Contact Vision Correction - Indications and Contraindications

Contact correction is widely used for myopia. It is known that patients with high myopia, especially with anisometropia, do not tolerate full spectacle correction well, and the resulting optical aberrations are not compensated by glasses. Contact lenses are well tolerated by myopia of almost any degree and myopic anisometropia with any differences in refraction of both eyes. They help restore binocular vision, improve the condition of the eye's accommodative apparatus and ensure high visual performance, whereas when wearing glasses, one has to be content with only tolerable correction (with a difference in the optical power of spectacle lenses of no more than 2.0 diopters).

Thus, myopia is the type of ametropia in which contact lenses have absolute advantages over glasses.

Relative indications for contact vision correction can be considered hyperopia and hyperopic anisometropia. If minus glasses reduce the size of the image on the retina, then plus glasses, on the contrary, increase it, therefore patients with hyperopia adapt worse to contact lenses, which, as is known, do not change the size of the image.

With astigmatism, it is not always possible to achieve maximum visual acuity with the help of glasses. Contact lenses successfully compensate for corneal deformations. When using lenses, patients do not complain of asthenopia. Toric soft contact lenses and gas-permeable hard contact lenses provide a clear, high-contrast image.

Contact lenses are also used to correct aphakia.

In keratoconus, the average visual acuity of patients using contact lenses is approximately 2 times higher than with spectacle correction. In addition, patients who wear contact lenses have stable binocular vision. Gas-permeable rigid contact lenses are used to correct vision in the initial stages of keratoconus. With severe corneal deformation, the selection of contact lenses is difficult, and if it is possible to do so, their tolerability is reduced. In this case, keratoplasty is indicated.

Cosmetic masking contact lenses are successfully used for congenital and post-traumatic eye lesions. In the case of iris coloboma or aniridia, contact lenses with a colored peripheral zone and a transparent pupil not only provide a cosmetic effect, but also reduce light scattering on the retina, eliminate photophobia and improve visual acuity.

Hard contact lenses are used to correct presbyopia, but in recent years bifocal and multifocal soft contact lenses have appeared.

Contact correction is used in the treatment of amblyopia. For this purpose, cosmetic contact lenses with a shaded (opaque) pupil zone or high-power lenses are prescribed to switch off the better-seeing eye. This creates conditions for connecting the amblyopic eye to visual work.

Contact lenses can be used for therapeutic purposes in various corneal diseases. As practice shows, soft contact lenses saturated with medicinal preparations are very effective in the treatment of eye diseases and injuries (in bullous keratopathy, non-healing corneal ulcers, dry eye syndrome to maintain corneal moisture, for the rehabilitation of patients after keratoplasty and eye burns).

However, there are objective medical contraindications to contact correction. First of all, these are inflammatory diseases of the anterior segment of the eye. Wearing contact lenses causes a deterioration in the condition of patients with parasitic eye diseases (especially with widespread mite blepharitis - demodicosis). Contact lenses should be prescribed with special caution to patients with dry eyes. In this case, it is better to use highly hydrophilic soft contact lenses (with a moisture content of more than 55%), and also use special moisturizing drops when wearing lenses.

Contact lenses are contraindicated in cases of obstruction of the lacrimal ducts and dacryocystitis.

In cases of iterigium and pinguecula, the selection of contact lenses is difficult due to mechanical obstacles to their movement on the cornea. In these cases, it is recommended to perform preliminary surgical treatment.

Finally, mental illness is an absolute general contraindication to wearing contact lenses.

Basic principles of contact lens selection and their features. The main criteria for choosing hard contact lenses over soft contact lenses are the presence of pronounced astigmatism (more than 2.0 D), a small palpebral fissure, a small corneal diameter, and intolerance to soft contact lenses. It should be emphasized that hard contact lenses are easier to care for, they cause fewer complications, and they can be used for a longer period of time.

To select the optimal parameters of corneal hard contact lenses, it is necessary to determine the overall diameter of the lens, the diameter of the optical zone, the optical power of the lens and the shape of its inner surface. When selecting the overall diameter of hard contact lenses, the dimensions of the palpebral fissure, the position and tone of the eyelids, the degree of protrusion of the eyeball, the diameter and shape of the cornea are taken into account. The overall diameter of hard contact lenses should be 1.5-2 mm less than the horizontal diameter of the cornea. The inner surface of corneal hard contact lenses has three zones: the central, or optical, sliding zone and the marginal zone. The diameter of the optical zone of the lens should be greater than the width of the pupil, determined in diffuse light, so that the displacement of the lens during blinking does not lead to a noticeable shift of the optical zone of the lens beyond the pupil. The sliding zone should correspond as much as possible to the shape of the cornea in this place and is designed to hold the hard contact lenses on the cornea due to the forces of capillary attraction. The lower the lens pressure on the cornea in the sliding zone, the higher the lens tolerance. The shape of the lens edge is determined empirically. It should ensure the formation of a meniscus of tear fluid and not cause discomfort to the patient. To hold the lens on the eye due to capillary forces, the gap between the lens and the cornea should be small enough, close in thickness to the natural tear film.

The optical power of a contact lens is determined based on the results of a clinical refraction study of the eye: it is equal to the spherical component of refraction + 1/2 the value of the cylindrical component. The final optical power of the lens is determined using a trial lens, the optical power of which is closest to the clinical refraction value. To the trial lens

Different spectacle lenses from the set are applied to obtain maximum visual acuity. When correcting myopia, a spectacle lens of minimum dioptric power is selected to obtain the greatest visual acuity, and when correcting hyperopia and aphakia, a lens of maximum dioptric power is selected.

To calculate the refraction of a contact lens, the optical power of the spectacle lens with which the patient had maximum visual acuity is added to the optical power of the trial lens. The success of selecting contact lenses depends on the following factors: the correspondence of the inner surface of the lens to the shape of the cornea, the centering of the lens and its mobility.

When selecting rigid contact lenses, they strive to achieve maximum compliance of the inner surface of the lens with the shape of the cornea, taking into account the preservation of a certain thickness of the tear fluid layer between the lens and the cornea in different zones. The tear fluid is tinted with a 0.5% fluorescein solution and, using a slit lamp in the light of a blue light filter, the distribution of fluorescein under the test lens is assessed and the need for changes in the design of the individually manufactured lens is determined.

It is extremely important to study the sensitivity of the cornea, the condition of the lacrimal organs, the production of lacrimal fluid, and the time of tear film rupture. Then the size of the eye slit, the turgor of the eyelids, and the diameter of the pupil are determined.

After a routine ophthalmological examination, they begin to select the lens shape and its optical power. Using an ophthalmometer, they determine the radius of curvature of the cornea in the main meridians and decide on the type of lens.

Under local anesthesia (0.5% dicaine solution), hard contact lenses from a trial set of lenses are placed on the eye, the optical power of which and the design parameters (total diameter, diameter of the optical zone and shape of the inner surface) correspond to the parameters of the corrected eye to the greatest extent. The position of the lens on the eye, its mobility, and the distribution of fluorescein under the lens are assessed. If the design of the lens selected from the trial set is optimal, an individual hard contact lens is made.

To finally confirm the correct selection of gas-permeable hard contact lenses, it is necessary to observe the patient for 2-3 days, gradually increasing the lens wearing time daily. During the adaptation period, the lens can be modified. At the end of the trial period, the manufactured lens is given to the patient, after instructing him on the rules of use and wearing regimen.

Spherical soft contact lenses, due to their elasticity, are effective only in the absence of significant changes in the shape of the cornea, since they largely repeat its irregular shape (for example, with astigmatism over 2.0 D). The selection of soft contact lenses is quite simple and is based on the results of ophthalmometry. There are special tables of correspondence between the radii and refractions of the cornea, the optical power of spectacle lenses and soft contact lenses. The thickness of soft contact lenses is selected taking into account the individual characteristics of the eye. With moderate astigmatism or reduced production of tear fluid, thicker soft contact lenses are prescribed (thin, highly hydrophilic soft contact lenses dehydrate faster and do not correct astigmatism). After the initial selection of soft contact lenses, the position of the lens on the eye, its mobility and the subjective sensations of the patient are assessed.

The correct position of the lens can be checked using the "shift test", where the lens is moved along the cornea by 1/3-1/2 of its diameter: if the fit is good, the lens should slowly return to the central position.

The recommended duration of wearing soft contact lenses during the adaptation period: in the first 3 days - 1-2 hours per day, in the next 3 days - up to 3 hours per day, from the 2nd week - for 4 days for 4 hours per day, then 3 days for 5 hours per day, from the 3rd week the time of wearing soft contact lenses is increased daily by 1 hour, bringing it up to 12 hours per day.

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