Visual Acuity

Myopia and myopic astigmatism in laser eye surgery

09 Dec 2021
Refractive errors in human eyes, more commonly known as myopia, astigmatism and hyperopia, are the most common cases of visual impairment 1 2,3. For example, the total number of people with myopia in the world has reached 1950 million in 2010 and could exponentially reach more than 4700 million by 2050. These data indicate that by 2050, about half of the world's inhabitants could suffer from myopia. This is primarily due to a change in genetics and the way of life of a modern person 2 3 4. As you may notice, myopia and myopic astigmatism tend to get more commonly treated in laser eye surgery.

Near-sightedness. Reasons

The main defining quality factors of vision are the eye lens (optical power more than 20 Diopters), curvature of the cornea (optical power more than 40 Diopters) and the length of the eyeball 5. In the emmetropic (clearly seeing) eye, light is focused in a deepening on the retina - the so-called “fovea“ (see figure below). In the fovea a clearly focused image is created, which is transmitted along the nerve fibers to the optic nerve.

Myopia, more commonly known as near-sightedness, is a condition of the eye in which the refractive power of the cornea is stronger than required to project an image on the retina, i.e., the cornea is more convex (too steep) 5. The greater the curvature of the cornea is, the greater its optical power (low and high myopia), and therefore the image from distant objects is projected not on the retina but in front of it (see figure below). As a result, the image from objects becomes blurry. Depending on the complexity of the myopia, nearby objects may be seen more clearly. Hyperopia or farsightedness is manifested in the indistinct perception of nearby objects. In this case, the image could have been projected behind the retina of the eye, but due to the lack of the ability to project it on the retina, it is simply scattered on the retina, which leads to a blurry image. In refractive surgery, myopia or general refractive astigmatism of the eye is treated by reshaping the anterior surface of the cornea (see figure below).

Myopia, astigmatism and laser eye surgery

Fig. 1. The principle of poor vision: Myopia

More often, myopia is formed due to the fact that the eye grows in adolescent period. So in most cases, myopia is formed before the age of 20 years old 6. Progressive myopia can be slowed down for children under 18 years old with the help of soft contact lenses, also special so-called orthokeratological lenses or special ophthalmic gels. Some soft contact lens (SCL) interventions for school-aged children can slow the progression of myopia (0.2 ~ 0.3 D per year) and reduce the axial length growths by about 0.10 mm per year 7. Treatment effect can last 24 months or longer. However, the mechanism of the effect of orthokeratology to slow down myopia is still unclear, although various associations and hypotheses are periodically being published 8.

Myopic Astigmatism

The cornea of a healthy eye has the same curvature in all directions, representing a flat spherical surface (X, Y in the figure below on the left).

Myopia, astigmatism and laser eye surgery

Fig. 2. Principle of poor vision. Myopic astigmatism

It should be assumed that during blinking due to the pressure of the eyelids on the cornea, as well as due to friction between eyelids and cornea, the ideal spherical shape of the cornea is disturbed. This explains one of the situations how astigmatism can occur 9 10. The cornea, as it were, acquires a shape of a cylinder.

An example of a simple correct direct myopic (near-sightedness) astigmatism is shown on the right in the figure above. It can be seen that the cornea has different curvature in two directions. In this case, there is a greater refractive power (and increased curvature) along the vertical Y meridian than along the horizontal X and this is called a with-the-rule astigmatism. With this astigmatism, the X and Y meridians are perpendicular to each other and their refractive power does not change throughout the meridian (symmetric) and is called “regular” 11. With such myopic astigmatism, the image from the stronger main meridian Y (greater curvature) is focused in front of the retina, while the image from the other major meridian X lies on the retina.

On a topographic map during a diagnostic examination, such astigmatism looks like a 90° rotated butterfly (see green colour in Fig.2).

Correction of astigmatism

The main meridians of astigmatism are determined by the optical power of refraction of the cornea. In this case, X and Y directions. Their optical power is designated by the method of their determination (keratometry) for small (K1) and large (K2) curvature of the cornea (see figure below). The difference between large and small curvature is called toricity. In this case, the visual anomaly occurs due to the too strong optical power of the Y meridian, which is more than X by two diopters.

Myopia, astigmatism and laser eye surgery

Fig. 3. Relation between eyeglass prescription and astigmatism

When we write a prescription for glasses or contact lenses, we use a format that allows us to describe the required optical power for the correction of myopia or hyperopia by a Sphere and for correction of astigmatism by a Cylinder with its optical axis: 0D Sph. -2D Cyl. Axis 0o. Thus, the glasses will have optical power of -2 Diopters in only one vertical direction.

Astigmatism and laser eye vision correction

The most common methods of dealing with myopia are glasses and contact lenses 12. But these conservative non-surgical methods of dealing with ametropia (myopia, hyperopia, astigmatism) are only temporary. In order to get rid of glasses, a surgical method for adults is required (see Treatments to improve vision). There are several reasons for this. One of the main ones is the ability to get rid of the wearing glasses and the painful wearing contact lenses. Wearing glasses may not be possible for certain professions, sports or other active hobbies such as fishing and hunting. Secondly, not everyone can wear contact lenses. They can be uncomfortable, not tolerated by eyes, and even unsafe in case of intense rejection by the eye 5. In the third, glasses can be associated with unacceptable aberrations (displacement of the visual rays) as the so-called glare (blindness of the patient in darkness and twilight).

When the two meridians of the cornea are at right angles, ametropia can be corrected with glasses (correct astigmatism). When the main meridians are not perpendicular, there are irregularities in them or the refractive power changes along the meridian, astigmatism is considered irregular and individual (personalized) laser correction is required (see Method of personalized (individual or customized) vision correction). Irregular astigmatism cannot be corrected with glasses or contact lenses, as the power varies along the major meridians. From a clinical point of view, the treatment of astigmatism is important in both laser vision correction and cataract surgery. Uncorrected astigmatism can lead to blurred vision, halos, etc.

The main goal of refractive surgery is to safely and predictably create a stable and desired refractive effect without introducing new optical problems. To correct myopia, it is necessary to reduce the refractive power of the eye, either by reducing the curvature of the corneal surface, or by introducing a silicone intraocular lens (IOL) of the required power. There are several surgical techniques available to treat ametropia. These techniques are broadly divided into two groups 12: those related to surgery on the cornea (refractive surgery of the cornea) and those related to surgery on the lens of the eye (lens refractive surgery) 12.

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