What is perimetry?
Perimetry measures both the limits of the visual field perceived by the unaided eye (visual field) and the acuity of perception. In contrast to the central visual field, which provides the highest visual acuity, the outer part of the visual field is mainly used for orientation in and perception of the surroundings. Therefore, it is important for the examination that the eye under examination fixates a point and does not move.
There are several methods of perimetry:
- automatic static perimetry: it is the most frequently used. The patient gives a signal via a button each time he perceives a luminous point at the edge of his visual field. 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 move from the outside toward the central field of vision. The patient reports as soon as he sees the light spot moving into his visual field.
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 is perimetry performed?
Perimetry can detect visual field defects, often before the person being tested is aware of them. The cause of such a visual field defect (scotoma) can lie in the eye itself or in the optic nerve, but also in the area of the transmitting nerve pathways in the visual center of the brain.
There are different forms of visual field loss such as central scotoma, hemianopsia (half-sided loss) or quadrant anopsia (quadrant loss).
The most common medical reasons (indications) for perimetry are:
- unexplained visual disturbances
- Glaucoma
- Retinal detachment (Ablatio retinae)
- Macular degeneration
- Lesions of the visual pathway due to brain tumors, strokes or inflammation
- Follow-up of already known visual field loss
- Assessment of visual acuity (e.g. for professional certificates)
What is done during a perimetry?
Finger perimetry
The patient fixes the tip of the examiner’s nose. The examiner 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 estimate the limits of the visual field. The patient reports each time he detects a movement of the fingers.
Static perimetry
The patient’s head rests on a chin and forehead support of the perimetry device and fixes a central point in the middle of the inside of a hemisphere. Points of light now light up at various points on the hemisphere. If the patient registers a light spot, he reports this by pressing a button.
If the patient does not notice a light signal, this is repeated later at the same position with a higher light intensity. In this way, not only the limits of the visual field but also the sensitivity of vision are determined and displayed in a visual field map.
Kinetic perimetry
Subsequently, the intensity and size of the light marks are reduced so that isopters can also be determined for weaker light signals.
What are the risks of perimetry?
Perimetry does not pose any health risks. However, since it is an examination method that requires high concentration, headaches and burning eyes may occur due to the exertion.
What do I have to consider during perimetry?
The results of this examination are highly dependent on the patient’s cooperation. Therefore, it is important to be awake and rested for perimetry. In addition, known visual deficiencies must be compensated for before the visual field map is collected so that the values are not distorted, especially for visual sensitivity.
