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TYPES OF VISION DEFECTS:

1. SHORT-SIGHTEDNESS (MYOPIA)

It is a vision defect in which a blurred image of an object located in the distance appears on the retina. The optical system of the eye focuses light beams in front of the retina, which is caused either by too long axis of the eyeball or by too high refractive power of the optical system.

There are 3 degrees of myopia:

• low up to – 3 D (diopters) causes the eye to see well only at a distance of 1 m to 30 cm;
• moderate up to – 6 – 8 D causes the eye to require correction both at distance and close up;

• High myopia is an untreatable, genetic degenerative disease that begins in early childhood, with visual defects increasing from minus a dozen to minus several dozen diopters over the course of life and characterized by progressive retinal atrophy.

Low and medium degree myopia and astigmatism up to 2.0 D can be fully corrected:

• Glasses, provided that the difference in the size of the defect in both eyes, as well as the amount of astigmatism, does not exceed 2-3 diopters. Correcting a higher defect with glasses would cause too large a difference in the size of the image from both eyes and great discomfort when viewing binocularly.

• Contact lenses, which have the advantage of not reducing the image as is the case with spectacle correction, can therefore fully compensate for the so-called different eyesight, i.e. defects in both eyes greater than 2 – 3 D. However, generally available soft contact lenses cannot correct astigmatism from 1.0 D cyl and above. In these cases, individually selected toric contact lenses are used.

• Refractive surgery, i.e. a laser procedure that reduces the thickness of the cornea and weakens its optical power. Indications and contraindications for such procedures are based on thorough qualifying tests and must be individually discussed with the operator. read more »

High myopia results from the existence of an eyeball that is too long from childhood, which continues to grow throughout life and causes the eye’s shape to cease to be spherical and become elongated and cylindrical. A parallel beam of light entering the eye looking into the distance is focused well in front of the retina. Therefore, it is necessary to weaken the optical power of the eye so that the refraction is weak enough and the image can be focused at the bottom of the eye. 3 methods are used:

• Glasses, however, with lenses with a power higher than -10.0 D (diopters), they reduce the image of the viewed object to such a large extent that its registration by the atrophically altered retina may be difficult. Similarly, a high degree of astigmatism is difficult to correct, as is significant anisometropia in both eyes (see above).
• Contact lenses that do not reduce the image so much can fully compensate for both high refractive errors and anisometropia. However, correcting astigmatism requires toric lenses, which are individually selected and ordered from the manufacturer with an individual order.
• Refractive surgery for high myopia most often involves implanting an intraocular lens (IOL) with such power that the optical system focuses the image on the retina. In this case, the most rational solution is to remove the lenses, which worsen myopia, in both eyes – especially when they begin to become cloudy due to cataracts – and replace them with inserts of artificial intraocular lenses (IOL). Indications and contraindications for this procedure are based on thorough qualifying tests and must be individually discussed with the operating surgeon.

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2. FARSIGHTEDNESS (HYPERMETROPIA)

This is a vision defect that causes a blurred image of an object located on the retina, especially when it is close up, which is why it is most often called farsightedness. The optical system of the eye does not focus light beams on the bottom of the eye, but behind the eye. This is caused either by too short an eyeball axis or by too high refractive power of the optical system.

There is a mechanism of accommodation in the eye, i.e. increasing the optical power of the eye by changing the power of the lens, which is normally necessary when viewing an object close to the eyes. Farsightedness occurs physiologically in young children because their eyeballs are still growing and until about 3 years of age they are smaller than the eyes of adults. However, children, as well as adolescents, have a very high ability to accommodate due to the flexibility of the lens. The contraction of the accommodative muscle located around the equator of the lens releases the tension of the ligaments stretching on the lens, causing an immediate, elastic thickening of the lens. In a child, it can easily eliminate hyperopia of 7-8 diopters when looking into the distance and increasing the strength of the accommodative contraction additionally adjusts the eye to looking up close.

The hypermetropic eye of the so-called “long-sighted person” sees poorly both close and far, but the ease of increasing the optical power of the eye thanks to accommodation means that hyperopia is hidden in children, and at a young age it is only partially revealed.

Excessive accommodation in children may cause accommodative convergent strabismus. In turn, in young people, with a weakening ability to accommodate, its chronic tension may cause a permanent contraction of the accommodative muscle, which does not relax even when looking into the distance. This condition induces eye pain and headaches, and the permanently thickened lens causes the increased optical power of the eye to paradoxically simulate myopia, which may be the reason for using incorrect correction with glasses.

Children and adolescents require complete pharmacological removal of the ability to accommodate before being prescribed corrective glasses. For this purpose, the doctor most often prescribes 1 drop of 1% atropine solution into the eyes for 3-4 days, 3 times a day, and only after this period he or she detects the actual vision defect and then adjusts the glasses to it. Accommodation paralysis after atropine lasts for several days, during which reading and other forms of precise work at close range are difficult.

In some cases, especially in older adolescents, in the absence of strabismus or suspicion of accommodation spasm, the doctor may decide to eliminate it with other pharmacological agents, the effect of which lasts only a few hours. In special cases, however, they may not fully affect the accommodation ability.

With age, the ability to accommodate decreases and farsightedness can only be compensated by strengthening the optical system of the eye with focusing lenses with a “+” sign. These are usually glasses or contact lenses.

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3. ASTIGMATISM

It is quite a common vision defect. Many people with myopia and hypermetropia also have some degree of astigmatism. Astigmatism occurs when the cornea is shaped more like a rugby ball than a sphere.

As a result, people with astigmatism have distorted and blurred vision because the beams entering the eye are refracted at different angles. People with a higher degree of astigmatism have blurred vision not only at long distances, as in myopia, but also up close.

4. AGE-RELATED FARSIGHTEDNESS (PRESBYOPIA)

Presbyopia, or age-related farsightedness, is the result of the physiological process of hardening and decreased elasticity of the lens, which becomes more and more difficult to change its shape despite the tightening of the accommodative muscle. Normal-sighted people, i.e. those who do not have any vision defects, notice the first symptoms of a weakening of the eye’s ability to adapt to seeing at close range (30-50 cm) around the age of 40-45. The so-called near point gradually moves further away, which results in the reading text being moved further and further away from the eyes.

To compensate for presbyopia, it is necessary to use an additional focusing lens (with a “+” sign) for visual work at close range. In hyperopic people, this results in the prescription of stronger plus lenses for near vision than those used for distance vision. People with low myopia work at close range without glasses, and people with medium and high myopia use weaker diverging lenses (“-“).

Lack of accommodative ability causes discomfort when looking at intermediate distances. Selected far-sighted glasses (distance 5 – 6 m) and the other ones, enabling sharp vision at a specific close distance, e.g. 30 – 50 cm for reading, or 70 cm for working at a computer monitor, do not provide a sharp image of objects located at intermediate distances. Varifocal lens glasses, the so-called progressive glasses, equipped with lenses whose power varies along the entire optical space, help solve this problem. Thus, the optical power of a lens located in the middle of its height corresponds to the power needed for looking into the distance, and that located in the lower part, used for looking close, corresponds to the power needed for reading from a distance of 30 cm.

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