There are many variations on the same general theme. The most common technique uses a programmed instrument (scanner) that has a pattern of light appearing at different locations of your field of vision, and you push a button each time a light flashes. The resulting chart provides a map of the locations that you detected. People with glaucoma will typically be able to detect fewer than half the flashes. Visual field testing is painless and safe. It is very important to get the results right the first time so that you know the locations of defects so that if you repeat the test you will be able to detect the changes.
We look for early changes in the peripheral vision which occur before any damage to the optic disc or to the central vision can be detected.
Glaucoma is often called a blindness wolf in sheep clothing because it usually causes no symptoms. It is usually discovered during a routine eye examination. In glaucoma, there is initially a damage to the nerve fibres of the optic nerve at the periphery of the field of vision. This is not noticeable, but visual field examination can detect these changes in a patient's vision before damage occurs to the central vision. The early stage of glaucoma damage is known as glaucomatous field loss. It is usually not apparent to the patient and can only be detected by an ophthalmologist or optometrist using appropriate equipment.
There are three types of visual field test. The first uses static light stimuli, where a tiny light is blinked in different areas of the field of vision.
The other tests use dynamic light stimuli, with a different shape displayed in different locations. The Humphrey field analyser is an example of a visual field test that uses dynamic light stimuli.
A new type of visual field test is oculo-kinetic perimetry or OKP. This test is usually preferred by patients and gives just as good result or better than standard automated perimetry (SAP).
On a visual field test, areas that appear darker are areas of visual loss. The dark areas are where light stimuli failed to be seen. In all of the following illustrations, blue represents better vision and red represents worse vision, as shown by the colours used in the illustrated maps above.
By careful observation of the positions of the dark areas, the pattern of the visual loss can be determined. Below are examples of different glaucoma-related patterns of visual loss seen on a visual field test.
Sometimes, visual loss may show up as a dark area which does not extend to the limits of the field of vision on a visual field test. This is known as a scotoma.
The next illustration shows a normal visual field test. The illustration on the right shows a visual field test from a person with glaucoma. They have a central scotoma in their vision, which is not symmetrical about their midline.
Glaucoma causes damage to the optic nerve head in a characteristic pattern. Sometimes this can be seen directly with the help of a microscope, as shown in the photograph on the right. The characteristic changes to the optic nerve head and retina are known as glaucomatous optic neuropathy. It shows diffuse atrophy of the retinal nerve fibre layer. At the centre of the optic disc,