An optical assistant (OA) is often asked to perform pre-screening checks on the eye for patients, and visual field testing may be one of those checks required.
The visual field is the total extent of the vision as seen by each eye – and visual field screening is included in the eye examination for most adults, and occasionally children if a problem is suspected. It is used to detect any abnormality in the visual field, and to monitor a pre-existing ocular condition, such as glaucoma. It can even detect certain types of brain tumours.
A visual screening test should be performed on each eye separately. This is because there is an overlap between the right and left visual fields when both eyes are open; so the eye not being tested should be covered with an occluder or eyepatch. One exception is for certain visual screening tests for commercial drivers where the test is performed on both eyes at the same time- this is called a Esterman Visual Field test .
Some patients are anxious about the procedure, and putting the patient at ease is an important role for the OA. Reassurance and a clear explanation of how the test works should be provided. It is a good idea to inform the patient at this stage that you are there to perform the test, but that you are not trained to interpret the results, and the optometrist will go through the results with them as part of the sight test.
Preparing for the test
The eye is more sensitive to light in darker conditions, and so the test is conducted in a dimly lit room, and several minutes should be allowed for the eye to adapt to the darkened room. Depending on the type of field test required, it can take between six and 12 minutes to perform the test, which can seem like an eternity to some patients. They may also worry that something is wrong as it is taking some time, and so you should inform the patient the approximate time the test is likely to take.
It is important that the patient is sitting comfortably, and time should be taken to adjust the chair and instrument height. The patient should be sitting with their head in an upright position, and not tilted as this can adversely affect the results. The room should be a comfortable temperature, and free from any distractions. Ensure the chin and headrest have been sanitised with anti-bacterial wipes before the patient uses the machine.
Undertaking the test
The patient looks at a screen, which is at a close distance, and so if they are presbyopic they need to wear their reading spectacles in order to see the screen clearly. If the patient wears multifocal lenses, these should not be worn for the field test, as the reading part of the lens will not be in the right position to see the screen when looking straight ahead. It may be possible to use ready readers, which are close to the patient’s reading prescription-check with the optometrist or dispensing optician first..
If you need to use lenses in a trial frame to perform the test and If the patient has astigmatism, the sphere plus half the cyl is the best compromise, but if in any doubt again check this with either the optometrist or a dispensing optician. It is important that the patient opens their eyes fully in order for the full extent of the visual field to be tested. However, this can be a problem if the patient has a ptosis – a drooping upper eyelid. In this case, you should instruct the patient to open their eyes as wide as they can. If this is still insufficient, offer the patient some micropore tape can be used to temporarily hold up the lid.
Once in the correct position, with their chin on the rest and forehead against the brow bar, the patient should be directed to look at the central spot (usually a red dot) and instructed to remain focused on the central spot at all times. The patient holds a clicker device, and a series of faint spots of light are flashed across the screen. When the patient sees a light, they press the clicker.
If the patient appears tired, or fidgets a lot, make a note of this, as this can affect the results. Once the patient has completed the test, they proceed to their eye examination, where the optometrist will discuss their results.
In next month’s article, we will look at different field plots and some common errors encountered.