Your Visual Field Test Results

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A visual field test is a way to measure your entire visual field—the range of what you can see above, below, and on either side of you while your gaze is fixed on a central point directly in front of you. It can identify scotomas, or blind spots, as well as loss of peripheral (side) vision that is an indicator of glaucoma. There are several types of visual field tests. The simplest are done as a standard part of annual comprehensive eye exams. Your eye doctor may have you repeat the test or have additional testing depending on the results. More frequent visual field tests often are performed to monitor the progression of glaucoma.

Annual visual field testing is generally covered by vision insurance and may be covered at least twice per year for those who've been diagnosed with glaucoma. Those who are uninsured should discuss costs of the tests up front with their physicians.

What Is a Visual Field Test?

Your visual field is measured in degrees from the central fixation in four quadrants: temporal (toward your ear), nasal (toward your nose), superior (upper, or above center), and inferior (lower, or below center). A normal visual field measures about 90 degrees temporally, 50 degrees superiorly and nasally, and 60 degrees inferiorly. One eye will be tested at a time since visual fields of each eye overlap.

Different parts of the nervous system and brain control different parts of your visual field, and so certain patterns of vision loss can point to regions along the vision pathway—from retina to optic nerves to cortex of the brain—where there may be a problem such as structural damage. For example, increasing congruity (similarity between both eyes) generally indicates a defect that is located in higher cortical areas of the brain, and a loss of function in the lower visual field can point to problems with fibers of the upper retina or nerve head.

If you have abnormal results, there may be a problem with your central nervous system or an undiagnosed health problem. Also, a visual field test may detect central or peripheral retinal disease, some eyelid conditions including ptosis (drooping eyelid), optic nerve disease, and diseases affecting the visual pathways within the brain. Visual pathways carry information from the eye to the visual part of the brain, where information is processed into vision.

Potential causes of an abnormal visual field test result include:

Types

The methods for evaluating a person's visual field range from simple screening tests that can be performed in a few minutes during a regular vision check-up to more complicated and comprehensive computerized tests. Different patterns of vision loss are found with diseases of the eye, optic nerve, or central nervous system.

Confrontation Visual Field Test

For a confrontation visual field test, the doctor or technician sits facing the person being tested at eye level. The tester will instruct the person to cover one eye and with the other eye focus directly on one of the tester's eyes while they hold up one, two, or three fingers in various quandrants of the visual field. The goal of the person being tested is to respond correctly to how many fingers the tester holds up without taking their gaze off the tester's eye. The visual field of both eyes are tested in this way.

Static Automated Perimetry

Static automated perimetry uses a machine to quantify how well a person is able to detect flashing lights of varying size and brightness in different quadrants of the visual field while looking directly and steadily at a central point within a dome. The intensity or size of the light is increased until the patient sees it and responds by pressing a button.

The results of this test are largely dependent on the test taker. For that reason, it typically is repeated two and sometimes three times in a single session. Because automated visual field testing machines are computerized, the results of individual tests can be tracked and certain statistics calculated to rule out error.

Kinetic Perimetry

Kinetic perimetry involves points of light that are fixed in size and intensity. While the person being tested fixes their sight on a central point, a light is first presented in the periphery of their field of vision and moved along a vector toward the central point until the individual detects it and presses a button. This method may be performed using either a tangent screen or via a manual technique called Goldmann kinetic perimetry. Kinetic perimetry may be used for neurological field deficits, such as visual changes caused by a stroke or from optic neuritis.

Frequency Doubling Perimetry

Frequency doubling technology (FDT) perimetry is performed with a compact machine that uses flickering images of varying intensities. As the images appear in the person's field of vision, they press a button. This type of machine is also used to test for glaucoma.

Amsler Grid

The Amsler grid is a simple pattern of squares with a dot in the middle that's commonly used to test for central vision field problems. The patient focuses on the dot with one eye at a time to see if any areas of the grid appear distorted, blurry, dark, or blank, which can indicated a visual field deficit. The Amsler grid is often used to test for macular degeneration.

Interpreting Results

Automatic perimetry results are typically presented in a series of charts, including:

  • Gray-scale map: Darkened areas in the gray-scale chart plot out potential depressions or scotomas where there may be vision loss or blurred vision.
  • Decibel scale: Perimetry uses the decibel (dB) as a unit of measurement; the numeric scale shows a range of sensitivities at different test locations. The range of the scale depends on the type of test and the patient's age but may go from 0 dB (unable to see intense light) to up to 32 dB.
  • Mean deviation (MD): This gives the average difference between the patient's overall visual field sensitivity compared to "normal" age-matched controls. The mean-deviation value becomes more negative as the overall field worsens. Normal values are typically within 0dB (no difference from controls) to -2dB. Below -2dB may indicate a visual deficit.
  • Total deviation (TD): These charts show all portions of a person's visual field that differ from age-matched controls. The numbers show the difference in decibels between the patient's test results and those values expected for the person's age. Fully black squares in the TD probability plot are more likely to indicate abnormal vision than lighter gray shading.
  • Pattern deviation (PD): This shows more localized deviation results and the degree to which the shape of a patient's field differs from normal controls. Pattern deviation charts can be useful in tracking changes in a glaucoma-related deficit.
  • Visual field index (VFI): This is similar to the mean deviation and gives a percentage for overall vision. A VFI of 100% indicates perfect vision while 0% means there is no measurable vision.

Follow-Up

If you have abnormal results on a visual field test, your doctor may order additional tests or refer you to your primary care doctor or a specialist, such as a neurologist or endocrinologist, for further evaluation. The next stages of testing might involve blood tests—to screen for diabetes, hypertension, or hyperthyroidism, for example—or brain imaging.

Monitoring Glaucoma

Most types of glaucoma begin with loss of peripheral vision. The visual field test is used often by eye doctors to diagnose, determine the severity of, and monitor glaucoma. Your doctor will probably order a visual field test as soon as glaucoma is suspected and use the data to determine the severity and progression of the disease.

Visual field testing to monitor glaucoma may be performed two or more times throughout the year. This may seem repetitive and unnecessary, but repeating the test often helps your eye doctor evaluate the progression of the disease and to ascertain if any changes need to be made to your medication.

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