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An ophthalmic perimeter is a device that is used by physicians to study the properties of a patient’s field of vision. The term “field of view” refers to clinical ophthalmology, and refers to the area of space that the human eye perceives when the gaze is fixed. The field of view refers to the functional properties of the peripheral parts of the retina.
The state of the visual field significantly affects a person’s ability to navigate in the surrounding space. Why might this setting be changed? His condition is affected by diseases of the visual analyzer system, for example, the retina, the visual pathway or the optic nerve, the central nervous system.
Field of view, field boundaries, blind spot: what do these categories mean
The field of view in medicine refers to the totality of all points in space that can be simultaneously perceived by a fixed eye that has fixed one central point. This point is projected onto the retina in the area of the corpus luteum. All other points of the field are located in the peripheral parts of the retina. At the exit of the optic nerve from the eye, where there are no light-perceiving elements of the retina, there is a physiological scotoma, the so-called blind spot.
How can the field of view change? Its functional abilities of perception can be narrowed, and in certain cases, some parts of the field “fall out”. So, if a patient has bilateral blindness in half of the visual field, this condition is called hemianopia. The lesion can develop against the background of diseases of the optic nerve, cerebral cortex, visual pathways.
The narrowing of the field is measured in degrees. The magnitude of the blind spot or spots is determined by applying special grids, and is specified in linear terms or degrees.
To obtain more accurate and specific digital data on the size of the field of view, on the presence of its defects, on the boundaries of blind spots, ophthalmologists use special devices, including the perimeter. Identification and study of the central spot is performed by a special scotoma meter.
The peripheral boundaries of the visual field normally depend on the anatomical features of the structure of the eyelids, bones and the eyeball, since from above they are determined by the upper eyelid and superciliary arches, from below – by the side of the nose. The normal field of view, on average, has a limitation from above to 55 degrees from the fixation point, from below and along the inside – up to 90 degrees.
It should be noted that such data are averaged, standard, and cannot be taken as the only normal ones.
Assessing the boundaries of the field, the doctor takes into account the fact that the full visual acuity of the eye is observed only in the center, and the closer to the periphery of the retina, the less sharpness becomes.
Examination of the visual field: how and why it happens
Clinical practice of treating patients with visual impairments cannot do without a study of the visual field, since according to its results, the doctor can identify and clarify a number of general and eye diseases.
In the process of topical diagnosis of lesions of the central nervous system, if necessary, to clarify the localization of basal tumors, hemorrhages and foci of inflammatory processes, doctors use the results of examining the patient’s visual field. If the lesion is localized in the area of the Turkish saddle, a person has a loss of temporal fragments of the visual field of both eyes. If the pathological processes are vascular in the same area, both eyes may experience loss of the inner halves of the visual field. If the patient has two left or two right halves of the fields falling out at the same time, this indicates the location of the pathology behind the Turkish saddle. The preserved central part of the visual field on the left and right eyes indicates the formation of a lesion in the occipital part of the cerebral cortex, or in the zone of visual radiance.
If a patient loses half and the center of the field, this makes it possible to detect a focus of pathology in the tractus zone. Narrowing of the size of the field of view of a concentric nature, combined with the appearance of a central blind spot, indicates the presence of retrobulbar neuritis.
There are also cases when pronounced violations of the field size are accompanied by the formation of inflammation in the retina, for example, if the patient has retinitis, exudates, retinal hemorrhages. Characteristic changes in the field of view are also observed in glaucoma, retinitis pigmentosa, even in hysteria.
Thus, visual field studies are of significant value for clarifying complex and hidden pathologies localized in the brain and cranium.
Among all field survey methods, the control method is the simplest – it is carried out without the use of special devices.
The doctor sits opposite the patient at a distance of 1 meter. The subject fixes the gaze of his right eye in the area of the left eye of the doctor, and vice versa. The other eye must be closed. The doctor slowly moves his right hand in all directions from the central fixed point of view, so that the hand is always at the same distance from the patient and the doctor himself. It should be noted the moment when the hand simultaneously disappears from view of both. Under the condition of a normal field of vision for both the doctor and the patient, they will no longer see the hand at the same time, otherwise we can talk about narrowing the field of view of the patient.
Such a technique cannot be the basis for making a diagnosis, but provides only indicative information.
Other ways to measure this category:
- kinetic perimetry;
- static;
- approximate definition of hemianopsia;
- definition of central scotomas and metamorphopsias.
Kinetic perimetry
It is carried out using desktop or projection perimeters. For a desktop device, daylight is required, for a projection device, reduced artificial lighting is required. During diagnostics, an object with a diameter of 1 to 5 millimeters is slowly moved along the length of the perimeter arc in the direction from the periphery to the center. If a white object is selected, the subject must determine the moment when the marker appears in the field of view. Chromatic (colored in other colors) objects are identified by color.
Norm for an adult:
- outer borders: 90 degrees;
- internal: 55 degrees;
- upper: 55 degrees;
- lower: 60 degrees.
Permissible deviation – no more than 10 degrees in both directions.
In preschoolers, the peripheral boundaries of the visual field can be determined, on average, 10 degrees narrower than in adults.
Static perimetry
The method was first proposed in 1939. It allows you to identify the thresholds of light sensitivity of the retina in decibels, at those points of the study that suffer from glaucoma. Such a measurement is carried out by special computer perimeters.
Approximate definition of hemianopia
Does not require medical equipment. The patient is asked to split any oblong object used in the process into two parts with his index finger. For this purpose, a pencil, ruler or pen will do. If a person has a limitation in the fields of view, one of the sides of the object is visually cut in length, not falling completely into his field of view. If in this case you ask him to put his finger exactly in the center of the object, in reality it will be shifted either to the left or to the right.
Definition of central scotomas and metamorphopsies
Produced with the Amsler test – a special grid of squares intersecting horizontal and vertical lines. There is a cross in the table, which the patient must fix with his eyes during the examination. By the way he perceives this mark – clearly or with distortion, one can judge that he has metamorphopsia.
Ophthalmic perimeter: what it is and how it works
A device called “ophthalmic perimeter” is an analyzer that allows the oculist to examine the patient’s visual field indicators, disturbances in it, and shifts in its boundaries. Based on such data, the attending physician can diagnose eye diseases, the state of visual acuity, and the presence of functional pathologies.
The principle of operation of the device is based on the stimulation of vision analyzers by light exposure, resulting in specific reactions that reflect the state of the visual organs.
Thanks to the operation of the device, physicians were able to diagnose pathological changes at the very beginning of their appearance, and often the changes detected by the ophthalmic perimeter are the only symptoms and manifestations of certain diseases, on the basis of which a patient can be diagnosed.
For the purposes of determining and clarifying the diagnosis, two types of perimeters are used:
- for kinetic examination;
- for static perimetry.
The main difference between them is that in the first case the marker (label) moves, and in the second case the markers appear and disappear sequentially.
In addition, devices can be:
- manual;
- automatic.
The simplest perimeters include an arc of some radius, in the center of which the eye under study is located. The visual axis of the eye corresponds to the axis of rotation of the arc, as well as its radius.
In the perimeter, test objects of a reflective or self-luminous type can be used. Changing the color, brightness and size of the marker occurs due to the replacement of light filters and diaphragms.
The most common today are automatic perimeters, due to the use of which the duration of the examination procedure is significantly reduced. In such devices, an LED or a halogen lamp serves as a marker.
In addition, in automatic perimeters, the program provides for the presence of several test programs that make it possible to evaluate the peripheral and central areas of the visual field and detect pathological changes in them, which are typical, for example, for glaucoma or retinal diseases.
Modern models of automatic devices expand the diagnostic capabilities of most types of pathologies, as they are equipped with unique software, the function of automatically tracking the fixation and position of the eyes of the subject, as well as the ability to change the parameters of the test by pressing just a few buttons.
Ophthalmic perimeters are combined with a desktop computer or laptop, so the data obtained can be synchronized with a digital storage medium, saved and printed at any time.
Automatic ophthalmic perimeters are used by doctors in two systems – in the field of central and peripheral vision. The devices have the ability to work in fast, reduced or full modes.
Apparatus design: perimeter components
The PNR perimeter is a device that allows you to determine the range of the field of view in the conditions of an ophthalmological office. Its design is represented by a base, an arc, a chin rest, electric indicators, test objects, for example, LEDs.
Modern models are equipped with a convenient push-button control that can be used to control the on/off, as well as change the brightness and color of the test markers. In this case, the installation of the arc in the appropriate position in relation to the central disk scale is done manually.
The introduction of LED objects in ophthalmic perimeters has significantly improved the quality of research, and also extended the “life” of devices, since LED elements do not fade during operation, while maintaining the desired level of brightness and saturation.
The most accurate devices, according to ophthalmologists, are spherical and projection perimeters, which can calculate and calculate even the smallest deviations in the functioning of the visual fields.
Ophthalmic perimeters are widely used in the process of studying the state of the visual organs. Their use is characterized by simplicity, high efficiency and information content of research, which is why perimeters are installed in clinics, eye rooms, and specialized scientific institutions.
The choice of device should be based on a study of the reputation and experience of the manufacturer, on the compliance of the proposed design with modern technical developments. You also need to pay attention to the presence of a warranty period and industry certificates confirming the quality of the device.