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Revolutionary strategies lead to success of correcting astigmatism
Astigmatism is a commonly encountered refractive error in clinical practice, accounting for a high and significantly increasing prevalence worldwide. The presence of high degrees of astigmatism leads to negative impact on visual quality, such as the development of amblyopia and myopia, which require treatment.
However, correcting astigmatism to achieve satisfactory outcomes can be made more difficult by various challenging aspects of this refractive error, including the natural repair mechanism of the cornea, distinctive properties of individuals and the potential risk of IOL rotation.
With advances in technology and instrumentation enhancing our ability and accuracy of measuring, analyzing and treating astigmatism, astigmatism correction strategies have been improved markedly in recent years.
Methods of correction
Pathophysiologically, astigmatism results from asymmetry, typically in the anterior cornea, but can also result from the posterior cornea or lens.
In addition to non-surgical methods of correction, such as glasses and contact lens, laser surgery, phakic lenses and incisional management are methods for correcting significant clinical astigmatism that can result in excellent uncorrected visual acuity.
Of all of the methods of astigmatism correction, laser surgery has its advantages in precision and accuracy, especially for moderate refractive errors. Multiple series of clinical trials have demonstrated reduction of astigmatic cylinder after both PRK and LASIK. Additionally, modern customized laser treatments such as wavefront treatment technology and eye tracker features in excimer laser software have provided important advantages that have greatly improved the management of astigmatism.
It is also worth mentioning that the development of transepithelial PRK makes laser procedures safer and easier to perform than conventional PRK. Transepithelial PRK also improves slow epithelial healing compared with traditional PRK and can help avoid the risk of reopening incisions in pseudophakic eyes that is seen during LASIK.
Preferred lenses
For high degrees of astigmatism, toric or phakic lenses are preferred. Toric IOLs enable a lens-based surgical approach to the treatment of astigmatism for patients who need cataract surgery. To ensure satisfied outcomes using toric IOLs, centration and accurately measuring the axis before surgery are key factors. Instead of just marking the axis under the slit lamp and referring the conjunctival vessels, recent technology platforms are currently available to assist surgeons with the alignment. With these helpful advancements, correcting astigmatism using toric IOLs is stepping into a new era in terms of precision.
For patients who do not need phacoemulsification, phakic lenses can be considered — most commonly, the Visian Implantable Collamer Lens (STAAR Surgical). However, these lenses have disadvantages like secondary cataract and iris atrophy, and they have a tendency to rotate back if 15° off the horizontal axis. Evolutions of the lens are warranted to improve these disadvantages if the lens is to achieve widespread use.
Keratotomy
Surgical management of astigmatism with incisions is one of the traditional refractive procedures in ophthalmology. Nevertheless, incisional keratotomy, such as astigmatic keratotomy, remains relevant today in the management of astigmatism ranging from 0.5 D to 1.5 D by placing the limbal relaxing incision in the steep meridian.
Manual keratotomy is the traditional way to perform incisional correction, and its correction outcomes vary from case to case. This variation is associated with multiple factors, including the corneal elasticity, depth, length and width of incision. Recent research reported that a femtosecond laser can be used for incisional astigmatism management, and laser-induced keratotomy has demonstrated the ability to treat high levels of astigmatism with accurate incisions after penetrating keratoplasty.
A greater array of methods to correct astigmatism are available now than ever before. They are able to target different amounts of astigmatism with greater accuracy, improving visual quality and patient satisfaction. Currently, researchers are working for further improvement, since as the potential safety and effectiveness of modern astigmatism management continue to increase, both patient and surgeon expectations will surely rise, as well.
References:
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Nagy Z, et al. A new era in cataract surgery: laser refractive cataract surgery. Presented at: American Academy of Ophthalmology Subspecialty Day: Refractive Surgery; October 22, 2011; Orlando, FL.
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For more information:
Dennis S.C. Lam, MD, FRCOphth, can be reached at State Key Laboratory in Ophthalmology, Sun Yat-Yen University, 54 South Xianlie Road, Guangzhou 510060, People’s Republic of China; +852-3997-3266; fax +852-3996-8212; email: dennislam.gm@gmail.com.
Disclosure: Prof. Lam has no relevant financial disclosures.