Cataract: Back to basics
September 2022
by Ellen Stodola
Editorial Co-Director
In the spirit of the “Back to basics” theme of this issue, Steven Schallhorn, MD, and Cathleen McCabe, MD, discussed the important topic of identifying astigmatism prior to cataract surgery: how to determine the level of astigmatism, when to address it, and how even low levels may have an impact.
Dr. Schallhorn said the most important thing is taking the time to measure the corneal astigmatism. “That’s the critical component,” he said, adding the clinicians might get tripped up if they don’t see much astigmatism in patients’ glasses and choose to disregard it. “If a patient has very little astigmatism in their prescription, eyecare providers often don’t think about measuring corneal astigmatism, so that is the most important basic element,” he said. “The act of measuring is the most important first step.” A patient can have a diopter or less of manifest astigmatism and have much more than that in corneal astigmatism. The corneal astigmatism primarily determines the post-cataract procedure manifest astigmatism.
Source: Steven Schallhorn, MD
As far as measuring goes, Dr. Schallhorn said there is great technology for this. “We can now measure the total power of the cornea, combining both the anterior and posterior cornea components,” he said. “This is especially important if the corneal shape has been altered, such as patients who have undergone laser vision correction.”
Dr. McCabe said that this is an important topic, particularly because many people find it intimidating. She stressed that one of the most important things is to ensure the health of the ocular surface when performing and analyzing measurements. “I think the simplest thing is to carefully examine the patient. During the cataract evaluation, some practices will do biometry, topography, Ks, and axial length on the same day,” she said. “If you do that, make sure you do it at the beginning before anything has touched the eye. What we decided to do is separate those visits, so we can make sure that every patient is at least using lubricating drops prior to biometry and has a greater likelihood of having an ocular surface that is pristine and not dry.”
For accurate measurements, Dr. McCabe suggested having a way to reliably and accurately obtain the corneal curvature/Ks, noting there are several technologies for this. She also said it’s important to have an idea about the shape of the astigmatism, if it’s irregular, indicating a complex cornea, or regular astigmatism. “You don’t have to get super sophisticated about that, but you do need a picture of the central curvature of the cornea that tells you whether the astigmatism is regular or not,” she said. This can be determined with a topographer or even manual keratometry, looking for clear, sharp, and orthogonal mires. Having this basic information, Dr. McCabe said, helps set surgeons up for successful astigmatism treatment.
Then you have to decide how to treat. There are several reasons that many physicians lean toward putting in a toric lens, if it’s indicated, she said. First, there is long-term stability and predictability of toric IOLs. “We don’t have to factor in healing of the arcuate incision, healing that’s individual to the patient,” Dr. McCabe said. Second, this option doesn’t impact ocular surface health, while arcuate incisions and cutting through the corneal nerves can worsen dry eye in the postoperative period.
For these reasons, Dr. McCabe said she uses a toric lens when indicated, however, she noted that, in the U.S., low power torics are not available. “There are lower levels of astigmatism that I still think are important to treat, especially if we’re putting in a diffractive optic, and in those cases where it’s a lower level of astigmatism, I’ll do arcuate incisions,” she said.
“I think allowing patients to have the best quality of vision at distance is what they find to be most important,” she said. “We can provide an increased independence with excellent distance vision for most patients, and that fundamentally depends on accurate and universally applied astigmatism correction.”
The effect of residual astigmatism was the subject of an extensive study1 in which Dr. Schallhorn participated, looking at different amounts of astigmatism to determine what effect they have.
“Above a relatively low level, you should consider addressing it to achieve the best unaided postoperative vision and maximize patient satisfaction,” he said.
Dr. Schallhorn’s study found that even low levels of postoperative astigmatism can impact unaided vision and patient satisfaction after surgery. This includes 1 D or 0.75 D, but even down to 0.25–0.5 D. He called this conclusion a “wakeup call” for physicians to pay closer attention to corneal astigmatism and how to best address it.
[template id=14580]He also said that industry will play a big role in helping to develop better ways of correcting astigmatism, especially low levels. “Moderate to high levels of corneal astigmatism can be effectively addressed with toric IOLs, but if we want to raise the bar and improve outcomes, we need to refine methods to treat lower levels, both on the clinical side and on the industry side.” For example, Dr. Schallhorn said this could mean further developing and obtaining regulatory approval for toric IOLs for low levels of astigmatism. Whether it’s addressing the astigmatism on the cornea/limbus (incisional techniques, laser vision correction, or other methods) or with a toric IOL, Dr. Schallhorn said it’s important to improve the accuracy and predictability of correcting astigmatism.
Dr. McCabe agreed with the results of this study and said in her own experience, small amounts of residual astigmatism can impact quality of vision, especially when using more sophisticated optics that split light. “I’ve been treating those low levels of astigmatism with my own nomogram until recently when we had more validated nomograms,” she said. “I know low levels of astigmatism will decrease the quality of distance vision with a diffractive optic. If I think it’s universally important to treat in the setting of diffractive optics, then I also think it’s important to address all levels of astigmatism to improve quality of vision at distance with all lenses. Therefore, I treat all low levels of astigmatism when I’m trying to reach a refractive target that allows for independence from glasses. I think that does provide better outcomes than we would get if we were not so focused on reducing residual astigmatism.”
At this point in the evolution of delivering excellent outcomes with cataract surgery, it’s not reasonable to ignore astigmatism, Dr. McCabe said. “It’s a fundamental part of how we can improve the visual function and quality of vision for our patients.”
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A surprising, incidental finding from Dr. Schallhorn’s study was that leaving patients slightly hyperopic led to slightly better outcomes and patient satisfaction. “What it showed, which needs to be studied in greater detail, was that a low level of hyperopia resulted in better uncorrected vision and happier patients than if you leave those patients slightly myopic,” Dr. Schallhorn said. The important caveat is that it was in patients who wanted good distance vision in that eye; of course, this is not for patients in whom you’re targeting myopia.
In those patients where the physician wants to hit zero refractive error and give the best uncorrected distance vision, the findings from the study suggested that leaving patients slightly hyperopic is better than leaving them slightly myopic, he said, further clarifying that this is in reference to when the surgeon is deciding between lens power options with half diopter increments in which the estimated postop refraction straddles emmetropia. Previously, he would default to leaving the patient slightly myopic. He reiterated that this needs to be investigated further to understand in greater detail what it means and how should it drive practice.
The size of the study was its strength, Dr. Schallhorn said. It requires large sets of data to accurately assess patient satisfaction and patient-reported outcomes because of the inherent variability in patient responses.
Dr. McCabe said that she usually aims for as close to plano as possible. “I’ve found that allows the patient to have the best quality of vision,” she said. “Unfortunately, right now, we don’t have a way of targeting in less than half diopter increments of power.” Dr. McCabe said that when it’s within a half diopter, she generally will target closest to plano or a little on the myopic side, but she added that there are certain optics that work better with a little residual hyperopia, like the Synergy (Johnson & Johnson Vision).
About the physicians
Cathleen McCabe, MD
Medical Director
The Eye Associates
Bradenton, Florida
Steven Schallhorn, MD
Professor of Ophthalmology
University of California
San Francisco
San Francisco, California
Reference
- Schallhorn SC, et al. Effect of residual astigmatism on uncorrected visual acuity and patient satisfaction in pseudophakic patients. J Cataract Refract Surg. 2021;47:991–998
Relevant disclosures
McCabe: Alcon, Bausch + Lomb, Carl Zeiss Meditec, Johnson & Johnson Vision, LENSAR, Rayner
Schallhorn: None
Contact
McCabe: cmccabe13@hotmail.com
Schallhorn: scschallhorn@yahoo.com
