What are visual field tests?
Firstly, a visual field test is a simple and painless test which measures your peripheral, or all-around, vision.
During it, an optical assistant will ask you to cover one eye, place your chin on a rest and gaze at a central point of a screen, which is usually white.
While you continue to stare at the centre of the screen, small flashes of light will briefly appear in your periphery, and they may be faint and hard to see. In some variations of the test, the optical assistant will ask you how many flashes you see each time — this may be 1, 2, 3 or 4.
For other tests, you may be asked to press a button each time you see a light. After the test, the optical assistant will consequently use your answers to build up a picture of how well you see in each aspect of your vision, for each eye.
Is there anything I can do at home before?
Most of us are able to detect faint lights in all parts of our vision, apart from in each eye’s natural blind spot; this is found at the optic nerve head. It’s an area of reduced sensitivity that the visual field test is designed to detect.
A quick and simple test you can do at home to find your eye’s natural blind spot is to cover one eye and stare straight ahead at a fixed object. Slowly move your fingertip horizontally across your vision. You’ll find it when the tip of your finger seems to disappear. When testing your right eye, for example, your fingertip will disappear when your finger is just to the left of the object you are fixating on. It’s this area of reduced sensitivity that the visual field machine is able to detect.
What does a visual field test show?
The visual field machine will complete an analysis of the responses given during the test and collate these results in a visual field plot.
Your optometrist then uses this information to compare your visual field test to others of a similar age, as well as looking for any loss of sensitivity across the visual field. This is split into two types: diffuse and localised.
What is a diffuse visual field defect?
A diffuse visual field defect occurs when the overall sensitivity of the eyes is lower than average.
This type of defect is commonly caused by a cataract, but it could also be due to not wearing the correct glasses during the test. As such, the visual field results must be compared with the results of the other tests carried out during an eye examination.
What is a localised visual field defect?
A localised field defect occurs when a particular area of the visual field has reduced sensitivity, therefore making it harder for that person to detect the lights presented in that location.
There are certain patterns of localised visual field defects that are typical of different eye diseases, including glaucoma.
When could it indicate glaucoma?
If you have a glaucoma-suspicious visual field defect, your optometrist will investigate this thoroughly by looking closely at the results of other tests that have been carried out.
These tests include:
Optical coherence tomography (OCT)
By bringing together all this information, the optometrist can assess whether the visual field defect is likely to be caused by glaucoma, or possibly another eye condition.
You can read more about these tests here.
What does an abnormal visual field test result mean?
An abnormal visual field test could indicate early signs of glaucoma, but further tests will need to be done to confirm this.
It’s important to note that many people who are not familiar with a visual field test, or perhaps did not understand the test fully, can find it difficult on their first attempt.
Sometimes the optometrist will ask for the test to be repeated in order to see if there is a consistent defect. It’s generally only repeatable field defects that may be a cause for concern.
Visual field defects in glaucoma
Visual field defects in glaucoma are caused by damage to the sensitive nerves at the back of the eye, in the layer known as the retina. This is most often due to a slow and painless rise in pressure inside the eye, which is why your optometrist will also check the pressure in each eye.
People with early signs of glaucoma may just have lowered sensitivity to light in one part of their visual field. Or for those whose glaucoma is further progressed may have an arc-shaped area where the eye is less sensitive to light.
Visual field defects can spread over time and could lead to tunnel vision, where only a small central area of vision is left.
Fortunately, there is a range of ways to treat glaucoma and prevent this from happening. The earlier visual field problems are detected the better, as treatment can slow or stop the progression of sight loss.
If you have been diagnosed with glaucoma and given treatment, you will be asked to continue the treatment and return for regular checks. Your visual fields will be assessed every time you have a check-up so the ophthalmologist can see if the treatment is working.
For more information on the detection and the diagnosis of glaucoma, you can learn more in our glaucoma resource.