What causes an increase in intraocular pressure?

Both the anterior (between the cornea and the iris) and posterior (between the iris and the lens) parts of the eye are filled with a water-like fluid called the aqueous humor. As new aqueous humor is constantly being produced, this rate of production has to be balanced by an equal drainage rate. The two pathways through which aqueous humor drains are the trabecular meshwork and the uveoscleral drainage.^[1]

In open-angle glaucoma, there is reduced outflow of aqueous humor through the trabecular meshwork because of the stiffening of the inner wall region. In angle-closure glaucoma, the iris narrows the anterior drainage angle, causing the outflow of aqueous humor to be reduced from both the trabecular meshwork and the uveoscleral drainage.^[2] The reduced outflow of aqueous humor in open-angle or angle-closure glaucoma causes the build-up of aqueous humor inside the eye, leading to an increase in intraocular pressure.