How a Visual Field Test Works

A patient being tested for defects in her visual field

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Have you ever been asked to take a computerized visual field test? You may not be familiar with the formal name of the test, but you probably remember taking it. The test requires the patient to look into a machine, hold their head still, and detect a series of flashing lights without moving their eyes. The patient is instructed to tap a button whenever a flash of light is detected. The light flashes vary in intensity and appear in all quadrants of the visual field. Visual field testing is a way to measure your entire visual field, or how much you can see to each side while focusing your eyes on a central point (peripheral vision). Conducting a visual field test is called perimetry.

One measure of your visual function is to read letters on a visual acuity chart. This is a measure of your central vision and is the most critical part of your vision. However, that is only one a measure of your visual function. Another aspect is your overall visual field, sometimes referred to as peripheral vision. Although many people mistake it as simply a peripheral vision test, a visual field test is actually designed to measure the overall field of vision as it is interpreted by the brain in four neurological quadrants. Different parts of your brain control different parts of your visual field. Results of a visual field test can sometimes help physicians made a diagnosis.

Measuring Your Visual Field

There are different ways to conduct a visual field examination. The most common way to measure the four quadrants of a visual field is to perform "confrontation neurologic" visual fields. This is the most common way to measure it. Both optometrists and ophthalmologist perform confrontation visual fields with this method during a comprehensive eye examination. It is performed by having the doctor or technician sit at eye level with the patient. One eye is covered. The other eye focuses directly on the technician's eye and either one, two or four fingers are held in each of the four quadrants. The patient is not allowed to move their eye or look at the fingers but must respond with how many fingers the technician is holding up. After all four quadrants are tested, the other eye is measured.

When a visual field deficit is discovered with the finger counting method or if the physician suspects visual field changes, a more formal method will be used called automated perimetry. An automated perimeter is a computerized instrument that measures the field with different lights of different sizes and brightness. An automated perimeter is able to conduct several different types of field tests in a standardized fashion. A threshold test measures an individuals "just barely detectable" vision and quantifies how sensitive a patient may or may not be of detecting points that are considered normal.

These maps of visual sensitivity are very important in diagnosing diseases of the visual system. Different patterns of visual loss are found with diseases of the eye, optic nerve central nervous system.

Potential Causes of Abnormal Results

Once a visual field defect is detected, doctors will usually repeat the test a couple of times to confirm the results. An automated visual field test is a very powerful tool. However, the results are somewhat dependent upon the test taker. Once a person has completed at least one test, they usually perform better the second time around. Final assessments are usually not made until after the patient has repeated the test at least twice and sometimes even a third time. Because automated visual field testing units also have a computer in them, the reliability of the test can be tracked. Certain statistics are calculated to rule out user error and give the doctor some measure of reliability.

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