 2006

Introduction

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 Procedure

AK 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.

Indications

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 Anatomy

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.

Contraindications

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.

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