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The study of binocular vision can be carried out by various methods, among which the study using a 4-point color test (test with a color device) is generally accepted.
The subject observes 4 multi-colored circles (2 green, white and red), glowing through filter glasses (with one red and one green glass). The colors of the circles and lenses are chosen in such a way that one circle is visible only with one eye, two circles – only with the second, and one circle (white) is visible with both eyes.
The patient sits at a distance of 5 m from a direct and strong light source. He puts on filter glasses: the right eye is covered with red glass, and the left eye is green. Before starting diagnostic manipulations, the quality of the filters is checked. To do this, one by one cover the eyes with a special shield, while the patient first sees two red circles with his right eye, and then three green circles with his left eye. The main examination is carried out with the eyes open at the same time.
There are three options for the results of the examination: binocular (normal), simultaneous and monocular vision.
Sokolova’s method (1901)
The method consists in asking the patient to look into a tube with one eye (for example, a sheet turned into a tube), a palm is applied to its end from the side of the open eye. In the presence of binocular vision, the impression of a “hole in the palm” is created, through which a picture is perceived, which is visible through the tube. This is because the image seen through the hole in the tube is superimposed on the image of the palm in the other eye.
With the simultaneous nature of vision, the “hole” does not coincide with the center of the palm, and with monocular vision, the “hole in the palm” phenomenon does not appear.
The experience with two pencils (they can be replaced with ordinary sticks or felt-tip pens) is indicative. The patient should try to align the tip of his pencil with the tip of the pencil in the doctor’s hands so that a clearly straight line is formed. A person with binocular vision easily performs tasks with two eyes open and misses when one eye is closed. Missing is noted in the absence of binocular vision.
Other, more complex methods (prism test, Bogolin striped glass test) are used by an ophthalmologist.
Strabismus according to the Hirschberg method
The magnitude of the angle of strabismus is simply and quickly determined by the Hirschberg method: a beam of light is directed into the eyes of the subject and the location of light reflections on the cornea is compared.
A reflex is fixed in the eye and observed near the center of the pupil, or coincides with it, and in the eye that squints, it is determined in a place corresponding to the deviation of the visual line.
One millimeter of its displacement on the cornea corresponds to a strabismus angle of 7 degrees. The larger this angle, the farther from the center of the cornea the light reflex is shifted. So, if the reflex is located on the edge of the pupil with its average width of 3-3,5 mm, then the angle of strabismus is 15 degrees.
A wide pupil makes it difficult to accurately determine the distance between the light reflex and the center of the cornea. More precisely, the angle of strabismus is measured on the perimeter (Golovin’s method), on the synoptophore, with a test with prism cover.
Subjective method for determining binocular vision
To determine the level of refraction of light in the eyes by a subjective method, you need a set of lenses, a trial spectacle frame and a table for determining visual acuity.
The subjective method for determining refraction consists of two stages:
- determination of visual acuity;
- applying to the eye in the frame of optical lenses (first +0,5 D, and then -0,5 D).
In case of emmetropia, a positive glass worsens the Visus, and a negative glass first worsens it, and then does not affect it, since accommodation is turned on. With hypermetropia, “+” glass improves Vizus, and “-” glass first worsens, and then, with a large accommodation voltage, it is not displayed on Vizus.
In young patients with visual acuity equal to one, two types of refraction can be assumed: emmetropia (Em) and hypermetropia (H) of a weak degree with the participation of accommodation.
In elderly patients with visual acuity “one”, only one type of refraction can be assumed – accommodation is weakened due to age.
With visual acuity less than one, two types of refraction can be assumed: hypermetropia (high degree, accommodation cannot help) and myopia (M). In hypermetropia, a positive glass (+0,5 D) improves Visus, and a negative glass (-0,5 D) worsens Visus. In myopia, positive glass worsens visual acuity, while negative glass improves it.
Astigmatism (different types of refraction in different meridians of one eye) is corrected with cylindrical and spherical cylindrical lenses.
When determining the degree of ametropia, the glass changes for the better Visus with it (1,0).
At the same time, in hypermetropia, refraction determines the largest positive glass, with which the patient sees better, and in myopia, the smaller negative glass, with which the patient sees better.
A different kind or degree of refraction in both eyes is called anisometropia. Anisometropia up to 2,0-3,0 D in adults and up to 5,0 D in children is considered tolerable.
Objective methods for determining binocular vision
Skiascopy (shadow test), or retinoscopy is an objective method for determining the refraction of the eye. To carry out the method, you need: a light source – a table lamp; mirror ophthalmoscope or skiascope (concave or flat mirror with a hole in the middle); skiascopic rulers (this is a set of cleaning or diffusing lenses from 0,5 D-1,0 D in ascending order).
The study is carried out in a dark room, the light source is placed on the left and somewhat behind the patient. The doctor sits down 1m from him and directs the light reflected from the skiascope into the eye being examined. In the pupils, a light reflex is observed.
Slightly rotating the glass handle, the reflected beam is moved up and down or left and right, and the movement of the skiascopic reflex in the pupils is observed through the opening of the skiascope.
Thus, skiascopy consists of 3 points: obtaining a red reflex; obtaining a shadow, the movement of which depends on the type of mirror, the distance from which it is examined, on the type and degree of refraction; shadow neutralization with a skiascopic ruler.
There are 3 options for the skiascopic reflex (shadows against the background of the red reflex):
- the skiascopic reflex moves in accordance with the movement of the mirror;
- it moves opposite to the movement of the mirror;
- there is no shadow against the background of the red reflection.
In the case of the coincidence of the movement of the reflex and the mirror, we can talk about hypermetropic vision, emetropic or myopic to one diopter.
The second variant of the movement of the skiascopic reflex indicates myopia of more than one diopter.
Only with the third variant of the movement of the reflex do they conclude that myopia is one diopter and the measurements are stopped at this point.
When examining an astigmatic eye, skiascopy is performed in two main meridians. Clinical refraction is calculated for each meridian separately.
In other words, binocular vision can be examined in many ways, everything directly depends on the brightness of the symptoms, on the patient’s complaints and on the professionalism of the doctor. Remember, strabismus can only be corrected in the early stages of development and it will take a long time to do so.