Methods for the study of refraction

The most common subjective method of investigating refraction is a method based on determining the maximum visual acuity with correction. Ophthalmological examination of the patient regardless of the expected diagnosis begins with the application of this diagnostic test. In this case, two problems consistently solve: determine the type of clinical refraction and evaluate the degree (magnitude) of clinical refraction.

The maximum visual acuity should be understood as the level that is achieved with correct, full-fledged correction of ametropia. With adequate correction of ametropia, the maximum visual acuity should approximate to the so-called normal and is designated as complete, or corresponding to the "unit". It should be remembered that sometimes due to the peculiarities of the structure of the retina, the "normal" visual acuity can be more than 1.0 and be 1.25; 1.5 and even 2.0.

Method of conducting

To conduct the study, a so-called spectacle frame, a set of test lenses and test objects for assessing visual acuity are needed. The essence of the technique is reduced to determining the influence of test lenses on visual acuity, while the optical power of the lens (or those - with astigmatism) of the lens, which ensures maximum visual acuity, will correspond to the clinical refraction of the eye. The basic rules of the study can be formulated as follows.

• With visual acuity equal to 1.0, it is possible to assume the presence of emmetropic, hypermetropic (compensated tension of accommodation) and mild-miopic refraction. Despite the fact that in most textbooks it is recommended to start the study with a lens eye to the eye with the force of +0.5 D, it is advisable to first use the lens -0.5 Dpt. With emmetropia and hypermetropia, such a lens under conditions of cycloplegia will cause vision impairment, and under natural conditions, visual acuity can remain unchanged because of compensation of the force of the lens by the accommodation voltage . With weak myopia, regardless of the state of accommodation, an increase in visual acuity can be noted. At the next stage of the study, a +0.5 diopter lens should be placed in the trial frame. With emmetropia, in any case, there will be a decrease in visual acuity, with hypermetropia in conditions of disabled accommodation, its improvement will be established, and with safe accommodation, vision may remain unchanged, since the lens compensates for only part of the latent hypermetropia.
• With a visual acuity of less than 1.0, one can assume the presence of myopia, hypermetropia and astigmatism. The study should begin with an eye to the eye lens -0.5 Dpt. With myopia, a tendency to increase visual acuity will be noted, and in other cases vision will either deteriorate or remain unchanged. In the next stage, the use of a lens of +0.5 Dptra will reveal hypermetropic refraction (vision or remains unchanged or, as a rule, increases). In the absence of a tendency to change the visual acuity against the background of correction with spherical lenses, one can assume the presence of astigmatism. To clarify the diagnosis, it is necessary to use special lenses from the test set-the so-called cylinders, in which only one of the sections is optically active (it is located at an angle of 90 ° to the cylinder axis indicated on the astigmatic lens). It should be noted that the exact subjective definition of the type and especially the degree of astigmatism is a laborious process (despite the fact that special tests and techniques are proposed for this). In such cases, the basis for establishing the diagnosis should be the results of objective studies of refraction.
• After establishing the type of clinical refraction, the degree of ametropia is determined, while changing the lenses, they achieve maximum visual acuity. In determining the magnitude (degree) of ametropia, the following basic rule is adhered to: from a number of lenses that equally affect the visual acuity, with a myopic refraction, choose the lens with the lowest absolute force, and for the hypermetropic refraction, choose the lens with the greatest absolute power.

It should be noted that in order to determine the maximum visual acuity, trial contact correction using a rigid contact lens that corrects not only ametropia, but also aberration of the anterior surface of the cornea can be used. In polyclinic conditions, instead of this test, it is recommended to carry out a test with a diaphragm. At the same time, in the process of subjective examination of refraction, visual acuity is determined with test spectacle lenses and a diaphragm 2.0 mm in diameter, which are simultaneously placed in a trial rim. However, the described method has a number of difficultly removable disadvantages. First, in the course of the study, one has to focus on the level of visual acuity, the reduction of which can be caused not only by the presence of ametropia, but also by pathological changes in optical media and the neuro-receptor apparatus. In addition, the method is not applicable in the absence of contact with the patient (for example, in young children), as well as simulation and aggravation. In these cases, objective methods of investigating refraction are more informative, in particular, skiascopy, conventional and automatic refractometry, and ophthalmometry.

More accurate data on clinical refraction can be obtained with the help of special instruments - refractometers. In a simplified form, the principle of operation of these devices can be represented as recording of reflected light signals from the retina, the focusing of which depends on the type and degree of clinical refraction.

In conventional refractometers (Hartinger, Rodenstock), the setting, setting the required position and type of the test-mark instrument is done manually. In recent years, these devices in the clinic are practically not used.

More advanced in terms of objectification of the study are automatic refractometers in which the analysis of the reflected infrared light from the retina is carried out automatically with the help of a special electronic unit. The peculiarities of the refractive index technique on these devices are detailed in the instructions to each of them. The main thing is that the refractive examination on automatic refractometers is usually carried out by the average medical staff, and the results are printed out on a special form on the following basic parameters: the magnitude of the spherical ametropia, the magnitude of the astigmatism, the position of one of the main meridians. Despite the relatively high cost of automatic refractometers, in recent years they have gradually become an integral part of the staff equipment of the office of the ophthalmologist.

The general lack of refractometers of various types - the so-called instrumental accommodation - is a phenomenon due to which the data obtained in the study may have a shift towards myopic refraction. The reason for this is the impulse to the accommodation voltage, caused by the location of the optical part of the device at a small distance from the eye being studied. In some cases, the objectification of refractometric data requires cycloplegia. In the latest models of automatic refractometers, devices are provided that reduce the possibility of the emergence of instrumental accommodation.

The methods described above are designed to determine the clinical refraction of the eye.

Ophthalmometry

In foreign terminology - keratometry  - an objective method of studying only refraction of the cornea. The essence of the method is reduced to the measurement of mirror images projected onto the cornea of the instrument's test stamps (ophthalmometer), whose dimensions, other things being equal, depend on the radius of curvature of the anterior surface of the cornea. During the study, the position of the main meridians of the cornea (in degrees), as well as the optical force (in diopters) and the radius of curvature of the anterior surface of the cornea (in milliliters) in the indicated meridians are determined. It should be noted that between the last indicators there is a clear dependence: the smaller the radius of curvature of the cornea, the greater its optical strength.

In some models of automatic refractometers there is a unit, with which during the research in parallel with clinical refraction (i.e., total refraction of the eye), the refraction of the cornea is assessed.

Although, based on the results of ophthalmometry, it is impossible to judge the clinical refraction of the eye as a whole, but in a number of situations they can be important and even fundamental.

• In the diagnosis of astigmatism, the results of ophthalmometry can be used as a starting point. In any case, they should be clarified, if possible, with the help of refractometry and necessarily by subjective examination of refraction. The latter circumstance is associated with the possible influence on the parameters of the general astigmatism of lens astigmatism.
• The data obtained in ophthalmometry (in particular, on corneal refraction), along with the length of the anteroposterior axis, are used in various formulas to calculate the parameters of refractive operations (for example, radial keratotomy) and the optical strength of intraocular lenses (IOLs) used to correct ametropia various genesis (for example, hypermetropia, usually arising after the removal of cataracts ).
• Precise determination of the radius of curvature of the anterior surface of the cornea is necessary when choosing such an important parameter of contact lenses as the base radius of their posterior (facing the eye) surface. This measurement is necessary, conditionally speaking, to achieve congruence of the front surface of the cornea and the back surface of the contact lens.
• Informativeness of ophthalmometry is quite high in cases of abnormal corneal astigmatism, which is usually acquired - is formed due to various lesions of the cornea (traumatic, inflammatory, dystrophic, etc.). In this study, a significant increase or, conversely, weakening of the refraction of the cornea, a violation of the mutually perpendicular arrangement of its main meridians, a distortion of the shape of the mirror image of the test marks on the cornea can be revealed during the study.

With the help of ophthalmometry, it is possible to study the refraction of the cornea only in the central (diameter 2.5-3 mm) zone. Meanwhile, even in the absence of astigmatism, the shape of the whole surface of the cornea differs from the spherical and geometrically conditionally can be represented as a paraboloid of rotation. In practical terms, this means that, even within one meridian, the radius of curvature of the cornea changes: gradually increases in the direction from the center to the periphery of the cornea, and the refraction of the cornea decreases accordingly. Knowing the parameters of the cornea in paracentral and even peripheral areas is necessary in a number of clinical situations: when choosing contact lenses and keratorefractive operations, determining the degree of influence of various diseases of the cornea on its refractive properties, etc.

Keratotopographic methods for studying the refraction of the entire surface of the cornea

Research methods that provide an assessment of the curvature and refraction of the entire surface of the cornea are called keratotopographic, since with their help one can get an idea of the relationship between the refraction of different parts of the cornea (conditionally topography).

An approximate evaluation of the refraction of the entire surface of the cornea can be carried out using such a simple method as keratoscopy, during which an image of concentrically arranged circles is projected onto the cornea using an uncomplicated device (keratoscope). The keratoscope is a disk with illuminated alternating white and black concentric circles. If the cornea has a shape close to spherical, the image is formed from the correctly arranged circles. With astigmatism, these images take the form of an oval, and with an incorrect astigmatism their ordered arrangement is disrupted. With the help of a keratoscope, only a qualitative assessment of the sphericity of the cornea can be obtained.

Photokeratography study

Photokeratography study of corneal topography provides for the mathematical processing of photokeratograms (images of mirror images of circles). In addition, the measurement of the refraction of different sections of the cornea can be performed with a conventional ophthalmometer equipped with a special attachment for changing the fixation of the patient's gaze (the so-called fixation holometry).

However, the most informative method for studying corneal refraction is computerized keratotopography. Special instruments (keratotopographs) provide the possibility of conducting a detailed objective analysis of refraction and curvature at various sites of the cornea. In keratotopografah laid several computer programs for processing the results of the study. There is also a particularly clear variant of data processing with the help of so-called color mapping: the color and intensity of coloring of different zones of the cornea depends on the refraction of the latter.

The question of the consistency of the application of subjective and objective methods of investigating refraction is important. Obviously, in the presence of automatic refractometers, objective refractometry can precede a subjective evaluation of refraction. However, it is subjective tests that should be of fundamental importance not only in establishing the final diagnosis, but also in selecting an adequate method for correcting ametropia.