Overview of visual field test
A visual field test is a method of measuring an individual’s entire scope of vision, that is their central and peripheral (side) vision. Visual field testing maps the visual fields of each eye individually and can detect blind spots (scotomas) as well as more subtle areas of dim vision. The visual field test is a subjective examination, so the patient must be able to understand the testing instructions, fully cooperate, and complete the entire visual test to provide useful information.
Who needs visual field testing?
The visual field test is an important part of regular eye care for people at risk of losing sight of disease and other problems. People with the following conditions should be monitored regularly by an ophthalmologist, who will determine how often a visual field test is required:
- Multiple sclerosis
- Pituitary gland disorders
- Central nervous system problems (such as a tumour that may compress the visible parts of the brain)
People with diabetes and high blood pressure have a higher risk of developing blocked blood vessels in the optic nerve and retina. They may need a visual field test to detect any effects of these conditions on their vision.
If your field of vision is limited, your ability to drive may be at risk. If you are concerned about vision loss or your ability to continue driving, talk to your eye doctor.
Types of the visual field test
Confrontation visual field testing: The doctor faces the patient and asks the patient to look straight ahead. The doctor will present stationary or moving targets in the patient’s peripheral (side) visual fields. While maintaining a straight-ahead gaze, the patient lets the doctor know when he/she can see the target in the peripheral vision. The target may be a small disc on a stick, but most commonly the target is the doctor’s hand holding up one or two fingers.
Amsler grid: This is a printed image of a grid with a dot in the centre. The patient is asked to look at the dot, one eye at a time, and note whether the grid lines surrounding the dot appear distorted, faded, or partially missing. This test is most often used to detect central visual field defects.
Static automated perimetry (such as Octopus or the Humphrey Field Analyzer): Pinpoint flashes of light of varying size and brightness are projected within a large white bowl. The patient is asked to look at the centre of the bowl and press a button each time light is seen in the peripheral vision. The machine collects the data and uses sophisticated software to analyze the results.
Kinetic perimetry (such as Goldmann perimeter): Moving targets of various light sizes and intensities are shown and the patient indicates when they become visible in the peripheral vision. The resulting data is used to map the full visual field. The full, normal range of the visual field extends approximately 120° vertically and nearly 160° horizontally.
Frequency doubling perimetry: This test utilizes varying intensities of a flickering image to analyze the visual field. It is particularly useful in detecting early glaucoma field loss.
What happens during the visual field test?
The technician or professional will comfortably sit you in front of the machine and use the appropriate lenses to correct any eyeglass correction you need. He or she will give you instructions on how to take the test. During the test, the technician will check to make sure you are looking straight ahead in the fixation light, check to make sure your upper eyelid is high enough not to block your view, and may gently reposition you if your head has turned too much.
The automatic measurement results are typically displayed in a series of charts, including:
- Grayscale Map: Dark areas in a grayscale graph plot possible depressions or osteomas where there may be vision loss or blurred vision.
- Decibel scale: Uses the ocean decibel (decibel) as the unit of measurement. The digital scale shows a range of sensitivities at different test sites. The scale depends on the type of test and the age of the patient but may go from 0 dB (unable to see intense light) to 32 dB.
- Total deviation (TD): These plots display all parts of a person’s visual field that differ from the age-matched controls. The numbers show the difference in decibels between a patient’s test results and those expected values for a person’s age. All-black squares in a TD probability chart are more likely to indicate abnormal vision from light grey shading.
- Pattern deviation (PD): This further shows results for positional deviation and the degree to which the patient’s field shape differs from the normal controls. Pattern aberration charts can be useful in tracking changes in glaucoma-related deficits.
- Visual field index (VFI): is similar to the average deviation and gives a percentage of the overall vision. A VFI of 100% indicates perfect vision while 0% means no measurable vision.