Your eye is a pressurized ball. It has to be, because a firm globe holds its shape, keeps the cornea curved correctly and keeps the retina pressed smoothly against the back wall.
That pressure comes from fluid that is made continuously inside the eye and has to leave at the same rate. Almost all of the leaving happens through one structure, a ring of spongy tissue no wider than a few sheets of paper are thick, called the trabecular meshwork.
It is a small piece of anatomy with an outsized role, because when it stops letting fluid through easily, pressure climbs and the optic nerve pays the price.
Where the meshwork sits
Find the edge of your colored iris, where it meets the white of your eye. Just behind that junction, in the recess where the iris root meets the back of the cornea, is what eye doctors call the drainage angle. The meshwork lines that angle as a complete circle around the eye.
It is not a single hole. It is layered, sieve-like tissue with progressively finer openings, and fluid works its way inward through those layers. Beyond the last layer sits a thin circular channel, Schlemm's canal, which collects the fluid and passes it into small vessels that carry it back into the bloodstream.
The tissue is alive, not plumbing. Cells in the meshwork clean debris out of the pathway, remodel the material around them and respond to how much they are being stretched. That maintenance role matters later.
Pressure is a balance, not a leak
Fluid is produced behind the iris at a fairly steady rate, all day and all night. It flows forward through the pupil, washes across the back of the cornea, and reaches the angle. The pressure inside your eye is simply the result of how easily that fluid gets out compared with how fast it is coming in.
Some resistance is normal and necessary. Without it, the eye would soften and the optics would suffer. The meshwork is designed to resist, and healthy eyes settle at a pressure that keeps the globe firm without straining anything.
Trouble starts when resistance rises. Production does not slow down to compensate, so the fluid backs up and pressure rises until a new balance is reached at a higher level. That higher level is what gets measured at your eye exam.
Why the resistance goes up
There is rarely one dramatic cause. More often the pathway becomes gradually less efficient, and several things can push it that way.
- Aging changes. The number of active cells in the meshwork tends to fall over the years, and the tissue stiffens.
- Loose pigment. Granules rubbed off the back of the iris can accumulate in the openings.
- Flaky deposits. In some eyes a whitish material builds up on structures in the front of the eye and settles in the drainage pathway.
- Inflammation. Cells and protein from an inflamed eye can clog the sieve, sometimes temporarily.
- Steroid response. Some people's pressure rises on steroid medication, including drops, creams and inhalers, through changes in the meshwork itself.
- Injury. Blunt trauma can scar or tear the angle, sometimes years before pressure becomes a problem.
Family history matters too, and so does ancestry, which is why those questions come up in an eye exam even when nothing hurts.
Open-angle glaucoma starts here
In the most common form of glaucoma, the angle looks open and normal on examination. The iris is nowhere near the meshwork. The obstruction is microscopic, spread through the tissue and the outflow channels, and it is a matter of degree rather than a plug you could point to.
Because the change is gradual, nothing feels different. There is no ache, no redness and no blur in the early years. Damage begins at the optic nerve head, where the bundle of nerve fibers leaves the eye, and the first losses are in the peripheral field, where you are least likely to notice them.
This is the essential point about open-angle glaucoma. By the time a person notices missing vision, a great deal of nerve tissue has already been lost, and lost nerve fibers do not come back. Everything about managing the disease is aimed at protecting what remains.
Pressure numbers are not the whole story
It would be tidy if a single pressure reading sorted people into safe and unsafe. It does not. Some eyes sustain damage at pressures inside the usual range, a pattern often called normal-tension glaucoma, while other people run higher readings for years without the optic nerve suffering.
Pressure also fluctuates through the day, so one measurement is a snapshot. Corneal thickness affects the reading as well. That is why an assessment looks at the nerve itself, the field of vision and scans of the nerve fiber layer, not just the number.
Treatment, when it is needed, works by improving outflow or reducing production, and the choice among the available options belongs to your eye doctor. Some approaches target the meshwork directly. None of that is a decision to make from an article.
Common questions
Can I feel high eye pressure?
Usually not. Chronic elevation is silent. A sudden, severe rise can cause pain, redness, blurred vision, halos and nausea, and that combination is an emergency needing same-day care.
Does eye pressure relate to blood pressure?
They are separate measurements of separate systems, and a normal blood pressure reading tells you nothing about your eye pressure. Only an eye exam measures the latter.
Can the meshwork be repaired or cleaned out?
There is no way to restore aged tissue, but several procedures are designed to improve outflow through or around it. Whether any of them fits your eye is a conversation with an eye doctor.
The meshwork does its work silently, and so does the damage when it starts to fail. That combination is exactly why routine exams carry weight, especially if glaucoma runs in your family. If you are overdue, you can find a local eye doctor who takes your vision plan and have your pressure and optic nerve checked properly.