Glaucoma is a group of eye diseases that cause progressive damage to the optic nerve, which transmits visual information from the eye to the brain. This damage is associated with elevated fluid pressure inside the eye, known as intraocular pressure (IOP). Glaucoma definitively affects peripheral vision, characteristically targeting side vision first. This makes it a “silent thief of sight” due to the lack of early symptoms. If IOP is not managed, peripheral vision loss progresses, eventually leading to a loss of central vision and potential blindness.
What Glaucoma Does to the Optic Nerve
The underlying mechanism of vision loss in glaucoma is the gradual death of the delicate nerve fibers that form the optic nerve. These fibers act as communication cables, carrying signals from the light-sensing retina to the brain. The most common form of the disease, primary open-angle glaucoma, involves a failure of the eye’s internal drainage system, called the trabecular meshwork.
When the eye fluid, or aqueous humor, cannot drain properly, the intraocular pressure (IOP) builds up inside the eye. This sustained pressure causes mechanical stress on the optic nerve head. The mechanical compression and reduced blood flow disrupt the ability of the nerve fibers to transport essential nutrients and signals, leading to their eventual death.
This nerve damage is permanent. The optic nerve fibers responsible for peripheral vision are often structurally more vulnerable to this mechanical stress, which explains why side vision is typically affected earliest. The damage results in a characteristic hollowing, or “cupping,” of the optic nerve head, a sign eye doctors look for during an examination.
The Progression of Peripheral Vision Loss
Glaucoma begins to affect vision by creating small, isolated blind spots, known as scotomas, in the visual field. These initial blind spots are typically located in the periphery and are usually so subtle that the patient is entirely unaware of them. The brain is remarkably skilled at filling in these missing patches of visual information, allowing a person to maintain a seemingly normal view of the world.
The progression of damage often follows a specific pattern, beginning with the outer field of vision and slowly moving inward toward the center. As the disease advances, these individual scotomas merge and grow, causing a noticeable constriction of the visual field. This narrowing of the side vision is the stage referred to as “tunnel vision,” where the person can only see what is directly in front of them, like looking through a straw.
Because central vision, which is responsible for sharp focus like reading or recognizing faces, is usually preserved until the disease is very advanced, the patient may not seek treatment until significant, irreparable damage has occurred. The silent, gradual nature of this peripheral vision loss underscores why glaucoma is such a threat to eyesight.
How Doctors Measure Vision Loss
Detecting and monitoring this progressive vision loss requires specialized clinical tools, as it is often missed during a standard eye chart test. One primary method is the visual field test, or perimetry, which provides a functional map of the patient’s entire visual field. This test uses a bowl-shaped instrument that flashes small lights of varying intensity in different locations, requiring the patient to press a button when a light is seen.
The results of the visual field test reveal areas of reduced sensitivity and the characteristic patterns of blind spots associated with glaucoma. Optical Coherence Tomography (OCT) provides an image of the optic nerve head and the retinal nerve fiber layer. OCT measures the thickness of the nerve fibers, allowing doctors to track structural thinning over time, which can often detect damage before it is visible on a functional visual field test.
These tests are important for establishing a baseline and monitoring for subtle changes that indicate disease progression. Discrepancies between the structural OCT scans and the functional visual field tests are common, requiring the ophthalmologist to correlate both pieces of information to determine the true stage of the disease and the rate of vision loss.
Preserving Remaining Vision Through Treatment
Any vision lost due to optic nerve damage cannot be recovered. The philosophy of treatment is focused on preventing further damage and preserving the patient’s existing peripheral and central vision for life. This makes early and consistent intervention important.
The main approach to halting progression is lowering the intraocular pressure (IOP), which is the only treatable risk factor for most forms of the disease. Treatment typically begins with prescription eye drops that reduce the production of fluid or increase its outflow from the eye. If eye drops are insufficient, laser procedures or surgical options are used to create better drainage and achieve a lower, target IOP.
Reducing the pressure by a significant percentage, often 25% or more, can slow the rate of vision loss. The long-term success of treatment depends heavily on patient compliance, as consistent daily use of medication is necessary to maintain the reduced IOP and protect the remaining nerve fibers from further compression.