A visual field test is an eye examination which measures central and peripheral (side) vision. It requires the patient understand the testing instruction, fully cooperate, and complete the entire test in order to provide useful information.
Usually, it is done to detect damage to the optic nerve caused by glaucoma and for detection of central or peripheral retinal disease, eyelid conditions such as ptosis or drooping, optic nerve disease, and diseases affecting the visual pathways within the brain.
The test is done by a computerized assessment in an optometrist’s, ophthalmologist’s or neurologist’s office one eye at a time with the opposite eye completely covered to avoid errors.
During the test, the patient must look straight ahead at all times in order to avoid testing the central vision rather than the peripheral and when the patient sees a light or movement of various intensities and at different locations, he/she must push a button.
Visual field testing can be grouped into few categories:
- Amsler grid – It tests macular vision in and around the center of the retina and detects specific conditions such as macular degeneration. Duration of the test: 1 minute.
- Confrontational – The test is one on one by a doctor or technician as a screening tool, in which peripheral finger movements are brought from the far periphery to the near periphery in four quadrants. Duration of the test: 2 minutes.
- Kinetic perimetry – The test is done with moving light sources and requires a dedicated trained technician throughout the entire test. Duration of the test: 20 minutes.
- Static perimetry – The test is done with a device with fixed light sources, either stationary pinpoint light sources or projected dots within a large white bow and run by the onboard computer. Duration: 8 minutes.
- Frequency doubling analysis – The test utilizes varying intensities of a shimmering grid of light in standardized sectors of the peripheral and central visual field. Duration: 10 minutes.
All testing devices incorporate an internal computer with the ability to store, print, and transmit important patient data and results can be interpreted as:
- Fixation errors – The number of times the patient looks away from the central target and this is a key indicator of patient cooperation or fatigue.
- False positives – The number of times the patient pushes the button when, in reality, a light source is not visible.
- False negatives – The number of times the patient fails to push the button when, in reality, there is a light source visible.
- Points tested – Indicates the total number of separately illuminated testing points, and therefore data points presented to the patient for testing.
- Reliability index – The overall reliability of the patient's testing for each eye; poor reliability can indicate patient fatigue, insufficient understanding of the test, or poor vision for other reasons such as cataracts.
- Standard deviation – The difference in peripheral field acuity when compared to a normative database that tells the doctor whether or not a particular part of the peripheral field is normal, depressed, or absent.
- Visual field map – The final basic report which indicates the patient's visual field anywhere from the central 10 degrees all the way out to the farthest reaches of the field at 90 degrees.
Visual field testing is usually covered by virtually all health-care insurance plans, but it can be limited to one time per year for patients who are called glaucoma suspects and two visual field tests per year for patients who have already been diagnosed with glaucoma.