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; Antonio Pascotto, MD, Consulting Ophthalmologist, Pascotto, Istituto per la Salute degli Occhi, Clinica Mediterranea, Italy; Sergio Claudio Saccà, PhD, Professor of Ophthalmology, Department of Neurological and Visual Sciences, Ospedale San Martino, Italy; Mauro Fioretto, MD, Professor of Ophthalmology, University Eye Clinic of Genova; Head of Ophthalmology Department, Hospital of Casale Monferrato, Italy; Vincenzo Orfeo, MD, Head, Operating Unit, Clinica Mediterranea, Naples, Italy
Contributor Information and Disclosures

Updated: Oct 21, 2009

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. AK 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 Thornton, Buzard, Price, Grene, Nordan, and Lindstrom.^1,2,3,4,5

History of the Procedure

Corneal transplant surgery and radial keratotomy surgery both stimulated the development of astigmatic keratotomy.

Troutman, in writing about corneal transplant astigmatism, discussed the benefits of corneal relaxing incisions to decrease residual astigmatism.⁶ When radial keratotomy was first developed, it was discovered that radial incisions had very little predictable effect on astigmatism. A profound astigmatic effect occurred only when the direction of the incisions was turned 90 degrees.

Troutman’s work included the development of wedge resection for very high astigmatism. While wedge resection helped in cases of perhaps greater than 10 diopters of astigmatism, many patients with lesser degrees of astigmatism still needed help. The donor-recipient interface creates, in essence, a new limbal architecture. The same rules apply to incisions made inside the donor-recipient interface as to the untouched normal limbus.

Four schools of thought developed.

Nordan proposed a relatively simple method of straight transverse keratotomy, with target corrections in the range on 1-4 diopters.⁴

Lindstrom developed a technique, as well as a nomogram, including an age factor.⁵ This technique eventually evolved into the Astigmatism Reduction Clinical Trial (ARC-T) study.³

Thornton’s technique involved making paired arcuate incisions, following a curve on the cornea, with differing optical zone sizes.¹ Incisions were generally placed at the 7.0 mm and 8.0 mm optical zones. Multiple pairs of incisions were included on his nomogram. Others, including Chayez, Chayat, Celikkol, Parker, and Feldman, have recommended optical zone sizes as small as 5.0 mm.

Nichamin developed an extensive nomogram for AK at the time of cataract surgery; however, the popularity of toric intraocular lenses (IOLs) has diminished the frequency of AK at the time of cataract surgery.

Indications

AK is helpful with a wide range of refractive problems. AK may 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 physiologically similar to limbal relaxing incisions. Patients with corneal transplants may be afflicted with irregular astigmatism. Although most congenital astigmatism is regular astigmatism, following transplantation, one quadrant may be especially steep or flat, and this is known as nonorthogonal astigmatism. It may occur where a segment of the donor-recipient interface has healed too tightly or where it has inadvertently slipped. AK, used in conjunction with high-quality corneal topography, allows an individualized approach to each case.

In addition to its usefulness in patients who have undergone corneal transplantation, AK may be helpful for patients who suffer from mixed astigmatism. When a patient requests vision correction surgery and his or her refractive error has a spherical equivalent approaching zero (eg, -1.00 + 2.00 X [any axis]), LASIK may not be necessary.

Corneal relaxing incisions couple. The concept of coupling means that when a tangential or arcuate incision is placed, two events occur. Along the meridian of the incision(s) and central to the incision(s), flattening of the cornea occurs. Commensurately, the meridian 90 degrees away steepens. The combination of flattening of the steeper axis with steepening of the flatter axis yields the total amount of astigmatism correction.

In the patient who has undergone previous LASIK surgery, AK offers an option. If the patient develops significant astigmatism in the years following his or her procedure, AK may be preferable to lifting a well-healed flap.

For patients presenting for LASIK with high astigmatism, the combined treatment of LASIK with AK may provide a more satisfactory result than LASIK alone. Since the minor axis of astigmatism treatment with the excimer laser 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. This may make the laser correction more precise.

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 1 diopter of topographical astigmatism should be considered for AK at the time of their cataract surgery.

The use of AK becomes more important when using multifocal IOLs because good simultaneous uncorrected near and distance vision can be obtained only with a nearly spherical cornea. At the time of this most recent revision, toric multifocal IOLs are not available in the United States.

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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