Visual Field Testing
Visual Field Tests Provide Important Information to Diagnose and Monitor Many Eye Diseases
Visual field testing is a sophisticated, automated computerized vision test that measures both central and peripheral vision/visual function. It is essential in monitoring glaucoma and it is a useful ancillary test to diagnose, document or treat neurologic or retinal disease. It is also used to document vision loss or obstruction for functional purposes.
Confrontation Manual Fields
There are several ways to test someone’s visual field manually. Confrontation fields is the most common method used during routine eye examinations and is a very gross test meaning it is only able to detect significant defects. It would never be used to follow glaucoma or even to quantify vision loss only to detect that there may be some vision loss. During this test the doctor will sit about arm’s length away from the patient and have the patient cover the eye that is not being tested. If the patient is covering the left eye they will look at the doctor’s left eye. The doctor will then hold up fingers in the peripheral vision halfway between patient and doctor. The patient should then tell how many fingers are being held up without looking directly at the fingers. The doctor will repeat this test in all four quadrants for both eyes.
Description of a Visual Field Test
Computerized perimeters give a much more comprehensive report and picture of the visual field along with any defects in the field. These just require the operator to set up the machine with patient name, age and eye glass prescription and the machine runs the test. It can even detect if the patient’s eye is moving around too much or not holding proper fixation on the assigned target. The most common machine used is the Humphrey Visual Field Analyzer (HFA) and there are various versions of this machine. It consists of a hemispheric bowl which is what the patient looks into during the test. Various small points of light will appear on the screen and the patient must look straight ahead and click a button when they see the flash of light. The flashes will vary in brightness and the test will determine the least amount of brightness necessary for the patient to see the flashes at each point otherwise known as the threshold. The machine will test each point at a certain brightness level two times and if the point is missed both times it is recorded as a miss and the point becomes brighter. There are several different threshold patterns and the doctor will choose the test required based on the patient’s condition. The report obtained from the HFA contains comprehensive information. Concentration by the patient is very important during visual fields. Patient reliability is reported by numbering false positive and negative errors as well as fixation losses and short term fluctuation. It also gives the mean deviation or overall depression or elevation of the visual field when compared to a reference field. This would commonly be affected by something like a cataract. The pattern standard deviation reports localized field depressions ignoring any overall depressions. The glaucoma hemifield test compares the superior field to the inferior since glaucoma tends to affect one more than the other. The total deviation plot represents the difference in decibels between the patient’s field and many age related normal fields. There are other smaller versions of this visual field analyzer that use different technology but give the same detailed reports. The picture below is a visual field printout representing a normal visual field.
Uses for Visual Field Testing
A visual field analyzer is most commonly used to monitor or diagnose glaucoma. Glaucoma is a disease characterized by loss of optic nerve fibers. There are several types of glaucoma and all of them start with loss of peripheral vision. The goal when treating glaucoma is to lower the intraocular pressure to a level which prevents further loss of optic nerve fibers. The disease is never cured but the aim is to stop progression. The progression or stability of the disease must always be monitored by evaluating the visual field as well as the intraocular pressure, appearance of the optic nerve and nerve fiber layer. Many of these tests also have a screening option which is a less detailed field test. The screening is useful to discover a problem, but once a person is diagnosed as having glaucoma only the threshold test should be performed.
Visual Field defect above shows a classic arcuate scotoma
(curvilinear field loss) characteristic of glaucoma
In addition to glaucoma, visual field analyzers are able to detect other diseases involving the retina, optic nerve and brain. Visual field loss due to retinal disease could be caused by anemia, blood vessel occlusion, diabetes, hypertension, retinal detachment, retinitis pigmentosa, macular degeneration and many more. Conditions affecting the optic nerve include inflammation, blood vessel occlusion, aneurysms, atrophy or a tumor just to name a few. There are countless problems which could occur in the brain and create visual field loss including tumors in various locations, aneurysms, strokes or migraine auras.
Visual field tests are also used to measure functional vision for documentation purposes. For example, many people experience something called dermatochalasis over time which is the protrusion of orbital fat from behind the eye into the eyelids. This causes the eyelids to sag and if it progresses can obstruct vision. Insurance will cover a blepharoplasty (eyelid lift) if the vision loss is able to be quantified and documented with a visual field test. In this case the doctor will do the test while the eyes are relaxed and repeat it with the eyelids taped up. If there is a significant improvement in the visual field then the surgery is warranted.
A visual field test is extremely valuable because it is a noninvasive, rapid method of gathering a wealth of information regarding a person’s ocular and neurological health.