The inability to see objects outside of your direct line of sight is known as peripheral vision loss (PVL), often described as “tunnel vision.” Peripheral vision is the side vision that allows you to detect motion and spatial awareness without shifting your gaze. It serves a protective function, alerting you to movement and obstacles while central vision focuses on detail. PVL is a symptom indicating a problem with the eye structures or the brain pathways responsible for sight.
How the Visual System Processes Peripheral Information
The retina, a light-sensitive layer at the back of the eye, contains specialized cells that capture visual information. Rods are the photoreceptor cells primarily responsible for peripheral vision, functioning best in low-light conditions. Rods are concentrated in the outer regions of the retina and are highly sensitive to motion, providing broad environmental awareness.
In contrast, cones are clustered in the fovea, the retina’s center, and process sharp, detailed central vision. Signals from the rods travel through retinal cells to the optic nerve. The optic nerve transmits this information to the visual cortex at the back of the brain for processing.
Eye Diseases Causing Gradual Vision Loss
Damage localized within the eye structure often leads to a slow, progressive loss of peripheral sight. Glaucoma is a leading cause, where elevated pressure inside the eye gradually damages the optic nerve. This pressure impedes the nerve’s ability to transmit signals, and the resulting damage typically begins by eroding the outermost edges of the visual field. Because the loss is so slow, it often remains unnoticed until the condition is advanced, leading to the characteristic “tunnel vision” effect.
Another condition is Retinitis Pigmentosa, an inherited degenerative disorder that primarily affects the retina’s photoreceptors. It initially causes the breakdown of the rods, which are essential for peripheral and night vision. Individuals often first report difficulty seeing in dim light, and over time, the visual field progressively constricts from the sides inward. This genetic condition slowly deteriorates the peripheral retina, leading to irreversible loss of side vision.
Sudden Loss and Neurological Factors
Peripheral vision loss can be sudden, temporary, or stem from issues outside the eye, such as in the brain or circulatory system. A stroke interrupts blood flow to the brain and can damage the visual cortex, the area responsible for processing sight.
This damage often results in a visual field cut, known as hemianopsia, where the same side of the visual field is lost in both eyes. Such loss is typically sudden and may be permanent, requiring immediate medical attention. Temporary visual changes, including a loss of side vision, can occur during a severe migraine with aura, but this disturbance usually lasts for a short period before normal vision returns.
Advanced Diabetic Retinopathy, a systemic condition, causes peripheral vision loss when high blood sugar damages the blood vessels supplying the retina. The resulting vascular damage can cause abnormal vessel growth or blockages, leading to vision deterioration that affects the peripheral field.
Necessary Medical Testing and Diagnosis
If you notice any loss of side vision, an evaluation by an eye care professional, such as an ophthalmologist or optometrist, is necessary.
The Visual Field Test, also called perimetry, is the primary diagnostic tool for assessing the extent of vision loss, mapping your entire field of vision by asking you to press a button when faint lights appear in a bowl-shaped instrument while you look straight ahead. Other tests provide information about the underlying cause. Tonometry measures the intraocular pressure to check for glaucoma, and ophthalmoscopy allows the doctor to examine the retina and optic nerve for signs of damage. If a neurological cause like a stroke is suspected, imaging tests such as a CT or MRI scan of the brain may be required to identify any lesions or damage to the visual pathways.