Intraocular pressure (IOP) is defined as the pressure exerted on the eye by the fluids it contains. Like blood pressure, IOP is measured in millimeters of mercury (mm Hg).
Distribution of intraocular pressure in the general population
Normal IOP is about 12–22 mm Hg. The use of the term "normal" does not signify "correct" but instead indicates the "statistically" normal range. In most people, IOP varies over a diurnal cycle; the highest level occurs in mid-morning and the lowest level occurs at night, during sleep.
: occurring in a cyclical pattern over a 24-hour period.
The most important contributor to IOP is the aqueous humor. The eye maintains normal IOP by balancing the amount of aqueous fluid produced by the ciliary process and the amount drained out through the trabecular meshwork and .
Although both production and drainage are important to regulating IOP, the primary method of physiologic regulation is by increasing or decreasing the amount of aqueous fluid that drains out of the canal of Schlemm. IOP is regulated to a lesser extent by altering the amount of aqueous humor produced by the ciliary processes.
Together, the conventional and uveoscleral drainage routes for aqueous humor provide balanced IOP. The majority of resistance to outflow of aqueous humor is provided by the trabecular meshwork of the conventional drainage route.