October 2018


Revisiting astigmatism
The true measure of astigmatism

by Chiles Aedam R. Samaniego EyeWorld Asia-Pacific Senior Staff Writer

This color LED topography illustrates classic against-the-rule astigmatism in a 70-year-old cataract patient. The anterior cornea measures 0.63 D, but accounting for the posterior
corneal astigmatism, the total cornea measures 0.84 D, making this a toric IOL candidate.
Source: Tal Raviv, MD

Ophthalmologists share their preferred approaches to measuring astigmatism

In preparation for cataract surgery, we owe it to our patients (and to ourselves) to be as accurate as possible when making measurements that will determine their ultimate IOL power and axis,” said Kendall Donaldson, MD, medical director, Bascom Palmer Eye Institute, Plantation, Florida. Thus, the measurement of astigmatism is an essential component of the preoperative evaluation of cataract patients.
Preeya Gupta, MD, Duke Eye Center, Durham, North Carolina, agreed. “Managing astigmatism is an important part of cataract surgery, especially for patients who have high visual needs, meaning they want to see well without glasses,” she said. “If you’re not assessing astigmatism preoperatively, you’re not going to be able to provide your patients with the highest refractive outcomes.”
Vance Thompson, MD, Vance Thompson Vision, Sioux Falls, South Dakota, called astigmatism “the most common refractive error.”
While the goal is to treat the astigmatism intraoperatively, he noted that surgical healing variables and some of the limitations to current measurements mean it is not unusual for patients to need enhancement postoperatively.
In these cases, measuring astigmatism preop serves to reduce the extent to which the refractive outcomes need to be fine-tuned later on, in order to, as Dr. Thompson likes to say, “take the football in for a touchdown.”
EyeWorld reached out to Dr. Donaldson, Dr. Gupta, and Dr. Thompson, as well as Tal Raviv, MD, founder and medical director, Eye Center of New York, to discuss their preferred approaches to measuring astigmatism.

Series of measurements

Beginning with an optical biometer such as the IOLMaster (Carl Zeiss Meditec, Jena, Germany) or LENSTAR (Haag-Streit, Koniz, Switzerland), Dr. Donaldson recommends a series of measurements that includes topography and tomography before refractive cataract surgery in order to “help meet modern day high patient expectations for outcomes after cataract surgery,” she said.
Dr. Donaldson evaluates the typical manifest refraction, their current glasses prescription, and measurements made using the IOLMaster, Pentacam (Oculus, Wetzlar, Germany), and Galilei (Ziemer, Port, Switzerland).
Topography is also a routine part of Dr. Gupta’s preoperative evaluation. “We do a topography on everyone,” she said. “The purpose of topography is to look at the pattern of astigmatism. We want to make sure that it is regular astigmatism as opposed to irregular astigmatism.”
Astigmatism, she said, may have a variety of causes, ranging from dry eye disease, surface features such as pterygium, Salzmann’s nodules, and anterior basement membrane dystrophy, to corneal ectasia, keratoconus, and pellucid marginal degeneration. Treatment in these cases should manage the root cause.
Dr. Thompson reiterated the importance of topography as a routine part of preop evaluation. “I don’t think anyone should go into surgery without a corneal topography,” he said. Conditions such as undiagnosed keratoconus and anterior basement membrane dystrophy affect the quality and reproducibility of the astigmatism measurement, he said.
Dr. Thompson noted that topography reduces the uncertainty of irregular measurements taken by an inexperienced technician who may have the patient stare too long, allowing the tear film to break up before acquiring measurements. “A topography that looks beautiful eliminates all those variables,” he said. Moreover, “if the topographic astigmatism agrees with the keratometry, I know I’m going into surgery with accurate numbers.”
Once the astigmatism has been identified as regular, Dr. Gupta proceeds with biometry, currently using the IOLMaster 700 and the LENSTAR. “I like to use two devices because I like to compare the repeatability and accuracy of the measurements,” she said.

Multiple devices

These doctors’ routines for preop evaluation indicate that no single instrument is enough. According to Dr. Raviv, the routine use of multiple devices to measure astigmatism prior to surgery “speaks to the limitations of our technologies.”
“No keratometry measurement is perfectly repeatable, and inter-device agreement varies considerably,” he said. 
While he said that the accuracy and repeatability of optical biometry has dramatically improved their refractive outcomes, as have the latest generation of IOL formulas such as the Barrett and Hill-RBF, Dr. Raviv admitted that “[t]he variability in our K measurements and inability to accurately measure the posterior cornea still remain a challenge.” 
“In fact, Graham Barrett’s latest formula improves its refractive outcomes by allowing multiple device K readings instead of just one,” he said.
Dr. Raviv routinely uses three different devices—the LENSTAR, Cassini Total Corneal Astigmatism (Cassini Technologies, The Hague, the Netherlands), and a Topcon autorefractor/keratometer (Topcon Medical Systems, Oakland, New Jersey)—and more, if indicated. “For irregular eyes, we’ll also utilize a Placido disc-based topographer.”
Each device has its strengths and weaknesses. “For IOL calculations, LENSTAR or IOLMaster 700 Ks are the best, auto-Ks are pretty accurate,” Dr. Raviv said. “Sim Ks from Placido-based devices are more approximate and shouldn’t be plugged into formulas except in extreme cases such as keratoconus when other keratometers fail. I find that Ks from Scheimpflug topographers are the least ideal for IOL calculations.”
“In an ideal situation, all of these measurements will be consistent,” Dr. Donaldson said. “However, in some cases … the measurements will differ. In those cases, we need to determine why there is a discrepancy and treat the underlying cause. The most common cause is ocular surface disease.”

Ocular surface matters

“As part of our standard cataract evaluation, my technicians perform an Ocular Surface Disease Index questionnaire, tear osmolarity, and MMP-9 test,” Dr. Donaldson said. “This allows us to assess both signs and symptoms of ocular surface disease in preparation for surgery. In addition, if measurements are inconsistent or if irregularities are detected during topography or tomography, the patient should be brought back for additional measurements to ensure reliability before surgery.”
In case of ocular surface disease, Dr. Donaldson initiates treatment and brings the patient back for follow-up evaluation a few weeks later.
“I think it’s important when you’re assessing astigmatism to not forget the ocular surface,” Dr. Gupta said. “At my clinic I routinely look at osmolarity and meibography, but a simple thing that you can also do is use a little fluorescein strip to highlight any corneal staining that might alter the pattern of the astigmatism.”
Dr. Thompson also cautioned against using measurements from untreated dry eye, but added that surgeons and their technicians should think about acquisition time; as noted earlier, the technique used to measure astigmatism using any device may introduce variables. “It’s not only about the ocular surface but also about technician comfort and speed of acquiring information so the tear film doesn’t break up and negatively affect our measurements,” he said.
Again, the use of multiple devices can highlight the presence of ocular surface problems. Dr. Raviv noted that variable measurements among different devices may indicate one of three possibilities: “poor wettability” of the cornea from ocular surface disease or dry eye; an irregular corneal surface that destabilizes the overlying tear film; or a subtle ptosis, enough to create variability between blinks. 
“I address variable Ks from ocular surface disease by priming the ocular surface (with preservative-free tears, steroid drops, LipiFlow [Johnson & Johnson Vision, Santa Ana, California], IPL [intense pulsed light], immunomodulator, as indicated) and repeating measurements a few weeks later,” he said. “With epithelial basement membrane dystrophy patients, there is the option to perform a superficial keratectomy, especially if an advanced technology IOL is requested.”

Adjustability and true measurements

In the future, Dr. Donaldson looks hopefully toward IOLs that are adjustable postoperatively and that will be less dependent on preoperative measurements. The Light Adjustable Lens (RxSight, Aliso Viejo, California), for instance, already allows some surgeons to fine-tune their postoperative refractive results.
Postop adjustability certainly makes preop astigmatism measurement “less impactful, but at the same time we want to go into surgery with the most accurate measurement so we either don’t need to adjust IOL power postoperatively or it’s a smaller adjustment,” said Dr. Thompson, who has experience with the LAL as a principle investigator in the RxSight United States FDA monitored clinical trials.
Dr. Gupta also thinks that preop astigmatism measurement will remain valuable despite the ability to adjust postop. Among other things, LALs are only currently available as a monofocal. Preop astigmatism management will thus remain critical for patients who need presbyopia-correcting IOLs. In addition, there may be patients who are unable to come back and forth for additional measurements and for the lock-in procedure after LAL implantation.
More critical developments may come in terms of how physicians measure astigmatism through the entire cornea. “There’s a key concept called total corneal astigmatism, which is a combination of the anterior and posterior corneal astigmatism,” Dr. Gupta said. “We are still looking for that device that gives us true total corneal astigmatism … potentially even have devices that measure the true value as opposed to an approximated value.”
At the moment, devices only provide posterior corneal astigmatism measurements based on demographic assumptions, Dr. Thompson said. Future technologies will hopefully give surgeons true, direct measurements of both the anterior and posterior corneal astigmatism.
Dr. Raviv said that he sees work toward improving posterior keratometry and total corneal astigmatism measurements being done via color LED and swept-source OCT. In addition, more topographers are beginning to noninvasively measure tear breakup time and other dynamic tear film-related variables, “which may help us know when to trust our Ks and when not to, as well as better monitor our dry eye treatments.”
In any case, Dr. Donaldson said, “in the shorter term, cataract surgery is becoming more customized, requiring extensive attention to detail and high-level, time-consuming preoperative evaluations to help meet the expectations of a population that thinks that cataract surgery will provide them vision that compares with LASIK and other corneal refractive procedures of the 21st century.”

Editors’ note: Dr. Donaldson has financial interests with Alcon (Fort Worth, Texas), Johnson & Johnson Vision, and Bausch + Lomb (Bridgewater, New Jersey). Dr. Raviv has financial interests with Cassini Technologies and Johnson & Johnson Vision. Dr. Thompson has financial interests with Johnson & Johnson Vision, Alcon, Avedro (Waltham, Massachusetts), Bausch + Lomb, RxSight, and Carl Zeiss Meditec. Dr. Gupta has no financial interests related to her comments.

Contact information

: kdonaldson@med.miami.edu
Gupta: preeya.gupta@duke.edu
Raviv: TalRaviv@EyeCenterofNY.com
Thompson: vance.thompson@vancethompsonvision.com

The true measure of astigmatism The true measure of astigmatism
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