Correction of vision with eyeglass lenses

The main task of any correction of ametropia ultimately amounts to creating conditions for focusing images of objects on the retina. Depending on the principle of action, the methods of correction of ametropia can be conditionally divided into two large groups: methods that do not change the refraction of the main refracting media of the eye, spectacle and contact lenses, or the so-called traditional correction means; methods that change the refraction of the main refractive media of the eye, surgical.

With myopia, the main goal of correction is to reduce refraction, with hypermetropia - its amplification, and with astigmatism - an uneven change in the optical power of the main meridians.

In a number of cases, when choosing the correction method for ametropia, the term "intolerance" of correction should be used. This term is collective: it combines a complex of objective and subjective symptoms, in the presence of which the application of this or that method of correction is limited.

It is necessary to distinguish the direct effect of correction on visual acuity and visual performance - the "tactical" effect of optical correction, as well as the effect on refractive dynamics and certain painful eye conditions (asthenopia, spasm of accommodations, amblyopia, strabismus) - a strategic effect. The second effect is to a certain extent realized through the first.

Despite achievements in the field of contact and surgical correction of vision, glasses remain the most common method of correction of ametropia. Their main advantages include accessibility, practical absence of complications, the possibility of modeling and changing the strength of the correction, as well as the reversibility of the effect. The main drawback of glasses is due to the fact that the spectacle lens is located at a certain (about 12 mm) distance from the top of the cornea and, therefore, does not make up one optical system with the eye. In this regard, spectacle lenses (especially the so-called high refractive indices) have a significant effect on the magnitude of the retinal image, that is, formed on the retina, images of objects. Weakening refraction scattering (negative) lenses reduce them, and amplifying, collecting (positive), on the contrary, increase. In addition, spectacle lenses of high refraction can change the field of view.

Depending on the optical effect, stigmatic, or spherical, astigmatic, or aspheric, and prismatic spectacle lenses are distinguished. In the astigmatic lenses (cylinders), the axis and the optically active cross section are perpendicular to the axis. The refraction of the rays occurs only in the plane of the active cross section. According to the number of optical zones spectacle lenses are divided into monofocal and multifocal (two zones or more).

When examining a patient for the purpose of prescribing glasses, two closely related tasks must be solved: to determine the static refraction of each eye; choose an adequate optical correction, which depends on the state of static and dynamic refraction, the age of the patient, monocular and binocular tolerance of glasses, as well as indications for their purpose.

It is advisable to adhere to the following examination procedure:

• determining the visual acuity of each eye;
• Specification of the type and degree of ametropia with the help of a subjective method (preliminary automatic refractometry is possible), based on determining the maximum visual acuity with correction (a significant increase in visual acuity will indicate a predominant effect of refraction on this indicator);
• in pre-school children and patients with amblyopia, medication cycloplegia and the definition of refraction with the help of objective and subjective methods in conditions of the disabled accommodation;
• clarification of maximum visual acuity with the help of trial contact correction or a sample with a diaphragm;
• selection of glasses, taking into account the following general rules for the appointment of spectacle lenses for various types of ametropia and their tolerability, taking into account the results of trial wearing glasses for 15-30 minutes (reading, walking, moving eyes from one subject to another, head and eyes); while taking into account the quality of the binocular tolerance of the glasses for both distance and near.

Indications for the appointment of glasses for farsightedness are asthenopic complaints or reduced visual acuity of at least one eye. In such cases, as a rule, a permanent optical correction is appointed depending on subjective tolerance with a tendency to the maximum correction of ametropia. If, with asthenopia, such correction does not improve, then for visual work at a close distance, more powerful (1.0-2.0 D) lenses are prescribed. With farsightedness of a low degree and normal visual acuity, you can limit the appointment of glasses for work only at close range.

To children of early age (2-4 years) with farsightedness more than 3.5 Dptr, it is advisable to prescribe glasses for permanent wearing by 1.0 dpts weaker than the degree of ametropia. In such cases, the meaning of optical correction consists in eliminating the conditions for the emergence of accommodative squint. If the child has stable binocular vision and high visual acuity without correction, to 6-7 goals, the glasses are canceled.

With myopia of mild to moderate degree, as a rule, a "submaximal" correction is recommended (corrected visual acuity within 0,7-0,8). In some cases, taking into account professional activities, full correction is possible. The rules of optical correction for near are determined by the state of accommodation. If it is weakened (a decrease in the reserve of relative accommodation, pathological types of ergographic curves, visual discomfort when reading in glasses), designate a second pair of glasses for working at close range or bifocals for permanent wearing. The upper half of the glasses in these glasses serves to view into the distance and completely or almost completely corrects myopia, the lower half of the glasses designed to work at close range, weaker than the top by 1.0; 2.0 or 3.0 D, depending on the subjective sensations of the patient and the degree of myopia: the higher it is, the more usually the difference in strength of the lenses intended for the distance and for near. This is the so-called passive method of optical correction of myopia.

With high-grade myopia, a permanent correction is prescribed. The power of lenses for distance and for nearness is determined depending on the subjective tolerance of the correction. With its intolerance, it is possible to solve the problem of contact or surgical correction of myopia.

In order to increase the accommodative capacity of the myopic eye, special exercises are performed for the ciliary muscle. If it is possible to achieve stable normalization of this ability, a full or almost complete optical correction is prescribed and for working at close range (an active method of correction of myopia). In these cases, the glasses will prompt the accommodation to be active.

With astigmatism of all types, constant wearing of glasses is shown. The astigmatic correction component is prescribed depending on subjective tolerance with a tendency to complete correction of astigmatism, spherical - in accordance with the general rules for prescribing glasses for farsightedness and nearsightedness.

With anisometropia, a permanent optical correction is prescribed taking into account the subjectively tolerated difference between the strength of the correcting lenses for the right and left eyes. The possibilities of spectacle correction anisometry are limited due to the fact that the magnitude of the image on the retina depends on the optical power of the spectacle lenses. Two images vary greatly in size and do not merge into a single image. With a difference in lens strength of more than 3.0 D, aniseiconia is noted (from the Greek anisos - unequal, eikon - image), which has a significant effect on the tolerance of glasses. In these cases, there are medical indications for the use of contact lenses and refractive surgery.

Prismatic lenses have the property of deflecting light rays toward the base of the prism. The main indications for the appointment of such lenses can be combined into three main groups:

• heterophoria (imbalance of the oculomotor muscles) with the phenomena of decompensation;
• double vision (diplopia) in the background of paresis of oculomotor muscles;
• some forms of friendly strabismus (in conjunction with other methods of treatment).

The prismatic effect can be achieved with the help of conventional glass prisms, the so-called Fresnel lenses (which are fixed by pressing on the back surface of a conventional spectacle lens), bifocal spherical prismatic glasses (FBS), and by shifting the center of the lenses in the frame.

Spectacle prisms made of glass with an optical power of more than 10.0 prism dioptres are not manufactured because of their large size and weight. Fresnel nozzles for glasses - thin plates of soft plastic - are light and easy to use. The displacement of the center of spectacle lenses by 1.0 cm provides a prismatic effect of 1.0 prism dioptre for each diopter of the optical power of a conventional spectacle lens. In the positive lens, the base of the prism is directed toward the center shift, and in the negative lens to the opposite. FBS, proposed by Ye. V. And Yu. A. Utekhin, can be used for unloading accommodation and convergence. In the lower part of the "minus" glasses for the left, an element for near is pasted, consisting of a combination of a "plus" sphere of 2.25 diopters and a prism with the power of 6.75 prism dioptres, whose base is turned towards the nose.

Correction of presbyopia is based on the use of positive (collecting) lenses when working at close range. According to various sources, the age at which it is necessary to select "presbyopic" glasses varies from 38 to 48 years and depends on the type and degree of concomitant ametropia, the type of labor activity, etc. Ultimately, the question of the expediency of prescribing the first presbyopic glasses decide individually taking into account the patient's complaints. As a rule, the first symptoms of presbyopia are the need to move the object away from the eye (as a result, the degree of tension of accommodation decreases) and the appearance of complaints on asthenopia by the end of the working day.

Various methods for determining the strength of spectacle lenses intended for the correction of presbyopia (including those providing for the study of the accommodation volume) are proposed. However, in clinical practice, the most common method is in which they are guided by the so-called age norms: the first glasses - +1.0 diopters are prescribed at the age of 40-43 years, subsequently they increase the strength of the glasses by approximately 0.5-0.75 Dpt every 5 -6 years. The final value of presbyopic correction by the age of 60 is +3.0 D, which makes it possible to perform visual work at a distance of 33 cm.

When combining presbyopia with ametropia, the force of the lenses is corrected by adding the force of the spherical lens (with the corresponding sign), which corrects the ametropia. The cylindrical correction component, as a rule, remains unchanged. Thus, with hypermetropia and presbyopia, the spherical component of glasses for distances is increased by the amount of presbyopic correction, and in myopia, on the contrary, is reduced.

Ultimately, when prescribing glasses to correct presbyopia, the test for subjective tolerance - reading the text with trial lenses for a certain period of time - is crucial.

In order to avoid the use of several pairs of glasses in presbyopia, combined with ametropia, it is advisable to designate bifocal and even multifocal glasses, the upper part of which is intended for vision in the distance, and the lower one for near. There is also a method that allows within one subjectively tolerated difference in the power of the lenses one eye to be corrected for vision in the distance and the other for near vision.

When combining presbyopia with a lack of convergence, it is advisable to use spheroprymatic lenses. The prism, whose base is turned to the nose due to the deflection of the rays in the nose, helps to reduce the degree of convergence. A small prismatic effect can be achieved by deliberately reducing the distance between the centers of the positive spectacle lenses in comparison with the interpupillary distance.

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