Our visual field (also called field of vision or field of view) is the area of space that we can see at one time, and includes both central vision and peripheral vision.
Reading an eye chart during an eye exam tests the clarity of our central vision — the part of our visual field we use to recognize faces and read. But measuring visual acuity with an eye chart reveals little or nothing about our peripheral vision, which is essential for safe driving, seeing traffic when crossing a street, performing well in sports, and avoiding obstacles and falls.
Also, vision problems from glaucoma and other conditions can start in our peripheral visual field before central vision is affected. Early detection of peripheral vision problems — particularly those caused by glaucoma — can help your eye doctor initiate treatment that can prevent central vision loss.
Types of Visual Field Tests
During a comprehensive eye exam, your eye doctor will perform one or more visual field tests to evaluate your peripheral vision. The term used to describe the measurement of your visual field is perimetry. Some visual field tests are simple screening tests; others are much more detailed and involve the use of sophisticated perimetry equipment. Since the visual field of each eye overlaps, visual field testing typically is performed on one eye at a time.
Examples of visual field tests include:
Confrontation Visual Field Testing
This is a very simple screening test where the extent of your peripheral visual field is compared with that of your eye doctor. Sitting directly in front of you at a distance of approximately arm’s length, your doctor will have you close one eye and look directly into his or her open eye.
Your doctor will then hold a small object (or hold up one or more fingers) at arm’s length far off to the side so it is out of sight, and then gradually move the object inward. You tell the doctor the moment you can see the object, which should essentially be the same time he or she sees it if you both have normal peripheral vision.
If your peripheral visual field appears limited during a confrontation screening test, your eye doctor typically will perform a more detailed visual field test or refer you to a specialist for advanced testing.
This type of visual field testing is more advanced than confrontation screening and tangent screen testing, and is more effective at detecting and monitoring visual field defects. (The technical term for a visual field defect, or “blind spot,” is scotoma.)
In the most commonly performed automated perimetry tests, you sit in front of a computerized instrument that has a large, concave (bowl-shaped) testing surface. Your chin and forehead are comfortably positioned on a headrest facing the inside of the bowl. As with tangent screen testing, one eye is covered and you focus on a central fixation target for the duration of the test, which generally takes only a few minutes per eye.
While you are focused on the fixation target, small lights will appear briefly (one at a time) in your peripheral field of view. As soon as you see a light appear, you use a small hand-held “clicker” button to acknowledge that you saw it. The examiner can adjust the number, position and brightness of the test lights, depending on the specific type of visual field test your eye doctor requests.
From your responses, the automated perimetry test creates a detailed map of the size and sensitivity of your peripheral vision.
An automated perimetry test often is recommended during or after your first comprehensive eye exam even if your eye doctor does not suspect you have a visual field problem. The results of this “baseline” visual field test can then be stored electronically and used for comparison purposes for future routine or specially ordered visual field tests to rule out glaucoma and other eye diseases.
Tangent Screen Testing
This visual field test is more detailed than confrontation screening, but not as detailed as automated perimetry (see below). In this test, one eye is covered and you stand or sit about three feet from a non-reflective black screen. During the testing, you keep your gaze fixed on a small bright target in the center of the screen.
As you continue to focus on the fixation target, the examiner uses a hand-held wand to move a small target from the periphery of the screen inward, and you tell him or her the moment you notice it in your side vision. Each point at which you notice the target is noted and the resulting visual field “map” is compared to what is expected for a person with a normal visual field.
Amsler Grid Test
Unlike the tests mentioned above, the Amsler grid test is a screening device used to evaluate the central visual field, not peripheral vision.
The Amsler grid typically is a small hand-held card, and has a central fixation dot in the middle of a grid pattern of fine horizontal and vertical lines. If, while you are looking at the fixation dot with one eye covered, any of the boxes or lines of the grid look blurry, wavy or broken, this could indicate a central visual field defect.
Amsler grid testing is performed primarily to rule out macular degeneration or other eye diseases that affect the macula, which is the most sensitive part of the retina that is responsible for central vision and color vision.
If an Amsler grid screening test suggests a problem exists, your eye doctor might recommend a more sensitive automated perimetry test that evaluates the central visual field or other testing of the macula.
What Causes Visual Field Defects?
As already mentioned, glaucoma is a primary cause of peripheral vision loss. Left untreated, glaucoma can eventually lead to central vision loss as well and even blindness, so early detection with routine eye exams that include visual field testing are essential.
Other diseases and conditions that can cause visual field loss include:
- High blood pressure
- Multiple sclerosis
- Optic nerve disease
- Brain tumors
- Detached retina
- Retinitis pigmentosa
- Macular degeneration (central visual field only)
If your eye doctor recommends special visual field testing, he or she will discuss with you the purpose of the testing and other details.
Article © 2012, Access Media Group LLC. Source: Visual Field Testing by AllAboutVision.com.