A visual field test, often called a perimetry test, is a non-invasive eye examination that maps the entire scope of your vision, including central and peripheral eyesight. This procedure assesses how well you can see objects outside of your direct line of sight. The results help eye care professionals identify abnormalities in the visual field. This test is a standard tool used to screen for and monitor conditions affecting the optic nerve and visual pathways.
The Role of Corrective Lenses in the Test
Patients generally do not wear their personal eyeglasses during the visual field test because the spectacle frames can block the light stimuli, creating artificial blind spots. The perimeter machine accounts for a patient’s refractive error. Instead of wearing personal glasses, the technician inserts specialized trial lenses directly into a holder on the machine to correct the patient’s distance prescription.
This correction ensures the patient’s vision is clear for the central fixation target and the lights flashing in the periphery. For those with presbyopia, an additional lens power may be added to ensure the central target remains sharp. Automated perimetry systems, such as the Humphrey Field Analyzer, use either manual trial lenses or advanced technology like a Liquid Trial Lens to quickly adjust the spherical correction. The Liquid Trial Lens can automatically correct prescriptions within a range of approximately plus or minus 8 diopters, covering the needs of most patients.
Patients who wear soft contact lenses may often keep them in during the test, as these lenses do not create the physical obstruction of a spectacle frame. Hard contact lenses or specialty lenses, however, might need to be removed because they can sometimes cause distortion or reduce contrast sensitivity, which could skew the test results. It is important to bring your most current eyeglasses or contact lens prescription to the appointment. This allows the technician to accurately calculate and apply the necessary lens correction to the machine. The goal is to correct the vision so any detected defect is a true sign of pathology, not an artifact from an uncorrected refractive error or a spectacle frame edge.
Understanding What the Test Measures
The visual field test provides a functional assessment of the entire visual system, from the retina to the visual cortex in the brain. The primary goal is to map out the sensitivity of the retina at different points, looking for areas of decreased vision known as scotomas or blind spots. These detected defects are then analyzed for specific patterns that correspond to damage along the visual pathway.
The test is routinely used to diagnose and monitor glaucoma, a disease that slowly damages the optic nerve and typically causes a progressive loss of peripheral vision over time. Because early glaucoma often has no noticeable symptoms, the visual field test is one of the most reliable methods for detecting subtle losses in side vision before the patient is aware of them. Detecting the characteristic patterns of vision loss helps the doctor determine the severity and progression of the disease.
Beyond glaucoma, the test is used to identify visual field defects caused by neurological conditions affecting the brain. Specific patterns of vision loss, such as those affecting the same side of the visual field in both eyes, can indicate damage from a stroke or a brain tumor. Conditions like multiple sclerosis, which can cause optic neuritis, and pituitary gland disorders are also monitored using this test. The results are commonly printed as a map, allowing the eye care professional to track changes over repeat visits.
Preparing for and Performing the Procedure
Preparation for a visual field test focuses on ensuring the patient is well-rested and alert to maximize the reliability of the results. It is recommended to minimize caffeine intake before the test, as stimulants can influence the patient’s reaction time and attention span. A good night’s sleep is also advised, as fatigue can significantly impact concentration and lead to unreliable results that may require repeating the test.
The procedure itself, typically using automated perimetry, involves sitting at a machine called a perimeter with your chin on a rest and your forehead against a band. The test is performed on one eye at a time, with the other eye covered by a patch. The patient is instructed to focus on a stationary target light directly in the center of a large, bowl-shaped screen.
While maintaining a steady gaze on the central target, the patient is given a handheld button and asked to press it every time they perceive a brief flash of light appearing in their side vision. The lights vary in brightness and position across the bowl, and some are intentionally so dim they are designed to be missed. The test requires unwavering concentration, as the machine tracks whether the patient’s eye drifts from the central target, known as a fixation loss. The technician may offer short breaks during the test to minimize fatigue and ensure the data collected is an accurate representation of the patient’s visual field sensitivity.