Best astigmatism management calls for complex treatment strategies

Among refractive errors, astigmatism poses the greatest challenge to surgeons. Correcting an irregular shape requires complex treatment plans for lasers, precise alignment strategies for IOLs and nomograms for relaxing incisions. Surgeons must take into account that long-term results might be jeopardized by the natural repair mechanism of the cornea, which leads to “filling in” irregular gaps, inducing regression and the potential threat of IOL rotation.

Often associated with other refractive errors, astigmatism requires a great deal of thought and never has a straightforward treatment, according to Ugo Cimberle, MD. However, technological advances have improved the chances of addressing astigmatism with precision and safety. Corneal topography, wavefront aberrometry and optical coherence tomography allow mapping of astigmatism and the creation of customized treatment plans, while new laser eye trackers with dynamic cyclotorsion compensation allow for precise delivery of laser ablation.

IOL technology offers premium toric models with increasingly higher performance. Many methods for accurate sizing and alignment of IOLs have also been developed.

Toric IOLs are a mainstay in astigmatism treatment and are preferable to incisional methods, according to surgeon Lisa B. Arbisser, MD.

“I’ve been using the toric IOL since the day it was approved,” Arbisser said. “When you have truly regular astigmatism of any significant magnitude, I prefer to go to a toric implant solution.”

The Alcon AcrySof Toric IOL was the first toric IOL approved by the U.S. Food and Drug Administration (FDA) in September 2005.

In addition, the femtosecond laser promises to revive incisional techniques. However, the surgeon’s mind remains crucial to coordinate, combine, correlate and interconnect information, according to Cimberle.

“If we find the right formula, we can give our patients a quality and clarity of vision they would never expect. Well-treated astigmatic patients are the happiest patients,” he said.

Paradigm shifts in laser surgery

Astigmatism is ubiquitous in refractive subspecialty practice and involves a variety of visual symptoms that require detailed analysis.

“Association of astigmatism and spherical refractive error is present in 90% of my laser-treated patients. In 15% of the cases, it is high astigmatism, above 2 D,” Cimberle said.

Off-axis wavefront aberrations, such as asymmetric astigmatism and coma, which may be present with a large angle kappa, have a high negative impact on visual quality. Blurry vision and image distortion may also be associated with asthenopic problems, such as headache, dizziness and fatigue.

“Any astigmatism above 1 D should be considered significant enough to have treatment,” Cimberle said.

In a population of 3,654 people between the ages of 49 and 97 years, investigators for the Blue Mountains Eye Study found that the prevalence of astigmatism increases with age, doubling its rate in people older than 80 years.

“About 30% of my cataract patients have astigmatism,” Günther Grabner, MD, said. “The sheer number of patients who will profit from astigmatic treatment is huge because, starting from 0.7 D, it reduces your uncorrected distance vision significantly.”

Cimberle defined astigmatism as “the optical transposition of an irregular corneal morphology that we name toricity. The section is an ellipse, often not asymmetric because the four hemimeridians have different curvature gradients.”

Corneal topographers and wavefront aberrometers, he said, have introduced a paradigm shift in the way astigmatism is evaluated.

“We can now see and analyze the morphology of the cornea, while in the past we could only address its optical transposition. We can also do precise wavefront analysis. Combining topography and aberrometry, we can produce accurate, individualized treatment plans for the laser,” Cimberle said.

Laser surgery for a high degree of astigmatism was first confronted with the problem of regression, Michael Assouline, MD, said.

“The cornea has a memory of shape, and when you change the curvature of one meridian too much, it tends to go back to where it was. With modern technologies, this problem has been overcome. The new laser algorithms have enabled us to produce ablation profiles with smooth transition zones and to work simultaneously on both meridians,” he said.

A second problem with using older lasers is that it is difficult to align the anatomical axis of astigmatism during ablation.

“This has been greatly improved with the use of eye trackers with capabilities for limbal and iris recognition, as well as pupil recognition,” Assouline said.

Iris registration has helped make alignment more accurate, according to Elizabeth A. Davis, MD, FACS, OSN Cataract Surgery Board Member. The Visx laser platform (Abbott Medical Optics) includes software that enables customized treatment, she said.

“In laser vision correction, there’s always a chance of under-correction for astigmatism,” Davis said. “Alignment of the astigmatic lens, I think, has been helped with iris registration. I use the Visx and we have that capability.”

However, iris registration is set at the beginning of a LASIK procedure and, thus, does not actively follow any instances of significant cyclotorsion, according to Davis.

“Every now and then you’ll see [cyclotorsion] in a patient, but large amounts are not overly common,” she said.

Managing cyclotorsion

Compensating for static and dynamic cyclotorsion is critically important in astigmatic correction, according to Massimo Camellin, MD. Torsional movements, particularly in hyperopic astigmatic patients, can be up to 15° or 20°. Positioning errors can add a further 10°, leading to a possible 30° of rotation.

“With a 30° rotation, there will be no astigmatic correction. Surgical error should never be beyond 5° for good results,” Camellin said.

Precise topography mapping is the first step to achieve this level of accuracy. The patient’s head needs to be perfectly vertical with no rotation, the light in the room has to be adequately calibrated as required by the topographer and at least four consecutive measurements must be taken to achieve the lowest standard deviation, Camellin said.

“Topographic data are then translated into wavefront data. Individual aberrations need to be isolated and quantified separately,” he said.

Low-order aberrations, ie, sphere and cylinder, are included in the treatment algorithm, and compensation by internal aberrations is assessed.

The nomogram needs to be further adjusted to include the patient’s subjective refraction, carefully measured by using small increments, with both natural and dilated pupils, according to Camellin.

“Optical zone diameter and transition zones are then set accordingly,” he said. “Low-toricity corneas can be addressed by smaller-diameter ablations, but large optical zones are generally better because we need to distribute the treatment over the entire cornea to obtain a regular profile.”

According to Assouline, one of the pitfalls in laser correction of astigmatism is related to the size of the optical zone.

“For high degrees of cylinder, since you are removing a different amount of tissue in the steep and flat meridians, the optical zone will be oval, with the small axis on the horizontal meridian. In some patients, especially hyperopes, the fixation point will not be very well-centered in this small optical zone because of the angle kappa. If you don’t take this into account, you’ll have severe problems with quality of vision,” he said.

Another issue with optical zone size might occur with LASIK because, if the hinge is positioned on the longer diameter of the ablation, the periphery might be undercorrected, again with a significant impact on quality of vision and efficacy of correction due to occurrence of coma aberration.

Surface ablation with PRK or LASEK might therefore be better indicated than LASIK. Regression can be effectively prevented with the use of mitomycin C, Assouline said.

Both Cimberle and Camellin use the Schwind Amaris laser platform for astigmatic correction.

Grabner performs LASIK with the iFS femtosecond laser (Abbott Medical Optics) and the Mel 80 excimer laser (Carl Zeiss Meditec), and he is in the process of acquiring the Schwind laser.

Phakic lenses

Camellin said he has gradually reduced the range of correction he performs with laser.

“Over 8 D of myopia, I now prefer phakic lenses, and I never treat with laser hyperopic astigmatism over 3 D or 4 D. Steepening a flat cornea leads to creation of a peripheral step, which ends up being filled by the epithelium,” he said.

Spectral-domain OCT now has facilities for epithelial mapping, allowing for close monitoring of epithelial reaction and the prevention of regression with corticosteroids. However, phakic lenses are a safer option in these patients.

“I use the Visian toric ICL (STAAR Surgical) with success,” Camellin said.

Assouline also uses the Visian toric.

“It is very comfortable, predictable and accurate in the correction of even high degrees of astigmatism. If accurately sized, it is stable in the sulcus. It doesn’t rotate, which is, of course, extremely important with toric lenses,” he said.

Cimberle uses the Visian ICL, but not the toric model.

“I prefer avoiding problems that may come from axis alignment. For high degrees of correction, which may not be safely treated with laser, I implant the spheric lens, wait 2 to 3 months, and then address the astigmatism and any residual spherical error on the cornea with laser because the astigmatism is ‘on the cornea.’ Should I have to remove the ICL at any time to do cataract surgery, I’d have the astigmatism already treated. These patients are young and I don’t need to achieve the full result with just one treatment. As a matter of fact, there are quite a few of my patients who are so happy just after the phakic lens correction that they don’t want me to laser them,” he said.

Davis gave the Visian toric phakic ICL (STAAR Surgical) and Verisyse phakic IOL (Abbott Medical Optics) high marks; however, neither implant is FDA-approved.

Toric IOLs for cataract surgery

Toric IOLs are more reliable than other astigmatism correction methods, according to Davis.

“I like toric IOLs better,” she said. “They’re much more predictable in terms of outcome. You also don’t see the regression of effect that you can get with limbal relaxing incisions.”

In patients who need cataract surgery, toric IOLs are an option that has encountered high appreciation among surgeons.

“With just one treatment, they provide the advantages of a clear lens, astigmatic correction and even some near vision because the toric component helps gain a small degree of multifocality. They are very good lenses and work extremely well,” Cimberle said.

“Toric IOLs are the true premium lenses, in my opinion. They give amazing results. A variety of them are available; some are customizable, like the Tecnis toric (Abbott Medical Optics) and the At Lisa (Carl Zeiss Meditec). I use both but prefer At Lisa because the plate haptics allow for better stability,” Camellin said.

Grabner uses toric versions of the AcrySof (Alcon), the Tecnis, the Rayner T-flex and the HumanOptics IOLs.

“All of them are good lenses with different injectors. I don’t really have preferences,” he said. “I implanted a fair number of them in patients with astigmatism more than 1.5 D who want to get rid of glasses. However, I tend not to use them in very old patients who have had glasses all their life. I prescribe new bifocal spectacles, and they are happy, unless they want to play sports like skiing, golf or use the bicycle a lot.”

Arbisser said she uses the AcrySof and Tecnis toric IOLs. However, both lenses have pros and cons, she said.

“There are trade-offs with either lens,” Arbisser said. “I’m a believer in blue filtering, which you can’t have with the AMO [Tecnis].

The Tecnis has less chromatic aberration and fewer glistening than the AcrySof lens, she said.

“I do believe that glistenings cause forward scatter, which doesn’t necessarily affect the patient’s vision,” Arbisser said. “I think they are both based on the same principle and should work well. One is on the front of the lens, the other on the back of the lens. Whether that makes a difference, I don’t really know, optically.”

In addition, the Tecnis has a square posterior edge and rounded anterior edge, whereas the AcrySof has an anterior square edge.

“There’s really no reason to have an anterior square edge,” Arbisser said. “The purpose of the square edge is to prevent [posterior capsule opacification].”

Arbisser expressed a desire for the Sulcoflex IOL (Rayner) to be available in the U.S.

“We desperately need a better sulcus lens,” she said. “The Sulcoflex is certainly popular in Europe. It even has multifocality and could potentially be used to add multifocality for pseudophakes.”

Alignment and stability are crucial issues with toric IOLs. A 10° axis deviation reduces astigmatic correction by one-third, 20° by two-thirds, and lens misalignment greater than 30° will increase the net astigmatic error. Manufacturers are working to further improve features to reduce misalignment.

“Toric IOL markings that should help us align the lens are not expanded enough toward the center to be of comfortable use with small pupils. Manufacturers should think that not all patients are well-dilated, and it would be good to have markings that extend more toward the center. Sometimes the haptics give you a clear indication of where the lens is but not all of the haptics,” Assouline said.

He also noted that very similar designs do not behave in the same way for stability. Lens material and volume, as well as overall length, are likely to contribute to the different outcomes.

“The ideal lens would have a hydrophobic surface and hydrophilic core, high volume and long length to make sure that the design will indent into the periphery of the capsule in the equator and be stable. But maybe a smaller lens will induce less rotation because it puts less pressure. We don’t know yet what the winning combination is,” Assouline said.

A wide variety of new technology platforms, such as Callisto eye (Carl Zeiss Meditec), ORange (WaveTec Vision) and iTrace (Tracey Technologies), are available to assist surgeons in IOL alignment.

Cimberle and Camellin use their own topography-based methods.

“I take a photo on the slit lamp, carefully orientating the patient’s head. I take blood vessels as reference points and then, using a program I developed with CSO, which superimposes a goniometer to the cornea, I re-track these points on the topography map and draw the lens axis. This allows me to keep within a maximum of 5° error,” Camellin said.

“On the slit lamp, I draw four reference points at the limbus. I retrace these points on the topography map, see where the toric axis is, measure the angle and know exactly where I must place the lens,” Cimberle said. “The Casia OCT (Tomey) uses a similar system, but it’s easy to do it just with a topographer that visualizes the limbus.”

Rotation of toric IOLs is rare, according to Arbisser.

“Mostly when there is rotation, it appears to be in myopes, theoretically with great big bags,” she said.

Some surgeons theorize that rotation is attributable to asymmetrical zonules, she said.

“Most of the time, when a lens rotates, the best way to fix it is to wait a week or so for a little bit of shrinkage of the bag to take place and then re-rotate it,” Arbisser said. “It’s very uncommon that it will rotate again. But the one way to be absolutely certain that it’s locked in, of course, is to forward-capture the optic, which would be a better thing to do, no doubt, with an AMO toric due to the fact that is has the opti-edge and it won’t have a square anterior edge.”

Incisional techniques

Incisional techniques are another traditional way of correcting astigmatism. Instability, imprecision, wound gape and scarring have generated problems with limbal relaxing incisions performed in the past, and not all surgeons currently use them.

According to Assouline, the use of limbal relaxing incisions is still viable if patients are carefully selected.

“Currently this approach is not precise enough compared to toric IOLs, at least with more than 1.5 D. For less than this, a simple pair of limbal relaxing incisions will do the trick. For higher astigmatism, there is the automated relaxing incision device by Moria, but femto is more efficient and precise in terms of depth and extension. The only precaution is to make sure you are dealing with regular astigmatism. Irregular will be subject to scarring and ectasia,” he said.

Arbisser said she prefers toric IOLs over incisional approaches to correct regular astigmatism. The cornea has wound-healing issues, and incisional approaches depend on nomograms that vary widely from patient to patient, she said.

Femtosecond laser arcuate incisions are more accurate than blade-created incisions but are less reliable than other approaches such as toric IOLs, according to Davis.

“Femtosecond lasers make pretty good incisions, but the nomograms aren’t perfect and you’ve still got the issue of regression of effect,” she said. “It’s still plagued by the same issues as with LRIs. It’s certainly more precise than blade-made incisions but not as predictable in terms of outcomes and stability of outcomes as the toric IOL.”

Intrastromal astigmatic keratotomy (AK) with a femtosecond laser may be a promising option. Grabner performs AKs with both the iFS laser and the Catalys laser (OptiMedica) for cataract surgery.

“They are safer as compared to anterior penetrating incisions; very precise as to the placement, shape and different options of inclination, and it is very fast surgery. For simultaneous treatment with cataract, it only takes 2 to 3 seconds,” he said.

The range of potential correction is lower compared with penetrating incisions, but effective reduction of corneal astigmatism between 0.5 D and 2.5 D is achieved in a safer way. Results are fairly stable over 1 year, Grabner said.

An international study to compare penetrating and intrastromal incisions is currently underway, but final results are not yet available. A study recently published in the Journal of Cataract and Refractive Surgery evaluated the effects of intrastromal incisions before cataract surgery, after cataract surgery and as a standalone procedure and found “excellent safety profile, rapid recovery, and stability of vision without the known risks associated with incisions that penetrate Bowman membrane.”

“We are currently working at improving nomograms,” Grabner said.

Arbisser said she performs AK with the Catalys laser but has not yet moved to intrastromal ablation.

“I personally have not decided to go to intrastromal [ablation],” Arbisser said. “I think it will be interesting to see where we go with that. It’s very tempting not to break the epithelium but, then again, we don’t want to break the endothelium.” – by Michela Cimberle and Matt Hasson