Perimetry is a non-invasive (non-penetrating into the body) diagnostic ophthalmic procedure used to determine the visual field. The visual field is the area that can be perceived from the outside world without moving the eye away from a central point. In contrast, the field of view is the area that can be registered with maximum eye movement but with the head still. The determination of the field of vision is important, for example, in eye muscle paresis (eye muscle paralysis). In addition to visual acuity, the visual field is of great importance for correct visual function. Especially during locomotion (e.g. walking or driving a car) it serves for orientation and timely recognition of newly emerging dangers. Losses in the visual field are called scotomas (Skòtos, Greek = shadow) and are caused by various diseases such as glaucoma (“green star”). Since small absolute scotomas are “filled in” by the brain like the physiological “blind spot“, they often cannot be registered by the patient and can only be determined with the help of perimetry. In relative scotomas, vision is merely reduced, so they are usually perceived as gray, washed-out areas.
Indications (areas of application)
- Unclear visual disturbances: Perimetry should be performed for orientation disorders, loss of brightness, nyctalopia (night blindness), or reading disorders. There are numerous conditions that can cause scotomas:
- Glaucoma (“green star”): due to increased intraocular pressure, nerve fiber damage occurs, resulting in visual field loss. The scotomas occur only when a large part of the nerve fibers (over 30%) has already perished and are therefore signs of an advanced stage of the disease.
- Ablatio retinae (retinal detachment): sectoral visual field loss.
- Macular degeneration (group of diseases of the human eye that affect the macula lutea (“the point of sharpest vision”) – also called “yellow spot” – of the retina and are associated with a gradual loss of function of the tissues located there): central visual field defects.
- Retinopathia pigmentosa (decline of visual receptors): concentric narrowed visual field.
- Lesions of the visual pathway: depending on the localization of the visual pathway lesion (damage), there are different types of visual field loss (eg, hemianopsia/hemifacial loss). Possible causes:
- Brain tumor
- Aneurysm (vascular dilation)
- Apoplexy (stroke) – usually quadrant or hemiparesis.
- Trauma
- Follow-up of known scotomas (e.g., in the context of glaucoma).
- Expert opinion / suitability assessment: the binocular visual field (sum of the visual fields for the left and right eye) is crucial for the assessment. For example, scotomas may not overlap for the purpose of granting fitness to drive.
Contraindications
There are no contraindications to the use of the procedure. However, care should be taken to ensure that the patient has sufficient compliance (cooperation).
The procedure
The simplest procedure to determine the visual field is finger perimetry. In this procedure, the physician sits opposite the patient and, by moving his finger, checks where peripheral perception stops when the patient’s gaze is centrally fixed. Apart from this simple but crude method, there are several types of perimetry available today, using different techniques and devices. All methods are based on the fact that the patient must fixate a fixed point and then give a signal as soon as he perceives an emerging light mark. Different sizes, brightnesses and colors of the light marks can be tested. In all measurement procedures, care must always be taken to ensure that the conditions are kept constant. There must be a standardized brightness of background and light mark, refractive errors of the eye should be compensated and especially for follow-up examinations the pupil width should remain the same. It must also be kept in mind that perimetry is a subjective measurement procedure and depends on the patient’s cooperation, attention, fatigue, and erroneous information.
Examination Technique
Perimetry is always performed monocularly (on one eye). The head is fixed in the center of the perimetry device with a chin and forehead support.The examiner is usually given a signal button to indicate when the light marks become visible.
- Kinetic perimetry
- Device: hollow sphere perimeter according to Goldmann.
- The examinee holds his eye in the center of the hollow sphere and fixates a point in the center of the hemisphere surface, with a distance of 33 cm between the eye and the fixation point. The physician is behind the device and can observe through a telescope whether the patient keeps the eye still. At the same time, he uses a mechanical lever system to move light marks from the periphery of the hemisphere toward the center. As soon as the light marks become visible to the patient, he emits a signal. The points where a certain light mark is perceived for the first time are points with the same retinal sensitivity. These points are determined in a radial (ray-like) arrangement and connected afterwards. The connecting line between the points is called an isopter. Subsequently, the light marks are gradually reduced in intensity and size, so that they can be perceived less and less in the periphery. The lower the luminance of a point, the more central runs the isoptere for this point, because the brightness perception of the retina decreases towards the periphery.
- Static perimetry
- Device (nowadays): computer-controlled perimeter.
- The examinee holds his eye in the center of a hemisphere-like, but computer-controlled device and fixes a central point. At various points in the visual field, the computer briefly illuminates a light mark. If this is registered by the patient, he signals this by pressing a button. If the light mark remains unnoticed, it appears again later at the same location with a higher luminosity until it is finally perceived. In this way, the stimulus thresholds of different points on the retina are determined. The results can be displayed as a grayscale or color printout.
- Combatimetry
- Device: combatimeter
- Kampimetry is an older examination method. The patient fixes a point in the center of a black screen, his visual field is tested by advancing bright stimulus marks. The modern variant is noise field kampimetry. The patient is shown a flicker image, at the sight of which he can perceive his scotomas themselves and, if necessary, mark them with a computer mouse.
- Grid according to Amsler
- This examination method is very simple and is used to detect central scotomas and metamorphopsia (image distortion). The patient looks at a central point of a grid and can see if there are gaps in the grid (in scotomas) or distortions of the lines (in metamorphopsia) by looking at the straight lines and drawing them in if necessary.
Possible complications
No complications are expected with perimetry.
