Module 2: Aberration/ Astigmatism Management

Aberrations — especially astigmatism — are common in patients with cataracts, which is why it is so important to correct them with whatever means possible. Managing aberrations goes beyond simply marking the patient and properly aligning a toric IOL. Here, Karolinne Maia Rocha, MD, PhD, shares her expertise in aberration and astigmatism management, and how surgeons new to refractive cataract surgery can achieve patient satisfaction.

Q. What is the most important aspect of aberration and astigmatism management that surgeons new to refractive cataract surgery should understand?

Karolinne Maia Rocha, MD, PhD: The days of leaving astigmatism and other aberrations uncorrected after cataract surgery are over, especially in terms of premium cataract surgery. From first-generation multifocal IOLs to the latest extended depth of focus (EDOF) IOLs, it has become ever more important for surgeons to minimize aberrations and correct refractive error. Even residual astigmatism as low as 0.5 D or 0.75 D can significantly degrade a patient’s visual quality postoperatively.

Q. How do aberrations affect quality of vision?

Rocha: The point at which light rays meet the pupil is called the eye’s wavefront. In a perfect eye free of aberrations, light rays travel through the eye and meet at a singular point on the retina. Deviations of this ideal pattern are called wavefront errors. Aberrations are measured in root-mean-square magnitudes, which decrease with pupil size and are limited by diffraction. Wavefront aberrations are divided into lower-order and higher-order types. Lower-order aberrations, such as defocus and astigmatism, are easily corrected with IOLs, glasses or contacts, whereas higher-order aberrations, such as coma and spherical aberration, require greater intervention.

Because the eye is a closed system, examining light rays as they enter the pupil is a difficult task. Therefore, simulators are the surgeon’s best tool to understand how aberrations affect patients’ vision. Adaptive optics simulation helps clinicians to both generate and measure the eye’s aberrations.

My colleagues and I published a study on the effects of Zernike wavefront aberrations on visual acuity using adaptive optics technology.¹ When generating higher-order aberrations — especially those near the center of the Zernike pyramid, such as coma, spherical aberration and secondary astigmatism — we found that they caused greater distortion of visual quality than the trefoil, quadrafoil and pentafoil aberrations at the periphery. We also know that correcting higher-order aberrations can enhance visual performance by improving retinal image quality, and patients can gain lines of vision.

Q. In terms of patient selection, in whom would you avoid implanting a premium IOL?

Rocha: It is important for surgeons to consider total higher-order aberrations and look for coma and spherical aberrations first, as patients with significant coma may not be good candidates for some of the premium IOLs, including multifocal, diffractive bifocal and EDOF IOLs. As a rule of thumb, I would not recommend multifocal IOLs for patients with aberrations higher than 0.5 µm in a 6-mm pupil. In addition, patients with irregular astigmatism are typically not candidates for premium IOLs.

Q. Can aberrations be beneficial in some patients? If so, how?

Rocha: Specific amounts of spherical aberration can increase depth of focus. In a separate study published in 2009, my colleagues and I again used an adaptive optics simulator to generate positive and negative spherical aberrations, and determined that spherical aberration can extend depth of focus up to 2 D. However, spherical aberrations higher than 0.6 µm for a 6-mm pupil size will affect visual quality.²

Q. How much does previous refractive surgery influence corneal aberrations and your overall astigmatism management plan?

Rocha: Myopic ablations from past refractive surgery can change the shape of the cornea to become more oblate, thus inducing positive spherical aberrations. For these patients, I recommend surgeons implant a monofocal IOL with negative spherical aberrations, which will greatly enhance modulation transfer function and provide high-quality distance vision.^3,4 However, patients will require glasses for near and intermediate vision.

Hyperopic ablations, on the other hand, create a hyperprolate corneal shape that generates negative spherical aberrations. These patients are great candidates for neutral, aberration-free IOLs to take advantage of the enhanced depth of focus that negative spherical aberrations can provide. At the 2017 American Society of Cataract and Refractive Surgery Annual Symposium and Congress, my colleagues and I presented a study of patients who had undergone laser anterior ciliary excision.^5-7 Patients with highly aberrated corneas who received aberration-free IOLs had good distance and near vision, or pseudoaccommodation, because of residual higher-order aberration.

Finally, refractive surgery may cause decentered ablations that can possibly result in coma. As mentioned above, surgeons should be wary of coma, as patients with severe cases may not be candidates for multifocal IOLs.

Q. What methods do you suggest surgeons use to measure astigmatism before refractive cataract surgery?

Rocha: A wealth of technology and toric calculators are available to measure astigmatism, so knowing where to begin can be challenging for new refractive cataract surgeons.

Surgeons should no longer rely solely on measuring anterior corneal curvature, as this will rarely provide an accurate snapshot of total astigmatic error. I highly recommend surgeons use a toric calculator that accounts for both anterior and posterior corneal curvature, as these calculators provide the most accurate measurements for total corneal astigmatism, in my experience.

Koch and colleagues published two studies in 2012 and 2013 on how posterior corneal astigmatism contributes to the accuracy of toric power calculations.^8,9 Using the Galilei combined Placido-dual Scheimpflug analyzer (Ziemer) to measure only anterior corneal curvature, eyes that had with-the-rule astigmatism tended to be over-corrected by an average of 0.5 D. In addition, eyes that had against-the-rule astigmatism were under-corrected by an average of 0.3 D. These errors may seem insignificant, but even minimal over- and under-corrections can have detrimental effects on postoperative visual quality. Therefore, refractive cataract surgeons should make every effort to avoid refractive surprises.

I rely heavily on the Baylor nomogram, which measures anterior and posterior corneal astigmatism. According to the Baylor nomogram, the threshold for implanting a toric IOL is 1.7 D in eyes that have with-the-rule astigmatism, and only 0.4 D in eyes that have against-the-rule astigmatism, especially if spectacles have more against-the-rule astigmatism. I always double-check my calculations using the ASCRS online toric calculator that incorporates the Barrett toric algorithm. In general, I tend to recommend a toric IOL for patients with 1.5 D or more of with-the-rule astigmatism, and I will perform a limbal relaxing incision (LRI) for patients with less with-the-rule astigmatism. Likewise, my cutoff for recommending a toric IOL is 0.5 D or more of against-the-rule astigmatism.

Q. How much does surgically induced astigmatism (SIA) affect overall outcomes, and how can surgeons keep it under control?

Rocha: Many factors affect SIA, including the patient’s age; existence of preoperative astigmatism; corneal biomechanics; and the type, size and location of incisions. Edema around the incision and sutures can also induce astigmatism. To reduce SIA, I recommend using small incisions and no sutures. I place my incisions on the steepest axis as determined by topography, and may even add an LRI on the opposite side of the incision as necessary.

Using vector or polar analysis, SIA calculators can help surgeons measure their personal SIA by subtracting a patient’s preoperative keratometry readings from the postoperative readings. This resulting SIA is added to the next patient’s preoperative keratometry readings to predict his or her postoperative results.

Q. What level of postoperative astigmatism is considered acceptable? At what level does it become clinically significant?

Rocha: Some IOLs are more tolerant of residual astigmatism than others. The new Symfony toric EDOF IOLs (Johnson & Johnson Vision [formerly Abbott Medical Optics]) feature a diffractive echelette design that can handle some residual astigmatism at distance, but at the possible expense of losing some near vision. Multifocal and diffractive bifocal IOLs do not tolerate residual astigmatism as well, and astigmatism as low as 0.5 D to 0.75 D can affect visual acuity.¹⁰

Q. What is your preferred toric IOL, and do you recommend surgeons use it to correct astigmatism and other aberrations?

Rocha: My premium-channel practice has increased significantly since adopting Symfony toric IOLs because most patients with astigmatism also want good vision at all distances, rather than a monofocal toric option. I certainly recommend surgeons use this IOL, but with the precaution that EDOF and other premium IOLs require meticulous measurements, again, without the presence of irregular astigmatism.

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