Glaucomatous visual field
We normally see a wide area of the space in front of us. Without moving our eyes, we see not only what is straight ahead, but some of what is above, below, and off to either side. Most people are familiar with this as "peripheral vision". The entire area that we see is called the visual field. The visual field is all the space that an eye can see at any given instant.
Vision is usually best right in the middle of the visual field. That is why we turn our eyes toward objects that we want to see better. The farther away from the center of our vision an object is, the less clearly we can see it. When an object moves far enough to the side, it disappears from our vision completely.
The visual field is described as being an island of vision surrounded by a sea of darkness. The outer aspect of the visual field extends approximately 60 degrees nasally, 90-100 degrees temporally, 50-60 degrees superiorly and 70-75 degrees inferiorly. The macula corresponds to the central 13 degrees of the visual field; the fovea to the central 3 degrees. A scotoma is either an absolute (total loss of vision) or a relative (partial visual loss) defect in the visual field. The blind spot is a normal scotoma. This is a location with no photoreceptor cells, where the retinal ganglion cell axons exit the eye to form the optic nerve.
A visual field test is used to determine whether the visual field is affected by diseases that cause local scotoma or a more extensive loss of vision or a reduction in sensitivity (threshold).
Visual field is used to detect:
- central or peripheral retinal diseases
- optic nerve diseases
- diseases affecting the visual pathways within the brain
Perimetry is the study of the visual field and a perimeter is an instrument designed for perimetry.
A visual field test measures two things:
- How far up, down, left and right the eye sees without moving.
- How sensitive the vision is in different parts of the visual field.
Visual field testing can be performed by a number of different techniques including confrontation (at the bedside), tangent screen, Goldmann kinetic perimetry, and automated static perimetry.
How the Test is Performed
Confrontation visual field exam: This is a quick and basic check of the visual field. The health care provider sits directly in front of you. You will cover one eye, and stare straight ahead with the other. You will be asked to tell when you can see the examiner's fingers. If the patient is able to report the number of fingers properly as compared with the visual field of the practitioner, the normal result is recorded as "full to count fingers" (often abbreviated FTCF).
Tangent screen or Goldmann field exam: You will sit about 3 feet from a screen with a target in the center. You will be asked to stare at the center object and let the examiner know when you can see an object that moves into your side vision. This exam creates a map of your entire peripheral vision.
Automated perimetry: You sit in front of a concave dome and stare at an object in the middle. You press a button when you see small flashes of light in your peripheral vision. Your responses help determine if you have a defect in your visual field. If you can't see flashes of light in an area of your field of view, then you may have a scotoma indicating vision loss.
How to Prepare for the Test
No special preparation is necessary.
How the Test Will Feel
There is no discomfort with this test.
The test has no risks. Perimetry is a simple, non-invasive technique that can be applied to almost all ages with no adverse reactions although on occasions it can be a little redious.
- Altitudinal field defects, loss of vision above or below the horizontal – associated with ocular abnormalities
- Bitemporal hemianopia, loss of vision at the sides (see below)
- Central scotoma, loss of central vision
- Homonymous hemianopia, loss at one side in both eyes – defect behind optic chiasm
Automated perimetry provides most sensitive and reproducible results. The sensitivity of confrontation techniques is about 20% for detection of arcuate field defects and about 70% for detection of hemianopia when compared to Goldmann and automated perimetry. Confrontation techniques have a high specificity (97%) and positive predictive value (96%) for detection of visual field defects when confirmed with standard automated perimetry (SAP). Frequency doubling technology (FDT) perimetry, which was developed as a screening test for glaucoma, was found to have a similar sensitivity and specificity to SAP in patients with optic neuropathy, but not in patients with hemianopia due to an inability to define the vertical limit of these defects.