Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk from glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg).
Intraocular pressure is mainly determined by the coupling of the production of aqueous humor and the drainage of aqueous humor mainly through the trabecular meshwork located in the anterior chamber angle.
An important quantitative relationship is provided below:
- IOP = F / C + PV
Where F = aqueous fluid formation rate, C = outflow rate, PV = episcleral venous pressure. The above factors are those that drive IOP.
Measured values of intraocular pressure are influenced by corneal thickness and rigidity. As a result, some forms of refractive surgery (such as photorefractive keratectomy) can cause traditional intraocular pressure measurements to appear normal when in fact the pressure may be abnormally high.
Current consensus among ophthalmologists and optometrists define normal intraocular pressure as that between 10 mmHg and 20 mmHg. The average value of intraocular pressure is 15.5 mmHg with fluctuations of about 2.75 mmHg.
Influencing factors 
Daily variation 
Intraocular pressure varies throughout the night and day. The diurnal variation for normal eyes is between 3 and 6 mmHg and the variation may increase in glaucomatous eyes. During the night, intraocular pressure may not decrease despite the slower production of aqueous humour. In the general population, IOP ranges between 10 and 21 mm Hg with a mean of about 15 or 16 mm Hg (plus or minus 3.5 mm Hg during a 24-hour cycle).
Fitness and exercise 
Studies have been conducted on both healthy and sedentary individuals to determine if intraocular pressure could be reduced with exercise. Some forms of exertion have been found to result in a decrease in intraocular pressure. Exercises studied included; walking, jogging, and running. These studies indicate the possibility that other exercise may also reduce intraocular pressure such as yoga, tai chi, pilates, and aerobics. However, some other forms of exercise may raise IOP.
Other factors 
Intraocular pressure also varies with a number of other factors such as heart rate, respiration, fluid intake, systemic medication and topical drugs. Alcohol consumption leads to a transient decrease in intraocular pressure and caffeine may increase intraocular pressure.
Taken orally, glycerol (often mixed with fruit juice to reduce its sweet taste) can cause a rapid, temporary decrease in intraocular pressure. This can be a useful initial emergency treatment of severely elevated pressure.
Ocular hypertension is the most important risk factor for glaucoma.
Intraocular pressure may become elevated due to anatomical problems, inflammation of the eye, genetic factors, or as a side-effect from medication. Intraocular pressure usually increases with age and is genetically influenced.
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