Perimetry
Valeria Dahm is a freelance writer in the medical department. She studied medicine at the Technical University of Munich. It is particularly important to her to give the curious reader an insight into the exciting subject area of medicine and at the same time to maintain the content.
More about the experts All content is checked by medical journalists.With perimetry - also called visual field measurement or visual field examination - the visual field is measured in ophthalmology and neurology. How to do this, what types of perimetry there are and when exactly the examination is necessary, you can find out here!
What is a perimetry?
With perimetry, both the limits of the field of vision perceived by the immobile eye and the sharpness of perception are measured. In contrast to the central field of vision, which offers the highest visual acuity, the outer part of the field of vision is primarily used for orientation in and perception of the environment. It is therefore important for the examination that the eye to be examined fixes a point and does not move.
There are several methods of perimetry:
- automatic static perimetry: it is the most commonly used. The patient uses a button to give a signal as soon as he perceives a point of light flashing on the edge of his field of vision. In addition to the location, the computer also records the strength of the stimulus, i.e. the brightness.
- kinetic perimetry: Here the points of light migrate from the outside to the central field of view. The patient reports as soon as he sees the point of light wandering into his field of vision.
- Finger perimetry: This is the simplest method, but it only allows a rough estimate of the field of view. While the patient fixes a point with his eyes, the doctor moves a finger into the field of vision from the outside (also inside, above and below). The patient reports as soon as he perceives this.
In each of these three methods, the untested eye is covered so that it cannot compensate for deficits in the other eye and thus falsify the examination result.
When do you do a perimetry?
Perimetry can be used to determine deficits in visual perception, often before the person to be tested even perceives them. The cause of such a loss of visual field (scotoma) can lie in the eye itself or in the optic nerve, but also in the area of the nerve pathways in the visual center of the brain.
There are different forms of visual field defects such as central scotoma, hemianopia (half-side loss) or quadrant anopia (quadrant loss).
The most common medical reasons (indications) for perimetry are:
- unexplained visual disturbances
- Glaucoma (glaucoma)
- Retinal detachment (ablatio retinae)
- Macular degeneration
- Lesions of the visual pathway caused by brain tumors, strokes, or inflammation
- Follow-up if the visual field defect is already known
- Assessment of eyesight (e.g. for professional certificates)
What do you do with perimetry?
At the beginning of each field of view measurement, one eye is usually covered with an eye patch. Then one of the following methods of perimetry is used.
Finger perimetry
The patient fixes the nasal tip of the examiner. He now spreads his arms and moves his fingers. If this is perceived by the patient, the examiner moves his hands into different positions so that he can assess the limits of the field of view. The patient reports whenever he detects a finger movement.
Static perimetry
The patient's head rests on a chin and forehead rest of the perimetry device and fixes a central point in the middle of the interior of a hemisphere. Points of light now light up at various points on the hemisphere. If the patient registers a point of light, he reports this by pressing a button.
If the patient does not notice a light signal, it is repeated later at the same position with a higher light intensity. In this way, not only the limits of the field of view, but also the sensitivity of vision are determined and displayed in a field of view map.
Kinetic Perimetry
With kinetic perimetry, points of light with constant brightness slowly move from the periphery to the center. The patient presses a button as soon as he perceives the signal. This process is repeated from different directions until the points at which the patient has recognized the light mark can be connected to form a line - the isopter. The retinal sensitivity is the same everywhere along this line.
The intensity and size of the light marks are then reduced so that the isopters can also be determined for weaker light signals.
What are the risks of perimetry?
Perimetry does not pose any health risks. However, since this is an examination method that requires high concentration, the exertion can lead to headaches and sore eyes.
What do I have to consider when performing perimetry?
The results of this examination are highly dependent on the cooperation of the patient. This is why it is important to be awake and rested for perimetry. In addition, known visual impairments must be compensated for before the visual field map is collected so that the values, especially for the sensitivity of vision, are not falsified.
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