You are in: eMedicine Specialties > Ophthalmology > REFRACTIVE DISORDERS Astigmatism, Limbal Relaxing IncisionsArticle Last Updated: Oct 16, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Antonio Pascotto, MD, Consulting Ophthalmologist, Istituto Diagnostico Varelli, Clinica Mediterranea, Napoli, Italy Coauthor(s): Sergio Claudio Saccà, PhD, Professor of Ophthalmology, Department of Neurological and Visual Sciences, Ospedale San Martino, Italy; Mauro Fioretto, PhD, Professor and Program Director, Department of Ophthalmology, Ospedale Santo Spirito, Casale Monferrato (AL), Italy; Vincenzo Orfeo, MD, Head, Operating Unit, Clinica Mediterranea, Naples, Italy Editors: Richard W Allinson, MD, Associate Professor, Department of Ophthalmology, Texas A&M University Health Science Center, Scott and White Clinic; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute, Florida; 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: LRIs, peripheral corneal relaxing incisions, corneal incisions, corneal relaxing incisions, CRIs, refractive error, diseases of refraction, refractive disorders, AK, astigmatic keratotomy, radial keratotomy, RK, arcuate keratotomy, Arc-T incisions, image distortion, blurring, corneal curvature, curvature of the cornea, corneal astigmatism, lenticular astigmatism, scleral limbal corneal incision, SLIC, surgical treatment of astigmatism, opposite limbal relaxing incisions, OLRIs, pre-existing astigmatism, PEA, cataract surgery, astigmatism correction, phacoemulsification, refractive surgery, surgically induced refractive change, SIRC INTRODUCTIONSection 2 of 12
  Astigmatism is present when the cornea is not spherical; that is, it is steeper in one meridian than in the meridian 90° away. The cornea with astigmatism may be thought of as being shaped like a football rather than being shaped like a basketball. Limbal relaxing incisions (LRIs) are a modification of astigmatic keratotomy (AK), a procedure to treat astigmatism. LRIs are placed on the far peripheral aspect of the cornea (the limbus), resulting in a more rounded cornea (see Image 1). Astigmatism is reduced, and uncorrected vision is improved. LRIs can correct astigmatism up to 8 diopters (D); however, the use of LRIs is routinely reserved for 0.5-4 D of astigmatism. LRIs can be performed either at the time of cataract surgery or as an independent procedure. Although LRIs are a weaker corrective procedure compared to corneal relaxing incisions (CRIs), LRIs produce less postoperative glare and less patient discomfort. In addition, these incisions heal faster. Unlike CRIs, making the incision at the limbus preserves the perfect optical qualities of the cornea. LRIs are also a more forgiving procedure, and surgeons often get excellent results, even with early cases. History of the ProcedureFor a century, ophthalmologists have known that modifications in the curve of the cornea result in a change in the power of vision of this central window of the eye. Initially performed on injured eyes, this finding prompted Dutch and Japanese ophthalmologists to create incisions in healthy eyes to improve vision. During the mid-1970s in Russia, Fyodorov began treating myopia (nearsightedness) in patients using radial or spoke cuts through 90-95% of the cornea. Technically referred to as radial keratotomy (RK), this procedure weakened the peripheral cornea and caused it to balloon in shape, thereby flattening the central cornea (see Image 1). Results, although not as accurate as laser correction, were fair but occasionally unstable, both visually and anatomically. LRIs are a modification of RK and are often performed with RK or cataract surgery. Arcuate keratotomy (Arc-T incisions) is a similar procedure to RK but is used only to correct astigmatism. This procedure involves the use of small relaxing incisions around the midperiphery of the patient's cornea. Arc-T incisions are commonly used for patients with only astigmatism or with high degrees of astigmatism. ProblemAstigmatism is the inability of the cornea to properly focus an image onto the retina, resulting in a blurred or distorted image. This occurs because of an improperly shaped cornea. The dome-shaped cornea is the outermost part of the eye. It is a transparent layer that covers the iris (the colored part of the eye), the pupil, and the lens. The cornea bends light and helps to focus it onto the retina. Specialized cells (photoreceptors) in the retina detect light and transmit nerve impulses via the optic nerve to the brain where the image is formed. Any incorrect shaping of the cornea results in an incorrect focusing of the light that passes through that part of the cornea. The bending of light is called refraction, and focusing problems with the cornea are called diseases of refraction or refractive disorders. Usually, the cornea is spherically shaped, like a basketball; however, in astigmatism, the cornea is elliptically shaped, like a football. An astigmatic cornea has a long meridian and a short meridian. Generally, these 2 meridians have a constant curvature and are perpendicular to each other (regular astigmatism). Irregular astigmatism may have more than 2 meridians of focus, and they may not be 90° apart. Therefore, a point of light passing through an astigmatic cornea has 2 points of focus, instead of 1 sharp image on the retina, resulting in blurry vision. What the blur looks like depends upon the amount and the direction of the astigmatism. A person with myopia or hyperopia (farsightedness) may see a dot as a blurred circle; a person with astigmatism may see the same dot as a blurred oval or a frankfurter-shaped blur. Some cases of astigmatism are caused by problems in the lens of the eye. Minor variations in the curvature of the lens can produce minor degrees of astigmatism (lenticular astigmatism). In these patients, the cornea is usually spherically shaped. The advent of phacoemulsification, foldable intraocular lenses (IOLs), and improved incision designs has decreased the incidence and extent of surgically induced astigmatism in patients with cataracts. FrequencyResearch on the causes of refractive errors has become an active field of study during the past few years. In a Montes-Mico study to evaluate the prevalence of astigmatism in infancy and childhood, the overall prevalence of astigmatism (³1.00 D of cylinder) decreased in relation to increasing age (2-12 y), from 44.3% to 5.2%. These results do not vary from those observed in other populations. Such factors as race, nutrition, and environmental condition did not vary from the literature regarding the prevalence of astigmatism in infants and children. Cataract surgery has been shown to cause astigmatism. The smaller the incision, the less the astigmatism induced following cataract surgery. Mean induced astigmatism following phacoemulsification varies from 0.55-1.29 D. According to the Swedish Cataract Register, extracapsular cataract surgery has been associated with higher levels of astigmatism (mean of 1.26 D) compared to phacoemulsification (mean of 0.55 D) in the same series. Approximately 15-20% of patients with cataracts have more than 1.5 D of keratometric astigmatism, refractive astigmatism, or both (Hoffer, 1980). EtiologyRegular astigmatism can be caused by the following: the weight of the upper eyelid resting on the eyeball that creates distortion, surgical incisions in the cornea, trauma or scarring to the cornea, the presence of tumors in the eyelid, or a developmental factor. Scarring or keratoconus can cause irregular astigmatism. PathophysiologyMost people have some degree of astigmatism, which is usually present at birth and is believed to be hereditary. Astigmatism can also result from ocular injuries or conditions (eg, keratoconus, chalazion). Astigmatism tends to occur with other refractive disorders; for example, about one half of patients with myopia are also astigmatic. ClinicalThe main symptom of astigmatism is blurring. Patients may experience headaches and eyestrain. Parents may notice possible astigmatism in a child when the child can see some parts of a pattern or a picture more clearly than other parts (eg, lines going across may seem clearer than lines going up and down). INDICATIONSSection 3 of 12
LRIs have gained widespread acceptance among cataract surgeons. LRIs are often combined with a cataract operation to reduce preexisting astigmatism, thereby resulting in better postoperative vision without glasses. LRIs can be used in individuals whose primary refractive error is astigmatism. RELEVANT ANATOMYSection 4 of 12
The keratometry (K) reading, performed with a keratometer (see Image 2), is useful in determining indications for surgery. In diopter readings, the higher number is the steepest. Determining the nature of the corneal toricity is important, whether the astigmatism is with-the-rule, against-the-rule, or oblique and whether the astigmatism is corneal or residual. In with-the-rule astigmatism, the vertical meridian is steeper than the horizontal meridian; in against-the-rule astigmatism, the horizontal meridian is steeper than the vertical meridian. Advances in corneal topography have greatly facilitated the understanding of corneal toricity. Differences in power graphically represent the differences in corneal contour not only along the principal meridians but also along the entire central cornea, demonstrating the differences in asphericity between the principle meridians. CONTRAINDICATIONSSection 5 of 12
Relative contraindications to LRIs are as follows:
WORKUPSection 6 of 12
Lab Studies
Imaging Studies
Other Tests
Diagnostic Procedures
TREATMENTSection 7 of 12
Surgical TherapyAfter numbing the patient's eye with anesthetic drops, the procedure can be completed within a few seconds. The procedure is safe and is not associated with glare or starburst, which often occurs with true corneal incisions (eg, RK, AK). Furthermore, the cornea is usually stable within a week, indicating that visual fluctuations have typically resolved by that time. The forgiving nature of this procedure is because of the placement and the length of the incision. Placing the incision precisely on axis is not as critical because the incision is 9 mm in length. Since the incision produces less effect than CRIs, significant overcorrections are rare. The amount, the axis, and the symmetry of the corneal cylinder are determined by keratometry and topography. The refractive cylinder is also considered in phakic patients. The surgical keratometer is used to confirm results. The globe is fixated with the modified Fine-Thornton ring, with a diamond blade placed just inside the limbus (see Image 7). The trifacet tip of the diamond blade allows for better control when making the incision; side-cutting edges allow for clockwise and counterclockwise cutting ability. The appropriate incision length is achieved by visually following the degree marks on the ring. A diamond blade should never be held against the side of the degree gauge because it causes damage to the blade and affects the quality of the incision. LRIs are made using a micrometer knife (see Image 8). For most patients, a blade setting of 600 mm is used; however, a blade setting of 500 µm is recommended for patients older than 80 years and for those patients with corneoscleral thinning. LRIs are placed in the steep axis at the limbus, just anterior to the palisades of Vogt. A 6-mm incision is required for each diopter of astigmatism up to 2 D; to correct astigmatism between 2-3 D, LRIs of 8 mm in length are used. For astigmatism greater than 4 D, LRIs are combined with CRIs to attain adequate correction. LRIs are used to correct the first 4 D; the remaining astigmatism over 4 D is corrected by CRIs. Compared to older patients, younger patients require longer incisions to achieve the same effect. Planning the strategy around corneal topography, keratometry, and surgical keratometry is always important. The same rules used for phakic patients apply to pseudophakic patients. However, the axis and the amount of astigmatism in pseudophakic patients are determined by refraction only. Topography is used to determine the symmetry of the astigmatism. Preoperative DetailsPatients receive extensive education and counseling about the procedure from both the staff and the ophthalmologist. Performing routine medical tests before LRIs is unnecessary because such tests do not increase the safety of this procedure. Preoperative medical tests can be ordered if indicated by a finding on the history or during the physical examination, even if a patient is not scheduled for surgery. Refractive error, current vision prescription, and best-corrected vision are determined. The outside of the eye is examined for infections, eyelid abnormalities, or tear duct problems. Intraocular pressure is checked to rule out glaucoma. The curvature and the thickness of the cornea are measured. The length, number, and depth of the incisions can be determined using the Gills and Gayton nomograms. This is a starting point nomogram that titrates surgery by length and number of LRIs. However, the length and placement can vary based on topography and other factors. The goal is to reduce cylinder power and to absolutely avoid overcorrecting with-the-rule astigmatism, because against-the-rule astigmatism must be minimized. In cases with 0.5 D or less of cylinder, only an astigmatically neutral cataract incision is used. Intraoperative DetailsThe scleral limbal corneal incision is generally used for cases of with-the-rule astigmatism and low against-the-rule astigmatism. When using the scleral limbal corneal incision in conjunction with against-the-rule astigmatism, the LRI can be moved slightly into the cornea, or, alternatively, the LRI could be placed opposite the scleral limbal corneal incision. For patients who have with-the-rule astigmatism or oblique astigmatism, the scleral limbal corneal incision is made temporally, and the LRIs are placed at the steep axis. The placement of the LRI should be customized to the topography. In cases of asymmetric astigmatism, the LRI in the steepest axis can be elongated slightly and then shortened the same amount in the flatter of the 2 steep axes. Paired LRIs do not have to be made in the same meridian. If the topography reveals nonorthogonal astigmatism, each LRI is placed at the steepest portion of the bow tie. Patients with low ( <1.5 D) against-the-rule astigmatism (180°) receive only a single LRI in the steep meridian, placed opposite to the cataract incision. However, if astigmatism is greater than 1.5 D, a pair of LRIs must be used. In against-the-rule astigmatism cases, one pair of LRIs may be incorporated into the cataract incision. The length of the LRI is not affected by the presence of the cataract incision. In low with-the-rule astigmatism cases, a single 6-mm LRI (0.6 mm in depth) is made at 90°. The LRI can be independent of the cataract incision in with-the-rule astigmatism cases (if the cataract incision is temporal and the LRI is superior). A 6-mm relaxing incision generally corrects about 1 D of astigmatism for a 73-year-old patient. For more astigmatism, paired LRIs are used. If the patient has 2-3 D vision, the pair of LRIs can be extended up to 8 mm. Four diopters can often be corrected or substantially reduced with a pair of 10-mm LRIs. LRIs can be effective in substantially reducing astigmatism even in high astigmatism cases. Two LRIs can be placed and lengthened to 10 mm to correct the first 4 D. CRIs may then be added to provide additional correction. When used, CRIs are at 99% depth at the 8- or 9-mm optical zone, 2 mm in length for every diopter over 4 D. Added CRIs are reserved only for cases with higher levels of preexisting astigmatism, being careful not to overcorrect with-the-rule astigmatism cases. A conservative approach to astigmatism is always better. Making a long incision at the limbus is better than making a small relaxing incision on the cornea. Contributing pathology, such as corneal scarring or keratoconus, is often present in cases of high astigmatism. In these cases, the surgical effect is less predictable, so be conservative in planning surgery, preferring undercorrection. The quality of vision can be improved substantially in these high astigmatism cases, even with residual cylinder. Undercorrections are likely to occur; overcorrections are unusual. If an overcorrection occurs, the LRI can be sutured without creating any irregular astigmatism. LRIs lend themselves to enhancement. For example, if an undercorrection occurs with a single 6-mm LRI, this incision could be extended to 8 mm or a second LRI could be performed. Postoperative DetailsPatients usually experience little, if any, postoperative discomfort. Other than a scratchy eye sensation during the first day, patients report no adverse effects. Antibiotic drops are used 4 times a day until the epithelium heals. A series of mild-to-moderate astigmatic cases ( <3 D) were analyzed to evaluate the efficacy of the surgical approach discussed above. The mean preoperative keratometric cylinder of 3.82 D was reduced to a mean of 1.64 D postoperatively. Mean percent reduction of keratometric cylinder was 58%. All cases were reasonably improved, most without the use of CRIs. Of those cases, 71% had 20/40 or better uncorrected vision. Follow-upRefraction may fluctuate for about 2 months, and, at that time, enhancement may be performed if necessary. A conservative approach to surgery is best to avoid overcorrection. COMPLICATIONSSection 8 of 12
In spite of the complications outlined below, the chance of having this surgery without complications is 99%. If complications do develop, treatment is possible, and the final result is generally excellent. Surgical perforation of the cornea If a microperforation occurs during the incision, the procedure may be completed later. If a microperforation occurs, a suture may be required for closure. However, with AK or LRIs, perforation is very uncommon because of the relative thickness of the cornea. Damage to the iris and the lens could theoretically occur in a perforation, but it is extremely rare. Infection If the incision becomes infected and particularly if a perforation occurs, the eye is at risk of endophthalmitis (an infection involving the structures inside the eye). This condition requires intensive treatment with topical; local; and, in some cases, systemic antibiotics. A risk of complete vision loss exists following endophthalmitis. To minimize the risk of infection, full sterile precautions are taken and prophylactic antibiotic eye drops are used. Overcorrection and undercorrection The exact outcome is always uncertain because of the variability in individual healing response. Some patients may be overcorrected, while other patients are undercorrected. Further surgery may be necessary to reverse overcorrection or undercorrection. Instability of refraction In the immediate postoperative period, refraction fluctuates until about 2 months. Occasionally, a hyperopic shift occurs over several years. Changes in atmospheric oxygen may cause changes in the cornea, and increasing hyperopia may occur. This finding has been reported in mountaineers. These changes are more noticeable in patients who have undergone RK than in patients who have undergone LRIs. Weakening of the cornea Obviously, if the cornea has several deep incisions, it may be more prone to rupture if a direct blow is received. This finding has been reported in patients who have undergone RK but not in patients who have undergone simple AK or LRIs. OUTCOME AND PROGNOSISSection 9 of 12
Although LRIs are easy to perform, it is a surgical procedure, and, thus, the outcome cannot be guaranteed. This treatment may even be considered investigational by some medical professionals and their associates because long-term adverse effects are not yet known. As with any surgical procedure, LRIs should be undertaken only after careful consideration of the likelihood of success and the possible risks. Thorough professional advice from a qualified eye surgeon is required before any eye treatment is undertaken. Predictability can be defined in several ways, but a percentage approach in achieving visual goals is favorable, with 20/20 uncorrected vision being ideal and 20/40 uncorrected vision being acceptable. Uncorrected vision of 20/40 allows driving without glasses. The percentage of patients with an uncorrected visual acuity of 20/40 is 90-95%. The safety of the procedure is judged on the basis of the chance of a possible complication. Separating expected adverse effects of surgery and healing from complications is important. Some individuals can have a poor or excessive healing response. Immediately after surgery, some patients experience discomfort, which is usually controlled through the use of medications. Serious complications are extremely rare. Infection is the most worrisome complication, and, fortunately, it usually can be eliminated with antibiotics. Other possible complications include overcorrection or undercorrection. Most complications are treatable with either medications or further surgery. According to a recent study performed by Arraes et al, in a group of 16 patients, a significant reduction in preoperative astigmatism was observed in the first postoperative month in 2 (opposite sides) LRIs of the 6-mm group (57%, topographic astigmatism; 87%, refractional) and in 2 (opposite sides) LRIs of the 8-mm group (50%, topographic astigmatism; 65%, refractional), and the reduction was maintained with no significant alteration until the 12th postoperative month. The 1 side LRI of the 6-mm group did not yield a significant reduction in astigmatism, but no significant alteration occurred until the 12th postoperative month. In addition, no complications, such as postoperative discomfort, glare, anisocoria, diplopia, incision infection, and corneal thinning or ectasia, occurred. In a study performed by Kaufmann et al, patients having 1.5 D or more of keratometric astigmatism and having cataracts were randomly assigned to 2 surgical techniques: on-axis incisions (OAIs), consisting of a single clear corneal cataract incision centered on the steepest corneal meridian, or LRIs, consisting of 2 arcuate incisions straddling the steepest corneal meridian and a temporal clear corneal incision. Six weeks postoperatively, the flattening effect was 0.41 D (median and interquartile range, 0.15-0.78 D) in the OAI group and 1.21 D (range, 0.43-2.25 D) in the LRI group (P=0.002). After 6 months, the flattening effect was 0.35 D (range, 0.00-0.96 D) and 1.10 D (range, 0.25-1.79 D), respectively (P=0.004). These results demonstrated that the amount of astigmatism reduction achieved at the intended meridian was significantly more favorable with the LRI technique, which remained consistent throughout the follow-up period. FUTURE AND CONTROVERSIESSection 10 of 12
LRIs may have a better role in the future. The immediate recovery of vision observed after LRIs and the excellent quality of vision achieved have proved advantageous to many patients. Two LRIs of 8 mm and 6 mm that aimed to correct preoperative astigmatism of 2-3 D and 1-2 D, respectively, were found to be safe and effective, with a stable effect in the first postoperative follow-up year, according to Arraes et al and others. The 1 LRI of 6 mm that aimed to reduce 1 D of preoperative astigmatism was not effective, but it did not induce any significant postoperative complications. One frustration with any of the refractive surgical procedures now in use is that 10% of patients have residual refractive errors outside the optimal range; that is, their vision may be undercorrected or overcorrected postoperatively. In these patients, to fine-tune their vision to 20/40 or better, the same procedure can be repeated or a different procedure may be performed. A new procedure under investigation involves the use of the holmium laser to correct low degrees of astigmatism. The laser acts as a thermocoagulator, shrinking collagen in the area of treatment and steepening the cornea. LRIs will be a useful option for many patients who are dependent on spectacles or who have poor natural vision. Proper patient selection, counseling, and choice of the appropriate procedure allow eye surgeons to achieve the best results for patients who choose to undergo this surgery. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
Astigmatism, Limbal Relaxing Incisions excerpt Article Last Updated: Oct 16, 2006 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
