Visual field testing, or perimetry, is a test of your optic nerve function, whereas the HRT nerve scan measures your optic nerve form.
Tests of your optic nerve form and function need to be considered along with your clinical exam at the office to help decide whether a change in your treatment is required. This test is usually repeated approximately every four months as a baseline assessment and if there is a confirmed defect. It can be done less frequently if results have been stable. A change on just one visual field test often needs to be confirmed by repeat testing and by correlating with the HRT nerve scan and the clinical findings. No ancillary test result on its own is enough to confirm progression of glaucoma as there can be fluctuations from visit to visit or other artifacts.
The part of the back of the eye that is directly damaged from glaucoma is the ganglion cells within the retina. These ganglion cells all meet by contributing their axons to form the optic nerve which connects your retina to your brain. Images from the outside world are captured by the retina and sent to the brain along the optic nerves.
The visual field test maps out any damaged areas to your retina by having you look straight ahead at a darkened background and have you click a button every time you see a light appear somewhere in this bowl. The machine actually tests many points in your field of vision with different intensities of light to determine the faintest light that you can still see at any given point and produces a visual field map of these results. There are certain patterns that are characteristic of damage to the ganglion cells (glaucoma) that Dr. Schertzer would be looking for when you perform this test. If these patterns of missing light correspond with areas that show structural change on your HRT nerve scan and clinical examination of your optic nerve, then this would suggest that your glaucoma is not stable and a change in treatment would be needed.
(c) 2009/2016 Rob Schertzer
Read more about Visual Field Testing in our Knowledgebase.