I often find that my patients struggle to perform visual field testing and the results I get are not useful as they have lots of fixation losses, false positives, and false negatives. What can I do?
Instrument and patient setup are very important but often forgotten parts of the visual field analysis Inadequate setup of the patient or the instrument or both is the most common cause for artefactual visual field defects. Artefactual visual field defects may look like disease-related visual field defects but are caused by the process of assessing the visual field and not because of a disease of the eye, the pathway to the brain, or the brain. Artefactual visual field defects take the practitioner’s time because they have to be reviewed and a decision made as to whether it is a genuine visual field defect or an artefactual one. They also take the patient’s time as the visual field test will usually need to be repeated.
Visual fields test are often performed by clinical assistants which is fine as long as the clinical assistant understands the importance of patient and equipment set up and know what to do.
To gain repeatable and meaningful visual field results it is extremely important that the room illumination is always kept the same and is in accordance with the recommendations for the instrument being used.
It can take up to half an hour for the eyes to adapt completely to darkness when a person enters a dim room from a very bright environment. Most adaptation, however, occurs in the first five minutes. It is important therefore to set the room and background luminance to the levels you are using for the test before you set up the patient so that they have time to adapt.
The patient should be seated as comfortably as possible, squarely facing the screen, usually with their chin on a rest and forehead against a bar. Some instruments have a monitor that allows you to line up the eye; others have a mark for the level with the outer canthus. Each person is unique in size, and the chair, instrument, and chin rest will all need to be adjusted to obtain the correct position. It is extremely important that the patient is seated comfortably.
Take your time to explain what is going to happen to the patient. This should include where they should look, what their responses should be, and approximately how long the test will take. For example:
‘This is a test to see what you are seeing out of the sides of your eyes. Throughout the test, it is important that you look at the central spot(s) all the time. Always look at the spot straight ahead. The machine will flash a small light in your side vision. When you see a flash of a small light press the button on this handset. Sometimes you will go for a period without seeing any lights in your side vision, this is all part of the test. I’ll test the right eye first and then the left eye. If you need a rest at any time just let me know.’
Most visual field tests are set at 33cm from the patient’s eyes. It is a near visual task and the patient must be corrected for that distance using full aperture trial lenses. It is not good practice to use the patient’s reading prescription because this may be set for a longer working distance. Measure the patient’s distance correction. If they have a cylindrical power <1.00DC calculate the best vision sphere of the distance correction and add a near addition according to their age to obtain the best trial lens for visual field testing:
<35 years no addition
40-44 years +1.50DS
45-49 years +2.00DS
50-54 years +2.50DS
55-59 years +3.00DS
60-64 years +3.50DS
>64 years +4.00DS
For example, a 56-year-old patient, distance spectacle prescription +1.00/-0.50 x 90 Best Vision Sphere = +0.75 plus +3.00DS for near, therefore use +3.75DS for the trial lens or if the cylinder is > 1.00 use the exact distance spectacle prescription (including a cyl) plus the near add as above.
If previous tests indicate one eye has a better visual field or if visual acuity testing indicates a better eye then test the better eye first. Put the trial lens in the holder and line it and the patient up. Bring the trial lens as close to the patient’s eye as possible without touching the lashes. If a cylinder is used, the axis is recorded from the perspective of looking at the patient.
When the test is completed for each eye, check the reliability indices: fixation losses, false positives, and false negatives. Then decide if any visual field defect is genuine or artefactual.