Perimetry

PERIMETRY (from the Greek peri - around and metron - measure), one of the methods for studying peripheral vision, based on the projection of the spherical surface of the retina onto a spherical and concentric outer surface; in practice, this method is carried out using a special device called a perimeter. The idea of the method itself belongs to Purkinje (Purkinje), who proposed it as early as 1825, but it was practically implemented in the form of a special device only in 1857 by Aubert and Forster (Aubert, Forster), of whom the latter introduced a number of improvements to the initial model and introduced the device into general use under the name of the perimeter, which is why the latter is usually called by his name. Landolt (Landolt, 1872) significantly simplified and improved Forster's perimeter. Although over time the device underwent numerous modifications and improvements introduced by various authors, Forster's and Landolt's basic principle remained unchanged. The main part of the perimeter in modern models (Figure 1) is an arc made of a blackened metal strip 5 cm wide, measuring half or a quarter of a circle described by a radius of 30 cm. Degree divisions are marked on the inner or outer surface of the arc, starting from its center, which is taken as 0 and at the same time serves as the fixation point for the subject, for which it is marked with a small white circle. With its apex, the arc is attached to a vertical post on a massive cast-iron base in such a way that at the point of attachment it can rotate around its horizontal axis. The inclination that the arc assumes during these rotations in relation to the vertical, in other words, the position of the meridians, is noted on a special circle mounted on the same post behind it, using a special pointer rotating together with the arc. A. N. Maklakov (1884) proposed taking two intersecting arcs at right angles in the center instead of one, so that the field could be examined in two meridians at once. As a test object for perimetry, a small white square is used, which is moved along the inner surface of the arc either simply by the investigator's hand at the end of a thin black stick or using a special mechanism with cord transmission on a special carriage-type device. The latter is a combination of three circles, of which the lower rotating one is divided into a series of sectors colored in primary colors (for white and color perimetry), the second above it is stationary,

Fig. 1. Forster's perimeter; modern model. with a quadrangular cutout on the front edge of 20 mm, and the third between the first two, also rotating, with a whole series of square cutouts on the edge ranging from 2 to 20 mm, regulating the size of the test object like a diaphragm. In view of the fact that the nodal

Figure 2. Perimeter in the form of a hemisphere; Scherk's model.

point of the eye being examined must coincide precisely with the center of the arc, which in turn requires a stationary position of the subject's head, on the opposite end of the cast-iron base in front of the arc there is a special column with a horizontal chin rest, which can be set at any height using a special screw, and in addition, in front of the rest, a vertical rod rises with a small platform on its upper end, against which the subject presses with the lower edge of the orbit. Of the numerous variants of the perimeter, more or less significantly differing from the model just described, the following deserve mention: 1. Hollow perimeters (for example, Scherk's), in which the arc is replaced by a hemisphere (Fig. 2); the inner surface of the latter is blackened, and the meridians and parallel circles are drawn in red paint; in the vertical meridian, the hemisphere is divided into two halves connected by hinges so that, as needed, either half can be turned aside. The test object at the end of a thin black stick is moved along the inner surface of the hemisphere by the investigator's hand, and the results are immediately marked with chalk. The positive aspects of this model consist, firstly, in eliminating impressions from extraneous objects distracting the subject's attention, and secondly, in the free movement of the test object not only in meridional but also in all other directions. 2. Perimeters without an arc (e.g., Helmbold's), an ingenious (Fig. 3) and very simple model in idea, which is easily constructed by the most primitive means. As a test object, a small white square is taken,

m Figure 3. Perimeter without an arc; Helmbold's model. mounted on a long, thin stick with a miniature loop at the very end. If, by hooking the loop onto a hook, you pull the cord so that the test object describes a 90° arc, the weight at the free end of the cord rises to a certain height, and the point on the back board against which it stops is marked with the number 90, and the entire distance between 0 and 90 is divided into 9 equal parts, each corresponding to 10°. To determine the boundaries of the visual field according to this scale, it is sufficient to note the divisions against which the weight will stop during movements of the test object in various directions or meridians, the deviations of which the investigator establishes approximately. 3. Portable perimeters. Of

Figure 4. Hand perimeter; Schweigger's model.

the numerous models, the most commonly used is Schweigger's hand perimeter, which in appearance resembles an ordinary perimeter in miniature (Fig. 4), with an arc of 15-20 cm radius; the entire device is held by the subject in their hands by a special handle, and the test object is moved along the arc manually. Elschnig slightly modified this model, making it collapsible. 4. Self-registering perimeters (for example, Hardy, Fig. 5). The advantages of self-registering perimeters are that they relieve the necessity of specifically

Figure 5. Self-registering perimeter; Hardy's model.

noting the boundaries of the visual field on the arc and drawing these boundaries on paper or a diagram, and also eliminate any possibility of errors during this manipulation, since the registration of the visual field is performed automatically. This is achieved by the fact that all movements of the test object by means of a cord and gears are transmitted to a special registering system located behind the arc and equipped with a special pointed pin that marks the position of the object by point punctures on the opposing diagram. The entire system rotates together with the arc, and the pin also performs lateral movements. 5. Luminous perimeters (e.g., Michel's, Fig. 6), intended for examination in the dark if for some reason it is undesirable or impossible to use daylight or artificial lighting, as well as for those cases when thanks to

the opacity of the transparent

media, e.g., the lens,

the subject cannot see

Figure 6. Luminous perimeter; Michel's model.

The advantages of such perimeters also include that the examination is always conducted under identical lighting conditions, and the subject's attention is not distracted by extraneous objects. Externally, these instruments resemble the self-recording perimeter just described, with the only difference being that on the inner surface of the arc, painted gray, there is a deep groove along which a small box containing a small electric lamp instead of the test object moves; the box is equipped with a series of diaphragms of varying widths and a set of colored glass plates in case color perimetry in the dark is necessary. The fixation point is a small red lamp at the center of the arc. In addition to those listed, there are still a whole range of perimeter models of various types that are not widely used. It should also be kept in mind that the perimeter is used not only for the study of peripheral vision but also for other purposes, such as determining the angle of strabismus, the position of a cysticercus in the eye, etc. The study of peripheral vision generally amounts to a comprehensive examination of the so-called visual field (see). For practical purposes, the most important aspects are: 1. Determination of the outer boundaries of the visual field, which under normal conditions depend on the configuration of the facial structures surrounding the eye (eyelids, bridge of the nose, nose, edges of the orbit), further on the position of the eye itself in the orbit, on the size of the pupil, etc. 2. Determination of partial defects or scotomas in the visual field, which develop under the influence of focal diseases of the visual apparatus. 3. Determination of color sensation on the periphery of the retina. The actual technique of perimetry is as follows: the patient sits with their back to the light, places their chin on a special stand of the instrument, and with the eye being tested, fixes the zero division at the center of the arc; the other eye is covered during this time. The doctor, positioned opposite the patient, must carefully ensure that the patient's eye does not move away from the designated mark throughout the examination. It is preferable to conduct the examination in diffused daylight, placing the instrument between two windows, set back somewhat from the wall, or even better under similar conditions but in a corner of the room. If necessary, artificial lighting can also be used, which should also be sufficiently strong and diffused. When determining the boundaries of the visual field for white color, with which perimetric research usually begins, the test object is slowly and silently moved along the perimeter arc from the periphery toward the center until the patient, while continuing to fixate on the central mark, notices the movement of 'something white' and reports this to the examiner. The division of the arc at which the test object first becomes visible to the subject corresponds to the outer boundary of the visual field at that location. By sequentially rotating the perimeter arc around its center by 45°, this type of manipulation is performed in at least 4 meridians, namely: the vertical, horizontal, and two intermediate oblique ones, starting at least with the vertical, and in each meridian, 2 points at opposite ends are determined. The 8 points found are plotted on the corresponding degree divisions of special diagrams (see Visual field), and by connecting them, the outline of the boundaries of the visual field of that eye is obtained. To determine suspected scotomas, the test object is moved along a series of meridians, 10-15° apart, throughout the entire length of the arc, starting from the outer boundaries of the visual field to the fixation point itself, and as soon as the object enters such a defect area, it will immediately disappear from the subject's view and reappear upon leaving this area. The degree divisions of the arc at which the object disappears and reappears are transferred to the diagram, and by connecting them, the outline of the scotoma itself is obtained. The examination of color sensation on the periphery of the retina is carried out on the same principle as the determination of peripheral perception of white color, with the only difference that instead of white objects, corresponding colored objects of the same size as the white ones are used, and the subject should not in advance know what color the object is, and the boundary of the visual field for a given color is considered to be the place where it is correctly recognized by the subject. For clinical purposes, it is usually sufficient to determine the boundaries of the visual field for blue, red, and green colors. Color perimetry is also a very important and precise method of research, as it often reveals pathological changes where the white object does not yet show them.

D. strupov.