REFRACTION OF A EYE

From Big Medical Encyclopedia

REFRACTION OF A EYE (late lat. refractio refraction) — the refracting force of optical system of an eye expressed in dioptries.

The river of as the physical phenomenon («a physical refraction») is defined by the radius of curvature of each refracting environment of an eye, indices of refraction of environments and distance between their surfaces. Therefore, the physical characteristic of R. of is caused by its anatomic structure (see. Eye, dioptrics ).

In clinic, however, not the absolute force of the optical (svetoprelomlyayushchy) apparatus of an eye, and its ratio with a length of an eye (a perednezadny axis), i.e. the provision of back main focus in relation to a retina matters, as makes clinical R.'s concept of.

Fig. 1. The diagrammatic representation of a path of rays in optical system of an eye at different types of a clinical refraction of an eye: and — the myopia, back main focus of an eye is located before a retina; — the emmetropia, back main focus is located on a retina; in — the hypermetropia, back main focus is located behind a retina.

Depending on the provision of back main focus (a point of refraction of the beams passing through optical system of an eye parallel to its optical axis) in relation to a retina distinguish three types of clinical R. of (fig. 1). 1. Back main focus matches a retina; such refraction is called proportional and is designated as emmetropia (see). 2. At an arrangement of back main focus ahead of a retina speak about a myopia or short-sightedness (see). 3. At an arrangement of back main focus behind R.'s retina of call a hypermetropia or far-sightedness (see). The last two types of R. of are disproportionate and unlike an emmetropia call them ametropias (see). Thus, an emmetropichesky eye is established to the parallel beams going from infinity, i.e. the refracting force of its optical system corresponds to length of its axis, focus of parallel beams matches precisely a retina, and such eye well sees afar. For sight close such eye needs to strengthen the refraction, as it can be reached by means of accommodation (see. Accommodation of an eye ). The short-sighted eye having as if the excess refracting force can see well close at this or that final distance depending on degree of a myopia, but for good sight afar needs use of the dispersing lens turning the dispersing beams going at a short distance into parallel. An eye with a gipermetropichesky refraction to parallel beams is not established but, on condition of inclusion of the accommodation is capable to see well afar. For examining of close located objects it is forced in a bigger degree to use the accommodation, and in case of its insufficiency it is necessary to resort to use of a converging lens of the corresponding force. At any kind of a clinical refraction of eyes has always only one most remote point in space, to a cut it is established (beams, coming from this point, are focused on a retina). This point is called further point of clear sight (see). For an emmetrichesky eye it lies in infinity, at a myopia — at some final distance ahead of an eye (that is closer, than degree of a myopia is higher); for a gipermetropichesky eye the further point of clear sight is imaginary since in this case on a retina only the beams which already have nek-ry degree of a convergence can be focused, and there are no such beams under natural conditions. Thus, the provision of a further point of clear sight defines a look a wedge, refractions and degree of an ametropia.

Optical system eyes (see) can as it is stated above, amplify due to accommodation. Depending on it distinguish a static refraction of an eye, i.e. a refraction at rest of accommodation, and dynamic — a refraction at inclusion of accommodation.

Depending on a form of the optical apparatus of an eye distinguish spherical R. when refraction of beams in an eye is identical in all meridians, and aspherical R. when in the same eye there is a combination of various refractions, i.e. refraction of beams is unequal on various meridians (see. Astigmatism of an eye ). Small degrees of an astigmatism (to 0,5 dptr) meet quite often, almost do not worsen sight therefore such astigmatism call physiological.

For R.'s definition in clinic use two methods: subjective and objective. At preschool and school age clinical R. is determined by in the conditions of a tsikloplegiya, i.e. against the background of switching off of accommodation, a cut carry out by means of an instillation in a conjunctival sac of each eye 0,5 — 1% of solution of Atropini sulfas, 0,25% of solution of Scopolaminum of hydrobromide, etc. by strictly certain technique. At more advanced age the issue of carrying out a tsikloplegiya is resolved individually.

The subjective method consists in selection of the corresponding corrective glass in the course of the research visual acuities (see); at this method use testimonies of the patient. Expression of a refraction and its degree at a myopia is the weakest of dispersing lenses, with the help the cut is reached the highest visual acuity afar; at a hypermetropia as an indicator serves the strongest of converging lenses at the highest visual acuity afar. At spherical R. correction is carried out spherical lenses, at an astigmatism — cylinder lenses (see. Points ). Carry to objective methods of definition of a refraction skiaskopiya (see) and refractometry of an eye (see).

The river of was studied by many researchers — G. Helmholtz, Cherning (M. H. E. Tscherning), A. Gullstrand, Listing (.). Century of Listing), V. K. Verbitsky, E. Zh. Tron, etc., but the reasons of development of its different types remain not quite found out. In the second half of 19 century the founder of the doctrine about a refraction and accommodation Dutch scientist F. Donders considered an emmetropichesky refraction norm, and an ametropia pathology. At the same time change of a perednezadny axis of an eye (its lengthening was considered as a major factor of development of ametropias at short-sightedness and shortening at a hypermetropia). Attached to changes in the refracting force of an eye minor significance. Allocation of these factors as the reasons of development of that or sometimes R.'s type of laid the foundation for the doctrine about existence of two types of ametropias: axial and refraction.

Shteyger's researches (A. Steiger, 1913) allowed to establish considerable variability of the optical apparatus of an eye and to explain emergence of different types of a refraction with an accidental combination of the varying elements of the optical apparatus, i.e. the refracting force and length of an axis of an eye. Further researches E. Zh. Tron, A. I. Da-shevsky, etc. these data confirmed. E.g., at an emmetropia, according to E. Zh. Tron, length of an eye varied ranging from 20,54 mm to 38,18 mm, and the refracting force ranging from 52,59 to 71,3 dptr, by data A. I. Dashevsky, the refracting force of eyes at an emmetropia changed ranging from 52,0 to 67,0 dptr. Along with it a certain consistent pattern in a combination of the basic elements defining a clinical refraction of an eye, namely, negative correlation between them i.e. the expressed tendency to a combination of longer axis of an eye with weaker refracting force, and, on the contrary, shorter axis with higher refracting force was determined.

It was established that the emmetropia is defined by an optimum combination of anatomo-optical elements of an eye. As for ametropias, E. Zh. Tron suggested to divide them into four groups: 1. The axial ametropia — the refracting force within the sizes observed at an emmetropia, but length of an axis of an eye is more, or it is less than the sizes, observed at an emmetropia (30,2% inspected fell to the share of this group of ametropias); 2. The refraction ametropia — length of an axis of an eye within the sizes observed at an emmetropia, but the refracting force is more, than at an emmetropia (3,7% inspected); 3. An ametropia of the mixed type — length of an axis of an eye and the refracting force are out of the limits observed at an emmetropia (3,4%); 4. A combinational ametropia — length of an axis of an eye and the refracting force do not exceed the limit of the sizes observed at an emmetropia (62,7%). Thus, the last type of an ametropia was the most frequent. It gives the grounds to consider an emmetropia and small degrees of a hypermetropia and a myopia as biological options during formation of a clinical refraction of an eye. Only extreme degrees of ametropias (over 6,0 dptr) can be considered as considerable deviations from biological options, and, as a rule, in these cases the axial factor prevails. Cases with the high progressing short-sightedness and heavy changes in covers of an eye (a sclera, a choroid and a retina) need to be regarded already as pathology and to carry out not only optical correction, but also to carry out the corresponding treatment.

According to A. I. Dashevsky, it is necessary to distinguish three groups of clinical R. of: emmetropia, proportional and disproportionate (axial) ametropias. Carry cases where the refracting force and length of an axis of an eye are that what can be observed also at an emmetropia, disproportionate — those to proportional ametropias, at to-rykh an emmetropia it is impossible. On the basis of studying of optical system of eyes fotooftalmometrichesky and fo-toanatomichesky by methods A. I. Da-shevsky adheres to the theory of so-called primary refraction of an eye of ii secondary, on a cut primary shape of an eye is spherical and only further there is a change of this form in secondary due to change of parameters of an eye (one, two or all three of its diameters) therefore develop both an emmetropia, and other types of clinical R. of. According to V. P. Odintsov, almost all newborns have a hypermetropia; among the persons which reached 25-year age, the hypermetropia is observed in 50 — 55%, an emmetropia — in 30 — 35% and a myopia in 15 — 20% of cases.

Now it is established that in the developed countries a certain tendency to growth of number short-sighted is observed that connect hl. obr. with usual work at a short distance, napr, reading, the letter.

The Japanese researcher Sato (I. Sato, 1957) among pupils of higher educational institutions established up to 70% of cases of short-sightedness. It is necessary to emphasize, however, that short-sightedness at school age (a so-called school myopia), as a rule, remains within low degrees at preservation of high visual acuity (with correction). Mechanism of development short-sightedness (see) it is interpreted in different ways. E.g., according to A. I. Dashevsky, the usual tension of accommodation at occupations at a short distance (initial «spasm» of accommodation) is fixed further, creating clinically a myopia. According to E. S. Avetisov, major importance in development of short-sightedness belongs to weakness of accommodation (inborn and acquired owing to various diseases) therefore the impulse to increase in length of an eye under laws of negative correlation is born.

Fig. 2. A refraction curve of Becha (solid line) showing distribution of different types of a clinical refraction of an eye at adults in comparison with a normal variation curve (dashed line). On abscissa axis the degree of a refraction expressed in dioptries is shown (E — an emmetropia; on a piece E — N are designated degrees of a hypermetropia, on a piece E — M — degrees of a myopia).

If to recognize that expression of refraction norm is not only the emmetropia, and and small degrees of an ametropia, then comparison of two curves is of great interest: the refraction curve A. Betsch characterizing optical system and received on the basis of numerous data (a research of 12 thousand eyes), and a normal variation curve, edges serves as expression of normal biological variability of parameters of an eye. More or less full coincidence of these curves is noted only at children's age. At adults the refraction curve differs from normal variation, first, in the peakedness a little, and secondly, a nek-eye shift towards a myopia (fig. 2). Extreme degrees of ametropias go beyond biological variability.

Analyzing various theories of emergence of R. of, it is possible to consider that in clinical R.'s formation as it is necessary to recognize participation and a role of both hereditary factors, and environmental factors.



Bibliography: Averbakh M. I. Ophthalmologic sketches, page 220, M., 1949; Avetisov E. S. Protection of sight of children, page 39, M., 1975; V. V. Wolves and Sh and l I e in V. G. General and military ophthalmology, L., 1980; D and sh e in - with to and y A. I. New methods of studying of optical system of an eye and development of its refraction, Kiev, 1956; Odintsov V. P. Course of eye diseases, page 59, etc., M., 1946; Tronas E. Zh. Optical bases of an ametropia, Saturday. in commemoration of the fortieth anniversary nauch. deyateln. zasl. the scientist M. I. Averbakh, page 489, M. — L., 1935; it, Variability of elements of the optical apparatus of an eye and its value for clinic, L., 1947; Betsch A. tJber die menschliche Refraktionskurve, Klin. Mbl. Augenheilk., Bd 82, S. 365, 1929; Steiger A. Die Entstehung der spharischen Refraktionen des menschlichen Auges, B., 1913.


M. L. Krasnov.

Яндекс.Метрика