An ideal optical system is stigmatic, meaning a point object focuses to a single point image. Conversely, in an astigmatic system, a point object focuses on the retina not as a point, but as a surface in the form of an ellipse, disc, or line. The perceived image is thus not clear. Astigmatic individuals may suffer from headaches, eye fatigue, or blurred vision.
Astigmatism is a common visual defect, similar to hyperopia and myopia, yet it retains its complexity. Ocular astigmatism is most often induced by the toricity of the cornea, whose curvature (inversely proportional to optical power) varies according to the meridians. This geometric deformation causes a difference in correction between these meridians, known as corneal astigmatism. This phenomenon can also appear in the lens, known as internal astigmatism. Internal astigmatism can compensate for corneal astigmatism, and the combination of the two results in total astigmatism.
There are two types of astigmatism: regular and irregular.
Regular astigmatism is characterized by meridians perpendicular to each other. It can be direct, inverse, or oblique depending on the orientation of the flattest meridian (least powerful). It can be corrected with glasses, contact lenses, or refractive surgery. It is mostly congenital and caused by corneal deformation.
Less commonly, there is irregular astigmatism, defined by axes that are not perpendicular to each other. This can be corrected primarily with rigid lenses and surgery. Its etiology often includes keratoconus, trauma, or degeneration.
Astigmatism affects foveal vision and, depending on its power, reduces visual acuity and contrast sensitivity. An uncorrected astigmatic person will see blurred at all distances, unlike a myope, hyperope, or presbyope. Images appear blurred, distorted, doubled, and imprecise. Astigmatism can also lead to eye fatigue or headaches.
Therefore, managing astigmatism is essential, starting with its quantification.
Accurate refraction is crucial for managing astigmatism:
A study published in the “Journal of Cataract & Refractive Surgery” revealed that nearly 50% of people with astigmatism are not properly corrected with appropriate glasses or contact lenses.
Indeed, astigmatism affects approximately 30% of the world’s population. It is one of the most common refractive errors, alongside myopia and hyperopia.
There are three main types of astigmatism:
This is why screening for astigmatism starts with a thorough eye examination.
Unfortunately, the lack of vision health experts and the inequality of their skills today leads to a decrease in the quality of eye exams conducted in stores.
The rapid evolution of refraction technologies improves the precision and repeatability of eye exams. Topography is interesting in certain cases of high astigmatism, particularly for adapting rigid lenses. With an objective analysis performed by an auto-refractometer or retinoscopy, the practitioner can perform a finer subjective refraction. Similarly, the automation of refraction is a real asset in managing visual disorders.
When conducting an eye exam, whether with trial frames or a phoropter, one should perform an examination using SACS.
SACS stands for Sphere, Axis, Cylinder, and Sphere. This process can be done in different ways depending on the chosen method.
The cross-cylinder is composed of two cylindrical lenses of equal power but opposite signs, mounted together to be quickly flipped. During the CCR, it is important to choose a line of acuity at 80% of the worst eye’s visual acuity or a point cloud. It is essential to let the subject choose between position 1, 2, and emphasize the similarity of both, which is possible and even expected. The spherical equivalent must also be considered.
This is the most commonly used test, in manual or automated eye exams.
Using chevrons helps to precisely determine the axis of astigmatism.
By rotating the test, the chevrons are aligned to appear identical. In this position, the bisector of the chevrons aligns with the focal axis on the retina. By orienting the chevrons, the main meridians of the eye are identified. In this position, the grids offer the most difference. The corrective lens will be a negative cylinder perpendicular to the sharpest and blackest view direction.
Nowadays, the MCC test is seldom used, being less simple and less intuitive.
The parent dial allows estimation of the astigmatism axes. It can be used from afar, up close, blurred or not, depending on the subject’s acuity.
It is generally used as a complement to another method.
This test is simple to use and understand, helping to unblock or approach an eye exam when astigmatism is significant.
However, it is not considered a reliable astigmatism test on its own.
When acuity remains poor despite optimal sphere and unsatisfactory astigmatism search, a cylinder can be estimated using an experimental table, and the subject is allowed to provide the axis in which they feel most comfortable. These tests are always refined with other, more precise tests like the CCR.
The stenopaeic slit acts as a safety measure when acuity decreases. Sometimes related to a pathological condition or a functional defect such as amblyopia, it is important during an eye exam to reschedule an appointment in case of significant fatigue or to refer the patient to an ophthalmologist if the problem is more severe. SiVIEW includes this safety measure in its algorithm to ensure a secure exam.
The tests previously mentioned are all compatible with trial frames.
The handheld CCR offers slight differentiation by the variety of powers offered. Indeed, the handheld CCR exists in +/-0.25 but also in +/-0.50, +/-1.00, +/-2.00.
This method is interesting for treating certain cases, notably for patients with low vision.
The SiVIEW solution automates the eye exam and follows a rigorous methodology developed by optometric experts. After determining the sphere providing the best possible visual acuity, the algorithm evaluates the cylinder. The chosen acuity line represents 80% of the best acuity. An axis and then a cylinder variation is implemented, naturally including the spherical equivalent. Once the axis and cylinder are validated, a new sphere check is performed.
SiVIEW’s integrated AI considers various parameters such as the patient’s age, anamnesis, and responses throughout the exam to determine the correction offering the best comfort.
At SiVIEW, our algorithm quickly evaluates the appropriate cylinder power based on patient responses. Even when choosing the test, the solution guides you to display the correct test at the right time based on the subject’s responses.
This procedure optimizes the quantification of regular astigmatism, ensuring precise and tailored care for astigmatic patients.
SiVIEW’s solution enables vision health professionals to perform an eye exam for most visual defects affecting vision. It allows detecting astigmatism, hyperopia, myopia, and presbyopia. With the report provided at the end of the exam, it is possible to decide calmly on correcting the visual disorder using glasses or lenses.
Astigmatism can be corrected in various ways depending on its severity and nature. Here are the main treatments for managing astigmatism:
Torical contact lenses: Specifically designed to correct astigmatism, these lenses have a particular shape that compensates for the difference in refractive power between the horizontal and vertical axes of the eye. They are available in both soft and rigid forms.
Corrective glasses: Glasses are a common solution for correcting astigmatism. Specially adapted corrective lenses for astigmatism have a unique shape that compensates for the difference in refractive power between the horizontal and vertical axes. For patients with conditions such as keratoconus, it is crucial to choose an appropriate technique, such as using rigid lenses or specific surgery.
Refractive surgery: Refractive surgery for astigmatism aims to modify the shape of the cornea to correct the visual defect. Common surgical procedures include radial keratotomy and various laser surgery techniques such as PRK, LASIK, and SMILE. LASIK is the reference surgical treatment for correcting astigmatism.
Importance of follow-up appointments: Scheduling regular appointments with an optometrist or ophthalmologist is important for monitoring the evolution of astigmatism correction and adjusting treatments accordingly. A comprehensive eye examination will assess the severity of your astigmatism and recommend the most appropriate correction method for each case of astigmatism.
References:
– https://www.gatinel.com/recherche-formation/astigmatisme/astigmatisme-definitions-et-formulations/
– https://www.2mcontact.com/blog/85_lentilles-toriques-ideales-pour-astigmatisme.html (image 1)
– https://www.gatinel.com/recherche-formation/astigmatisme/astigmatisme-et-erreur-daxe/
– STEVENSON DW. THE UNRELIABILITY OF THE ASTIGMATIC FAN OR CLOCK DIAL TEST. JAMA. 1909;LIII(1):8–12. doi:10.1001/jama.1909.92550010014002
– https://www.qualidoc.fr/specialites/ophtalmologie/astigmatisme/ (image 2)
– L’optique face à des difficultés de recrutement, 2018 https://www.acuite.fr/actualite/magasin/128892/loptique-face-des-difficultes-de-recrutement
Writer : Purneet Singh