Visual Field Testing
Most of us can remember having our vision tested, reading letters on a chart to assess our central vision. But as we can imagine, our central vision is only a small part of our overall vision. Our peripheral vision lets us see things around us; it helps us see a car in the lane beside us; it helps us not bump into doorframes or trip on cracks in the street as we walk.
Since we don’t focus using our peripheral vision, it can be much harder to measure, and we could lose a significant chunk of our peripheral vision before we even notice. Because of this, it is very important for people at risk of visual field loss to be routinely tested so these disorders can be picked up early and be well managed. Some conditions that affect peripheral vision include glaucoma, retinitis pigmentosa, diabetes, and strokes. Other than strokes, these conditions generally progress slowly, making it even more difficult to notice a loss of vision yourself.
If you are unsure whether you are at risk of any of these conditions, speak to your doctor or optometrist. There is no harm in assessing for these factors early; since they are not reversible, picking up problems early can help you manage your conditions in a way that will help you live your best life.
Peripheral vision loss is often thought to present as “tunnel vision.” The images often shown for conditions like glaucoma are a loss around the very outside of your vision, leaving the center mostly intact. This is generally not the case. True, you may lose the nerve fibers responsible for your vision around the outside, but there can be loss in multiple areas. Think of it like losing a few scattered pixels throughout the screen of your vision.
How do we test peripheral vision?
When we test your peripheral vision, you will be sat at a visual field machine, have one eye covered at a time, and be given a response button. The aim of the test is to determine what you can see in your periphery, so you will be asked to keep your gaze straight ahead and click the response button each time you catch the flash of light in your peripheral vision. These light flashes are of different intensities, so we can tell how sensitive your vision is in an area. It is very normal for people to feel like they are doing the test poorly and imagine lights or accidentally missing lights due to poor reaction time. Modern visual field testing machines monitor reliability and will allow for some error, repeating lights if the responses don’t seem to make sense. The machine also has built-in gaps to monitor if the patient is pressing the button in error and will alert the technician if a test is unreliable.
Should a visual field test show that you have not lost any peripheral vision, it is easy to relax and not undergo regular monitoring, but most conditions that affect peripheral vision are chronic. Once it is determined you are at risk of these conditions, it is important to undergo routine testing to pick up any changes early.
We can perform other scans (such as OCT) on the nerve fibers in the retina that give peripheral vision. These scans can determine how many nerve fibers a patient has compared to the average person their own age. Of course, some people are quite different to the average, some people are born with fewer nerve fibres, and other people would begin losing vision while still appearing “average” on these scans, making it important to continue to perform routine visual field tests once your Ophthalmologist or Optometrist has determined you to be at risk.