Refractive Anomalies. Emmetropia and ametropia

Static refraction is determined by the position of the posterior principal focus of the eye's optical system relative to the retina. With proportionate clinical refraction, or emmetropia (from the Greek emmetros - proportionate, opsis - vision), this focus coincides with the retina, while with disproportionate types of clinical refraction, or ametropia (from the Greek ametros - disproportionate), it does not coincide. With nearsightedness (myopia), the rays are focused in front of the retina, and with farsightedness (hypermetropia), they are focused behind it.

Theoretically, the disproportion of clinical refraction can be caused by two main reasons: the discrepancy between the physical refraction and the length of the eye and, conversely, the discrepancy between the length of the eye and the refraction. In the first case, ametropia is designated as refractive, in the second - as axial. High-degree ametropias are usually caused by significant deviations of the anteroposterior axis from the "normal" dimensions in the direction of increase (in myopia) or decrease (in hypermetropia).

In general, ametropia should be considered as a result of discrepancy between the optical and anatomical components of the eye. The length of the eye axis, which is more variable than its refractive power, is primarily "guilty" of such discrepancy. Based on this, we can say that the weaker the refraction of the eye, the shorter it is, the stronger the refraction, the longer the eye, i.e. the hypermetropic eye is short, and the myopic eye is long.

In clinical practice, the degree of ametropia is judged by the power of the lens that corrects it and artificially transforms the eye into emmetropic. As a result, myopic refraction, which should be corrected with a diverging lens, is usually designated by the minus sign, and hypermetropic by the plus sign. In the physical sense, myopia is a relative excess, and hypermetropia is a deficiency of the refractive power of the eye.

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

If the optical system of the eye is not spherical, then such refraction is called astigmatism (from the 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, two main mutually perpendicular sections, or meridians, are distinguished: in one of them the refractive power is greatest, in the other - least. General astigmatism consists of corneal and crystalline, although, as a rule, the main cause of astigmatism is a violation of the sphericity of the cornea.

Astigmatism is called regular if the refractive power remains practically constant in each of the main meridians, and the transition of refraction from one main meridian to another occurs smoothly and resembles a sinusoid, the most prominent points of which correspond to the main meridians. Regular astigmatism is usually congenital, and irregular astigmatism is most often a consequence of some diseases of the cornea and, less often, the lens. It should be noted that in clinical practice, cases of complete absence of astigmatism are very rare. As a rule, a detailed examination of "good-sighted" eyes (for example, using refracto- and ophthalmometry, which will be described below) reveals regular astigmatism within 0.5-0.75 diopters, which practically does not affect visual acuity, so it is called physiological.

In cases where the clinical refraction of both main meridians is the same, we speak of complex astigmatism. In mixed astigmatism, one of the meridians has hypermetropic refraction, the other - myopic. In simple astigmatism, the refraction of one of the meridians is emmetropic.

The course of rays in astigmatism is most successfully described by the Sturm conoid. The shape of the light scattering figure depends on the location of the conoid section 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 (CH) - both main meridians have hypermetropic refraction of different values, the retina is located in front of the focal lines;
• simple hypermetropic (H) - one of the main meridians has emmetropic refraction, the other - hypermetropic, the retina coincides with the anterior focal line;
• mixed (MN) - 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 myopic refraction of different values, the retina is located behind the focal lines.

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

The main meridians of the astigmatic eye are usually designated in accordance with the so-called TABO scale - a degree and circular scale, the reading of which is made counterclockwise (a similar scale is used in special trial frames designed for checking vision and selecting glasses).

Depending on the position of the main meridians, there are three types of astigmatism of the eye - direct, reverse and with oblique axes. With direct astigmatism, the direction of the meridian with the greatest refractive power is closer to the vertical, and with reverse - to the horizontal. Finally, with astigmatism with oblique axes, both main meridians lie in sectors remote from the specified 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 myopic refraction equal to -4.0 and -1.0 D, respectively, then the degree of astigmatism will be -4.0 1.0 = 3.0 D. In the case when the main meridians have hypermetropic refraction of +3.0 and +0.5 D, the degree of astigmatism will be: +3.0 - +0.5 = 2.5 D. Finally, with mixed astigmatism and refraction of the main meridians of -3.5 and +1.0 D, the degree of astigmatism will be: -3.5 - +1.0 = 4.5 D.

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

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