Refractive anomalies. Emmetropia and ametropia

Static refraction is determined by the position of the posterior main focus of the eye optical system relative to the retina. With commensurate clinical refraction, or emmetropia (from the Greek emmetros - commensurate, opsis - vision), this focus coincides with the retina, with disproportionate types of clinical refraction, or ametropia (from the Greek ametros - disproportionate), does not coincide. With myopia (myopia), the rays are focused ahead of the retina, and with hyperopia (hypermetropia) - behind it.

Theoretically, the disparity of clinical refraction can be caused by two main causes: the discrepancy between physical refraction of the length of the eye and, conversely, the discrepancy of the eye length of refraction. In the first case, ametropia is referred to as refractive, in the second - as an axial one. High-grade ametropia, as a rule, is caused by significant deviations in the size of the anteroposterior axis from the "normal" size in the direction of increase (in myopia) or a decrease (in gynermetropia).

In general, ametropia should be considered as a result of discordance between the optical and anatomical components of the eye. In such a licorrelation, the length of the axis of the eye, which is more variable than its refractive force, is primarily "guilty". Proceeding from this, we can say that the refraction of the eye is weaker, the shorter it is than the refraction is stronger, the longer the eye, that is, the hypermetropic eye is short, and the myopic eye is long.

In clinical practice, the degree of ametropia is judged by the strength of the lens, which corrects it and artificially turns the eye into an emmetropic one. Because of this, myopic refraction, which should be corrected with a scattering lens, is usually indicated by a minus sign, and a hypermetropic refraction is indicated by a plus sign. In the physical sense, with myopia, there is a relative excess, and with hypermetropia - a lack of refractive power of the eye.

With ametropia under conditions of maximum relaxation of accommodation, the image on the retina of an object located at infinity may be indistinct: each detail of the image forms on the retina not a point, but a circle, called the circle of light scattering.

In the event that the optical system of the eye is not spherical, this refraction is called astigmatism (from Greek astigmatism: a - negative prefix, stigma - point). With astigmatism, there is a combination of different refractions or different degrees of one refraction. In this case, there are two main mutually perpendicular cross sections, or a meridian: in one of them the refractive force is greatest, in the other - the smallest. The general astigmatism consists of corneal and lens, although, as a rule, the main cause of astigmatism is the violation of the sphericity of the cornea.

Astigmatism is called correct if in each of the main meridians the refractive force remains practically constant, and the transition of refraction from one main meridian to another occurs smoothly and resembles a sinusoid whose most prominent points correspond to the main meridians. Correct astigmatism is usually congenital, and the most frequent is a consequence of some diseases of the cornea and, more rarely, the lens. It should be noted that in the clinical practice very rarely there are cases of complete absence of astigmatism. As a rule, in a detailed study of "sight-seeing" eyes (for example, using refractive and ophthalmometry, which will be described below), the correct astigmatism is detected within 0.5-0.75 D, which practically does not affect the visual acuity, therefore it is called physiological.

In those cases when the clinical refraction of both major meridians is the same, they speak of complex astigmatism. With mixed astigmatism, one of the meridians has hypermetropic refraction, the other - myopic. With simple astigmatism, the refraction of one of the meridians is emmetropic.

The course of rays with astigmatism is best described by the conoid of Sturm. The shape of the light scattering pattern depends on the location of the section of the conoid by a plane perpendicular to the optical axis. In the eye, such a "plane" is the retina.

Depending on the position of the retina relative to the focal lines, the following types of astigmatism are distinguished:

• complex hypermetropic (HH) - both main meridians have hypermetropic refraction of different sizes, the retina is in front of the focal lines;
• simple hypermetroic (H) - one of the main meridians has emmetropic refraction, the other - hypermetropic, the retina coincides with the front focal line;
• mixed (MH) - one of the main meridians has hypermetropic refraction, the other - myopic, the retina is located between the focal lines;
• simple myopic (M) - one of the main meridians has emmetropic refraction, the other - myopic, the retina coincides with the posterior focal line;
• complex myopic (MM) - both main meridians have a myopic refraction of different sizes, the retina is located behind the focal lines.

The peculiarity of vision with astigmatism is that, depending on the refraction and location of the main meridians, the patient sees lines of different orientation differently.

The main meridians of the astigmatic eye are usually designated in accordance with the so-called TABO scale-gradient Ilo-Krug scale, which is counterclockwise (a similar scale is used in special trial frames intended for checking eyesight and selecting glasses).

Depending on the position of the main meridians, three types of astigmatism of the eye are distinguished: direct, inverse and with oblique axes. With direct astigmatism, the direction of the meridian, which has the greatest refractive force, is closer to the vertical, and in the opposite direction to the horizontal. Finally, with astigmatism with oblique axes, both principal meridians lie in sectors remote from the indicated directions.

The degree of astigmatism is judged by the difference in refraction in the two main meridians. The principle of calculating the degree of astigmatism can be illustrated by the following examples. If the main meridians have a myopic refraction equal to -4.0 and -1.0 D, respectively, then the degree of astigmatism will be -4.0 1.0 = 3.0 Dpt. In the case where the main meridians have hypermetropic refraction +3.0 and +0.5 D, the degree of astigmatism will be: +3.0 - +0.5 = 2.5 Dpt. Finally, with mixed astigmatism and refraction of the main meridians -3.5 and +1.0 D, the degree of astigmatism will be: -3.5 - + 1.0 = 4.5 Dpt.

To compare astigmatism with spherical types of refraction, the concept of "spherical equivalent" is used. This is the average arithmetic refraction of the two main meridians of the astigmatic system. So, in the above examples, this indicator will be -2.5, respectively; +1.75 and -1.25 diopters.

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