Peripheral vision. Peripheral and central vision: features Peripheral vision impairment

Central or form vision is carried out by the most highly differentiated area of the retina - the central fovea of the macula, where only cones are concentrated. Central vision is measured by visual acuity. The study of visual acuity is very important for judging the state of the human visual apparatus and the dynamics of the pathological process.

Visual acuity refers to the ability of the eye to separately distinguish two points in space located at a certain distance from the eye.

When studying visual acuity, the minimum angle at which two light stimuli of the retina can be perceived separately is determined. Based on numerous studies and measurements, it has been established that a normal human eye can separately perceive two stimuli at a visual angle in one minute.

This visual angle value is taken as the international unit of visual acuity. This angle on the retina corresponds to a linear value of 0.004 mm, approximately equal to the diameter of one cone in the central fovea of the macula. For separate perception of two points by an optically correct eye, it is necessary that on the retina between the images of these points there is a gap of at least one cone, which is not irritated at all and is at rest. If the images of the dots fall on adjacent cones, then these images will merge and separate perception will not work.

The visual acuity of one eye, which can separately perceive points that produce images on the retina at an angle of one minute, is considered normal visual acuity equal to one (1.0). There are people whose visual acuity is higher than this value and is equal to 1.5-2.0 units or more.

When visual acuity is above one, the minimum visual angle is less than one minute. The highest visual acuity is provided by the central fovea of the retina. Already at a distance of 10 degrees from it, visual acuity is 5 times less.

To study visual acuity, various tables with letters or signs of various sizes located on them are proposed. Special tables were first proposed in 1862 by Snellen. All subsequent tables were built on the Snellen principle. Currently, Sivtsev and Golovin tables are used to determine visual acuity.

The tables consist of 12 rows of letters. Each of the letters as a whole is visible from a certain distance at an angle of 50, and each stroke of the letter is visible at a visual angle of 10. The first row of the table is visible with normal visual acuity equal to 1.0 from a distance of 50 m, the letters of the tenth row from a distance of 5 m.

Visual acuity testing is carried out from a distance of 5 m and for each eye separately. On the right side of the table there is a number indicating visual acuity when tested from a distance of 5 m, and on the left there is a number indicating the distance from which this row should be seen by the person being examined with normal visual acuity.

Visual acuity can be calculated using the Snellen formula:

where V (Visus) is visual acuity, d is the distance from which the patient sees, D is the distance from which an eye with normal visual acuity should see the signs of a given row on the table.

If the subject reads the letters of row 10 from a distance of 5 m, then Visus = 5/5 = 1.0. If he reads only the first line of the table, then Visus = 5/50 = 0.1, etc. If visual acuity is below 0.1, i.e. the patient does not see the first line of the table, then the patient can be brought to the table until he sees the first line and then visual acuity can be determined using the Snellen formula.

In practice, they use the doctor’s extended fingers, taking into account that the thickness of the finger is approximately equal to the width of the stroke of the first row of the table, i.e. it is not the patient who is brought to the table, but the doctor who approaches the patient, showing spread fingers or Pole’s optotypes. And just as in the first case, visual acuity is calculated using the formula. If the patient counts his fingers from a distance of 1 m, then his visual acuity is 1:50 = 0.02, if from a distance of two meters, then 2:50 = 0.04, etc. If the patient counts fingers at a distance of less than 50 cm, then visual acuity is equal to counting fingers at a distance of 40, 30, 20, 10 cm, and counting fingers near the face. If even such minimal form vision is absent, but the ability to distinguish light from darkness remains, vision is designated as infinitesimal vision - light perception 1/infinity.

With light perception with correct light projection, Visus = 1/infinity proectia lucis certa. If the eye of the subject incorrectly determines the projection of light on at least one side, then visual acuity is regarded as light perception with incorrect light projection and is designated Visus = 1/infinity pg. 1. incerta. In the absence of even light perception, vision is zero and is designated as follows: Visus = 0.

The correctness of light projection is determined using a light source and an ophthalmoscope mirror. The patient sits down, as when examining the eye using the transmitted light method, and a beam of light is directed from different directions into the eye being examined, which is reflected from the ophthalmoscope mirror. If the functions of the retina and optic nerve are preserved throughout, then the patient says exactly from which side the light is directed at the eye (top, bottom, right, left).

Determining the presence of light perception and the state of light projection is very important for deciding the appropriateness of certain types of surgical treatment. If, for example, with clouding of the cornea and lens, vision equals correct light perception, this indicates that the functions of the visual apparatus are preserved and one can count on the success of the operation.

Vision equal to zero indicates absolute blindness. More accurately, the condition of the retina and optic nerve can be determined using electrophysiological research methods.

To determine visual acuity in children, children's tables are used, the principle of which is the same as for adults. The display of pictures or signs starts from the top lines. When checking visual acuity for school-age children, as well as for adults, the letters in the Sivtsev and Golovin table are shown starting from the lowest lines.

When assessing visual acuity in children, one must remember the age-related dynamics of central vision. At 3 years of age, visual acuity is 0.6-0.9, by 5 years of age it is 0.8-1.0 for the majority.

In the first week of life, the presence of vision in a child can be judged by the pupillary reaction to light. You need to know that the pupil of newborns is narrow and reacts sluggishly to light, so you need to check its reaction by shining a strong light on the eye and preferably in a darkened room. In the 2nd 3rd week - by briefly fixating the gaze on a light source or a bright object. At the age of 4-5 weeks, eye movements become coordinated and stable central gaze fixation develops. If vision is good, then a child at this age is able to hold his gaze for a long time at a light source or bright objects. In addition, at this age, a reflex of closing the eyelids appears in response to the rapid approach of an object to his face. It is almost impossible to quantify visual acuity even at a later age.

In the first years of life, visual acuity is judged by the distance from which he recognizes the people and toys around him. At the age of 3, and in mentally well-developed children even 2 years old, visual acuity can often be determined using children's tables. The tables are extremely varied in their content.

In Russia, tables by P.G. are quite widespread. Aleynikova, E.M. Orlova with pictures and tables with optotypes of Landolt and Pfluger rings. When examining vision in children, the doctor requires a lot of patience and repeated or multiple examinations.

Color perception, research methods and diagnosis of its disorders

The human eye distinguishes not only the shape, but also the color of an object. Color perception, as well as visual acuity, is a function of the cone apparatus of the retina and the nerve centers associated with it. The human eye perceives colors with wavelengths from 380 to 800 nm.

The richness of colors comes down to 7 colors of the spectrum, into which, as Newton showed, sunlight passed through a prism is decomposed. Rays longer than 800 nm are infrared and are not part of the human visible spectrum. Rays less than 380 nm are ultraviolet and do not cause an optical effect in humans.

All colors are divided into achromatic (white, black and all kinds of gray) and chromatic (all colors of the spectrum except white, black and gray). The human eye can distinguish up to 300 shades of achromatic color and tens of thousands of chromatic colors in various combinations. Chromatic colors differ from each other in three main ways: hue, brightness (lightness) and saturation.

Hue is the quality of color, which we denote by the words red, yellow, green, etc., and it is characterized by wavelength. Achromatic colors have no hue.

The brightness or lightness of a color is its proximity to white. The closer the color is to white, the lighter it is.

Saturation is the density of tone, the percentage of the main tone and impurities to it. The more basic tone a color has, the more saturated it is.

Color sensations are caused not only by a monochromatic ray with a certain wavelength, but also by a combination of rays with different wavelengths, subject to the laws of optical color displacement. Each primary color has a corresponding additional color, which when mixed with it produces white.

Pairs of complementary colors are at diametrically opposite points of the spectrum: red and green, orange and blue, blue and yellow. Mixing colors in the spectrum that are located close to each other gives the feeling of a new chromatic color. For example, mixing red with yellow produces orange, and mixing blue with green produces blue. The full variety of color sensations can be achieved by mixing only three primary colors: red, green and blue. Because There are three primary colors, then special elements must exist in the retina to perceive these colors.

The three-component theory of color perception was proposed in 1757 by M.V. Lomonosov and in 1807 the English scientist Thomas Young. They suggested that the retina contains three types of elements, each of which is specific only for one color and does not perceive another. But in life it turns out that the loss of one color is associated with a change in the entire color worldview.

If there is no sensation of red, then both green and purple colors become somewhat altered. 50 years later, Helmholtz, who came up with his theory of three-componentity, pointed out that each of the elements, being specific to one primary color, is also irritated by other colors, but to a lesser extent. For example, the color red irritates red elements most, but green and purple elements to a lesser extent. Green rays are strongly green, weakly red and violet. The violet color has a very strong effect on violet elements, and less strongly on green and red elements. If all three kinds of elements are irritated in strictly defined relationships, then the sensation of white color is obtained, and the absence of stimulation gives the sensation of black color.

Stimulation of only two or all three elements by two or three stimuli in varying degrees and proportions leads to the sensation of the entire gamut of colors found in nature. People with the same development of all three elements have, according to this theory, normal color perception and are called normal trichromats. If the elements are not equally developed, then there is a violation of color perception.

Color vision disorder can be congenital or acquired, complete or incomplete. Congenital color blindness occurs more often in men (8%) and much less often in women (0.5%).

The complete loss of function of one of the components is called dichromasia. Dichromates can be protanopes, with the loss of the red component, deuteranopes - green, tritanopes - violet. Congenital blindness to red and green colors is common, but violet color blindness is rare. The famous physicist Dalton suffered from protanopia, who in 1798 was the first to accurately describe red color blindness.

Some people experience a weakening of color sensitivity to one of the colors. These are color anomalies. Decreased perception of red color is called protanomaly, green - deuteranomaly and violet - tritanomaly.

Based on the severity of the color anomaly, anomalies of type A, B, and C are distinguished. Color anomalies A include forms that are more far from the norm, while C includes forms that are more close to the norm. An intermediate position is occupied by color anomalies B.

Achromasia—complete color blindness—is extremely rare. In these cases, no color tones are distinguished; everything is perceived in gray, as in a black and white photograph. With achromasia, there are usually other eye changes: photophobia, nystagmus, central vision is not higher than 0.1 due to aplasia of the fovea, nyctolapia (improved vision in low light).

Complete color blindness mostly manifests itself as a familial disorder with a recessive pattern of inheritance (color asthenopia). Color asthenopia in some people should be considered as a physiological phenomenon, indicating insufficient stability of chromatic vision.

The nature of color vision is influenced by auditory, olfactory, gustatory and many other irritations. Under the influence of these indirect stimuli, color perception can be suppressed in some cases, and enhanced in others. To diagnose color vision disorders in our country, we use special polychromatic tables of Professor E.B. Rabkina.

The tables are built on the principle of equalizing brightness and saturation. The circles of the primary and secondary colors have the same brightness and saturation and are located so that some of them are formed against the background of the rest of the figure or figure. The tables also contain hidden numbers or shapes that are recognizable to those who are color blind.

The study is carried out in good daylight or fluorescent lighting of the tables, because otherwise the color shades change. The subject is placed with his back to the window, at a distance of 0.5-1 m from the table. The exposure time of each table is 5-10 s. The test subject's readings are recorded and, based on the data obtained, the degree of anomaly or color blindness is determined. Each eye is examined separately, because very rarely unilateral dichromasia is possible. In children's practice, a young child is asked to use a brush or pointer to trace a number or figure that he distinguishes. In addition to tables, special spectral devices - anomaloscopes - are used to diagnose disorders and more accurately determine the quality of color vision. The study of color perception is of great practical importance.

There are a number of professions for which normal color perception is necessary. These are the transport service, fine arts, chemical, textile, and printing industries. The color discrimination function is of great importance in various fields of medicine: for infectious disease doctors, dermatologists, ophthalmologists, dentists; in knowledge of the surrounding world, etc.

Acquired color vision disorders are possible, which, compared to congenital ones, are more diverse and do not fit into any schemes. Red-green perception is disrupted earlier and more often, and yellow-blue perception is disrupted later. Sometimes it's the other way around. Acquired color vision disorders are accompanied by other disorders: decreased visual acuity, visual field, appearance of scotomas, etc. Acquired color blindness can occur due to pathological changes in the area of the macula, papillomacular bundle, damage to higher parts of the visual pathways, etc. Acquired disorders are highly variable in their dynamics. To diagnose acquired color vision disorders E.B. Rabkin proposed special tables.

9-11-2012, 13:04

Description

Central vision should be considered the central portion of visible space. This vision is the highest and is characterized by the concept of “visual acuity”.

Visual acuity- this is the ability of the eye to perceive separately points located at a minimum distance from each other, which depends on the structural features of the optical system and the light-receiving apparatus of the eye. The angle formed by the extreme points of the object under consideration and the nodal point of the eye is called the visual angle.

Determination of visual acuity (visometry). Normal visual acuity refers to the ability of the eye to separately distinguish two luminous points at a visual angle of 1 minute. It is much more convenient to measure visual acuity not by visual angles, but by reciprocal values, i.e. in relative units. Normal visual acuity equal to one is taken to be the reciprocal of the visual angle of 1 minute. Visual acuity is inversely proportional to the visual angle: the smaller the fenian angle, the higher the visual acuity. Based on this relationship, tables for measuring visual acuity are calculated. There are many versions of tables for determining the severity of fenium, which differ in the test objects, or optotypes, presented.

In physiological optics, there are the concepts of minimally visible, distinguishable and recognizable. The subject must see optotype, distinguish its details, recognize the represented sign or letter. Optotypes can be projected onto a computer screen or display. Letters, numbers, drawings, and stripes are used as optotypes. Optotypes are constructed so that from certain distances the details of the optotype (the thickness of the lines and the spaces between them) are visible at a viewing angle of 1 minute, and the entire optotype is visible at a viewing angle of 5 minutes. International optotype accepted broken Landolt ring. In Russian ophthalmology, the most common is the Golovin-Sivtsev table, which contains letters of the Russian alphabet and Landolt rings as optotypes. The table has 12 rows of optotypes. In each row, the sizes of the optotypes are the same, but they gradually decrease from the top row to the bottom. The magnitude of optotypes changes in arithmetic regression. Within the first 10 rows, each row differs from the previous one by 0.1 units of visual acuity, in the last two rows by 0.5 units. Thus, if the subject reads the third row of letters, then visual acuity is 0.3; fifth - 0.5, etc.

When using the Golovin-Sivtsev table, visual acuity is determined from 5 m. The lower edge of the table should be at a distance of 120 cm from the floor level.

First, the visual acuity of one eye (right) is determined, then the left eye. The second eye is covered with a shutter. From a distance of 5 m at a viewing angle of 1 minute, details of the optotypes of the tenth row of the table are visible. If the patient sees this row of the table, then his visual acuity is 1.0. At the end of each row of optotypes, the symbol V indicates the visual acuity corresponding to reading this row from a distance of 5 m. To the left of each row, the symbol D indicates the distance from which the optotypes of this row differ at a visual acuity of 1.0. Thus, the first row of the table with visual acuity equal to 1.0 can be seen from 50 m.

To determine visual acuity you can use Siellen-Deuders formula visus = d/D, where d is the distance from which the subject sees a given row of the table (the distance from which the study is carried out), m; D is the distance from which the subject should see this row, m.

Using the above formula, you can determine visual acuity in cases where the study is carried out in an office length, for example, 4.5 m, 4 m, etc. If the patient sees the fifth row of the table from a distance of 4 m, then his visual acuity is equal to: 4/10 = 0.4.

There are people with higher visual acuity- 1.5; 2.0 or more. They read the eleventh or twelfth row of the table. A case of visual acuity with the naked eye is described: the subject could distinguish the satellites of Jupiter, which are visible from the Earth at an angle of 1 second. If visual acuity is below 0.1, the subject must be brought closer to the table until he sees its first line.

Since the thickness of the fingers approximately corresponds to the width of the strokes of the optotypes of the first row of the table, you can show the examinee your spread fingers(preferably against a dark background) from different distances and, accordingly, determine visual acuity below 0.1 also using the above formula. If visual acuity is below 0.01, but the subject counts fingers at a distance of 10 cm (or 20, 30 cm), then visual acuity is equal to counting fingers at a distance of 10 cm (or 20, 30 cm). The patient may not be able to count fingers, but detects the movement of the hand near the face, this is considered the next gradation of visual acuity. The minimum visual acuity is light perception (vis = 1/-) with correct or incorrect light projection. Light projection is determined by directing a beam of light from an ophthalmoscope into the eye from different sides. In the absence of light perception, visual acuity is zero (vis = 0) and the eye is considered blind.

To determine visual acuity in children they use table by E. M. Orlova. It uses drawings of familiar objects and animals as optotypes. And yet, at the beginning of the study of visual acuity in a child, it is recommended to bring him close to the table and ask him to name the optotypes.

The table for studying visual acuity is placed in a wooden box open at the front, the walls of which are lined with mirrors on the inside. In front of the table there is an electric lamp, covered at the back with a screen for constant and uniform illumination (Roth-Roslavtsev apparatus). The optimal illumination of the table is provided by a regular 40 W incandescent lamp. A light fixture with tables is mounted on the wall opposite the windows. The lower edge of the illuminator is placed at a distance of 120 cm from the floor. The room where patients are waiting for an appointment and the eye room should be well lit. Currently, test mark projectors are increasingly used to study visual acuity. Optotypes of various sizes are projected onto the screen from a distance of 5 m. The screens are made of frosted glass, which reduces the contrast between the optotypes and the surrounding background. It is believed that such a threshold definition is more conducive to real visual acuity.

To determine visual acuity below 0.1, use optotypes developed by B. L. Polyak in the form of line tests and Landolt rings, designed to be presented at a certain close distance, indicating the corresponding visual acuity. These optotypes are specially created for military medical and medical-social examinations carried out when determining fitness for military service or disability group.

There is also an objective (independent of the patient’s testimony) way of determining visual acuity, based on optoclistic nystagmus. Using special devices, the subject is shown moving objects in the form of stripes or a chessboard. The smallest size of the object that caused involuntary nystagmus (seen by the doctor) corresponds to the visual acuity of the eye being examined.

When determining visual acuity certain rules must be followed.

Examine visual acuity monocularly (separately) in each eye, starting with the right.
During the test, both eyes should be open, one of them should be covered with a shield made of opaque material. If it is not there, then the eye can be closed with the palm (but not the fingers) of the subject. It is important that he does not press through the eyelids on the closed eye, as this can lead to temporary decrease in vision. The shield or palm is held vertically in front of the eye so that the possibility of intentional or unintentional peeking is excluded, and so that light from the side falls on the open palpebral fissure.
The study should be carried out with the correct position of the head, eyelids and gaze. There should be no tilting of the head to one or the other shoulder, turning the head to the right or left, tilting it forward or backward. It is unacceptable to squint. In case of myopia, this leads to increased visual acuity.
When researching, the time factor should be taken into account. In normal clinical work, the exposure time is 2-3 s, in control experimental studies - 4-5 s.
Optotypes in the table should be indicated with a pointer; its end should be clearly visible; it should be placed exactly with the exposed optotype at a certain distance from the sign.
The study should begin by showing the breakdown of optotypes in the tenth row of the table, gradually moving to rows with larger characters. In children and people with obviously reduced visual acuity, it is permissible to start testing visual acuity from the top line, showing from top to bottom one character in the line to the row in which the patient is mistaken, after which you should return to the previous row.

Visual acuity must be assessed according to the series in which all signs were named correctly. One error is allowed in the third to sixth rows and two errors in the seventh to tenth rows, but then they are recorded in the visual acuity record. Near visual acuity is determined using a special table, which is calculated at a distance of 33 cm from the eye. If the patient does not see the upper row of the Golovin-Sivtsev table, i.e. visual acuity is less than 0.1, then determine the distance with which he distinguishes the optotypes of the first row. To do this, the subject is brought closer to the table until he sees the first row, and the distance from which he distinguished the optotypes of this row is noted. Sometimes they use cut-out tables with optotypes of the first rad, which bring them closer to the patient.

The presence of vision in a newborn can be judged by the direct and friendly reaction of the pupils to light, with sudden illumination of the eyes - by the general motor reaction and the closure of the eyelids. From the second week, the newborn reacts to the appearance of bright objects in the field of vision by turning his eyes in their direction and can briefly follow their movement. At 1-2 months, the child fixes a moving object with both eyes for a long time. From 3-5 months, formal vision can be checked using a bright red ball with a diameter of 4 cm, and from 6-12 months - with a ball of the same color, but with a diameter of 0.7 cm. By placing it at different distances and attracting the child’s attention by swinging the ball, determine visual acuity. A blind child reacts only to sounds and smells.

You can roughly check visual acuity, which is of decisive importance in professional selection, labor and military examination.

Visual acuity may decrease depending on many reasons. They can be divided into three groups.

Most common reason is a refractive error (nearsightedness, farsightedness, astigmatism). In most cases, visual acuity is improved or completely corrected with the help of glasses.
The second reason for decreased vision- clouding of the refractive transparent structures of the eye.
Third reason- diseases of the retina and optic nerve, pathways and visual centers.

It should also be noted that throughout life, visual acuity changes, reaching a maximum (normal values) by 5-15 years and then gradually decreasing after 40-50 years.

VISUAL ACUITY STUDY

To study visual acuity, tables are used containing several rows of specially selected characters, which are called optotypes. Letters, numbers, hooks, stripes, drawings, etc. are used as optotypes. Even Snellen in 1862 proposed drawing optotypes in such a way that the entire sign was visible at a viewing angle of 5 minutes. and its parts at an angle of 1 min. The detail of a sign refers to both the thickness of the lines that make up the optotype and the space between these lines. All the lines that make up the optotype E. and the spaces between them are exactly 5 times smaller than the size of the letter itself. In order to eliminate the element of guessing the letter, to make all the signs in the table identical in recognition and equally convenient for studying literate and illiterate people of different nationalities, Landolt proposed using open rings of different sizes as an optotype. From a given distance, the entire optotype is also visible at a viewing angle of 5 minutes, and the thickness of the ring, equal to the size of the gap, at an angle of 1 minute. The examinee must determine which side of the ring the gap is located on.

In 1909, at the XI International Congress of Ophthalmologists, Landolt rings were adopted as an international optotype. They are included in most tables that have received practical application.

In the Soviet Union, the most common are the tables of S.S. Golovin and D.A. Sivtsev, which, along with the table made up of Landolt rings, include a table with letter optotypes. In these tables, for the first time, letters were selected not at random, but on the basis of an in-depth study of the degree of their recognition by a large number of people with normal vision. This naturally increased the reliability of determining visual acuity. Each table consists of several (usually 10-12) rows of optotypes. In each row, the sizes of the optotypes are the same, but gradually decrease from the first row to the last. The tables are designed to study visual acuity from a distance of 5 m. At this distance, details of the optotypes of the 10th row are visible at a viewing angle of 1 min. Consequently, the visual acuity of the eye that distinguishes the optotypes of this series will be equal to one. If visual acuity is different, then determine in which row of the table the subject distinguishes the signs. In this case, visual acuity is calculated using the Snellen formula:

where d is the distance from which the study is carried out, and D is the distance from which the normal eye distinguishes the signs of this row (marked in each row to the left of the optotypes).

For example, the subject reads the 1st row from a distance of 5 m. The normal eye distinguishes the signs of this series from 50 m. Therefore,

VISUS = 5M/50M = 0.1.

The change in the value of optotypes is carried out in an arithmetic progression in the decimal system so that when examining from 5 m, reading each subsequent line from top to bottom indicates an increase in visual acuity by one tenth: the top line is 0.1, the second is 0.2, etc. to the 10th line, which corresponds to 1. This principle is violated only in the last two lines, since reading the 11th line corresponds to visual acuity of 1.5, and the 12th - 2 units. Visual acuity corresponding to reading this line with distance of 5 m, is indicated in the tables at the end of each row, i.e. to the right of the optotypes. If the study is carried out from a shorter distance, then using the Snellen formula, it is not difficult to calculate visual acuity for each row of the table.

To study visual acuity in preschool children, tables are used where drawings serve as optotypes.

If the visual acuity of the subject is less than 0.1. then determine the distance from which it distinguishes optotypes of the 1st row. To do this, the subject is gradually brought to the table, or, more conveniently, the optotypes of the 1st row are brought closer to him, using cut tables or special optotypes of B. L. Polyak. With a lesser degree of accuracy, low visual acuity can be determined by using, instead of optotypes of the 1st row, a demonstration of fingers on a dark background, since the thickness of the fingers is approximately equal to the width of the lines of the optotypes of the first row of the table and a person with normal visual acuity can distinguish them from a distance of 50 m. Visual acuity is calculated using a general formula. For example, if the subject sees optotypes of the 1st row or counts the number of demonstrated fingers from a distance of 3 m, then his

VISUS = Z m / 50 m = 0.06.

If the visual acuity of the subject is below 0.005, then to characterize it indicate the distance from which he counts his fingers, for example:

VISUS = counting fingers at 10 cm.

When vision is so poor that the eye does not distinguish objects, but perceives only light, visual acuity is considered equal to light perception: VISUS = 1/? (one divided by infinity is the mathematical expression for an infinitesimal quantity). Light perception is determined using an ophthalmoscope. The lamp is installed to the left and behind the patient and its light is directed to the eye being examined from different sides using a concave mirror. If the subject sees light and correctly determines its direction, then visual acuity is assessed equal to light perception with correct light projection and is designated

VISUS =1/? proectia lucis certa (or abbreviated - 1/? p. I.e.)

Correct projection of light indicates the normal function of the peripheral parts of the retina and is an important criterion in determining indications for surgery in case of clouding of the optical media of the eye.

If the eye of the subject incorrectly determines the projection of light on at least one side, then such visual acuity is assessed as light perception with incorrect light projection and is designated

VISUS =l/? projectia lucis incerta (or abbreviated - 1/? p. 1. inc.)

Finally, if the subject does not even sense light, then his visual acuity is zero (VISUS = 0).

To correctly assess changes in the functional state of the eye during treatment, during examination of work ability, examination of military personnel, professional selection, etc., a standard method for studying visual acuity is required to obtain comparable results. To do this, the room where patients are waiting for an appointment and the eye room must be well lit, since during the waiting period the eyes adapt to the existing level of illumination and thereby prepare for the examination.

Tables for determining visual acuity should also be well, evenly and always equally illuminated. To do this, they are placed in a special illuminator with mirror walls.

For lighting, a 40 W electric lamp is used, covered with a shield on the patient’s side. The lower edge of the illuminator should be at a level of 1.2 m from the floor at a distance of 5 m from the patient. The study is carried out for each eye separately. The result for the right eye is recorded

VISUS OD =, for left VISUS OS = For ease of remembering

It is customary to examine the right eye first. Both eyes must be open during the examination. The eye that is not currently being examined is covered with a shield made of white, opaque, easily disinfected material. Sometimes it is allowed to cover the eye with the palm of your hand, but without pressing, since after pressing on the eyeball, visual acuity decreases. It is not allowed to squint your eyes during the examination.

Optotypes on the tables are shown with a pointer; the duration of exposure of each sign is no more than 2-3 s.

Visual acuity is assessed according to the row where all the signs were correctly named. Incorrect recognition of one character in the rows corresponding to visual acuity of 0.3-0.6 and two characters in the rows of 0.7-1.0 is allowed, but then after recording the visual acuity in brackets it is indicated that it is incomplete.

The organ of vision is the most important of all senses for humans. It allows you to obtain up to 90% of information about the world around you. The visual analyzer is strictly adapted to the perception of the visible part of the spectrum of light radiation reaching the Earth through the atmosphere with a wavelength of 380-760 nm.

Vision is a complex and not fully understood process. It can be represented schematically as follows. Rays of light reflected from objects around us are collected by the optical system of the eye on the retina. The photoreceptors of the retina - rods and cones - transform light energy into a nerve impulse due to the photochemical process of decomposition followed by resynthesis of the visual pigment chromoprotein, consisting of a chromophore (retinal) - vitamin A aldehyde - and opsin. The visual pigment contained in rods is called rhodopsin, and in cones - iodopsin. Retinal molecules are located in the discs of the outer segments of photoreceptors and, under the influence of light, undergo photoisomerization (cis- and trans-isomers), as a result of which a nerve impulse is born.

The rod apparatus is a formation that is highly sensitive to light at threshold and suprathreshold illumination - night (scotopic: from the Greek. skotos- darkness and opsis- vision), as well as in low light (0.1-0.3 lux) - twilight (mesopic: from Greek. mesos- average, intermediate) vision (determined by the field of view and dark adaptation). The cone apparatus of the retina provides daylight, or photopic (from the Greek. photos- light), vision (determined by visual acuity and color vision). The receptor (peripheral), conductive and cortical sections of the visual analyzer are involved in the formation of the visual image. In the brain, as a result of the synthesis of two images, an ideal image of everything visible to a person is created. Confirmation of the reality of a visual image is the possibility of its recognition by other signals: speech, auditory, tactile, etc.

The main functions of the organ of vision are central, peripheral, color and binocular vision, as well as light perception.

4.1. Central vision

Central vision should be considered the central portion of visible space. The main purpose of this function is to serve the perception of small objects or their details (for example, individual letters when reading a page of a book). This vision is the highest and is characterized by the concept of “visual acuity”.

Visual acuity ( Visus or Vis) - the ability of the eye to distinguish two points separately with a minimum distance between them, which depends on the structural features of the optical system and the light-receiving apparatus of the eye. Central vision is provided by the cones of the retina, occupying its central fovea with a diameter of 0.3 mm in the area of the macula. As you move away from the center, visual acuity decreases sharply. This is explained by changes in the density of neuroelements and the peculiarity of impulse transmission. The impulse from each cone of the fovea passes through separate nerve fibers through all parts of the visual pathway, which ensures a clear perception of every point and small details of an object.

Points A and B (Fig. 4.1) will be perceived separately provided that their images on the retina “b” and “a” are separated by one unexcited cone “c”. This creates a minimum light gap between two separate points.

The diameter of the cone "c" determines the value of maximum visual acuity. The smaller the diameter of the cones, the higher the visual acuity. The images of two points, if they fall on two adjacent cones, will merge and will be perceived as a short line.

Taking into account the size of the eyeball and the diameter of the cone of 0.004 mm, the minimum angles aOB and AOB are equal to G. This angle, which allows us to see two points separately, in physiological optics is called the visual angle, in other words, this is the angle formed by the points of the object under consideration (A and B) and nodal (O) point of the eye.

Determination of visual acuity (visometry). To study visual acuity, special tables are used containing letters, numbers or icons of various sizes, and for children - drawings (cup, Christmas tree, etc.). They are called optotypes (Fig. 4.2).

In physiological optics, there are the concepts of minimally visible, distinguishable and recognizable. The subject must see the optotype, distinguish its details, and recognize the represented sign or letter. Optotypes can be projected onto a computer screen or display.

The creation of optotypes is based on an international agreement on the size of their details, distinguishable at a visual angle of Γ, while the entire optotype corresponds to a visual angle of 5 degrees.

In our country, the most common method for determining visual acuity is the Golovin-Sivtsev table (Fig. 4.3), placed in a Roth apparatus. The bottom edge of the table should be at a distance of 120 cm from the floor level. The patient sits at a distance of 5 m from the exposed table. First, the visual acuity of the right eye is determined, then the left eye. The second eye is closed with a shutter.

The table has 12 rows of letters or signs, the size of which gradually decreases from the top row to the bottom. In constructing the table, the decimal system is used: when reading each subsequent line, visual acuity increases by 0.1. To the right of each line is the visual acuity, which corresponds to the recognition of letters in this row. On the left opposite each line is indicated the distance from which the details of these letters will be visible at a visual angle of G, and the entire letter - at a visual angle of 5". So, with normal vision, taken as 1.0, the top line will be visible from a distance of 50 m , and the tenth - from a distance of 5 m.

There are also people with higher visual acuity - 1.5; 2.0 or more. They read the eleventh or twelfth row of the table. A case of visual acuity equal to 60.0 is described. The owner of such vision could distinguish with the naked eye the satellites of Jupiter, which are visible from the Earth at an angle of 1".

If visual acuity is below 0.1, the subject must be brought closer to the table until he sees its first line. Visual acuity should be calculated using the Snellen formula:

where d is the distance from which the subject recognizes the optotype; D is the distance from which this optotype is visible with normal visual acuity. For the first row, D is 50 m. For example, the patient sees the first row of the table at a distance of 2 m. In this case

Since the thickness of the fingers approximately corresponds to the width of the strokes of the ontotins of the first line of the table, it is possible to show the examinee spread fingers (preferably against a dark background) from various distances and, accordingly, determine visual acuity below 0.1 also using the above formula. If visual acuity is below 0.01, but the subject is counting fingers at a distance of 10 cm (or 20, 30 cm), then Vis is equal to counting fingers at a distance of 10 cm (or 20, 30 cm). The patient may not be able to count fingers, but detects the movement of the hand near the face, this is considered the next gradation of visual acuity.

The minimum visual acuity is light perception (Vis = l/oo) with correct ( pioectia lucis certa) or incorrect ( pioectia lucis incerta) light projection. Light projection is determined by directing a beam of light from an ophthalmoscope into the eye from different sides. In the absence of light perception, visual acuity is zero (Vis = 0) and the eye is considered blind.

To determine visual acuity below 0.1, optotypes developed by B. L. Polyak are used, in the form of line tests or Landolt rings, intended for presentation at a certain close distance, indicating the corresponding visual acuity (Fig. 4.4). These optotypes are specially created for military medical and medical social examinations carried out when determining fitness for military service or group of disability.

There is also an objective (independent of the patient’s indications) method of determining visual acuity, based on optokinetic nystagmus. Using special devices, the subject is shown moving objects in the form of stripes or a chessboard. The smallest size of the object that caused involuntary nystagmus (seen by the doctor) corresponds to the visual acuity of the eye being examined.

In conclusion, it should be noted that throughout life, visual acuity changes, reaching a maximum (normal values) by 5-15 years and then gradually decreasing after 40-50 years.

The finer details the eye can perceive, the higher its visual acuity (visus). Visual acuity is usually understood as the ability of the eye to perceive separate points. The relationship between the size of the object under consideration and its distance from the eye is characterized by the angle at which the object is visible. The angle formed by the extreme points of the object under consideration and the nodal point of the eye is called the visual angle. Visual acuity is inversely proportional to the visual angle: the smaller the visual angle, the higher the visual acuity. The minimum visual angle that allows two points to be perceived separately characterizes the visual acuity of the eye being examined. For normal visual acuity equal to one (visus = 1.0), the reciprocal value of the visual angle G is taken. If this angle is greater (for example, 5"), then visual acuity decreases (1/5 = 0.2), and if it is smaller (for example, 0.5"), then visual acuity doubles (visus = 2.0), etc. Visual acuity of 1.0 is not a limit, but rather characterizes the lower limit of normal. Visual acuity is highest in the area of the central fovea of the macula, and as it moves away from it it quickly falls.

In order to eliminate the element of guessing the letter, to make all the signs in the table identical in recognition and equally convenient for examining literate and illiterate people of different nationalities, Landolt proposed using open rings of different sizes as an optotype. From a given distance, the entire optotype is also visible at a viewing angle of 5", and the thickness of the ring, equal to the size of the gap, is visible at an angle of 1". The examinee must determine which side of the ring the gap is located on. In the Soviet Union, the most common are Sivtsev tables, which, along with the table composed of Landolt rings, include a table with letter optotypes. In these tables, the letters were not selected randomly, but based on the calculation of their size and the angular dimensions of the parts. Each table consists of 10-12 rows of optotypes. In each row, the sizes of the optotypes are the same, but gradually decrease from the top row to the bottom. The distance from which the details of the optotypes of this series are visible at a viewing angle of 1" is indicated.

when examining from 5 m, reading each subsequent line from top to bottom indicates an increase in visual acuity by one tenth: the top line is 0.1, the second is 0.2, etc. until the 10th line, which corresponds to one. This principle is violated only in the last two lines, since reading the 11th line corresponds to visual acuity of 1.5, and the 12th - 2.0. Visual acuity corresponding to reading a given line from a distance of 5 m is indicated in the tables at the end of each row, i.e. to the right of the optotypes.

To study visual acuity in preschool children, tables are used where drawings serve as optotypes.

If the visual acuity of the subject is less than 0.1, then the distance from which he distinguishes the optotypes of the 1st row is determined. To do this, the subject is gradually brought to the table or, more conveniently, the optotypes of the 1st row are brought closer to him, using cut tables or special optotypes of B. L. Polyak. With a lesser degree of accuracy, low visual acuity can be determined by showing fingers on a dark background instead of the optotypes of the 1st row, since the thickness of the fingers is approximately equal to the width of the lines of the optotypes of the 1st row of the table.

If the visual acuity of the subject is below 0.005, then to characterize it they indicate at what distance he counts his fingers, for example: visus = counting fingers at 10 cm. When the vision is so poor that the eye does not distinguish objects, but perceives only light, visual acuity is calculated equal to light perception: visus = one divided by infinity. Determination of light perception is carried out using an ophthalmoscope. The lamp is installed to the left and behind the patient and its light is directed to the eye being examined from different sides using a concave mirror. If the subject sees the light and correctly determines its direction, then visual acuity is assessed equal to light perception with correct light projection.

Correct projection of light indicates the normal function of the peripheral parts of the retina and is an important criterion in determining the indication for surgery in case of clouding of the optical media of the eye.

For lighting, a 60 W electric lamp is used, covered with a shield on the patient’s side. The lower edge of the illuminator should be at a level of 1.2 m from the floor at a distance of 5 m from the patient. The study is carried out for each eye separately. For ease of memorization, it is customary to examine first the right eye, then the left eye. Both eyes must be open during the examination. The eye that is not currently being examined is covered with a shield made of white, opaque, easily disinfected material. Sometimes it is allowed to cover the eye with your palm, but without applying pressure. It is not allowed to squint your eyes during the examination.

Optotypes in the tables are shown with a clearly visible pointer, the end of which is placed exactly under the sign being exposed, but so that there is a sufficient gap between them. The duration of exposure of each sign is no more than 2-3 s.

Determination of visual acuity begins with the display of optotypes of the 10th row, showing them separately, and not in a row. This speeds up the research and eliminates the guessing of small characters based on similar outlines to larger ones.

For people with low vision, it is permissible to begin the study with large signs, showing from top to bottom one character in a row to the row where the person being examined is mistaken, after which the signs of the previous row are shown in a breakdown.

Visual acuity is assessed according to the series in which all signs were correctly named. It is allowed to incorrectly recognize one character in the rows corresponding to visual acuity of 0.3-0.6, and two characters in the rows corresponding to 0.7-1.0, but then after recording the visual acuity in brackets it is indicated that it is incomplete.

When selecting glasses for work, control expert studies, and determining visual acuity in bedridden patients, a special table for near vision is used, which is designed for a distance of 33 cm from the eye. The control here is both the correct recognition of individual letters and the free reading of the smallest text with the obligatory indication of the distance at which the study was carried out.

In infants, visual acuity is usually determined approximately by determining whether the child's eye fixes large and bright objects, or by using objective methods.

Objective methods for determining visual acuity are based on the appearance of involuntary optokinetic nystagmus when viewing moving objects. A table consisting of alternating black and white stripes or squares of different sizes, the angular dimensions of which are known, moves in the window of the nystagmus apparatus. Visual acuity is determined by the smallest amount of moving objects that cause nystagmoid eye movements. The appearance and disappearance of nystagmus is determined using a corneal microscope or by recording the biopotentials of the extraocular muscles on an electrocardiograph.