What is a visual field test?
Our brains are extremely clever in the way they process visual data from our eyes to make us feel the whole view around us is in focus all the time. In actual fact, much of the detail we see in our peripheral vision is an optical illusion created by our brains using the visual memory of what we have just looked at.
Whilst this function of our eyes and brain is extremely helpful, it can also mislead us to think our peripheral vision is normal when it is not. If you are missing a part of your peripheral vision, for instance if you have glaucoma, your brain just fills in the gaps. This can be dangerous when driving. For example, say for instance you have a visual field defect to the right-hand side and you glance slightly to the right then back straight ahead again, your brain will then fill in the gap you are missing to the right-hand side with vision memory. Now, say for instance a ball bounces into the road from the right, you won’t register it until it reaches your central vision looking straight ahead and so it bumps off your car. Now imagine if that ball was actually a person and you saw them too late. So, hopefully you can understand why it is important to monitor your visual fields accurately. In many ways visual field defects are actually more likely to cause an accident than poor vision clarity, and people are still legal to drive even with a visual acuity of around 20/40, which is half as good as normal. However, most patients with glaucoma who lose their driving licence, do so because of visual field defects identified by the DVLA.
So how does your optometrist and ophthalmologist measure your visual field?
Nowadays, automated visual field machines are used, but they need to be of the correct specification to do threshold VF test to keep you safe. The word threshold means the machine is working out the exact minimum level (threshold) that you can see in each part of the field. This is in contrast to a more basic ‘supra’ threshold screening test, which just identifies abnormal below a normal level. By actually measuring your exact threshold level accurately, the VF test helps your ophthalmologist monitor your peripheral vision for stability. OcuPlan only partners with optometrists that have the correct VF equipment to keep you safe and provides guidance on how these tests should be performed.
The accuracy of visual field testing is important when using these tests to monitor for glaucoma stability or progression. Most machines have reliability indices to help the clinician identify VF tests that may have low accuracy and may need repeating. Excessively long test duration greater than 8 minutes for a standard automated test and 5 minutes for a fast automated test may indicate the patient lost concentration or became tired. Repeating the test at that visit may not actually be sensible if tiredness is the problem. If other reliability indices are OK, then the test result should still be acceptable. If fixation losses are greater than 20%, false-positive errors are greater than 15% or false-negative errors are greater than 15%, it is sensible to repeat a VF test. If the reliability issues happen again, then it is likely that patient factors are the cause, and the test does not need to be repeated again at that time. Sometimes with more patient practice the reliability indices will improve. It is worth remembering that almost half of the lights shown during the test will not be seen even by a person with normal vision, because the test is trying to find the minimum level of what you can see. So, remember if you are doing a VF test, only press the button when you are sure you have seen a light and try not to second guess the test.