Open-angle glaucoma develops when fluid inside the eye drains too slowly, gradually raising pressure that damages the optic nerve. The core problem is increased resistance in the eye’s drainage tissue, but the full picture involves genetics, blood flow, immune activity, and age-related changes that together explain why some people develop the disease and others don’t.
How the Eye’s Drainage System Fails
Your eye constantly produces a clear fluid called aqueous humor that nourishes internal structures and then drains out through a spongy tissue called the trabecular meshwork. In a healthy eye, production and drainage stay balanced, keeping internal pressure between 10 and 20 mmHg. In open-angle glaucoma, the drainage pathway looks physically open (unlike closed-angle glaucoma, where it’s visibly blocked), but the tissue resists fluid outflow at a microscopic level.
The bottleneck sits deep in the drainage system, at or near the inner wall of a tiny channel called Schlemm’s canal. This endothelial lining contains microscopic pores that allow fluid to pass through cells from one side to the other. In glaucomatous eyes, the density of these pores is greatly reduced compared to healthy eyes. With fewer escape routes, fluid backs up and pressure climbs. The outer, more porous layers of the meshwork contribute almost no resistance on their own. A single pore 20 micrometers wide could theoretically carry the eye’s entire fluid output, so the problem isn’t in these coarser structures. It’s in the fine, innermost layer where cells have lost their ability to let fluid pass efficiently.
Age-related material does accumulate in the tissue over time, but research suggests this buildup alone has little direct effect on fluid dynamics. The real change is cellular: the endothelial cells lining Schlemm’s canal become less permeable, and the connective tissue just beneath them may fill with extracellular matrix that further chokes off flow.
Elevated Pressure and How It Damages the Nerve
When intraocular pressure rises, it compresses the optic nerve head, the spot where roughly 1.2 million nerve fibers exit the back of the eye on their way to the brain. This compression disrupts the two-way transport of essential molecules along these nerve fibers. Survival signals produced in the brain normally travel down to the retinal ganglion cells (the neurons whose fibers form the optic nerve). When that supply gets cut off, the cells begin a process of programmed self-destruction called apoptosis.
The dying cells don’t go quietly. As ganglion cells deteriorate, they trigger inflammatory signaling and oxidative stress. Mitochondria inside the cells malfunction, and multiple self-destruct pathways activate simultaneously. Genes that normally protect neurons get dialed down while genes promoting cell death and inflammation ramp up. This isn’t a single switch being flipped; it’s a cascade where each failing cell makes the local environment more toxic for its neighbors.
The Immune System Adds Fuel
The retina has its own resident immune cells called microglia. In a healthy eye, they maintain synaptic connections and clean up debris. In glaucoma, these cells shift into an aggressive, inflammatory state. Research in animal models shows that microglial activation appears months before any detectable nerve damage, suggesting these immune cells may help initiate the disease rather than simply responding to it.
Once activated, microglia release inflammatory molecules that create a toxic environment for surrounding neurons. They also begin actively engulfing parts of healthy nerve cells, a process that appears to be a harmful reactivation of a normal developmental mechanism (during fetal development, microglia prune excess neurons). The complement cascade, a molecular tagging system the immune system uses to mark cells for destruction, is one of the earliest detectable molecular changes in glaucomatous retinas. When researchers genetically removed a key complement protein called C1q in animal models, synaptic and dendritic integrity was preserved, confirming that this immune tagging contributes directly to nerve damage.
Blood Flow and the Vascular Theory
Not everyone with elevated eye pressure develops glaucoma, and some people develop it with pressures in the normal range (called normal-tension glaucoma). This points to a second major contributor: inadequate blood flow to the optic nerve.
Ocular perfusion pressure, the difference between blood pressure pushing into the eye and intraocular pressure pushing back, determines how well the optic nerve gets nourished. Multiple population studies have found strong links between low ocular perfusion pressure and both the development and progression of open-angle glaucoma. The relationship is more consistent than the link between systemic blood pressure alone and glaucoma, which has produced contradictory results across studies. Some research even suggests that chronically low blood pressure (hypotension) may be a more relevant risk factor than high blood pressure.
Each 10 mmHg increase in systolic blood pressure is associated with roughly 0.2 to 0.3 mmHg higher eye pressure, but this explains only about 5% of the variation in intraocular pressure across populations. The practical takeaway is that the balance between blood delivery and eye pressure matters more than either number alone. Poor perfusion starves the optic nerve of oxygen and nutrients, compounding whatever mechanical damage elevated pressure is already causing.
Genetic Risk Factors
About 5% of open-angle glaucoma cases trace to single-gene mutations with a high likelihood of causing disease. The most significant is the myocilin gene (MYOC), the first gene linked to open-angle glaucoma and still the most common known genetic cause. One specific myocilin mutation (Gln368Stop) has been found across nearly every studied population, including African American, European, Australian, and South American groups. Myocilin mutations were originally discovered in families with juvenile-onset open-angle glaucoma, a form that strikes earlier and often more aggressively.
A second gene, optineurin (OPTN), is linked primarily to normal-tension glaucoma. A specific mutation (Glu50Lys) carries the strongest evidence, and optineurin mutations may account for up to 1.5% of normal-tension cases. The remaining 95% of open-angle glaucoma likely involves complex interactions among many genes, each contributing a small amount of risk, combined with environmental and age-related factors.
Who Is Most at Risk
Race and ethnicity are among the strongest demographic risk factors. The Baltimore Eye Survey found that people of African descent had up to six times the prevalence of glaucoma compared to European Americans in certain age groups. Open-angle glaucoma is six times more likely to cause blindness in African Americans than in European Americans. People of Latinx descent also face higher prevalence, earlier onset, and faster progression compared to those of European background. These disparities reflect both biological differences in eye anatomy and genetics and systemic differences in access to screening and treatment.
Age is the other dominant factor. Both elevated intraocular pressure and the cellular changes underlying nerve damage become more common with aging. Family history matters too, particularly having a first-degree relative with the disease, which roughly doubles to triples your risk depending on the study.
Why It Goes Unnoticed for Years
Open-angle glaucoma typically destroys peripheral vision first. Because each eye’s visual field overlaps with the other, and because the brain compensates for small gaps, most people notice nothing until significant damage has occurred. There is no pain, no redness, no obvious warning. The disease is often discovered incidentally during a routine eye exam or, in some cases, only when central vision begins to decline.
The speed of progression depends heavily on pressure levels. Analysis of 177 untreated patients showed that those with pressures between 21 and 25 mmHg progressed from early visual field changes to end-stage disease in an average of 14.4 years. At pressures between 25 and 30 mmHg, that timeline shrank to 6.5 years. Above 30 mmHg, complete visual field loss took an average of just 2.9 years. These numbers underscore why early detection through regular eye exams changes outcomes so dramatically: the disease moves slowly enough to manage in most people, but only if it’s caught before irreversible damage accumulates.
Medications That Can Trigger It
Corticosteroids, whether taken as eye drops, oral pills, inhalers, or injections, can cause a form of open-angle glaucoma by directly interfering with the trabecular meshwork. Steroids stabilize cell membranes in ways that cause large sugar-protein molecules to accumulate and swell within the drainage tissue, physically increasing resistance. They also boost production of structural proteins like fibronectin, elastin, and collagen in the meshwork, thickening it further. On top of that, steroids suppress the meshwork cells’ ability to clean up debris, so waste products pile up in the drainage channels.
Steroids even reduce production of prostaglandins, natural molecules that help regulate fluid outflow. And in a notable genetic connection, glucocorticoids increase expression of the same myocilin gene that causes inherited forms of the disease. Steroid-induced glaucoma is considered avoidable because pressure typically returns to normal once the steroid is discontinued, though prolonged exposure can cause permanent damage.