Practice Essentials
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.
Background
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]
History of the Procedure
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.
Problem
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.
Epidemiology
Frequency
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.
Etiology
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.
Pathophysiology
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.
Presentation
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.
Indications
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.
Relevant Anatomy
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
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.
Prognosis
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.
Patient Education
Patient education is important when performing surgical correction of astigmatism. Some patients have some residual astigmatism postoperatively.
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Typical astigmatic keratectomy incisions.
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Wedge resection.
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Corneal marker indicating optical zone diameter and degrees of arc.
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LASIK for myopic with-the-rule astigmatism.
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LASIK for myopic against-the-rule astigmatism.
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LASIK for hyperopic with-the-rule astigmatism.
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LASIK for hyperopic against-the-rule astigmatism.
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Corneal topography of a central island.
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Corneal topographies of decentered myopic ablations.
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A properly aligned reticle. Note the position over the center of the pupil.
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Infectious keratitis following LASIK. Note hyperemia, which typically does not occur in cases of diffuse lamellar keratitis.
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Epithelial ingrowth following LASIK surgery.
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Diffuse lamellar keratitis - Stage 1.
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Diffuse lamellar keratitis - Stage 2.
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Diffuse lamellar keratitis - Stage 3. Note central clumping of interface cells.
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Diffuse lamellar keratitis - Stage 4. Note "cracked" appearance in central cornea following stromal melt.
