Vision correction with spectacle lenses

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

In myopia, the main goal of correction is to reduce refraction, in hyperopia, to increase it, and in astigmatism, to unevenly change the optical power of the main meridians.

In some cases, when choosing a method for correcting ametropia, it is necessary to use the term "intolerance" of correction. This term is collective: it unites a complex of objective and subjective symptoms, in the presence of which the use of a particular correction method is limited.

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

Despite advances in contact and surgical vision correction, glasses remain the most common method of correcting ametropia. Their main advantages include availability, virtually no complications, the ability to model and change the strength of correction, and the reversibility of the effect. The main disadvantage of glasses is due to the fact that the spectacle lens is located at a certain distance (about 12 mm) from the apex of the cornea and, thus, does not form a single optical system with the eye. In this regard, spectacle lenses (especially so-called high refractions) have a significant effect on the magnitude of the retinal, i.e., formed on the retina, image of objects. Scattering (negative) lenses that weaken refraction reduce them, while intensifying, collecting (positive) lenses, on the contrary, increase them. In addition, spectacle lenses of high refraction can change the field of vision.

Depending on the optical action, stigmatic or spherical, astigmatic or aspherical, and prismatic spectacle lenses are distinguished. In astigmatic lenses (cylinders), an axis and an optically active section located perpendicular to the axis are distinguished. Refraction of rays occurs only in the plane of the active 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, it is necessary to solve two closely related problems: determine the static refraction of each eye; select adequate optical correction, which depends on the state of static and dynamic refraction, the patient's age, monocular and binocular tolerance of glasses, as well as the indications for their prescription.

It is advisable to adhere to the following order of examination:

• determination of visual acuity of each eye;
• clarification of the type and degree of ametropia using a subjective method (preliminary automatic refractometry may be performed), based on determining the maximum visual acuity with correction (a significant increase in visual acuity will indicate the predominant influence of refraction on this indicator);
• in preschool children and patients with amblyopia, performing drug-induced cycloplegia and determining refraction using objective and subjective methods under conditions of switched-off accommodation;
• clarification of maximum visual acuity using a trial contact correction or a test with a diaphragm;
• selection of glasses taking into account the general rules for prescribing spectacle lenses for various types of ametropia and their tolerability, given below, taking into account the results of trial wearing of glasses for 15-30 minutes (reading, walking, moving the gaze from one object to another, head and eye movements); in this case, the quality of binocular tolerability of glasses is taken into account for both distance and near vision.

Indications for prescribing glasses for farsightedness are asthenopic complaints or decreased visual acuity in at least one eye. In such cases, as a rule, permanent optical correction is prescribed depending on subjective tolerance with a tendency to maximum correction of ametropia. If such correction does not provide improvement in asthenopia, then stronger lenses (by 1.0-2.0 diopters) are prescribed for visual work at close range. In case of low farsightedness and normal visual acuity, it is possible to limit the prescription to glasses for work only at close range.

For young children (2-4 years) with hyperopia of more than 3.5 diopters, it is advisable to prescribe glasses for permanent wear 1.0 diopters weaker than the degree of ametropia. In such cases, the meaning of optical correction is to eliminate the conditions for the occurrence of accommodative strabismus. If by the age of 6-7 the child retains stable binocular vision and high visual acuity without correction, glasses are canceled.

In case of mild to moderate myopia, "submaximal" correction is usually recommended for distance vision (corrected visual acuity within 0.7-0.8). In some cases, taking into account professional activity, full correction is possible. The rules of optical correction for near vision are determined by the state of accommodation. If it is weakened (reduced reserve of relative accommodation, pathological types of ergographic curves, visual discomfort when reading with glasses), a second pair of glasses is prescribed for working at close range or bifocal glasses for constant wear. The upper half of the lenses in such glasses is used for distance vision and completely or almost completely corrects myopia, the lower half of the lenses, intended for working at close range, is weaker than the upper by 1.0; 2.0 or 3.0 D depending on the patient's subjective feelings and the degree of myopia: the higher it is, the greater the difference in the power of lenses intended for distance and near vision. This is the so-called passive method of optical correction of myopia.

In case of high myopia, permanent correction is prescribed. The strength of the lenses for distance and near is determined depending on the subjective tolerance of the correction. In case of its intolerance, it is possible to decide on 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 capacity, full or almost full optical correction is prescribed for work at close range (active method of myopia correction). In these cases, glasses will encourage accommodation to active activity.

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

In case of anisometropia, permanent optical correction is prescribed taking into account the subjectively tolerated difference between the power of the correcting lenses for the right and left eyes. The possibilities of spectacle correction of anisometropia are limited due to the fact that the size of the image on the retina depends on the optical power of the spectacle lenses. The two images differ significantly in size and do not merge into a single image. If the difference in the power of the lenses is more than 3.0 D, aniseiconia is observed (from the Greek anisos - unequal, eikon - image), which has a significant impact 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 to 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 signs of decompensation;
• double vision (diplopia) against the background of paresis of the oculomotor muscles;
• some forms of concomitant strabismus (in combination with other treatment methods).

The prismatic effect can be achieved using conventional glass prisms, so-called Fresnel lenses (which are fixed to the back surface of a conventional spectacle lens by pressing), bifocal spheroprismatic glasses (BSPO) 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 diopters are not manufactured due to their large size and weight. Fresnel attachments to glasses - thin plates made of soft plastic - are lightweight and easy to use. Shifting the center of spectacle lenses by 1.0 cm provides a prismatic effect of 1.0 prism diopter for each diopter of optical power of a conventional spectacle lens. In a positive lens, the prism base is directed toward the center shift, and in a negative lens - in the opposite direction. The BSPO proposed by E. V. and Yu. A. Utekhin can be used to relieve accommodation and convergence. At the bottom of the "minus" glasses for distance, an element for near vision is glued, consisting of a combination of a "plus" sphere of 2.25 diopters and a prism with a power of 6.75 prism diopters, the base of which is facing the nose.

Correction of presbyopia is based on the use of positive (converging) lenses when working at close range. According to various sources, the age at which the need for selection of "presbyopic" glasses arises ranges from 38 to 48 years and depends on the type and degree of concomitant ametropia, type of work activity, etc. Ultimately, the question of the advisability of presbyopic glasses is decided individually, taking into account the patient's complaints. As a rule, the first symptoms of presbyopia are the need to move an object away from the eye (as a result of which the degree of accommodation tension decreases) and the appearance of complaints of asthenopia by the end of the working day.

Various methods have been proposed for determining the power of spectacle lenses intended for the correction of presbyopia (including those that involve studying the volume of accommodation). However, in clinical practice, the most common method is based on the so-called age norms: the first glasses - +1.0 D are prescribed at the age of 40-43 years, subsequently the power of the glasses is increased by approximately 0.5-0.75 D every 5-6 years. The final value of presbyopic correction by the age of 60 is +3.0 D, which provides the ability to perform visual work at a distance of 33 cm.

When presbyopia is combined with ametropia, an adjustment is made to the calculation of the lens power - the power of the spherical lens (with the corresponding sign) is added, which corrects ametropia. The cylindrical component of the correction, as a rule, remains unchanged. Thus, with hyperopia and presbyopia, the spherical component of the glasses for distance is increased by the amount of presbyopic correction, and with myopia, on the contrary, it is decreased.

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

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

When presbyopia is combined with convergence insufficiency, it is advisable to use spheroprismatic lenses. A prism whose base is turned toward the nose due to the deflection of rays toward 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 positive spectacle lenses compared to the interpupillary distance.

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