Review of Cornea & Contact lenses

RESHAPING THE OUTCOME:

CORNEAL ASTIGMATISM CORRECTION DURING CATARACT SURGERY

Surgeons have more options—and decisions—than ever before.

By Harmohina Bagga, MD, and Kerry K. Assil, MD

Release Date: November 2015
Expiration Date: November 1, 2018

Goal Statement:

This course reviews surgical treatment options for managing astigmatism, including complications.

Faculty/Editorial Board:

Dr. Harmohina BaggaDr. Harmohina Bagga, MD, is a board-certified, fellowship-trained cataract and glaucoma specialist, She joined the Assil Eye Institute in Los Angeles as head of its glaucoma services in March 2015.




Dr. Kerry K. Assil, MDDr. Kerry K. Assil, MD, is regarded as one of the world's foremost experts in refractive surgery, having made significant advances in the field with his numerous inventions. He has trained thousands of eye surgeons in the latest refractive surgical techniques and has authored more than one hundred textbooks, textbook chapters and articles on refractive surgery.



Credit Statement:

This course is COPE approved for 1 hour of CE credit. COPE ID is 47304-AS. Please check your state licensing board to see if this approval counts toward your CE requirement for relicensure.

Joint Sponsorship Statement:

This continuing education course is jointsponsored by the Pennsylvania College of Optometry.

Disclosure Statement:

Dr. Bagga and Dr. Assil have no financial interest in any products mentioned.


A limbal relaxing incision is commonly combined with a toric IOL when the need for astigmatism correction exceeds the toricity of currently available IOLs.
Photo: Uday Devgan, MD

Cataract surgery has evolved from a high-risk medical procedure that once required a hospital stay to the most commonly performed refractive procedure worldwide. Its ubiquity has given surgeons ample opportunity to refine the outcomes with great precision. Advancements in pre-and intraoperative diagnostic testing and surgical techniques, as well as increased availability of premium intraocular lens (IOL) implants, have escalated patient expectations, thus mandating consistently accurate management of pre-existing astigmatism in order to deliver on expectations.

When planning astigmatism correction, consider the magnitude and axis of the astigmatism, corneal pachymetry, incision location, degree of irregular or asymmetric astigmatism, desired entry site, corneal pathology, patient age and the status of the fellow eye. Since most eyes exhibit against-the-rule astigmatism over time, some surgeons advocate an under-correction of with-the-rule cylinder. Furthermore, mild astigmatism of about 1D with or against the rule with a spherical equivalent of -0.5D may aid in expanding the depth of focus.1

Research has suggested that residual with-the-rule astigmatism may favor better uncorrected distance acuity (as most visual stimuli are of a vertical nature), with residual against-the-rule cylinder improving uncorrected near vision.2,3

Measurement modalities also weigh into the equation. Corneal power is typically calculated using a keratometric refractive index of 1.3375D, which is the power of a hypothetical single refractive surface without factoring in the curvature of the posterior corneal surface. Since the advent of slit-scanning topography and Scheimpflug imaging, the impact of posterior corneal astigmatism has been revisited. Because of the opposing power of the anterior and posterior corneal surfaces, the vector difference between the magnitude of keratometric astigmatism and total corneal astigmatism is higher than 0.50D in up to 28.8% of eyes.4 The difference between the location of the steep meridian in keratometric astigmatism vs. the total corneal astigmatism is higher than 10 degrees in more than 17% of eyes.3 However, some reports did not identify this compensatory effect of posterior corneal astigmatism, but rather found that the total corneal astigmatism exceeded the keratometric astigmatism.5

PREOPERATIVE MEASUREMENT

No single method has been identified as being the most accurate for measuring true astigmatism. Though refraction is an important component of preoperative evaluation, the presence of a cataract will call into question its reliability due to subjective error. Accurate keratometry and corneal topography are essential in the planning and management of astigmatism; however, it is important to ensure absence of dry eye, excessive pooling of tears, blinking, fixation errors and misalignment to obtain good quality, reliable scans, since tear film abnormalities can introduce artifactual irregular astigmatism. Commonly used methods to assess preoperative astigmatism include keratometry (optical biometry, manual biometry, or both) and corneal topography (e.g., Placido-based, slit-scan imaging and Scheimpflug imaging). Both measurements need to be assessed and taken into consideration.

The Cassini Corneal Shape Analyzer (i-Optics) is a relatively new device designed to measure anterior and posterior corneal curvature to create a complete picture of the cornea. It uses multicolor LED ray-tracing technology to capture radial and circumferential measurements with an accuracy of less than 2µm, which results in precise corneal axis measurements to within three degrees (as compared with 13 degrees with Placido technology) and the magnitude to within clinical margins (2%).

One of the key elements in the precise management of astigmatism is prevention of the bias induced by cyclotorsion of the eye intra-operatively. The Cassini registers and digitally records conjunctival features and vessels, as well as the exact location of the visual axis and the magnitude of astigmatism. Intra-operatively, the Cassini can link to femtosecond laser-assisted cataract surgery platforms or 3D intraop- erative imaging and guidance to assist with the proper alignment and positioning of the incisions and IOL.

Intraoperative aber-rometry has minimized the guesswork in refining outcomes. The ORA (short for optiwave refractive analysis, Alcon) intra-operative aberrom-eter is a miniaturized wavefront device that attaches onto the bottom of nearly any operating microscope and works using a Talbot-Moiré waveform aberrometer to measure the optical power of the eye at the time of surgery. It takes into account anterior and posterior corneal astigmatism, as well as axial length, to generate IOL power and astigmatism measurements. It has proven to be an essential tool, especially in eyes with previous refractive surgery or in instances where topography and keratometry measurement do not correlate. Studies have reported more than a 50% increase in accuracy with use of the device in eyes with previous myopic LASIK or photorefractive keratec-tomy, compared with the surgeon’s best preoperative choice, the Haigis L formula and the Shammas IOL formula.6

Note, some important guidelines to obtain accurate measurements include ensuring a smooth corneal surface, maintaining physiologic IOP, absence of corneal edema from stromal hydration of the wound, thorough washout of dispersive vis-colelastic from the eye and absence of air bubbles/cortical matter.

TREATING ASTIGMATISM

There are a number of methods to reduce astigmatism prior to and during cataract surgery, depending on the magnitude of astigmatism. The simplest way is to position the cataract incision on the steep axis to induce wound flattening, which results in a decrease in cylinder. This technique works well in patients with pre-existing astigmatism of less than 1.0D7 However, some limitations with this approach include unpredictable healing of the corneal incision with inconsistent results, and difficulty placing the incision at certain axes, especially in patients with prominent brow or nasal bridge. Another technique, limbal relaxing incisions (LRIs), involves the placement of incisions corresponding to the steep meridian, which leads to corneal flattening and a reduction in astigmatic power. The advantage to this approach is the resulting coupling effect, in which the amount of flattening that occurs in the incised meridian is nearly equal to the amount of steepening that occurs 90 degrees away, with little resulting change in spheroequivalent. This obviates the need to make any change in implant power and, when coupled with intraoperative aber-rometry, can be used to treat up to 3.0D of preoperative cylinder.

Several nomograms are available to calculate accurate location and length of LRIs. The length is usually expressed in degrees of arc or clock hours, rather than chord length, to diminish overcorrections and undercorrections for unusually small or large corneas. Practitioners must keep in mind that the goal is to reduce the patient’s cylinder without overcorrecting or shifting the resultant axis. While some surgeons prefer an empiric blade depth setting, our preference is to use adjustable diamond blade depth settings based on pachymetry. Readings should be set at 90% depth of the perilimbal corneal thickness.

Table 1. Guidelines for Treating Astigmatism
  Magnitude of Astigmatism Treatment Options
1 <1.0D Incision rotation on steep axis
2 1D to 3.5D LRI
3 1.5D to 4.0D Toric IOL
4 >4.0D Toric IOL + LRI
  • Final treatment titrated based on ORA measurements.
  • Astigmatism up to 3D to 4D may still be treated with LRI and postoperative excimer laser ablation in patients with multifocal implants.

Patients who have undergone radial keratometry surgery require smaller sized incisions or alternate techniques such as toric IOLs or enhancement with PRK/LASIK to address astigmatism. Our preference is to perform the LRI after inserting the IOL; we use the ORA to confirm magnitude and axis of astigmatism and then re-measure the residual astigmatism, making depth or length refinements based upon the readings. To correct high levels of astigmatism, usually above 4.0D, LRIs may be used in conjunction with a toric IOL or excimer laser surgery. However, success of LRIs can be limited by lack of precision, variable healing response and regression.8-11

Femtosecond laser-assisted arcuate incisions offer the potential advantage of being more precise with improved accuracy and safety and enhanced reproducibility than manual incisions. The LensAR laser system with Streamline is the first femtosecond laser platform to offer wireless integration with the Cassini Corneal Shape Analyzer, allowing preoperative data transfer from the device. The iris registration feature automatically compensates for cyclotorsion and the need to manually mark the eye. Surgeons have the flexibility to use their preferred LRI nomogram. Because of its ability to perform OCT imaging of the cornea, incisions can not only be programmed to an exact 90% depth of the pachymetry readings, but can also be customized more precisely in terms of length and angulation. Additionally, surgeons have the option of titrating the correction of astigmatism by opening the incisions intra-operatively after obtaining measurements with ORA or postoperatively at the slit lamp. However, the wound healing characteristics and sustained, long-term efficacy of femtosecond laser-created incisions still needs to be determined. Additionally, the rate-limiting step for femtosecond laser arcuate incisions is its inability to perform reliable limbal incisions due to frequent presence of corneal opacification from arcus in this region (Table 1).

Toric IOLs are another option for astigmatism correction during cataract surgery, especially in cases of higher amounts of astigmatism. The ideal patients for toric IOL implantation are those with regular astigmatism; however, good outcomes have been reported in cases of mild to moderate irregular astigmatism with fairly symmetrical preoperative readings within the central 4mm cornea.12-14 It is important to take into account the vector sum of the preexisting corneal astigmatism and the surgeon-induced astigmatism when determining the power and meridian of a toric IOL.

Another factor to consider is the impact of effective lens position and of the spheroequivalent power of the IOL on the effective toricity of the IOL at the corneal plane. The effective toricity diminishes with increasing anterior chamber depth and lower IOL spherical power. Some of the nomograms like the Holladay IOL Consultant Program (Holladay Consulting) and the Tecnis Toric Calculator (Abbott Medical Optics) take these factors into account when calculating toric IOL power. Studies have reported comparable correction of astigmatism with LRIs and toric IOLs in mild to moderate astigmatism.15 Since multifocal toric IOLs are not yet available in United States, our preference is to perform LRIs for preoperative astigmatism of up to 3D, especially in combination with multifocal IOLs. We prefer toric IOLs with or without LRIs for astigmatism greater than 3.0D to 3.5D.

Correct intraoperative alignment of the IOL and good postoperative rotational stability are critical factors for achieving the desired astigmatic correction. When a toric IOL is misaligned, there is a reduction in the cylinder correction along the desired meridian and induction of cylinder at a new meridian. Bench studies suggest a 33% loss of astigmatic correction if the optic rotates 10 degrees.16

A limbal relaxing incision created by femtosecond laser. Photo: Walter Whitley, OD

Until recently, marking the eye with the patient in the upright, seated position before surgery to compensate for ocular torsion during surgery was a key step to achieve correct lens position. Even then, there was a higher potential for error since the measurements were generated from the anterior cornea. With intraoperative aberrometry, more accurate measurements, as well as confirmation of the correct axis after IOL implantation, has become possible. This allows intraoperative measurement of residual refractive astigmatism and, consequently, live adjustments of the toric IOL axis. However, as discussed earlier, several intraoperative variables—such as the use of an eyelid speculum, squeezing of the eyelids during surgery and the cataract surgery procedure itself—affect intraoperative measurements of aberrometry.

Postoperatively, the orientation axis of the toric IOL can be verified to confirm that no IOL rotation has occurred. Toric IOL misalignment can be determined by using the slit lamp biomicroscope (with rotating slit and full mydriasis), the postoperative residual refraction and keratometry values and wavefront aberrometry. Web-based calculators are available. Misalignment is determined by using postoperative manifest refraction, the amount of astigmatism at the corneal plane and the IOL axis to determine the magnitude of misalignment, direction of rotation needed and the residual astigmatism after IOL rotation. Realignment of a rotated toric IOL should be performed as soon as possible because adhesions form between the capsular bag and the IOL optic. We prefer to wait about a month, which allows enough time for refraction to stabilize but before fibrotic adhesion between the anterior and posterior capsules and IOL haptics. If IOL rotation is not expected to improve the residual astigmatism to <0.75D or less, our approach is excimer laser ablation or IOL exchange, depending on the magnitude of the refractive surprise and corneal thickness. Occasionally there are unsatisfactory outcomes with residual refractive error after cataract surgery. Secondary options for astigmatism correction include LRI augmentation, IOL exchange, PRK or LASIK.

In summary, cataract surgery offers a second window of opportunity to improve visual acuity with minimal spectacle dependence. Recent innovations in preoperative and intraoperative diagnostic techniques allow a more precise measurement of astigmatism. With careful planning and a treatment plan customized to the individual patient’s needs, expectations and ocular anatomy, it is possible to achieve excellent visual outcomes with cataract surgery.

References

  1. Savage H, Rothstein M, Davuluri G, et al. Myopic astigmatism and presbyopia trial. Am J Ophthalmol. 2003 May;135(5):628-32.
  2. Novis C. Astigmatism and toric intraocular lenses. Curr Opin Ophthalmol. 2000;11:47–50.
  3. Trindade F, Oliveira A, Frasso M. Benefit of against-the-rule astigmatism to uncorrected near acuity. J Cataract Refract Surg. 1997;23:82–5.
  4. Ho JD, Tsai CY, Liou SW. Accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement. Am J Ophthalmol. 2009;147:788–795
  5. Srivannaboon S, Soeharnila, Chirapapaisan C, Chonpimai P. Comparison of corneal astigmatism and axis location in cataract patients measured by total corneal power, automated keratometry, and simulated keratometry. J Cataract Refract Surg. 2012; 38:2088–93.
  6. Ianchulev T, Hoffer KJ, Yoo SH, et al. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology. 2014;121:56–60.
  7. Borasio E, Mehta JS, Maurino V. Torque and flattening effects of clear corneal temporal and on-axis incisions for phacoemulsification. J Cataract Refract Surg. 2006;32:2030-2038.
  8. Kaufmann C, Peter J, Ooi K, et al. Limbal relaxing incisions versus on-axis incisions to reduce corneal astigmatism at the time of cataract surgery. J Cataract Refract Surg 2005;31:2261–5.
  9. Budak K, Yilmaz G, Aslan BS, et al. Limbal relaxing incisions in congenital astigmatism: 6 month follow-up. J Cataract Refract Surg 2001;27:715–19.
  10. Bayramlar HH, Daglioglu MC, Borazan M. Limbal relaxing incisions for primary mixed astigmatism and mixed astigmatism after cataract surgery. J Cataract Refract Surg 2003;29:723–8.