Refractive focus also includes posterior corneal astigmatism, aberrometry for toric IOLs
Making the headlines in clinical diagnosis was the identification of a new layer in the cornea, considerations regarding posterior corneal astigmatism and toric IOLs, and the use of aberrometry to determine toric IOL corrective power.
By Lynda Charters; Reviewed by Uday Devgan, MD, Peter S. Hersh, MD, and Jonathan Talamo, MD
With clinical diagnosis being central to the ophthalmic practice, the year has proven to be especially noteworthy in the refractive surgery arena. Among the highlights are the identification of a new layer in the cornea, important considerations regarding posterior corneal astigmatism and toric IOLs, and the use of aberrometry to determine toric IOL corrective power.
The discovery of a heretofore-unrecognized layer in the cornea, referred to as Dua’s layer, was a noteworthy event, according to Uday Devgan, MD. Dr. Devgan is associate clinical professor, Jules Stein Eye Institute, UCLA School of Medicine, and chief of opthalmology, Olive View UCLA Medical Center, Los Angeles.
Harminder Singh Dua, MD, PhD, MBBS, the head of the Division of Ophthalmology and Visual Sciences at the University of Nottingham, United Kingdom, and associates recently reported their discovery in Ophthalmology (2013;120:1778–1785).
This new corneal structure, located between Descemet’s membrane and the corneal stroma, was found to be 15 μm thick when imaged by electron microscopy. Investigators noted that this layer is very strong and impervious to air.
The presence of Dua’s layer is highly relevant in lamellar dissection for corneal transplantation where air could be injected below it using the big bubble technique to help minimize the risk of tearing, Dr. Devgan said.
In diseases, such as keratoconus, a defect in Dua’s layer could account for the pathogenesis of corneal hydrops because it would allow passage of water from the aqueous.
Some debate surrounds the identification of Dua’s layer as expressed by Jester and colleagues (Ophthalmology 2013;120:1715-1717) and whether the new corneal layer is indeed a new layer that deserves the title, noted Peter S. Hersh, MD.
Dr. Hersh is the director, Cornea and Laser Eye Institute, Hersh Vision Group, Teaneck, and clinical professor of ophthalmology, and chief, Cornea and Refractive Surgery, Rutgers-New Jersey Medical School, Newark, NJ.
“We will see more arguments among pathologists about Dua’s layer and what it actually means-both with regard to the general description of the anatomy of the cornea and also the implications of it for lamellar techniques, such as deep anterior lamellar keratoplasty and Descemet's stripping automated endothelial keratoplasty,” he said.
The importance of the effect of the posterior corneal surface on the overall refractive astigmatism of the eye was acknowledged by Doug Koch, MD, and his associates from Baylor University, Waco, TX. Investigators reported their findings in the Journal of Cataract and Refractive Surgery (Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. EPub ahead of print 2013 Oct 26).
“We have all seen the patient who underwent IOL implantation with a perfectly positioned spherical IOL and absolutely no astigmatism apparent in our keratometry or corneal topography measurements, but has a cylindrical component to his refraction,” Dr. Devgan said.
This is due to the astigmatic effect of the posterior cornea that is not measured by routine keratometry, topography, or other methods that focus on the anterior corneal surface, he noted.
Dr. Koch and his colleagues noticed this in particular with patients with toric IOLs implanted who still had significant residual astigmatism even with a perfectly positioned IOL, Dr. Devgan explained. They devised the Baylor toric nomogram that takes into account the average posterior corneal astigmatism for patients with against-the-rule (ATR) and with-the-rule (WTR) astigmatism.
“Simply stated, we should aim to leave eyes with a small degree of WTR astigmatism as measured by our keratometry devices,” Dr. Devgan said. “When using the AcrySof toric IOLs (Alcon), which are spaced evenly in half-diopter steps at the corneal plane, I remember that I should add-up to the next toric T step for against-the-rule, while I should wimp down one toric T step for with-the-rule. By linking the add-up for ATR and wimp-down for WTR, it makes it easy to remember.”
Another option is to measure the posterior corneal surface accurately before surgery using corneal tomography devices, such as the Galilei Dual Scheimpflug Analyzer (Ziemer) or Pentacam (Oculus), and then use the total corneal power via ray tracing to determine the amount of astigmatism that needs to be addressed by the toric IOL, according to Dr. Devgan.
Finally, intraoperative aberrometry may be beneficial when taking aphakic refractions during surgery since the entire cornea, anterior and posterior, will be taken into account during the biometric analysis, Dr. Devgan said.
Dr. Hersh also noted that there is a debate about when to use toric IOLs, when to treat corneal astigmatism, and when the astigmatism is lenticular in nature.
“As we gain more knowledge about anterior and posterior corneal curvatures from Scheimpflug analysis, the increasing use of optical coherence tomography, and from intraoperative wavefront analysis,” he said, “these techniques are enhancing our ability to implant the appropriate lenses, whether they be toric or not, and also improving our ability to place the appropriate lenses in patients who have undergone previous LASIK or PRK procedures.
“It is making the problem of postoperative refractive errors after cataract surgery performed after LASIK far more manageable,” Dr. Hersh said.
Jonathan Talamo, MD, considers the use of intraoperative aberrometry to be an essential component in implantation of toric IOLs.
“Without aberrometry, the surgeon relies completely on the measurement of anterior corneal astigmatism and manual reference markings on the eye to calculate the corrective power of a toric IOL and align the IOL in the eye,” said Dr. Talamo, associate clinical professor of ophthalmology, Harvard Medical School, Boston. “With intraocular aberrometry, after the cataract is removed, the aphakic refraction is determined.”
The results obtained with toric IOLs probably have a great deal to do with the posterior corneal astigmatism, Dr. Talamo noted.
“Even though we can image the back of the cornea with tomography and optical coherence tomography, there is no good way to translate those measurements into anything meaningful about refractive astigmatism,” he said.
When measuring an aphakic refraction using the newest-generation aberrometer (VerifEye, WaveTec Vision), the measurement incorporates the posterior astigmatism into the equation, he said.
Practically speaking, in a patient with WTR astigmatism, the aberrometer shows that the astigmatism is lower than might be anticipated by looking at the anterior corneal measurements using topography or tomography alone, Dr. Talamo said. In patients with ATR astigmatism, the astigmatism tends to be greater when relying on topography or tomography alone.
Dr. Talamo and his colleagues conducted a study in which they compared the refractive results after toric IOL placement with and without the use of intraoperative aberrometry.
In that prospective non-randomized study, the results of which were presented at the 2013 meeting of the American Academy of Ophthalmology, they studied consecutive eyes with (n = 37) and without (n = 30) use of the ORA System (WaveTec Vision) intraoperatively. The preoperative characteristics of both patient populations were well matched.
The authors found that the absolute reduction in astigmatism after surgery was significantly lower in the eyes in which aberrometry was performed intraoperatively compared with the eyes in which it was not performed.
In addition, the likelihood that the patient would have an absolute astigmatism that was sufficiently low to facilitate an excellent visual result was also significantly greater in the eyes in which aberrometry was performed. The patients had a high chance of being within 0.25 or 0.5 D of the targeted correction with aberrometry, he noted.
“There was a highly statistically significant difference in clinically relevant outcomes,” Dr. Talamo said.
In addition to seeing a higher mean reduction in postoperative astigmatism in the aberrometry group (75% versus 57%), the use of aberrometry made it more than twice as likely (78% versus. 37%) that the postoperative refractive astigmatism would be within 0.5 D of target.
In the aberrometry group, the VerifEye axis was used along with conventional preoperative marking technique to guide initial rotational placement of the IOL, and additional rotations in 32% of eyes were needed to refine IOL position.
Astigmatic IOL power was changed during surgery in 25% of the aberrometry group based on aphakic refraction, while spherical power was changed about one-third of the time.
“This technology allows surgeons to have more confidence to be less aggressive in patients with WTR astigmatism and more aggressive in patients with ATR astigmatism when choosing toric IOL powers,” Dr. Talamo said.
Uday Devgan, MD
Dr. Devgan is a consultant to Alcon Laboratories.
Peter S. Hersh, MD
Dr. Hersh has no proprietary interest in the subject matter.
Jonathan Talamo, MD
Dr. Talamo is a consultant to WaveTec Vision.
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