How Treatments Lower Eye Pressure

Every glaucoma treatment in use today works the same way: it lowers the pressure inside the eye. They differ in how. A simple way to picture it is a sink. A faucet runs all day, drains carry the water away, and the water level is the eye pressure. Tap a treatment below to see which part of the sink it changes.

The numbers are examples for one imaginary eye, to show how the pieces fit together. They are not a prediction of what any treatment will do for you.

The faucet and the drains

• The faucet is the ciliary body, a ring of tissue behind the iris. It makes a clear fluid (aqueous humor) all day and night. It cannot be turned off, and the eye needs the fluid.
• The main drain is the trabecular meshwork, a fine filter in the angle where the iris meets the cornea. Most of the fluid leaves here.
• The side drain is a second, smaller route out through the wall of the eye (the uveoscleral path).
• The water level is the eye pressure. It settles wherever the amount coming in equals the amount going out.

In the most common kind of glaucoma, the faucet is normal. The main drain is partly clogged, so fluid backs up and the pressure rises. The What Is Glaucoma? page shows the same parts in a drawing of the eye.

Three ways to lower the level

Turn down the faucet. Some drops (timolol, dorzolamide, brinzolamide, brimonidine) make the eye produce less fluid. So does cyclophotocoagulation, a laser applied to the outside of the eye.

Open the drains. Prostaglandin drops (latanoprost and its relatives) open the side drain, and netarsudil relaxes the main one. The SLT laser treats the main drain so it clears itself. MIGS gives fluid a shortcut past the clogged part.

Add a new drain. A trabeculectomy or a tube shunt makes a new way out of the eye. These lower pressure the most and are used when the other steps are not enough.

Why there is a floor, and why adding more helps less

The main drain empties into small veins on the surface of the eye, and those veins have a pressure of their own, around 8 to 10 mm Hg. Fluid cannot run out through the main drain into veins that are at a higher pressure, so that vein pressure is a floor. Turning the faucet down, or opening the drains, brings the level toward the floor but not under it. It is the dotted line near the bottom of the sink.

Only a new drain gets past it. A trabeculectomy or tube shunt carries fluid somewhere other than those veins, which is why surgery can reach pressures that drops and laser cannot.

Treatments also give diminishing returns:

• The same part twice helps less the second time. A second drop that slows the faucet adds less than the first, because the eye never stops making fluid altogether. The same goes for two treatments on the main drain: they share the same clogged filter, and it can only be opened so far.
• Different parts add up better, which is why you may be on one drop that slows the faucet and another that opens a drain, or have a laser and still use a drop.
• The closer the level is to the floor, the less anything moves it. A drop that takes a pressure of 30 down by 7 may take a pressure of 16 down by 2.

How low is low enough?

There is no single safe number. "Normal" pressure is often quoted as 10 to 21 mm Hg, but a nerve that is already damaged usually needs a lower level than a healthy one. Your doctor sets a target pressure for each eye from the state of the nerve, how high the pressure was when the damage happened, and how fast things are changing, and adjusts it over time. The red zone in the picture is only a general guide.

What the sink leaves out

• Narrow angles. In some eyes the drain is fine but the iris covers it. That is treated differently, with a laser iridotomy or lens surgery.
• Pressure is not the whole story. Some people develop glaucoma at normal pressures, and blood flow and the strength of the nerve matter too. Lowering pressure is still the treatment that has been proven to protect vision.
• Pressure changes during the day, so one reading is a snapshot. This is one reason it is checked at every visit.

Questions worth asking

• What is my target pressure, and am I at it?
• Can drops or laser get me there, or is it below what they can reach?
• Which part of the sink does each of my treatments work on?
• If my pressure needs to come down further, what is the next step for me?