Do you know what refraction of the eye is and what causes refractive errors?

Refraction of the eye is the ratio between the length of the eyeball and the optical power of its refractive media. In a healthy eye, the image is projected directly onto the retina, because the retina lies at the focal point of the eye's optical system. This is also the prerequisite for sharp vision. However, if the rays converge outside the retina after refraction, one of the refractive errors of the eye develops.

Refractive errors

The basic sign of refractive errors is a problem with focusing at short or, conversely, at longer distances. To be able to focus, the eye needs a corrective aid – prescription glasses or contact lenses – or the person needs to undergo refractive laser eye surgery. Refractive errors include:

• short-sightedness
• long-sightedness
• astigmatism
• age-related long-sightedness

Short-sightedness (myopia)

A short-sighted person sees well at short distances, usually up to about five metres. At greater distances they focus with difficulty or, depending on the number of dioptres needed, cannot focus at all. The focal point of the eye's optical system is shifted forward, which causes the light rays to be refracted in front of the retina. Corrective aids, which are concave contact lenses or distance glasses with minus dioptres, shift the focal point backwards.

Long-sightedness (hypermetropia)

Long-sightedness is the opposite of short-sightedness. The focal point is too far back; light rays converge behind the retina. For a long-sighted person to be able to focus at closer distances as well, the focal point must be shifted forward with the help of corrective aids – convex contact lenses or prescription glasses for near vision.

Age-related long-sightedness (presbyopia)

Presbyopia can be described as a special, age-related form of long-sightedness that affects everyone after the age of 40. It is caused by wear of the human lens, which loses its elasticity with age and thus also its ability to accommodate. The result is an inability to focus at very short distances. It is also popularly known as short-arm syndrome. A person with presbyopia tries to hold the text in a book or on a mobile phone as far away from their eyes as possible, but often even the length of their arms does not help them read it; they still see it blurred. Reading glasses are used for correction.

Astigmatism

Astigmatism is a refractive error, in which the optical system of the eye does not have the same optical power in all directions. It is caused by the curvature of the cornea, which is greater in an astigmatic eye than in a healthy eye. Due to the different curvature of the cornea, the incoming light is also refracted differently, thus blurring part of the image. Astigmatism is often accompanied by another refractive error – short-sightedness or long-sightedness – and is stated in cylindrical dioptres. It is corrected with prescription glasses, contact lenses or laser eye surgery. 

Visual acuity testing is one of the most important parts of a complete eye examination

A comprehensive eye examination serves to precisely identify the vision problem and to make it possible to determine the most effective treatment. The basis of the examination is determining visual acuity, during which the objective and subjective refraction are established. Objective refraction is assessed with devices called autorefractometers, which determine the maximum dioptre value that the patient's eye can tolerate. Subjective refraction is determined using special glasses and an illuminated chart with symbols, i.e. the patient's subjective perception determines which dioptres are the most comfortable for them.

Surgery for refractive errors of the eye

Like other areas of medicine, eye microsurgery has undergone enormous development over the last 20 years. Although several of the traditional surgical methods are still used, mainly because of their affordability, they can hardly be compared with the modern ones.

Operations using the latest third-generation methods take place inside the cornea, so they are gentler and kinder to the eye, with a minimal risk of postoperative complications and a shorter healing time compared with surface methods.

The most modern method used for the permanent correction of astigmatism and short-sightedness is ReLEx SMILE 3D. It is a painless and quick operation - approximately 15 minutes - in which a laser creates a lenticule inside the cornea, which is removed through an incision only about 2 mm in size. The operation does not use an excimer laser, which burns the cornea, so the procedure is as safe as possible and vision stabilises within just a few hours after the operation.

Presbyopia is mostly treated with methods based on replacing the original worn lens with a new, artificial one. Among the most advanced methods of today we can include, for example, the FEMTO Z-Prelex method, in which a trifocal lens is implanted into the eye. Thanks to it, the patient will see well at all distances and will thus get rid of prescription glasses for distance and near, as well as reading glasses, for good. The artificial lens is also the best prevention against the development of cataract. During the procedure, only a femtosecond laser is used; no sharp surgical instruments, such as a scalpel, are needed.