October 2018

COVER FEATURE

Revisiting astigmatism
Correcting corneal astigmatism


by Ellen Stodola EyeWorld Senior Staff Writer/Digital Editor




This 68-year-old female wants a higher quality of uncorrected distance vision with cataract surgery of the left eye. The LENSTAR and Placido disc topography show good alignment in the quantitative values, with about 1 D of anterior corneal astigmatism around 165 degrees.


The posterior cornea on the Cassini LED topographer demonstrates a small amount of with-the-rule astigmatism, which ultimately leads to a total corneal astigmatism that is slightly more than 1 D.
Source: Elizabeth Yeu, MD


Experts discuss their preferences for managing corneal astigmatism

Correcting corneal astigmatism can be an important step in obtaining the best result for patients. This may be a factor both in cataract and refractive procedures. Several surgeons discussed how they determine corneal astigmatism, when they choose to correct it, and when they use different techniques—such as toric IOLs, LRIs, and femto AKs—to aid in the correction.
Michael Greenwood, MD, Vance Thompson Vision, Fargo, North Dakota, Brandon Baartman, MD, Vance Thompson Vision, Omaha, Nebraska, Elizabeth Yeu, MD, Virginia Eye Consultants, Norfolk, Virginia, and Kevin M. Miller, MD, chief of the cataract and refractive surgery division, University of California, Los Angeles, discussed the correction of corneal astigmatism.
Dr. Miller said that it is first important to identify whether a patient is presenting for refractive or cataract surgery. We handle these patients differently, he said.
For the refractive surgery patient, Dr. Miller measures the manifest astigmatism using a phoropter. “It doesn’t matter what the corneal astigmatism is,” he said. The manifest astigmatism is what we treat, even if doing so increases the corneal astigmatism.
For the cataract patient, it’s completely different. “We pay attention to what’s in the cornea and precisely measure the corneal cylinder. We can treat the corneal cylinder directly with relaxing incisions if it’s a small amount, or compensate for it by implanting a toric intraocular lens,” he said.
“For cataract patients, we measure the corneal astigmatism and we don’t particularly care about the manifest astigmatism,” Dr. Miller said, adding that you can measure the front corneal surface with a variety of devices and infer the posterior corneal astigmatism, or you can measure the astigmatism on the front and back surfaces directly.
Dr. Yeu said, “We’re very fortunate today that we have access to the Barrett toric calculator, and we have many years of experience and understand how refractive astigmatism affects vision.” She added that it’s important to differentiate between real and induced corneal astigmatism. “Identifying true corneal astigmatism will give us the best guidance in how to fix this at the time of surgery,” she said, adding that leaving more than 0.25 or 0.5 D on the table postoperatively can affect the quality of vision.

Determining and correcting total corneal astigmatism

Diagnostically, Dr. Yeu looks at several things. First, she uses a topographer to assess if it’s a good cornea capture. She uses the LENSTAR (Haag-Streit, Koniz, Switzerland) and captures between a 1.8 and 2.3 mm zone. Dr. Yeu also uses Cassini LED topography (Cassini Technologies, The Hague, the Netherlands) to see what is going on and to get instantaneous capture of the posterior cornea.
In terms of how much she corrects, Dr. Yeu said that particularly for patients younger than 65, she aims to leave residual 0.25 D of with-the-rule astigmatism. “It will protect them as they may drift and will also give them a great outcome,” she said.
For her patients who are 70 or older, she aims to leave them as astigmatically neutral as possible.
Dr. Yeu said her trigger point for treating astigmatism is when there is any cylinder more than 0.2 D, especially if the patient is looking for a refractive outcome. She added it’s important for surgeons to get comfortable doing femto AKs and LRIs.
For Dr. Greenwood’s preoperative measurements, he uses the LENSTAR and topography from the OPD-Scan III (Nidek, Fremont, California) to confirm that it is regular astigmatism. “My goal is to minimize any residual astigmatism, so I aim to correct the total amount of astigmatism,” he said. “I think that 0.50 D or more is visually significant.” Although there are some instruments that can estimate posterior corneal power, Dr. Greenwood thinks the most accurate way to measure total corneal power is intraoperative aberrometry, and for this, he is most familiar with using the ORA (Alcon, Fort Worth, Texas).
Dr. Baartman said when he is evaluating a patient prior to cataract or refractive surgery, he takes into account the topography, Scheimpflug measurement of the posterior cornea, and the corneal wavefront measurements and compares them all to the magnitude and axis of manifest astigmatism.
“When I’m considering toric lens placement, I like the Barrett toric calculator on the ASCRS website,” he said. “The amount of correction I would aim for depends on preoperative patient factors, such as pattern and regularity of the cylinder (e.g., is it with-the-rule or against-the-rule), and the presence of prior keratorefractive surgery or significant ocular surface disease.”
Dr. Baartman said he generally starts considering correction when the patient has manifest cylinder that is present on topography, and his jumping-off point is about 0.5 D. 
Dr. Miller will use a Scheimpflug device to determine total corneal cylinder. He added he has both the Pentacam (Oculus, Wetzlar, Germany) and Galilei (Ziemer, Port, Switzerland) devices. Dr. Miller said a decision will be made on how to proceed after calculating the total corneal astigmatism/cylinder.
He said he will treat any degree of astigmatism, and he likes to use the term “astigmatism management” when correcting small amounts because it’s not always possible to completely correct a patient’s astigmatism.
When determining how much astigmatism to correct, Dr. Miller said it depends on where you want to land. His postoperative “sweet spot” is 0.3 D with-the-rule.

Preference for LRIs, femto AKs or toric IOLs

Dr. Baartman said his choice among LRIs, AKs, and toric IOLs depends on the preoperative characteristics of each patient. “In general, I tend to favor femtosecond laser arcuate keratotomy incisions for patients with 0.5 D to 1.25 D of regular corneal astigmatism, especially if there is no significant contribution of the posterior corneal or lens and it shows up in the patient’s refraction,” Dr. Baartman said.
For patients with greater than 1.0 D to 1.25 D of corneal cylinder, this is when Dr. Baartman starts thinking about toric lenses. “Other patient characteristics I consider are presence of epithelial basement membrane dystrophy or significant ocular surface dryness, and if I’m considering incisional refractive surgery, I always evaluate the posterior corneal curvature to make sure we’re not looking at astigmatism developing from early or otherwise occult corneal ectasia,” he said. The orientation of the preoperative corneal cylinder is also something he considers, and he tends to be more conservative in treating with-the-rule astigmatism compared to against-the-rule.
Dr. Greenwood will base his decision on whether to choose an LRI, femto AK, or toric IOL on how much astigmatism is measured on the LENSTAR.
If it is less than 0.50 D, he does not correct the astigmatism. If it is between 0.5 and 1.25 D, he uses AKs with the femtosecond laser, and if it is greater than 1.25 D, he will use a toric IOL. “I use the Barrett toric calculator to aid in preop IOL selection, but I confirm it in the OR using aberrometry,” he said. “I use the Hill-RBF for non-toric IOL selection.”
Dr. Yeu said that she will always choose a toric if she can, particularly for anyone with more than 0.7 D of against-the-rule or with-the-rule of more than 1.25/1.3 D.
“I always go for femto AKs for a primary case,” Dr. Yeu said, adding that she uses the femtosecond laser at the time of surgery as it helps create a standardized capsulotomy and provides a more predictable refractive outcome.

Accuracy of LRIs and femto AKs

Dr. Greenwood said that he finds femto AKs more accurate in that they are more precise. “Manual techniques are good, but we will never be as precise as the laser, whether it is AKs or capsulotomies,” he said. “Of course, it all depends on how accurate your preop measurements are and how you are marking where to place your incisions.” Dr. Greenwood said that if your marks are not accurate, the placement of the incisions may not be at the axis you initially intended. “This is especially important as the eye can cyclorotate when laying supine.”
Prior to placing the IOL, Dr. Greenwood uses intraoperative aberrometry to help select the IOL power and measure the total corneal astigmatism. “If I did AKs and there is still some residual astigmatism, I can open them to get a greater effect,” he said. “If I chose a toric IOL, I will use the measurements to guide the toric power selection.”
Dr. Baartman said that he likes the accuracy of the femtosecond laser for placing astigmatic incisions at precisely the right position, depth, length, and centration. “On the femtosecond platform, OCT imaging allows me to be very specific in my intended depth of 90%, and I can feel confident in its safety,” he said. “I also appreciate the ability to open or not open the incisions, and I often make that decision with the use of intraoperative aberrometry.”

Advantages of toric IOLs

Dr. Baartman thinks toric lenses have the advantage of being able to treat larger amounts of refractive astigmatism without making large incisions on the cornea. “This can come into play when treating patients with ocular surface disease or a history of keratorefractive surgery, when incisional refractive surgery may not be best for the patient,” he said. “Particularly in those patients with larger amounts of cylinder, you have the ability to rotate a toric lens or employ an excimer laser to touch up residual astigmatism.” Ultimately, Dr. Baartman said that this helps preserve more of the corneal tissue and natural strength compared to strictly using an incision or keratoablative refractive surgery.
The biggest advantage is that they can correct higher amounts (greater than 1.5 D) of astigmatism compared to AKs or LRIs, Dr. Greenwood said. “I also like torics in the lower astigmatism range (1.0–1.5 D) because we are less dependent on patient healing and the response of the cornea to the incisions,” he said. “The other great thing about toric IOLs is with the intraoperative aberrometry I can take a measurement after I have made my cataract incisions and get real time feedback from the aberrometry to show me the steep axis and exactly where to place the IOL.” Physicians have learned that for each degree they are off axis, they lose 3% of the power of the toric, Dr. Greenwood said, adding that this may not be much on a lower powered toric, but in a high powered toric, that can be a big difference if off just a few degrees.

Editors’ note: Dr. Greenwood has financial interests with Alcon. Dr. Baartman, Dr. Miller, and Dr. Yeu have no financial interests related to their comments.

Contact information

Baartman
: brandon.baartman@vancethompsonvision.com
Greenwood: michael.greenwood@vancethompsonvision.com
Miller: kmiller@ucla.edu
Yeu: eyeulin@gmail.com

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