What is a visual field test?
A visual field test is a test of a person’s peripheral vision. Testing one eye at a time, the person is asked to maintain their gaze on a central target. Each location in their peripheral vision is then tested for visual sensitivity – this generates a map of that eye’s visual field. Multiple test targets are presented, some easier to see and some harder; the result being to determine what the eye can discern at each location of the visual field.
The position of targets on the visual field is measured in degrees of visual angle. The very center (the point of fixation) is 0 degrees of visual angle– and as we move farther from centre the visual angle increases. It is common to assess the central 24 degrees for each eye. This is normally achieved by testing 52 locations, at 6-degree intervals (see Figure 1, map of the visual field)
Once the visual sensitivity at each location are determined, these are compared to age-adjusted normal values; if the sensitivity is reduced compared to normal this is noted as a visual field deficit.
Figure 1. The 52 locations of the visual field
What can cause visual field defects?
Common eye diseases can cause visual field deficits:
- uncorrected short or long sightedness,
- cataracts,
- macular degeneration or
- glaucoma;
However the test is most important in the evaluation of glaucoma.
This is because those other conditions are best assessed by other forms of vision testing. In comparison, the visual field test is the most important test for glaucoma.
Diseases of the brain can also cause visual field deficits. A stroke or inflammation within the brain can result in changes of the visual field. Tumours or other lesions of the pituitary gland (located under the brain) can also affect the visual field.
Why is the visual field test so important for glaucoma?
Glaucoma, known as the sneak thief of sight, is due to painless, slow and irreparable damage to the optic nerve.
The optic nerve connects the eye to the brain and without it we cannot see. When glaucoma damage occurs, we are often unaware of it. This is because it predominantly affects peripheral vision initially, and affects central vision in more advanced stages.
We are generally not conscious of peripheral vision changes.
If we were to wait for glaucoma to be noticeable (when it affects central vision) we would be detecting it far too late.
That is why visual field testing, which can detect vision changes from glaucoma before a person notices it, is so critical for diagnosing glaucoma (see Figure 2).
Figure 2. The visual field of a patient with early glaucoma. The large rectangles represent portions of the visual field with reduced sensitivity (impaired sight). This person has lost sight in the upper, inner corner of their vision. Such loss is often asymptomatic and requires detection on visual field testing.
Visual field testing is also important for monitoring change in glaucoma over time. Once someone is found to have glaucoma, they need regular visual field tests to assess if the changes are stable or getting worse over time. This is critical to guide treatment decisions; whether the current treatment is sufficient to control the glaucoma, or whether further measures to lower the eye pressure are required.
As a glaucoma specialist Ophthalmologist in Melbourne, my patients are commonly performing visual field tests, often annually to monitor for glaucoma changes.
How is visual field testing performed?
Visual field testing has traditionally required a custom designed single-purpose machine. Such machines are bulky, expensive and to access require a visit to an eye clinic.
I am excited to announce that, through Australian health-tech company Eyeonic which I founded in 2020, we have developed an online visual field test, that does not require expensive equipment.
With the Eyeonic visual field test, visual field testing can be performed on any computer – all that is needed is internet access, however a webcam and volume are preferable.
Online visual field testing improves the experience of perimetry for my patients. It will also improve access and reduce barriers of cost to perimetry for glaucoma patients and suspects across the globe.
Online visual field testing improves the experience of perimetry for my patients. It will also improve access and reduce barriers of cost to perimetry for glaucoma patients and suspects across the globe.