Intraocular pressure

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).

Physiology
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.

Measurement
Intraocular pressure is measured with a tonometer as part of a comprehensive eye examination.

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.

Classification
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.

Ocular hypertension (OHT) is defined by intraocular pressure being higher than normal, in the absence of optic nerve damage or visual field loss.

Hypotony, or ocular hypotony, is typically defined as intraocular pressure equal to or less than 5 mmHg. Such low intraocular pressure could indicate fluid leakage and deflation of the eyeball.

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 indicate the possibility that Yog Asan such as 'Tratak' may actually reduce intraocular pressure. Other exercises include tai chi, pilates, and aerobics. However, some other forms of exercise may raise IOP.

Musical instruments
Playing some musical wind instruments has been linked to increases in intraocular pressure. One 2011 study focused on brass and woodwind instruments observed "temporary and sometimes dramatic elevations and fluctuations in IOP". Another study found that the magnitude of increase in intraocular pressure correlates with the intraoral resistance associated with the instrument, and linked intermittent elevation of intraocular pressure from playing high-resistance wind instruments to incidence of visual field loss. The range of intraoral pressure involved in various classes of ethnic wind instruments, such as Native American flutes, has been shown to be generally lower than Western classical wind instruments.

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.

Significance
Ocular hypertension is the most important risk factor for glaucoma.

Differences in pressure between the two eyes is often clinically significant, and potentially associated with certain types of glaucoma, as well as iritis or retinal detachment.

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.