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.
(c) 2009 Rob Schertzer