Opticians, how can you correct astigmatism in your patients?

Wednesday 17 July 2024
Corentin Auffret

An ideal optical system is stigmatic, meaning that an object point focuses on a single image point. Conversely, in an astigmatic system, an object point focuses on the retina not at a single point, but rather on a surface in the form of an ellipse, disk or line. The perceived image is therefore not sharp. People with astigmatism may suffer from headaches, eyestrain or blurred vision.

What is astigmatism? What causes astigmatism?

Astigmatism, like hyperopia and myopia, is a common visual defect, but this does not make it any less complex. Ocular astigmatism is most often induced by the toricity of the cornea, whose curvature (inversely proportional to optical power) varies along the meridians. It is this geometric deformation that causes a difference in correction between these meridians, known as corneal astigmatism. This phenomenon can also occur in the crystalline lens, in which case we speak of 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 perpendicular meridians. It can be direct, inverse or oblique, depending on the orientation of the flattest (least powerful) meridian. It can be corrected with glasses, contact lenses or refractive surgery. It is most often congenital, caused by a deformation of the cornea.

Irregular astigmatism, defined by non-perpendicular axes, is also rare. This can be corrected mainly with rigid lenses and surgery. Its etiology is often keratoconus, trauma or degeneration.

Impact on vision (blurred vision, headaches, eyestrain)

Ocular astigmatism affects foveal vision and, depending on its strength, reduces visual acuity and contrast sensitivity. A person with uncorrected astigmatism will see blurred images at all distances, unlike myopes, hypermetropes or presbyopes. Images appear blurred, distorted, doubled and imprecise. Astigmatism can also lead to eyestrain and headaches.

This is why managing astigmatism is so important, starting with its quantification.

The importance of good refraction for detecting astigmatism

Good refraction is essential for managing astigmatism:

A study published in the “Journal of Cataract & Refractive Surgery” revealed that almost 50% of people with astigmatism are not properly corrected with glasses or adapted contact lenses.

Astigmatism affects around 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:

Myopic astigmatism: One or both major meridians of the eye are myopic (if both meridians are myopic, they are myopic to different degrees).
Hypermetropic astigmatism: One or both major meridians are hypermetropic (if both are hypermetropic, they are hypermetropic to varying degrees).
Mixed astigmatism: one major meridian is myopic, the other hypermetropic.

That’s why screening for astigmatism starts with a good eye exam.

Unfortunately, the shortage of eye care experts and the inequality of their skills is leading to a decline in the number of quality eye exams carried out in-store.

Rapidly evolving refraction technologies are improving the accuracy and repeatability of eye exams. Topography is of interest in certain cases of high astigmatism, particularly for rigid lens adaptations. Thanks to objective analysis using an auto-refractometer or skiascopy, the practitioner can obtain a finer subjective refraction. Similarly, automated refraction is a real asset in the management of visual disorders.

The different solutions for quantifying regular astigmatism during an eye exam

When carrying out an eye exam, whether on a trial telescope or a refractor head, we need to perform a SACS examination.

SACS stands for sphere, axis, cylinder and sphere. This eye exam process can be carried out in a number of different ways, depending on the method chosen.

On refractor head :

1. The Rollover Cross Cylinder (RCC)

The crossed-over cylinder is made up of two cylindrical lenses of equal power but opposite sign, mounted together so that they can be quickly turned over. When using the CCR, it’s important to choose an acuity line of 80% of the worse eye’s visual acuity, or a point cloud. Remember to let the subject choose between positions 1 and 2, and insist on the similarity between the two, which is possible and even expected. The spherical equivalent should also be taken into account.

This is the most widely used test, for both manual and automated eye exams.

2. Square Chevron Patterns (MCC)

The use of chevrons makes it possible to accurately determine the axis of the astigmatism.

By rotating the test, the chevrons are arranged so that they appear identical. In this position, the bisector of the chevrons is on the same axis as the focal point on the retina. By orienting the chevrons, we obtain the orientation of the eye’s principal meridians. In this position, the test patterns offer the greatest difference. The corrective lens will be a negative cylinder whose axis is perpendicular to the sharpest and darkest direction seen.

Nowadays, the MCC test is rarely used, as it is less simple and less intuitive.

3. Parent dial

The parent dial allows you to estimate the axes of astigmatism. It can be used for distance, near, blurred or not, depending on the subject’s acuity.

It is generally used in conjunction with other methods.

The test is easy to use and understand, and can be used to clear the way for an eye exam when astigmatism is significant.

However, it is not, on its own, considered a reliable test for astigmatism.

4. Rotating cylinder / Pinhole slot

When acuity remains poor, despite an optimal sphere and an unsatisfactory search for astigmatism, we can estimate a cylinder based on the blocking acuity using the experimental table, and let the subject tell us the axis in which he or she feels most comfortable. These tests are always refined using other, more precise tests, such as the CCR.

The pinhole slit acts as a safety measure when acuity decreases. Sometimes linked to a pathological disorder or functional defect such as amblyopia, it is important during an eye exam to be able to reschedule an appointment in the event of significant fatigue, or to be able to redirect the patient to an ophthalmologist if the problem is more serious. SiVIEW’s algorithm incorporates this safety feature to guarantee a safe examination.

On test steady :

The tests described above are all compatible with test telescopes.

The hand-held CCR offers a slight differentiation in the variety of power ratings available. The hand-held CCR is available in +/-0.25, +/-0.50, +/-1.00 and +/- 2.00.

This method is particularly useful for treating patients with low vision.

How does SiVIEW handle astigmatism?

The SiVIEW solution automates eye exams, following a rigorous methodology developed by expert optometrists. After determining the sphere giving the best possible visual acuity, the algorithm moves on to evaluating the cylinder. The selected acuity line represents 80% of the best acuity. A variation of axis and cylinder is implemented, naturally including the spherical equivalent. Once the axis and cylinder have been validated, the sphere is checked again.

The AI built into SiVIEW takes into account various parameters such as the patient’s age, history and responses throughout the examination, and determines the correction offering the best comfort.

At SiVIEW, our algorithm enables you to quickly assess the appropriate cylinder power based on the patient’s responses. Even when choosing a test, the solution supports you in displaying the right test at the right time, based on the subject’s responses.

This procedure optimizes the quantification of regular astigmatism, guaranteeing accurate and appropriate management of astigmatic patients.

The SiVIEW solution enables eye care professionals to carry out an eye exam on the majority of visual defects that affect vision. It can detect astigmatism, hyperopia, myopia and presbyopia. With the help of the report provided at the end of the examination, it is possible to make a calm decision about correcting the visual defect with glasses or contact lenses.

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How is astigmatism corrected?

Astigmatism can be corrected in different ways, depending on its severity and nature. Here are the main treatments for astigmatism:

Toric contact lenses: Designed specifically to correct astigmatism, these lenses have a special shape to compensate for the difference in refractive power between the horizontal and vertical axes of the eye. They come in flexible and rigid versions.

Corrective glasses: Glasses are a common solution for correcting astigmatism. Corrective lenses specially adapted 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 a suitable technique, such as the use of 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 gold standard surgical treatment for the correction of astigmatism.

Importance of follow-up appointments: Scheduling regular appointments with an optometrist or ophthalmologist is important to monitor the progress of astigmatism correction and adjust treatments accordingly. A thorough eye examination will help assess the severity of your astigmatism and recommend the most appropriate correction method for each case of astigmatism.