Surgeons discuss how to address astigmatism during cataract surgery
Correcting astigmatism is an important part of cataract surgery, especially as the goals of modern cataract surgery have evolved to mirror that of refractive surgery, responding to heightened expectations of patients.
“Today, a patient’s goal is not just more clarity of vision with correction, but a more focused vision with little to no reliance on correction. Astigmatism, even as low as 0.5 D, impacts the outcomes with a perceivable effect on UCVA,” Healio/OSN Board Member Neda Shamie, MD, said.
In her opinion, the “entry point” into the modern-day approach to cataract surgery is to prioritize optimizing uncorrected visual acuity, which at the least requires the surgeon to consider addressing the patient’s corneal astigmatism.
“Addressing presbyopia with premium lens options may be considered a more ‘advanced’ option in modern cataract surgery, but astigmatic correction is an essential step toward optimized surgical outcomes. We have the tools and the technology to offer better outcomes for our patients and should embrace the opportunity to do so,” she said.
According to the degree of severity, astigmatism can be corrected by incisional techniques, toric lenses or, the newest option, the Light Adjustable Lens (LAL, RxSight). Excimer laser surgery may be used, if necessary, as a secondary enhancement procedure.
“The first goal is to assess corneal astigmatism through keratometry measurements and corneal topography/tomography. The second step is to determine how to address it. If it is less than about 1 D, limbal relaxing incisions (LRIs) can be performed in a safe and predictable manner using a validated nomogram. Beyond 1 D, LRIs become unpredictable and can cause corneal neuropathy, dry eyes or destabilize the cornea. When the corneal astigmatism measures more than 1 D against the rule or 1.5 D with the rule is when you have to consider lens-based correction,” Shamie said.
Toric lenses can correct up to 4 D of astigmatism, and with the pre- and intraoperative imaging modalities, intraoperative alignment tools and advanced IOL technology that are now available, the success rate has increased significantly, she said.
“And now there is also the LAL, which provides even better accuracy in correcting astigmatism as the light adjustment can correct up to about 2.5 D of astigmatism as measured postoperatively when the refraction has stabilized. This removes any concerns about unpredictable preoperative measurements, unpredictable effective lens position, shift in corneal measurements and rotation of a toric IOL,” Shamie said.
Limbal relaxing incisions
Incisional astigmatic treatment is the best option for low astigmatism of 0.75 D or less, according to Healio/OSN Section Editor Uday Devgan, MD. It is also the only option in the United States, where toric lenses that correct less than 1 D, such as the AcrySof IQ toric SN6AT2 (Alcon) and Tecnis toric ZCT100 (Johnson & Johnson Vision), are not available, “and for good reasons,” he said.
“With such a low degree of astigmatism, it is hard to find the precise axis, and the different measurement systems may give somewhat different answers. In addition, whatever size of incision you do, it has an astigmatic effect of 0.2 D to 0.5 D. If the patient has 0.75 D of astigmatism, steep at 180°, and you make your 2.75-mm phaco incision at the 180° meridian, then the patient may end up with just 0.25 D of astigmatism. No need for a toric IOL then,” Devgan said.
A first step to deal with low astigmatism is to perform the phaco incision on the steep axis, which is temporally for against-the-rule astigmatism, which is common in cataract patients. This has an immediate flattening effect. Paired with a small LRI, the effect will be greater.
“At the end of your case, with the IOL in the capsular bag, give a little more anesthesia with a sponge soaked in tetracaine. It is easy to think of LRIs with clock hours: 1 clock hour is 30°. I also use a fixation ring that is marked in 30° increments, which I line it up exactly with the reference mark and then perform a small arc with the LRI blade. At the end, I make sure that incision and LRI are totally sealed and watertight,” Devgan said.
This procedure is simple and comfortable for the patient, with recovery identical to regular cataract surgery.
The nomogram for LRIs developed 20 years ago by Kevin Miller, MD, is still the easiest and best, according to Devgan.
“Use a 500 µm depth blade, stay perpendicular, glide it smoothly for 1 clock hour, and with paired LRIs, you’ll correct about 0.7 D,” he said.
Alternatives to two paired LRIs are a phaco incision plus a single LRI or paired phaco incisions. The second phaco incision in this case is performed at the end of the case, opposite the original incision, with the IOL in the capsular bag and the eye still full of viscoelastic.
“It is a very easy and effective technique. You don’t need any additional instrument, just the keratome, and you perform the two incisions opposite to each other, both on the steep meridian of the corneal astigmatism. In most cases, doing the paired incisions for against-the-rule astigmatism corrects about 0.75 D and doing it for with-the-rule astigmatism will correct about 1 D — a very effective technique,” Devgan said.
Although 20% to 40% of patients with cataract and astigmatism are eligible for toric IOLs, only 7% receive them. A survey among members of the American Society of Cataract and Refractive Surgery showed that 95% of expert surgeons agree that treating astigmatism is critical, yet only 20% of their eligible patients are receiving toric IOLs.
“Some of the barriers are out of our hands, like those related to cost if patients may decline the recommendation if they are unable to afford the financial burden of an advanced lens. However, there are surgeons who simply don’t offer advanced lens options even when indicated, as in a patient with significant corneal astigmatism, as they may have not had experience in their training, may not appreciate or believe in the benefits to the patient, may have a misperception about a steep learning curve, or may want to avoid any increased education of their staff or an increased chair time with the patient,” Shamie said.
In reality, there is not much of a learning curve with a toric lens implant and not even much additional chair time, she said. Once astigmatism is assessed by corneal topography, most such patients do not need much explanation about astigmatism because they are typically familiar with the concept as it has been corrected by their glasses or contact lenses previously. They understand, accept and usually welcome the opportunity to treat their astigmatism at the time of their cataract surgery.
There are less common scenarios in which a patient’s corneal astigmatism has been essentially negated by their natural lenticular astigmatism such that their refractive error before cataract surgery has had little refractive astigmatism, Shamie said. These typically require a bit more chair time explaining the need for a toric when the patient insists that they have never had astigmatism. These cases are rare but even more reason to consider correcting the corneal astigmatism as these patients tend to be the most disappointed when the cataract surgery done with a monofocal lens implant leaves them with more astigmatism than before surgery.
The surgical technique modification is focused on steps to minimize the postoperative rotation of the toric IOL. An important added step is to irrigate and aspirate any ophthalmic viscosurgical device from behind the IOL optic and seat the IOL into the capsule with the lens rotated on axis.
“Using a silicone I/A tip, the risk of capturing or rupturing the capsule during the step of OVD removal from behind the optic is minimized,” Shamie said. “Leaving the eye on the softer side may also help keep the IOL from rotating, presumably as the capsule may ‘hug’ the optic and haptic of the IOL.”
Rotational stability is of primary importance with toric lenses. For every degree of rotation, 3% of cylinder power is lost, and realignment may be needed when rotation from the target axis is more than 10°.
“Fortunately, this is not a frequent event. It is reported in the literature in 0.65% to 3.3% of the cases and can be prevented by performing correctly the two maneuvers explained above,” Shamie said.
Small degrees of rotation might be addressed with LRIs, LASIK or PRK if necessary, or simply with spectacles or contact lenses.
To be successful with toric IOLs, careful preoperative assessment is required to make sure that corneal astigmatism is correctly measured, stable and regular, Nicole Fram, MD, said.
“We need to examine each patient with topography and/or tomography and look specifically at the Placido images to evaluate the quality of the surface,” she said. “Next, correlation with biometry to assess reproducibility of K readings is essential. Once we can confirm there are at least two to three diagnostics measuring the same power and axis of astigmatism, we can then use an online calculator to help plan for the toric IOL.”
Fram said the Barrett toric calculator is her preferred calculator for all posterior chamber IOLs.
“The most recent advancements in the online calculator have an integrated keratometry readings option. For years, we have looked at different K readings and gained a gestalt of the clinical picture. The integrated keratotomy option standardizes this approach. Once the data is uploaded, the calculator generates an integrated keratometry reading, which represents the mean of two or the median of three data points. When I am on the fence, looking at what the patient wore in their glasses prior to the cataract development can be helpful. The Barrett toric calculator is available at the ASCRS website. The calculator then allows the surgeon to accurately determine the power and axis of astigmatism necessary to reach our goals,” she said.
“For years we have been looking for at least two coinciding measurements, and now we can integrate them into our IOL calculation for astigmatic correction. Then you click on the button to calculate, and you have recommendations for any IOL as long as the A-constant is put in there accurately,” Fram said.
The Barrett calculator also makes a theoretical adjustment for posterior corneal astigmatism, a common cause, if not taken into account, for wrong IOL power and residual astigmatism.
“The Baylor group has demonstrated that the posterior cornea tends to have an against-the-rule contribution to the total astigmatism. This can cause us to overcorrect when we are correcting with-the-rule astigmatism and undercorrect when we are correcting against-the-rule astigmatism,” Fram said. “Using a calculator that accounts for this or intraoperative aberrometry that looks at total aphakic refraction can help us avoid these pitfalls.”
Positioning the lens
IOL misalignment is another cause of poor outcomes, more often due to wrong positioning from the start rather than postoperative rotation, in Fram’s opinion. Accurate corneal marking is a critical step for IOL axis orientation. It can be performed in an “old school” fashion by manual marking, which is still a good method in experienced hands. However, digital systems have freed surgeons from the dependence on marking and are able to provide a high degree of accuracy, Fram said.
“Intraoperative aberrometry with the ORA (Alcon) tells us where the steep axis is and at the same time enables us to measure the aphakic refraction. After the IOL is inserted and rotated into position to match the ORA recommendations, all the viscoelastic is removed from behind, and a second reading is taken to check the lens position. The eye should be pressurized, and the main incision should not be overhydrated. The first reading tells us the aphakic refraction and power recommendation, and the second pseudophakic reading helps the surgeon align the IOL position. Once the ORA tells us there is less than 0.5 D of residual astigmatism and no further rotation is recommended, you can finish the case and know you are on axis,” Fram said.
She also recommended performing a stress test at the end of the case, putting in either balanced salt solution or moxifloxacin to make sure the IOL does not wiggle off axis. If it does, take out more of the viscoelastic and make sure the IOL is not rotating. Going behind the lens to remove viscoelastic can add to stability in toric IOL cases.
Other digital marking systems include the Callisto, Verion and Lensar IntelliAxis. With the Callisto system, a reference image taken with the IOLMaster 700 (Carl Zeiss Meditec) is fed into the surgical microscope.
“You can see the blue marks on the steep axis, and you simply rotate the IOL to match them,” Fram said.
The Verion is a similar image-guidance system integrated in the Argos biometer (Alcon) and sending directly into the LenSx laser or a heads-up display. Finally, the Lensar IntelliAxis refractive capsulorrhexis system allows for precise axis marking directly into the capsulotomy. It has integrated Cassini and Pentacam technologies to find the steep axis based on iris registration.
Light Adjustable Lens
The LAL, which optimizes vision after implantation and healing, is the new frontier for astigmatic correction.
“It simplifies our preoperative decision-making, it spares us from looking at different topographies trying to figure out where we should align a toric lens, and it delivers better vision because we can precisely tailor the refractive power after the patient has healed,” OSN Refractive Surgery Section Editor John P. Berdahl, MD, said.
Adjustment after surgery makes a difference because surgeons cannot control healing, he said.
“Since we cannot control healing, we can only do our best to predict effective lens position. And if that lens is farther forward, patients end up farsighted, and if it is farther anterior, they end up nearsighted,” he said.
In addition, due to the unpredictable effects of healing on the eye, IOL rotation might occur, affecting the predicted outcomes of toric lenses.
Surgically induced astigmatism also depends on many factors that evolve unpredictably with healing and is therefore inconsistent. Looking at the published literature, the mean rate of surgically induced astigmatism is low but with a high standard deviation. What this means is that in some cases surgery induces more than 0.5 D of cylinder with perceived visual consequences.
About 40% of the premium lenses Berdahl implants in his practice now are LAL. He started within the FDA trial and continued to implant them after commercial rollout in July 2019.
“I find them most effective in post-refractive patients, in patients who are either good candidates for monovision or are prior monovision patients, and in patients with astigmatism. It has become my toric lens, with the exception of eyes that don’t dilate well or where the pupil is too small,” he said.
Predicting the future is easy but being right is hard, he said. What physicians have been trying to do so far with preoperative measurements is predict what that future outcome is going to be, hoping that they will end up with a plano manifest refraction.
“Now we can use the manifest refraction to make it plano. Just waiting a few weeks for the eye to heal up and then implanting the lens, you are going to have significantly better outcomes,” he said.
The LAL is implanted in the eye after standard cataract surgery, and the patient wears a pair of UV-blocking glasses for about 3 weeks until the eye heals. Then, by irradiation of UV light, polymerization of the silicone monomers in the lens matrix is induced, changing the curvature and power of the lens in a customized pattern.
“We see the patient back at least three times to achieve the desired power and optimal vision. During that time, the patient still wears the protective spectacles. Finally, we perform the ‘lock-in’ procedure. It is definitely more demanding in terms of postoperative time than a standard IOL, but it is worth it. Patients see spectacularly well with this lens, up to 12.5/20. The FDA data clearly shows that UCVA with the LAL is significantly better than with a monofocal lens, and cylinder is a lot less, 0 D in most cases,” Berdahl said.
When planning for cataract surgery, surgeons should always highlight the importance of correcting astigmatism, Shamie said.
“Patients may not be aware of how much the astigmatism can impact their satisfaction with surgical results, and we must spend time talking about this, especially to ensure that the patient’s expectations are addressed and considered. Residual astigmatism can cause significant visual disturbance. The subset of patients in which this is most critical are those who are getting multifocal or EDOF lenses because in those patients any amount of astigmatism can impact the visual outcome,” she said.
Shamie recently saw a patient who sought her advice after having had cataract surgery with a local surgeon who did not offer a toric IOL at the time of her initial surgery. She had nearly 3 D of corneal astigmatism and was not informed about all of her options, including having her astigmatism corrected. Unhappy with her surgical outcome, she sought care with Shamie, who performed an IOL exchange to replace the monofocal lens with a toric lens, giving her 20/20 uncorrected vision and an outcome that matched her initial hopes for improved vision.
“Not offering the patient a toric lens when it was indicated was a lost opportunity not only for the patient but also for the surgeon,” Shamie said.
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- For more information:
- John P. Berdahl, MD, can be reached at Vance Thompson Vision, 3101 W. 57th St., Sioux Falls, SD 57108; email: email@example.com.
- Uday Devgan, MD, can be reached at Devgan Eye Surgery, 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: firstname.lastname@example.org.
- Nicole R. Fram, MD, can be reached at Advanced Vision Care, 2080 Century Park East, Suite 911, Los Angeles, CA 90067; email: email@example.com.
- Neda Shamie, MD, can be reached at Maloney-Shamie Vision Institute, 10921 Wilshire Blvd., Suite 900, Los Angeles, CA 90024; email: firstname.lastname@example.org.
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