Intraocular pressure 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.
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
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 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.
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. Fitness and exercise
There is some inconclusive research that indicates that exercise could possibly affect IOP. 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 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.
Intraocular pressure has been measured as a secondary outcome in a systematic review
comparing the effect of neuroprotective agents in slowing the progression of open angle 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. References
External links www.allaboutvision.com What To Expect During
a Comprehensive Eye Exam www.emedicinehealth.com Ocular Hypertension
www.opt.indiana.edu Non-Contact Tonometry