The visual field represents the entire area an individual can see while their eyes are fixed on a single point, encompassing both central and peripheral vision. This scope of sight generally extends about 180 degrees horizontally and up to 120 degrees vertically. A visual field defect occurs when a portion of this field is lost, resulting in an area of reduced or absent sight.
Damage to any part of the visual pathway, which runs from the retina to the visual processing centers in the brain’s occipital lobe, can cause these defects. Since the loss can be gradual and often affects peripheral vision first, a person may not notice the defect until it is advanced or discovered during a routine eye examination. The specific size and shape of the vision loss provides important clues about the location and nature of the underlying problem.
Understanding the Patterns of Visual Field Loss
Visual field defects manifest in distinct geometric patterns determined by the location of damage along the visual pathway. One common pattern is a scotoma, an isolated blind spot or area of reduced vision surrounded by normal sight. A central scotoma involves the macula, causing a defect in the middle of the visual field. An arcuate scotoma is an arc-shaped area often associated with damage to the optic nerve fibers.
Concentric constriction, sometimes called “tunnel vision,” occurs when the entire outer boundary of vision shrinks, leaving only the central field intact. Neurological defects resulting from damage in the optic chiasm or brain often cause hemianopia, which is the loss of an entire half of the visual field.
Bitemporal hemianopia results in the loss of the outer (temporal) half of the visual field in both eyes, often caused by compression at the optic chiasm. Homonymous hemianopia means the vision loss is in the same half (right or left) of the visual field in both eyes, indicating a problem in the brain’s visual pathways behind the chiasm. A quadrantanopia is a smaller loss, affecting only a quarter of the field, such as the upper or lower quadrant.
Common Causes of Visual Field Defects
Visual field defects stem from issues in three primary locations: the eye, the optic nerve, or the brain’s visual centers. Glaucoma is a frequent cause, where increased pressure damages the optic nerve fibers, leading to a characteristic arcuate or nasal step pattern of peripheral vision loss that slowly advances inward. Retinal diseases, such as retinal detachment or retinitis pigmentosa, can also cause field loss, often presenting as an enlarging peripheral defect.
Damage involving the optic nerve typically results in unilateral defects that are often central or paracentral. Conditions like optic neuritis (inflammation of the nerve) or ischemic optic neuropathy (poor circulation to the nerve) cause this localized loss. Tumors or other compressive lesions near the optic nerve head can also cause specific patterns of loss.
Defects affecting both eyes symmetrically often point to a problem within the brain, specifically in the visual pathway after the optic chiasm. A stroke or transient ischemic attack (TIA) is a common cause, as the resulting lack of blood flow damages the visual cortex in the occipital lobe. Brain tumors and aneurysms can also compress these posterior pathways, leading to homonymous hemianopia or quadrantanopia.
Detection and Measurement of Field Loss
Visual field loss is identified and mapped primarily through a procedure called perimetry, which creates a detailed map of a person’s visual sensitivity. The most common method is automated static perimetry, which uses a bowl-shaped instrument called a perimeter.
During this test, the patient looks at a fixed central target while small lights of varying brightness appear randomly within the bowl. The patient presses a button each time a light is seen. The machine records the minimum brightness level required for detection at each point, generating a printout that represents areas of reduced sensitivity or blind spots. This helps the clinician diagnose and monitor the progression of conditions like glaucoma.
A quicker, less formal method is confrontation visual field testing, often used as a screening tool during a routine eye exam. In this procedure, the patient covers one eye and looks directly at the examiner. The doctor presents fingers or a target in the patient’s peripheral field, and the patient reports when they see the object, allowing the doctor to quickly check for gross defects.
Treatment and Living with Visual Field Changes
Management of a visual field defect involves two approaches: treating the underlying cause and helping the patient adapt to the resulting vision loss. If the defect is caused by a treatable condition, such as high intraocular pressure from glaucoma, treatment focuses on stabilizing the field by lowering the pressure with medications or surgery. When a tumor or aneurysm compresses the visual pathway, surgical removal or radiation therapy may be necessary to relieve pressure and potentially restore some function.
For fixed vision loss, low vision rehabilitation maximizes the use of remaining sight and improves daily function. Specialized low vision aids, such as magnifiers and electronic devices, help with reading and other close-up tasks. For people with constricted fields (tunnel vision), reverse telescopes are available that minify the image, allowing more of the visual field to be viewed at once.
Patients with homonymous defects often benefit from visual scanning training to move their eyes and head more effectively toward the blind side. Prism glasses can also be incorporated into spectacles to shift images from the blind field into the intact viewing area, improving awareness and mobility. Orientation and mobility instructors help patients with tasks like navigating environments safely and planning routes.