Visual field testing is functional testing, assessing both peripheral and central visual fields. With glaucoma, the peripheral visual field is usually lost first, gradually progressing more centrally, resulting in a “tunnel-like vision” and progressing to involve the central vision and finally resulting in blindness. Damage to the nerve layer on the optic disc (the optic disc is where all the fine retinal nerves in the eye collect into a bundle leading to the brain) will result in progressive increase in the cup/disc (C/D) ratio and progressive vision loss.
When there is damage to the optic nerve from “higher than desired” (or “target”) pressures, it is called glaucoma. Although, in general, glaucoma occurs in patients with higher than normal range pressure, meaning pressures outside the normal range for the general population (which is between 11 to 21), it can also occur when pressures are within the normal range for the general population, but too high for that particular patient. This is often referred to as “normal pressure” glaucoma. For example, if the normal pressure for a person is 11, but the pressures are now 18 or 19 then, although pressures are still in the normal range for the general population, they are significantly high for that particular patient.
Visual evaluation of the optic nerve clinically (by looking at it and recording the appearance), visual field testing and measuring the optic nerve thickness objectively, via devices such as the GDx or the OCT devices, are important parts of the glaucoma evaluation and follow-up. An abnormality of one or more of these tests may be suggestive of glaucoma and progression of any abnormality suggests progression of glaucoma.
It is believed that by the time typical early visual field defects are first identified, there may possibly be up to 50% damage of the optic nerve fibers. One of the earlier visual field findings is a called a nasal step. With this early visual field damage the patient is already considered to have “mild to moderate” glaucoma. Thus, careful and timely follow-up is required to make certain the eye pressure is no higher than the target pressure set (i.e., the pressure under which the patient's pressure should be kept in order to be safe), there is no progression of the optic disc changes, and importantly, there are no progressive changes in the visual field test. The interval of the testing is based on the severity and stability of the glaucoma.
Visual field testing and vision loss medical expert witness specialties include ophthalmology, ocular trauma, and neuro ophthalmology.