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Acoustic analysis highlights role of anisotropy in myopic astigmatism
The efficacy of PRK may be correlated with the release of sclerocorneal tensions, a study suggests.
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ROME – Irregular distribution of tension in sclerocorneal tissue may be a cause of myopic astigmatism, according to an acoustic analysis of eyes undergoing refractive surgery. This may have implications for the strategies used to treat astigmatism, researchers suggest.
“We used a special method of acoustic measurement to evaluate the mechanical tensions in the cornea and in the sclera of myopic-astigmatic eyes, and their changes after laser photoablation. We found a strict correlation between the degree of astigmatism and the degree of anisotropy. We also found that photorefractive keratectomy performed with the InPro broad-beam excimer laser (InPro Intraokulare Prothetik GmbH) decreases the degree of both astigmatism and anisotropy,” said Erika N. Eskina, MD, co-author of a poster presented at the winter meeting of the European Society of Cataract and Refractive Surgeons.
Electronic analysis
Fifty eyes of patients with direct axes compound myopic astigmatism were examined using an electronic analyzer of the tension of tissues based on the speed of sound traveling through them.
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“Our acoustic analyzer consists of a generator and two acoustic wave receivers. The distance between the generator and each receiver is 3 mm. The frequency of oscillation is in the sound range (6 kHz), and the wavelength is less than 3 mm. The system measures the speed of sound waves emitted by the generator and running in the surface layer of tissues — sclera and cornea in our case,” Dr. Eskina said.
She explained that when the sound waves encounter areas of high mechanical tension in the tissue, their speed is consequently increased, but in areas of low tension, the speed is reduced.
The speed of sound in the horizontal and vertical directions (Vx and Vy) is measured by applying the analyzer tip at different points of the cornea, and the time of wave delay is registered and measured by a computer connected to the receivers. Tissue anisotropy is then calculated by using a formula.
If the vertical tension (Y) is more than the horizontal tension (X), the anisotropy is positive; if less it is negative.
“We measured corneal anisotropy by applying the acoustometry unit on the center of the eye, and scleral anisotropy by placing it more peripherally, between superior and lateral rectus muscles,” Dr. Eskina said.
Significant correlation
“As a result of our analysis, we found that typical properties of the eyes with direct axes compound myopic astigmatism — as opposed to other types of astigmatism and simple myopia — are negative acoustic anisotropy of the cornea and a significant negative anisotropy of the whole eye,” Dr. Eskina said. “We also found a direct proportion between the degree of anisotropy and the degree of myopic astigmatism, which allows us to suppose that tension of eye tissues is one of the mechanisms that underlies the formation of myopic astigmatism.”
Different factors may contribute to generate this tension: the distribution of collagen fibers in cornea; the strength, elasticity or rigidity of the tissues; eye muscle tension; and other characteristics of the orbit anatomy, she added.
“We only see the result, which is a certain type of modification in the anisotropy of the tissues. Other combinations of the same factors will lead to other types of refraction and astigmatism,” she said.
Biomechanical changes
PRK treatment using the InPro broad-beam excimer laser was performed successfully in all eyes. Cases with stable refractive results underwent the same acoustic wave analysis 1 month and 6 months after surgery, to compare anisotropy results.
Correction of myopic astigmatism after laser surgery went hand-in-hand with a consistent reduction of anisotropy, which changed in the whole eye from –0.209 before surgery to –0.119 at 1 month and to 0.008 at 6 months, Dr. Eskina said.
“It is evident some modifications of the biomechanical properties of the eye occur after PRK. Maybe when we perform the ablation we thin the collagen fibrils that form tension, maybe we partially cut the circular ligament of the cornea and, as a result, the tension in this axis decreases. But the tension in the sclera changes too, which leads us to the idea that astigmatism is not just a corneal problem,” she said.
The results of this study raise a series of questions forecasting a new approach to refractive treatment.
“For the time being, we can only describe whatever refractive error is there and choose the best way to correct it. However, the findings of our study take us a small step ahead in investigating the reasons why a particular refractive situation has developed in an eye. Pathogenic orientation of refractive treatment is in the future, and it seems to me that our study is heading in that direction,” Dr. Eskina concluded.
For Your Information:
- Erika N. Eskina, MD, can be reached at the Clinic of Eye Laser Surgery “Sphere,” Studencheskaya Str., 26/3 Moscow, Russia 121 151; (7) 095-2490550; fax: (7) 095-2493886; e-mail: inprodoc@rinet.ru. Dr. Eskina has no direct financial interest in the products mentioned in this article, nor is she a paid consultant for any companies mentioned.