Glaucoma is a progressive disease that damages the optic nerve, which sends visual information from the eye to the brain. This damage can lead to permanent vision loss, developing so gradually that it goes unnoticed until later stages. The loss of sight occurs in specific patterns as the disease affects the nerve structures within the eye. This process is often, but not always, associated with an increase in the eye’s internal pressure.
What is a Wedge Defect?
A wedge defect is a pattern of vision loss that can occur in individuals with glaucoma. It appears during a visual field test, which maps out a person’s complete scope of vision. The defect is named for its characteristic V-shape that appears in the peripheral, or side, vision, with the point aimed toward the eye’s natural blind spot.
This type of visual field loss is a direct consequence of damage to specific nerve fiber bundles in the retina. In the early stages, most people are unaware they have this blind spot because the brain is adept at compensating for missing visual information by “filling in” the gap. This makes it feel as though the visual world is complete.
The defect is often located in the nasal part of the visual field, corresponding to damage to nerve fibers on the temporal side of the optic disc. Because central vision remains sharp until the advanced stages of glaucoma, daily activities like reading are not immediately affected. This preservation of central sight contributes to the silent nature of the vision loss, underscoring the importance of regular eye examinations.
The Connection Between Nerve Damage and Vision Loss
The shape of a wedge defect is directly linked to the anatomy of the Retinal Nerve Fiber Layer (RNFL). The RNFL is composed of millions of ganglion cell axons, which are long fibers that transmit visual signals from the retina to the optic nerve. These fibers are bundled together and travel from all parts of the retina to the optic nerve head, where they exit the eye.
These nerve fiber bundles do not travel in straight lines; instead, they follow a specific, curving path. The fibers from the upper and lower parts of the retina form sweeping arcs around the central macula, an area responsible for sharp, detailed vision. This specific arrangement is known as the arcuate pattern.
When glaucoma begins to damage the optic nerve, it affects these arcuate bundles first. The loss of a specific bundle of nerve fibers results in a corresponding blind spot in the person’s field of vision. Because of the arc-like trajectory of these fibers, the resulting area of vision loss takes on a wedge or arc shape, mirroring the anatomical pattern of the damaged nerves.
Diagnostic Tools and Procedures
Clinicians use functional and structural tests to identify a wedge defect and diagnose the underlying glaucoma. The primary functional test is automated perimetry, or a visual field test. During this procedure, a patient looks into an instrument and presses a button each time they see a small flash of light, which allows the device to map the entire field of vision. This process creates a detailed map that can reveal the precise location and size of any blind spots, including a wedge defect.
To complement the functional data, a structural imaging test called Optical Coherence Tomography (OCT) is used. An OCT scan is non-invasive and uses light to create high-resolution, cross-sectional images of the retina, allowing a doctor to see its distinct layers. This technology provides an objective measurement of the thickness of the Retinal Nerve Fiber Layer (RNFL).
By measuring the RNFL, an OCT scan can pinpoint where nerve fibers have thinned due to glaucoma. This structural damage directly corresponds to the functional vision loss detected by the visual field test. For instance, a thinning of the lower arcuate nerve bundle on an OCT scan would be expected to produce an upper wedge-shaped defect on the visual field map.
Managing Glaucoma to Prevent Progression
The management of glaucoma focuses on preventing further damage to the optic nerve and preserving remaining vision. Treatment cannot reverse vision loss that has already occurred, so a wedge defect is permanent. The goal of glaucoma management is to lower intraocular pressure (IOP), the main risk factor for disease progression. Controlling IOP reduces stress on the optic nerve, which helps prevent the wedge defect from worsening.
A common starting point for treatment is medicated eye drops. These prescription drops work by either increasing the outflow of fluid from the eye or decreasing the amount of fluid the eye produces, both of which lower IOP. A combination of drops may be needed to achieve the target pressure set by the ophthalmologist, and consistent daily use is necessary for the medication to be effective.
When eye drops are not sufficient or cause side effects, laser procedures are another option. A common procedure, Selective Laser Trabeculoplasty (SLT), targets the eye’s drainage system to improve fluid outflow. For more advanced cases, incisional surgery may be recommended, such as a trabeculectomy or the implantation of a drainage device, to create a new pathway for fluid to exit the eye.