Modern treatment strategies effective for congenital and irregular astigmatism
Over the last 2 decades, excimer laser correction of myopia has become a well-established and standardized refractive treatment. Corneal laser strategies for astigmatism and hyperopia followed shortly thereafter, achieving satisfactory outcomes. However, it is not rare to come across people who believe that astigmatism and hyperopia cannot be treated with refractive surgery. This misconception probably originates from the increased difficulty of measuring and managing these two refractive errors as compared with myopia and from the need to use more specific platforms for both measuring and treating them.
A cylinder component is often associated with spherical error, and in about two-thirds of cases, it is high enough to be specifically addressed, hence the necessity to optimize nomograms for simultaneous sphere and cylinder correction. Accurate preoperative assessment of the astigmatism is a mandatory first step. Cycloplegic refraction will be used to measure the refractive error, and corneal topography will provide accurate representation of the corneal component, giving further information on whether the astigmatism is regular or not, more or less symmetrical, and its orientation. In case of combined hyperopia, which may lead to amblyopia in some cases, evaluation of the quality of eye motion and binocular vision is recommended. It should be emphasized that only corneal astigmatism deserves to be treated and that we need to be aware of the risks of treating internal astigmatism that might, for instance, be related to the aging crystalline lens.
The advent of wavefront sensor has provided a major contribution to our understanding of the complex aberrometry of the astigmatic eye. We have been able to understand that regular astigmatism might be related to low-order aberrations (second order in the Zernike classification with two principal half-meridians) and to identify other forms of irregular astigmatism with more than two principal meridians, generating higher-order aberrations such as trefoil and quadrafoil.
Beyond their diagnostic capacities, imaging technologies have become guides to corneal laser treatment. Wavefront-guided customized photoablation allows us to address optical aberrations, and topography-linked treatment programs enable us to reshape irregular corneas.
Among the available excimer laser options, LASIK represents nowadays the gold standard, due to the faster visual rehabilitation and lesser discomfort compared with surface techniques. However, it is mandatory for LASIK patients to fulfill safety criteria in terms of corneal thickness, regularity and biomechanical strength. Not all astigmatic corneas are suitable for LASIK, and there may be cases of progressive, irregular astigmatism in which LASIK should be avoided due to the high risk of secondary ectasia. In fact, our list of risk factors has been growing in the past 5 years. On the other hand, in the cases that are suitable for refractive surgery, enormous progress has been made with the use of eye trackers that statically and dynamically compensate for cyclotorsion and pupil centroid shifts, and with the use of customized ablation profiles that can effectively address mixed astigmatism with a spectacular degree of precision, this can lead to unprecedented visual quality results.
In addition to the possibility of creating thin flaps with adjustable edges, the integration of femtosecond lasers has allowed the revival of relaxing incision procedures as a complement to other corneal procedures or to phakic or pseudophakic intraocular implants in case of astigmatism combined with a spherical component. We can now offer this option to our patients in case of residual astigmatism after IOL implantation as an alternative to laser enhancement.
Toric implants, in the form of phakic iris-claw and sulcus-fixated or pseudophakic monofocal or multifocal lenses, also represent an important advancement in refractive surgery. With just one step, we can achieve emmetropia. In addition to corneal topography, these lenses require preoperative detection of the horizontal axis and precise intraoperative alignment of the lens axis with the astigmatic axis. With the aid of a hand-held pendulum instrument for cornea marking and a goniometer to measure angular distances, or by using one of the many platforms available for toric IOL alignment, this crucial step of surgery can be successfully achieved.
Initially designed for the correction of regular astigmatism, toric implants can equally address irregular astigmatism related to keratoconus or corneal grafts whenever refraction is measurable. Otherwise, the implantation of intracorneal rings may be considered as an alternative procedure.
On the whole, we can be satisfied with the way we have learned to master astigmatic correction, thanks to the instruments that allow us to precisely measure and understand the complex nature of this refractive error and the constant progress of corneal and intraocular surgical techniques. For many years now we have been able to effectively address congenital regular astigmatism, while effective approaches to irregular astigmatism have been a more recent achievement, with a good rate of success if careful patient selection is performed.
This issue of Ocular Surgery News Europe Edition focuses on this complex topic and aims at providing readers with a review of the modern strategies for astigmatic correction, explained by leading experts. We hope that you will find useful insights for your daily practice.
- For more information:
- Béatrice Cochener, MD, PhD, is an OSN Europe Edition Associate Editor. She can be reached at University Hospital of Brest CHRU MORVAN, LaTIM (Laboratory of Medical Informations Treatment), Batiment IV, 5 Ave. FOCH, 29609 Brest. Cedex, France; +33-2-98-22-34-40; email: email@example.com.
Disclosure: Cochener has no relevant financial disclosures.