What is a visual field test?
We normally see a wide area of the space in front of us. Without
moving our eyes, we see not only what is straight ahead, but some of what is
above, below, and off to either side. Most people are familiar with this as
"peripheral vision." The entire area that we see is called the visual field.
Vision is usually best right in the middle of the visual field.
That is why we turn our eyes toward objects that we want to see better. The
farther away from the center of our vision an object is, the less clearly we can
see it. When an object moves far enough to the side, it disappears from our
vision completely.
A visual field test measures two things:
- How far up, down, left and right the eye sees without moving.
- How sensitive the vision is in different parts of the visual field.
Why do people need a visual field test?
The visual field test can help the doctor find early signs of
diseases like glaucoma that damage vision gradually. Some people with glaucoma
do not notice any problems with their vision, but the visual field test shows
that peripheral vision is being lost.
A visual field test can also help the doctor find out more about
the part of the nervous system that allows us to see. The visual part of the
nervous system includes the retina (the "film" in the camera-like eye),
the optic nerve (the "wire" that carries images from the retina to the
brain), and the brain itself. Problems with any part of this system can change
the visual field. There are well-known patterns in the test results that help
doctors recognize certain types of injury or disease. By repeating more visual
field tests at regular intervals, doctors can also tell whether the patient is
getting better or worse.
What happens during a visual field test?
There are several types of visual field tests, but they all have
one thing in common: the patient looks straight ahead at one point and signals
when an object or a light is seen somewhere off to the side.
If the patient turns the eye to look directly at the object or
the light, only the very center of the visual field will be tested. The tester
will explain to the patient exactly where to look so that the test is accurate.