Perimetry encompasses several ophthalmic procedures that serve to determine visual field limits as well as sensitivities of the visual system and play a role in particular for the fitness testing of occupational groups such as pilots. In each perimetry procedure, the person examined covers one eye and fixes a specific point in space with the open eye. In the course of the examination, various light stimuli appear at ever different points in space, which the person examined can either register or not perceive. Perimetric methods can be divided into kinetic and static methods; in kinetic methods, the light stimuli move from the patient’s peripheral visual field toward the center of the visual field, whereas in static methods they are each presented statically at one location and change only in intensity.
What is perimetry?
Perimetry is the ophthalmologist‘s systematic measurement of the visual field. In each perimetry, the person being examined covers one eye and fixes a specific point in space with the open eye. By perimetry, the ophthalmologist understands a systematic visual field measurement in which the outer and inner limits of the visual field as well as the sensitivities of the visual system are determined with a perimeter and light stimuli. Various individual methods fall within the scope of perimetry. One basic distinction is between kinetic and static examination methods. Besides finger perimetry, contour perimetry and threshold perimetry are among the best known methods. The former method is the fastest and simplest type of perimetry. While perimetric procedures were not automated in the beginning, they are nowadays increasingly controlled by machines. Hans Goldmann began to develop a kinetic perimeter with this goal in mind as early as 1945. About 30 years later, Franz Fankhauser developed a system that later became the first computer-controlled and static perimeter.
Function, effect and goals
Perimetry plays a role primarily for fitness tests. In this regard, the field of application is especially the flight fitness testing of pilots. Perimetric methods are also used to diagnose visual defects, however, because they can be used to distinguish whether a visual defect is related to the brain or the optic nerve. For this reason, perimetric methods have become standard for diagnosing eye diseases such as glaucoma. The individual steps of the procedure depend on which method is used. Ultimately, however, in the course of each perimetric method, optical stimuli are given one after the other, each of which appears at different points in space. One eye is always examined. The other eye remains covered and only takes its turn when the examination of the first eye has been completed. During the examination, the physician documents the patient’s perception of the stimulus and records the individual data of perception each depending on the location and intensity of the stimulus that appeared. The eye must remain static during the perimetric examination, i.e. the patient is asked to fixate on a single point in space, which he does not take his eyes off during the entire procedure. From the recordings, the physician creates a systematic visual field image, which he finally compares with a standard visual field. The differences between the above-mentioned individual procedures lie primarily in the effort involved. In the parellel test, also called finger perimetry, for example, the doctor and patient sit opposite each other and look at each other. The physician inserts an object from the peripheral visual field into the central visual field and compares his own perception with the patient’s perception. In static perimetry, on the other hand, the person being examined sits in front of a screen and fixes a point of light on the center of the screen with the open eye. In the course of the examination, the screen shows points of light at various locations, which the examinee marks as perceived by pressing a button. If the patient has not perceived a stimulus, the system increases the stimulus intensity. If this does not lead to the desired result either, the perimeter changes the location of the inserted stimulus. This procedure takes about ten to 20 minutes for each eye. At the end, the physician evaluates the data recorded in this way and compares the result with a standard finding.Unlike this static method, the light points in kinetic perimetry move from the periphery into the patient’s central field of vision. The system thereby measures the point in time from which the patient can see them. Both finger and contour perimetry belong to the kinetic methods. In contrast, threshold perimetry, which can only be performed with a high-tech electronic device, belongs to the static methods.
Risks, side effects, and hazards
The results of perimetry depend heavily on the cooperation of the person being examined. Thus, perimetric procedures are not absolutely objective procedures and sometimes yield questionable results in patients who are unwilling to cooperate. For example, perimetry of a child may be more unreliable than the same procedure on an adult examinee. For the patient, perimetric procedures are not associated with any risks or side effects, as all methods are non-invasive. However, since perimetric examinations require absolute concentration, some patients perceive the examination as extremely strenuous and sometimes much longer than it actually takes. Despite this subjective feeling, however, finger perimetry in particular requires little effort and is considered a particularly simple and time-saving examination method. By and large, however, physicians now use kinetic perimetry significantly less frequently than static procedures.
