Glaucoma causes vision loss by killing the nerve cells that carry visual signals from your eye to your brain. These cells, called retinal ganglion cells, have long fibers that bundle together to form the optic nerve. When they die, the visual information they were responsible for transmitting is permanently lost. The process is usually slow, starting at the edges of your visual field, which is why most people don’t notice it until significant damage has already occurred.
How Fluid Pressure Builds Inside the Eye
Your eye constantly produces a clear fluid that nourishes the lens and cornea, then drains out through a tiny mesh-like structure near the base of the iris. In the most common form of glaucoma (open-angle glaucoma), this drainage tissue gradually becomes clogged. The fluid itself isn’t produced any faster than normal. Instead, the tissue’s ability to let fluid pass through decreases over time. Abnormal proteins, collagen deposits, and thickened membranes accumulate in the drainage tissue, creating resistance that slows the outflow. The result is a slow, steady rise in the pressure inside the eye.
Normal eye pressure falls between about 10 and 21 mmHg. The traditional threshold for concern was anything above 21 mmHg, but many people develop glaucoma damage even within the “normal” range. That finding reshaped how doctors think about the disease: pressure matters, but it isn’t the whole story.
What Pressure Does to the Optic Nerve
The optic nerve exits the eye through a structure called the lamina cribrosa, a sieve-like plate of connective tissue at the back of the eyeball. This is the most vulnerable point. Pressure inside the eye pushes outward against this plate from one side, while the tissue pressure behind it pushes from the other. The nerve fibers threading through its tiny pores are caught in between.
When eye pressure rises, the mechanical strain on this plate increases. That strain compresses the nerve fibers passing through it and disrupts the movement of mitochondria along them. Mitochondria are the energy factories inside cells, and nerve cells depend heavily on a steady supply traveling down their length. When that transport slows, the nerve fibers begin to starve for energy. At the same time, the strain triggers remodeling of the surrounding connective tissue, which can further pinch and distort the nerve fibers over months and years.
The damage cascades from there. Nerve cells deprived of energy and growth signals activate a self-destruction sequence. Oxidative stress builds, inflammation kicks in, and the supporting cells around the nerve (astrocytes and microglia) shift from protective to harmful roles. The nerve cells die through a programmed process called apoptosis, not a sudden rupture, which is part of why the damage creeps forward so quietly.
Why Some People Lose Vision at Normal Pressure
In normal-tension glaucoma, eye pressure measures within the standard range yet the optic nerve still deteriorates. The leading explanation centers on blood flow. If the small arteries supplying the optic nerve don’t deliver enough oxygen, chronic ischemia (a slow, ongoing oxygen shortage) can produce the same pattern of nerve cell death seen in high-pressure glaucoma. Vascular conditions like low blood pressure, vasospasm, or narrowing of the carotid or ophthalmic arteries can all contribute. In one documented case, a patient with more than 90% blockage of the internal carotid artery developed progressive optic nerve damage that mimicked glaucoma, driven entirely by reduced blood supply rather than elevated pressure.
Even in standard high-pressure glaucoma, vascular problems likely play a supporting role. The combination of mechanical strain and inadequate blood flow makes the optic nerve especially vulnerable, which helps explain why two people with the same eye pressure can have very different outcomes.
How Angle-Closure Glaucoma Differs
In angle-closure glaucoma, the drainage angle between the iris and cornea physically narrows or shuts. The most common trigger, responsible for roughly 75% of cases, is pupillary block: the iris bows forward and seals against the lens, trapping fluid behind it. Pressure spikes rapidly. This can happen as a sudden emergency (acute angle closure, with severe pain, nausea, and blurred vision) or as a slow, intermittent process where the iris repeatedly brushes against the drainage tissue, forming scar-like adhesions that gradually seal the angle shut.
Open-angle glaucoma is three times more common overall, yet angle-closure glaucoma accounts for half of all glaucoma-related blindness worldwide. The reason is speed: pressure can climb high enough, fast enough, to cause extensive nerve damage in hours to days rather than years.
The Pattern of Vision Loss
Glaucoma doesn’t erase your vision uniformly. It follows the architecture of the nerve fibers in the retina. Each bundle of fibers curves in a specific arc from the retina to the optic nerve, and when a bundle dies, a corresponding patch of your visual field goes dark. The earliest defects typically appear as small blind spots (scotomas) in the mid-peripheral field, often above or below your central line of sight. You rarely notice these because your brain fills in the gaps using information from the other eye and from surrounding regions.
As the disease progresses, existing blind spots deepen and expand outward along the nerve fiber pathways. The most common pattern is a combination of a scotoma getting both darker and wider at the same time. Peripheral vision narrows progressively, sometimes described as tunnel vision in advanced stages. Central vision, the sharp detail you use for reading and recognizing faces, is typically the last to go. By the time central vision is affected, the vast majority of nerve fibers have already been lost.
Why the Damage Is Permanent
The retina and optic nerve are part of the central nervous system, just like the brain and spinal cord. Adult central nervous system tissue has severely limited capacity to regenerate. Once a retinal ganglion cell dies, no new cell replaces it, and its fiber in the optic nerve does not regrow. This is the same fundamental limitation that makes spinal cord injuries so difficult to reverse.
Several biological barriers stand in the way of regeneration. Inflammatory interactions with surrounding support cells actively suppress regrowth. Even in laboratory experiments where scientists have coaxed damaged nerve fibers to extend again, the fibers still face the challenge of navigating back to the correct targets deep in the brain and forming properly organized connections. Those connections also need to be wrapped in insulation (myelin) by specialized cells before they can transmit signals effectively. No current treatment can accomplish all of these steps in a living human eye.
This irreversibility is what makes early detection so critical. The global number of people living with glaucoma roughly doubled between 1990 and 2021, reaching an estimated 7.6 million cases. In many of those cases, substantial nerve damage was already present at the time of diagnosis because the gradual peripheral vision loss went unnoticed for years. Doctors look for a widening cup-to-disc ratio when examining the optic nerve: a healthy nerve typically has a ratio around 0.35, while a ratio above 0.9 suggests severe nerve fiber loss. Routine eye exams that measure pressure, inspect the optic nerve, and test the visual field remain the only reliable way to catch the disease before vision is irretrievably gone.