Management of astigmatism calls for complex strategies for best results

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 have to 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 having to deal with the potential threat of IOL rotation.

Technological advances have improved the chances of addressing astigmatism with precision and safety. On one hand, corneal topography, wavefront aberrometry and optical coherence tomography allow mapping of astigmatism and the creation of customized treatment plans. On the other hand, the 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. In addition, the femtosecond laser promises to revive incisional techniques.

“We have a wide choice of [treatment] options. A first selection is made depending on whether the patient is a cataract or just a refractive surgery patient. And in case of a young refractive surgery patient, any degree of astigmatism is treated. Even a small amount of astigmatism — for example, 0.25 D — is treatable,” Jerry Tan, MD, said. “If the patient is a cataract patient, then I will consider toric IOLs if the astigmatism is 0.75 D and above.”

A high prevalence

A large survey, the Singapore Epidemiology of Eye Disease Study, enrolled 10,033 Asian subjects older than 40 years and found a prevalence of nearly 60% of subjects with astigmatism of more than 0.5 D. Compared with the Tanjong Pagar Survey from the early 2000s, a significant increase was found.

Image: Tan J

“In Singapore, 85% of the population is myopic, and around 75% have astigmatism,” Tan said.

“Prevalence is likely to be growing because Asian cities are now highly polluted, and this causes more allergies in children. Allergies lead to frequent eye rubbing, which I believe is one of the main causes of astigmatism,” he said.

Large population studies and twin studies found that astigmatism is a highly heritable trait, with an estimated heritability ranging from 30% to 60%. Other studies have found that chronic mechanical trauma to the cornea may also contribute to the pathogenesis of this refractive error.

“Chronic rubbing causes release of inflammatory cytokines, which damage the collagen. A correlation between allergy or atopy, chronic eye rubbing and keratoconus has been demonstrated by studies,” Tan said.

The Blue Mountains Eye Study also found that the prevalence of astigmatism increases with age. In a population of 3,654 people between the ages of 49 years and 97 years, the prevalence of astigmatism doubled in people older than 80 years.

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,” Ugo Cimberle, MD, said.

Paradigm shifts in laser surgery

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.

OCT and Scheimpflug imaging add further opportunities to work on the entire corneal shape, introducing elements of structural analysis.

“Modern technologies allow us to obtain an accurate representation of the corneal shape, to evaluate whether the toricity is central or peripheral, if there are asymmetries, differences in curvature gradients, if there are deviations of the visual axis from the pupil axis. If we need to do a customized treatment, we translate topographic data into wavefront data and send our treatment plan to the laser,” Cimberle said.

Modern eye trackers, which can compensate for static as well as dynamic cyclotorsion during surgery and have capabilities for iris, limbus and pupil recognition, have also greatly improved the management of astigmatism.

“The Schwind laser has spectacular eye tracking facilities for rolling, cyclotorsion and axial movements. It makes surgery very safe and predictable,” Tan said.

Compensating for static and dynamic cyclotorsion is critically important in astigmatic correction, Massimo Camellin, MD, said. 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.

Treatment options

Treatment of hyperopic astigmatism is more challenging as compared with myopic photoablation. Hyperopes have a larger average angle kappa than myopes, and ablation zones need to be decentered from the pupil axis. In addition, lasering a flatter cornea leads to the creation of peripheral steps that need to be smoothened to avoid problems with halos and glare. Hyperopic eyes also need more precise cyclotorsion compensation.

“Hyperopic astigmatism requires customized treatments,” Camellin said.

Tan treats hyperopic astigmatism with plus cylinder to ablate less corneal tissue.

“Most of the ablation is in the periphery. Many problems come if you use large spot lasers, which make the optical zone too small and create problems with glare and halos. But if you use small spot lasers, such as the Alcon WaveLight or the Schwind, you can treat very high hyperopes, especially with the plus cylinder treatment that uses less tissue,” he said.

“When I see a hyperopic patient with astigmatism, I am happy because I can manipulate the laser program to do less of a hyperopic correction. A +4 D hyperope with no astigmatism is harder to treat than a +4 D with –1 D of cylinder. You take the cylinder, convert it to a plus cylinder correction, +1 D cylinder correction, ending up with +3 D and +1 D. You don’t need to treat as much hyperopia,” Tan explained.

New lasers that create smoother transition zones in the periphery and the use of mitomycin C have made surface treatments a valid alternative to LASIK in hyperopic astigmatism. On the other hand, the use of femtosecond lasers for thin flaps has enhanced the precision, safety and stability of LASIK.

“PRK has the disadvantage of slower epithelial healing in the elderly, especially in the presence of dry eyes, while LASIK is more difficult in pseudophakic eyes with clear corneal incisions. Application of the suction ring during femtosecond or microkeratome flap making can spontaneously reopen at the incision. That’s why I prefer scleral tunnel incisions when I do cataract surgery,” Tan said.

“Regression used to occur in the past with PRK due to the sharp transition zones, but now this problem has been overcome. LASIK not only has a significantly higher risk of iatrogenic ectasia, but it can also induce astigmatic changes due to the cut, though femto-LASIK has now minimized complications,” Cimberle said.

“I use both LASIK and PRK or LASEK, but I have more recently discovered the advantages of transepithelial PRK, an option offered by the Amaris (Schwind), particularly for complicated cases of irregular astigmatism in eyes damaged by previous LASIK surgery. We laser through the corneal epithelium without touching the eye. Irregularities like buttonholes and wrinkles are corrected in one step. Healing is faster, especially when I use autologous serum eye drops,” Tan said.

Phakic lenses

Camellin said that he has gradually reduced the range of correction 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 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.

“The best phakic IOL is still the ICL, much better than iris-claw lenses that have problems in the long term with iris atrophy and pupil dilation,” Tan said. “The only disadvantage in treating astigmatism with ICL is that you have to be very accurate in your axis position. By a 10° error, you lose 33% of your correction, which is not something a patient would accept,” Tan said.

Although the lens may be correctly positioned by the surgeon, it has a tendency to rotate if it is more than 15° off the horizontal axis, he explained.

“Probably it’s the structure of the sulcus, which is not a smooth line but has crypts, and depending on those, the ICL could be pushed in one or the other direction. Sometimes it’s because the patients are rubbing their eyes. Also, the ICL is best to be placed in the horizontal position, and the more horizontal it is, the more likely it will remain stable in that position,” Tan said.

Cimberle also 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 don’t want me to laser them,” he said.

For high refractive error, Günther Grabner, MD, uses the Artiflex toric iris-claw lens (Ophtec).

“I’ve used it since it was introduced in 2001. I have implanted approximately 300. Results are fast, and patients are very happy. It is a reversible procedure, which is a further advantage,” he said.

“The only problems we had were inflammation in two patients who did not use their topical treatment properly, and partial disenclavation in another two cases: a man who did Power Plate and a lady who did trampoline jumping. I wouldn’t do bungee jumping, either. Now I tell patients that these are the three things they should not do if they have this lens implanted,” Grabner said.

Toric IOLs for cataract surgery

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

“Sphere and cylinder are lower-order aberrations, and it has been surprising that methods of correcting astigmatism in postoperative cataract patients have been very slow to develop,” Tan said.

Aspheric IOLs were probably developed more quickly than toric IOLs because of the symmetry of correction. In other words, there was no axis influence on the implantation of aspheric IOLs.

“To implant a toric IOL, not only centration is important but the axis must be accurately measured and marked for perfect placement. Otherwise, with a rotational error of 10°, there is a loss of 33% of effectiveness of astigmatic correction,” Tan said.

In recent years, with the introduction of acrylic IOLs that have high rotational stability in the capsular bag, toric IOLs have become very precise and accurate and provide long-term stability of correction.

“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, which is a further advantage,” Camellin said.

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.

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.

“I use several platforms but find that the iTrace is the most elegant way of doing alignment,” Tan said.

“We have been using for 3 years the SMI Surgery Guidance system (SensoMotoric Instruments), which grabs the image of the frame and vessels of the eye before surgery and intraoperatively projects them into the microscope, using them as landmarks to guide toric IOL orientation,” Grabner said.

“It’s like a jet fighter shooting game: grab, track, compare, shoot. Very neat, fast and simple. Very helpful and precise,” Grabner said.

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.”

Incisional techniques

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

“I don’t believe in LRIs. They are very unpredictable, weaken the cornea, and in the day and age of toric IOLs and excimer lasers, they should not be the surgeon’s first choice of correction of astigmatism,” Tan said.

He told the story of a patient who was previously treated for 4 D of astigmatism with LRIs.

“She was so unhappy. I had to suture her LRI to reduce her astigmatism, and of course, you cannot be that accurate with your suturing. She still ended up with 2 D of residual astigmatism in the opposite axis. Then I waited for 6 months and slowly removed suture by suture until she could see better. She asked me to write a note to her eye doctor back home and to recommend that he not to do LRIs in the other eye,” he said.

He noted that most cataract surgeons are not cornea-trained surgeons. Even for cornea-trained surgeons, wedge resections and LRIs are not easy to perform and are not predictable.

“You need a surgical keratometer. And then the sutures loosen, wound healing is unpredictable, and it takes a long time to rehabilitate the patient. You can still cause a lot of astigmatism,” Tan said.

“Toric IOLs are so accurate nowadays, and lasers can perform accurate enhancements for low amounts of correction,” he said.

Intrastromal astigmatic keratotomies with a femtosecond laser might be promising, according to Grabner. He performs them with both the iFS laser (Abbott Medical Optics) and the Catalys laser (OptiMedica) for cataract surgery.

“They are safer as compared to anterior penetrating incisions, very precise as to the placement, shape, 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 ongoing international study is comparing penetrating and intrastromal incisions, 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. – by Michela Cimberle

References:

Attebo K, et al. Ophthalmology. 1999;doi:10.1016/S0161-6420(99)90251-8.

Fan Q, et al. PLoS Genet. 2011;doi:10.1371/journal.pgen.1002402.

How AC, et al. Arch Ophthalmol. 2009;doi:10.1001/archophthalmol.2009.134.

Nemeth G, et al. Eur J Ophthalmol. 2013;doi:10.5301/ejo.5000294.

Pan CW, et al. Invest Ophthalmol Vis Sci. 2013;doi:10.1167/iovs.13-11725.

Ruckhofer J, et al. J Cataract Refract Surg. 2012;doi:10.1016/j.jcrs.2011.11.027.

Rückl T, et al. J Cataract Refract Surg. 2013;doi:10.1016/j.jcrs.2012.10.043.

Vitale S, et al. Arch Ophthalmol. 2008;doi:10.1001/archopht.126.8.1111.

For more information:

Massimo Camellin, MD, can be reached at SEKAL, Via Dunant 10, 45100 Rovigo, Italy; phone/fax: +39-0425-411357; email: cammas@tin.it.

Ugo Cimberle, MD, can be contacted at CIDIEMME, Centro di Microchirurgia Oculare, Via Berlinguer 14, Ravenna, Italy; +39-054-440-4355; fax: +39-054-440-8956; email: cimberle@alice.it.

Günther Grabner, MD, can be reached at University Eye Clinic, Paracelsus Medizinische Privat-Universität, PMU, Universitätsklinikum, Müllner Hauptstrasse 48, A 5020 Salzburg, Austria; email: g.grabner@salk.at.

Jerry Tan, MD, can be reached at Camden Medical Centre, 1 Orchard Blvd. 10-06, Singapore 248649; +65-6738-8122 ; fax +65-6738-3822; email: info@jerrytan.com or catch-all@jerrytan.com.

Disclosures: Camellin, Cimberle and Grabner have no relevant financial disclosures. Tan is a consultant for Schwind.

 

 

Will the femtosecond laser revive widespread use of relaxing incisions at the time of cataract surgery?

Femtosecond laser has removed inconsistencies of procedure

Limbal relaxing incisions have been performed for more than a decade using a calibrated diamond knife. Keys to success are cylinder less than 2 D, exact calculation taking into account the effect of the surgeon-induced astigmatism, precise placement on the desired axis at a specified distance from the limbus as well as accurate arc length. In addition, the incision must be about 90% deep to generate the desired correction.

Considering how many variables there are to consider, it is not surprising that the femtosecond laser has arrived to save LRIs like a knight in shining armor.

The new laser system calculates all of these variables, and the surgeon simply has to drag and drop the incisions on the screen for the laser to perform them. The consistency and precision of laser LRIs simply cannot be matched by a surgeon, however skilled. The suction ring is placed, the applanation cone is applied, and the laser creates the keratotomy in seconds. All that remains is to open the incisions with a Sinskey hook or an iris repositor. The incision can even be opened at the end of cataract surgery or on the slit lamp later.

In a retrospective comparative case study, Bahar et al compared the outcomes of Abbott Medical Optics IntraLase-enabled astigmatic keratotomy and manual astigmatic keratotomy Only the femtosecond group achieved a statistically significant improvement. In a prospective randomized study, Hoffart et al compared the effectiveness of arcuate keratotomy with femtosecond laser vs. mechanical keratome. A wider spread of angle of error and an almost significant difference of mean absolute angle of error suggested a larger misalignment of treatment during mechanized astigmatic keratotomy.

An even newer advance is femtosecond intrastromal incisions, which do not break the epithelium. The future is bright; the future is femto.

 

Cyres K. Mehta, MS, FASCRS, is an OSN APAO Edition Board Member. Disclosure: Mehta has no relevant financial disclosures.

Predictability, efficacy higher with toric IOLs, laser photoablation

The femtosecond laser has certainly improved the quality and reproducibility of astigmatic keratotomy. The depth and arc length, as well as the centration and symmetry of the two incisions, can be precisely calculated. Refraction, corneal topography and corneal pachymetry are performed directly in the area of the intended incisions and then programmed into the laser. Similar to the LASIK flap, you can now have the accuracy that you could never achieve by mechanical methods, and this adds a lot to the procedure.

However, there are variables that we are still unable to control, such as elasticity gradient and strength, which vary between individuals and in relation to age. The biomechanical and functional response of the cornea to the treatment, both in the short and long term, remains, to some extent, unpredictable.

Femtosecond lasers are becoming an indispensable tool in any modern practice. The added capability for relaxing incisions is an additional bonus. However, astigmatic keratotomy remains a rough approach to astigmatic correction. It also requires calculating the different spherical equivalent resulting from what you remove from the cylinder and add to the sphere by performing the corneal cuts. Nowadays we have toric IOLs, which work wonders to correct astigmatism in our cataract patients. With careful patient selection and accurate preoperative measurements, they provide perfect outcomes and no surprises. And we have excimer lasers with reliable nomograms and potentiated eye trackers for astigmatism, which can be used for primary procedures or for astigmatic enhancement after IOL implantation. Both toric IOLs and laser procedures are, at present, more reliable methods of addressing astigmatism, with still more accurate and predictable results and fewer complications.

 

Vikentia Katsanevaki, MD, PhD, is an OSN Europe Edition Board Member. Disclosure: Katsanevaki has no relevant financial disclosures.