What is a Computerized Visual Field Test?
A standard automated perimetry (SAP) test measures the retina's sensitivity threshold to light at various points. By comparing your personal results to data of healthy individuals of the same age, it helps identify areas of abnormally decreased sensitivity (scotomas).
How is the test performed?
The test is performed sitting in front of a dome-shaped instrument. One eye is covered, and with the other eye, you look at a central fixation target. Throughout the test, flashes of light of varying intensities will appear in different locations in your visual field. Every time you notice the light - even if it is very faint - you press a hand-held button to indicate that you detected the stimulus.
The Importance of Comprehensive Evaluation: Why is 20/20 vision not enough?
Many tend to think that if they can read the small letters at the optometrist (visual acuity), their vision is entirely normal. In reality, the situation is more complex. Visual function comprises many components beyond visual acuity, such as color vision, visual field, contrast sensitivity, spatial vision, and 3D vision. In the early stages of glaucoma, functional damage tends to be localized and may predominantly affect the peripheral visual field, practically having no effect on a visual acuity test.
How can there be visual field damage without feeling it?
The brain and eyes' compensatory mechanisms are the reason it's hard to notice the damage. This is a tremendous but simultaneously cruel ability. The brain knows how to "fill in" the missing information by combining data from the healthy areas and the other eye. Because the damage progresses gradually, the brain learns to compensate for "holes" in the visual field so that it interferes less with daily life. While this amazing ability helps cope with the damage, it also prevents the patient from realizing there's a problem, and often patients only start to "feel" the glaucoma when there's significant irreversible damage.
What types of visual field tests exist?
There are several testing devices, and each can be configured with different tests depending on field size, testing strategy, stimulus size, and more. The most common test today is the "24-2" which tests the central and intermediate visual field. Since the resolution of this test is not always high enough to detect central damage, a "10-2" test focusing on the 10 central degrees of vision is often performed in parallel or alternately.
Does it matter where the test is performed?
Absolutely. For reliable glaucoma monitoring, it is best to perform the test on the same type of device and under the same conditions. Using different machines or software versions can create "noise" that makes it harder to identify changes. Performing the test on the same machine allows the creation of a trend analysis graph that helps evaluate and identify long-term changes.
What should I know before the first test?
This is not a "pass/fail" test: Don't try to "guess" where the light will appear.
Concentration and rest: The test requires focus. If you feel tired, you can pause for a moment to rest.
Optical correction: The test is performed with near-vision corrective lenses (if needed) to ensure the result reflects the optic nerve's condition, not a refractive error.
How long does the test take?
Testing each eye typically takes a few minutes. Together with preparation, the whole process takes about 20-30 minutes. Note that the duration is also affected by the type of field and strategy used, and lack of cooperation can prolong the test.
How do I know if the test is normal?
Interpreting the test is a task for a glaucoma specialist. The printout includes complex statistical indices (like MD and PSD) and probability maps. A normal test is one where the sensitivity at all points is within the normal range for your age, but we always need to consider stability compared to past tests.