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Humphrey Visual FieldsThe visual field test is designed to assess the sensitivity of the eye as well as the extent of the peripheral vision. The visual field test is mainly used to monitor damage to the nerve fibre at the back of the eye caused by glaucoma, though other ocular conditions may also be investigated. There are different types of visual field test - your consultant will request an appropriate test for your condition.
Having placed your chin on a chin-rest, you are presented with a series of flashing lights on a bowl-shaped screen. The lights may vary in brightness and will appear in different positions in your field of view. You will be asked to press a button each time you see a flash of light and the machine records what you see. The machine checks each point several times, so if you make a mistake the machine should be able to correct for this.
This varies from test to test. Typically a 24-2 SITA test takes about 5 minutes per eye and a 30-2 about 7 minutes per eye. Full threshold tests can take up to twice as long.
You may wear contact lens for the test, though if you are having other tests or a consultation afterwards, it may be best to wear glasses instead, if possible. If you use reading glasses, please bring them along.
Humphrey SITA and Full Threshold Testing
The Humphrey Visual Field is an industry standard test which allow us to look for defects in the sensitivity of the eye. For example, the central vision of the eye may be good, but the periphery of the visual field may be severely damaged. It is important to detect change in the visual field in order to maintain the function of the eye. As a good eye is able to compensate for poor vision in the other eye, the effects of visual field damage are often unnoticed by the patient until there is significant damage to the vision. The Humphrey visual field tests is designed to pick up the earliest changes to the function of the eye.
The test works by displaying flashes of light around a bowl with a fixed background light level. The machine aims to find the dimmest light that the eye is able to detect at various locations in the visual field.
The Humphrey Visual Field Analyser has become an industry standard machine for the diagnosis and monitoring of glaucoma and other ocular and neurological diseases. Like all visual field tests, the machine tests the patient's response to a visual stimulus. The machine may be set to run one of many optimised programs.
Threshold
testing of visual fields identifies the limit of the sensitivity of
the eye at programmed locations in the visual field.
The most
common Humphrey test is the 24-2, which tests the central 24 degrees of
vision. The Humphrey 10-2 test permits monitoring of patients with
very restricted fields and the 30-2 has more peripheral points.
The following strategies are available for the 24-2, 10-2 and 30-2 tests:
Full Threshold
- makes the least assumptions about the patient's vision. The
bightness of the stimulus is varied at each location in order to find
the threshold value. The results are based on the last point seen at
each location, so patient fatigue may affect the accuracy.
SITA Standard
- (Swedish Interactive Threshold Algorithm). With the benefit of
research into visual fields, the SITA test is able to make predictions
of threshold values by analysing the patient's previous responses. By
analysing the data in this way the length of the test is reduced,
often less than half the length of a similar full-threshold test. The
results are calculated using all of the data collected , so errors may
be identified and corrected automatically.
SITA Fast - the fastest threshold test. Best used with reliable subjects.
Stimulus Size
- Full threshold tests may be carried out using different stimulus
sizes. These equate to the stimulus sizes used by the Goldmann
Perimeter. Humphrey tests are normally carried out with stimulus size
III, but patients with poor vision may be tested with a size V
stimulus. Please note the SITA tests are only possible with a size III
stimulus.
Stimulus Colour
- the Humphrey test is normally carried out using a white stimulus
against a white background. However, the stimulus colour may be changed
when monitoring colour dependent changes.
Using the Institute of Ophthalmology's Progressor software, we are able to monitor progression of visual fields using linear regression analysis. The software is able to identify progressing points from a series of Humphrey 24-2 Full Threshold or SITA examinations and marks them according to clinical significance. (30-2 fields may also be used, but the outer points are ignored). It is also possible to create a simulated binocular field from a pair of visual field tests.
For further information about Progressor, click here.
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