The visual field is the area the eye sees when it is looking at some stimulus. The dimensions of the visual field with monocular vision (looking with one eye) are:
- the nasal part, the area toward the nose: 60 degrees (horizontal line)
- the temporal part, the area around the ears: the normal limit is 100 degrees (horizontal line)
- the superior part, the area from the center of visual filed upwards: the normal limits of this visual field are 60 degrees (vertical line)
- the inferior part, the area from the center of visual field downwards: the normal limit is 75 degrees (the vertical line)
These dimensions are certainly average values and depend on the individual facial anatomy, primarily of the frontal, maxillary, nasal, and zygomatic facial bones. The position of the eyelids, hairstyle, the prominence of the brow ridge, or the nose can also limit the visual field. When a person looks with both eyes, two monocular visual fields overlap, and you get a stereoscopic vision, which is 120 degrees in the horizontal dimension.
The extreme temporal periphery of the binocular visual field can be seen monocularly.
The image on the retina is upside down. Therefore, the projection of the superior visual field (or the upper half) suits the lower image of the retina and vice versa.
The normal visual field is described as the island of vision in the sea of darkness. The sea of darkness is the zones of the visual field with no perception of light, for example, a part of the visual field limited by the anatomic boundaries of the face. There is a gradual increase in the sensitivity to light culminating in the part of the eye called the fovea. The densest concentration of cones – the cells detecting light – is in the fovea, ∼16,000 cells/mm2, and it decreases towards the retina periphery where it is ∼300 cells/mm2 on 32 degrees of eccentricity.
With aging, the height of the island decreases as a result of a reduced ocular media transparency and the pupil size.
Blurry images in the vitreous body, corneal scars, cataracts, or another blurring of the media decrease the sensitivity and light transmission. Usually then comes a general reduction of sensitivity.
Damages occurring on the retina such as pigmentary retinopathy, glaucoma, toxic retinal damages as a consequence of medication use bring to circumferential damage and loss of peripheral boundaries of the visual field.
When the doctor interprets the visual field, he also has to know the visual pathway. Depending on the place of the damage, different specific images and visual defects are projected.
The defects in the visual field can be a manifestation of various diseases of the retina and different neurological diseases. It is advisable to implant a visual field at the age of 35 because at that age a doctor most often diagnoses glaucoma. Sometimes patients come with symptoms of headache or blurry vision, but sometimes it can be completely asymptomatic.
The diagnostic procedure is non-invasive, painless, and requires only patience and concentration.
PREPARATION OF THE PATIENT
It is very important to find a comfortable position before the test. Sit with your back straight, put the chin, and the forehead to the delimiter of the instrument. An uncomfortable position can influence the result of testing. Depending on what eye is being tested, you will have to move your head a little to the right of left. During testing, the nurse will occasionally check the position of your head.
The inadequate position brings to false-negative results. Before placing the head, an eye patch is put on the untested eye so it doesn’t influence the testing results. The patient must blink normally during testing so the cornea doesn’t dry. In case there is a droopy eyelid or ptosis (usually with older patients), it can be the reason for the presence of peripheral scotoma or defect. This can be prevented by patching the eyelid. Contemporary perimeters have the control of head fixation and movement, so they warn with alarm signals if the position is inadequate.
Before the test starts, adequate correction of the refractive error is also important so that the stimulus can be perceived. Patients who wear contact lenses are advised to always have them during the test. In the case of high myopia, the frame of the glasses brings to circumferential peripheral defect.
During testing, you should look into the fixed target. Sometimes light stimuli varying in intensity will occur. You will notice them as you look at the central target. Whenever you notice a stimulus, press the button on a handheld remote control. Sometimes the stimulus will be barely visible. That is customary because we are testing the limits of your sensitivity. You should press the button when you are at least 50% certain you noticed the stimulus.
To estimate the results correctly, sometimes the doctor will ask you to repeat the test. You shouldn’t get discouraged because of this. This type of diagnostics requires full concentration and cooperation.
The advantages of the digital perimetry are that the received data are compared with regular findings and data from persons of the same age deposited in the perimeter software. Sophisticated analyses give numerical data as well as the data presented in graphic scale and color maps depending on the type of the instrument.
Compared data are also shown in probability scales, i.e. special indexes presenting the degree of deviation from regular values. Sometimes they are the key factor in diagnostics. The instrument can do comparative analyses of several consecutive tests at different time intervals to identify progression.
The visual field is a standard part of the examination in diagnostics and monitoring of glaucoma. Glaucoma is defined as optic neuropathy that causes characteristic changes on the optic nerve and defects in the visual field. Unfortunately, 50% of the nerve fibers are damaged before visual field defects are manifested and measured by perimetry.