What is intraocular pressure? IOP refers to the pressure in the eye, which is the pressure of the contents of the eye on the inner wall of the eye. normal IOP refers to the level of IOP that does not cause damage to the optic nerve, usually 10-21 mmHg, with a difference of no more than 5 mmHg between eyes and a fluctuation of no more than 8 mmHg for 24 h. An increase in IOP can cause damage to the optic nerve by compressing it. Why does IOP rise? IOP depends mainly on the amount of fluid in the eye (atrial fluid), and if there is more atrial fluid, the IOP will be higher. Atrial fluid plays a role in nourishing the eye. Atrial fluid is produced by the ciliary process of the ciliary body, flows into the anterior chamber through the pupil, and is then discharged through the trabecular meshwork of the anterior chamber angle. In general, the production and discharge of atrial water maintain a dynamic balance, that is, the amount of atrial water produced and the amount of atrial water discharged are equal in a certain period of time. If atrial aqueous discharge decreases or atrial aqueous production increases, both can lead to an increase in intraocular pressure and cause glaucoma.