EyeNet Magazine


 
Clinical Update: Refractive
Optimizing Control of Astigmatism and Aberrations
By Lori Baker Schena, Contributing Writer
 
 

While cataract and kerato­refractive surgery have come a long way in the last decade, the treatment of astigmatism and aberrations continues to challenge ophthalmologists. Illustrating that point is the sheer amount of attention these problems garner, said Louis D. “Skip” Nichamin, MD, a cataract and refractive surgeon in private practice in Brook­ville, Pa. “Every year the journals receive literally hundreds of papers on how we measure aberrations and what is the best way to correct them. Through this research, we are slowly advancing the understanding of refractive surgery.”

Topography vs. Wavefront

“The reason aberrations continue to be a challenge is quite simple,” said Simon P. Holland, MD, clinical professor of oph­thalmology at the University of British Columbia in Vancouver. “Humans are daytime animals and we require neuro­adaptation to filter out aberrations, especially for night vision. With refractive surgery, you change the optics, which in turn causes certain aberrations we can measure. A lot of patients don’t adapt to these changes.”

Dr. Holland noted that while aberrations have become less of an issue with the advent of wavefront technology, in many patients it is still difficult to obtain consistent readings. In these situations, topography can serve as an excellent complementary tool, he said.

Dr. Holland pointed to patients who had abnormal wavefront measurements but normal topography. “Wavefront treatment would not be appro­priate because this approach would induce aberrations you don’t want,” he said. “These are patients who may be ideal candidates for a topographical approach, which is more anatomical.”

Topographic neutralization. In his practice at Pacific Laser Eye Centre in Vancouver, Dr. Holland and colleague David T. C. Lin, MD, are referred a large number of keratoconus patients and patients who had undergone unsuccessful refractive surgery. “When we try to obtain wavefront readings on these individuals, we sometimes cannot get reproducible results,” Dr. Holland noted. These patients typically cannot tolerate contact lenses, do not experience good quality of vision with their glasses and see poorly at night.

“In this situation, we may perform topography-guided PRK, which address­es corneal irregularities without the need for wavefront readings,” Dr. Holland said. “This is very challenging, however, because by doing this treatment, we generally create a refractive error.” To address that issue, they have developed an approach they titled the “topographic-neutralizing technique,” which involves topographic smoothing and refractive adjustment all in one treatment. The refractive outcome is based on algorithms.

This transepithelial topography-guided PRK incorporating the topographic-neutralizing technique requires intense planning up front. First, Dr. Holland explained, the surgeon makes a plano treatment plan with the Allegro Topolyzer (Alcon). “Second, we neutralize the cylinder by predicting the effect on astigmatism and sphere. We then create a myopic treatment in the center of the cornea to compensate for the initial curve we created to induce astigmatism.” To calculate the final treat­ment, he adds the manifest refraction.

Keratoconus patients. Dr. Holland noted that topographic neutralization is a valuable technique to improve predictability in topography-guided PRK for keratoconus patients. In one study authored by Drs. Holland and Lin, 32 keratoconus eyes underwent transepithelial topography-guided PRK with the Allegretto Eye-Q 400 Hz excimer laser. All of the eyes had a best-corrected visual acuity of 20/40 or better, were contact lens intolerant, and had residual stromal thickness greater than 300 µm. Patients also signed consent forms that addressed the possibility of a corneal transplant in the future.

Prior to PRK, the topographic neutralizing technique was performed. After surgery, mitomycin C 0.02 percent was applied for 15 seconds, and a bandage contact lens was placed until full re-epithelialization. Patients used standard steroid-tapering drops for three months and were seen in follow-up at 12 months. At that time, 77 percent of patients had achieved uncorrected visual acuity of 20/40 or better, and 82 percent achieved 20/20 BCVA. “In our practice, most patients can benefit from wavefront optimized treatment,” Dr. Holland said. “However, for those outliers with keratoconus and irregular corneas, where astigmatism remains an issue, we can turn to topography using this topographic neutralization approach.”

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Variability in Measures

Howard V. Gimbel, MD, MPH, in private practice in Calgary, Canada, said astigmatism is a key challenge not only in corneal refractive surgery but also with intraocular lens placement. “With the increasing use of toric phakic intra­ocular contact lenses as well as toric intraocular lenses, surgeons must pay attention to the axis of astigmatism as well as the cylinder power when placing the lens,” Dr. Gimbel said.

Topographic measurements are key tools in measuring astigmatism, but the variability among instruments had been a concern for Dr. Gimbel. “In our practice we are fortunate to have three different topographical devices, yet we found that they produce slightly different results,” he explained. “We wanted to determine if this was caused by instru­ment variability or the way the patient was positioned.”

To determine what caused the differences, 1,151 consecutive eyes in Dr. Gimbel’s practice underwent corneal topography by Orbscan, OPD-Scan and Pentacam and manifest refrac­tion before refractive surgery between 2005 and 2007. Initial results showed that the differences in cylinder power measured by two of the instruments (Orbscan and Pentacam) was significant. In addition, two machines agreed quite well in the mean absolute difference in axis, but the third (Pentacam) varied from those, and all three were different from the manifest refraction. “These results indicate that inter­instrument variability adds to the challenges of determining the true measurement of astigmatism in the refractive surgical population,” Dr. Gimbel said. “The next step is to determine if patient positioning and facial asymmetry are factors as well.”

With just a tilt of the head. In another study, Dr. Gimbel and colleague Peter Lee, MD, presented data from a sample patient that may begin to provide insight into the effects that patient positioning may have on measuring astigmatism. A sample patient before head tilt measured cylinder power of 1.5 D on the Orbscan, 1.4 D on the OPD and 1.2 D on the Pentacam, with a cylinder axis of 79 degrees on the Orbscan, 96 degrees on the OPD and 13 degrees on the Pentacam. In contrast, after head tilt, the same patient registered 1.4 D on the Orbscan, 1.4 D on the OPD and 3.8 D on the Pentacam, with a cylinder axis of 72 degrees on the Orbscan, 99 degrees on the OPD and 11 degrees on the Pentacam. “Patient positioning and facial asymmetry also impact the true measurement of astigmatism,” Dr. Gimbel said. “With these data, it appears we have just scratched the surface in terms of clinical implications.”

Reconsidering the data. Indeed, Dr. Gimbel said that he is in the process of reanalyzing the study data on different levels. For instance, while the study involved a large number of patients with astigmatism, it did not consider the amount of astigmatism. “We are looking at the data again to determine if it is the low powers in the cohort that gave rise to this difference in axis, particularly compared to the manifest,” Dr. Gimbel said. “Another area of future inquiry is to do repeated tests on one instrument. This takes time, and we need to staff up to accomplish this. From this point, we can take it another step and ask the techs to be sure to watch for head tilt and compare the topography results with those from the phoropter.”

In spite of the variations, Dr. Gimbel noted that current laser and intraocular technology allows for minimization of astigmatism. “With a number of the lasers there is iris registration and one can match the position of the patient under the laser to the topography that has been done, rather than to the manifest refraction,” he said. “If you can trust the topography, that approach should work.”

A more crucial issue involving astigmatism focuses on implanting toric IOLs for cataract surgery or refractive lens exchange procedures. In these instances, the manifest refraction cannot be utilized because of the effect of lenticular astigmatism; once the lens is removed from the eye, the cylinder power will be based only on the corneal cylinder. “This is where topography and autokeratometry must be utilized in selecting cylinder power and correctly placing the toric IOL in the refractive lens exchange or cataract surgery procedure,” Dr. Gimbel said.

Keep a record. He urged physicians to “record your numbers and record your outcomes, so that you can better analyze your data and address the astigmatism issue, even if you are in private practice. I always encourage physicians to do research in their own practice, whether or not they work in a university setting. We can all contribute answers to these questions, and we can do better for every patient. Physician expectation should always be higher than patient expectation.”
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Drs. Gimbel and Nichamin report no financial interests. Dr. Holland reports interests in Alcon and WaveLight.

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