Intraocular pressure

Intraocular pressure (IOP) is the fluid pressure inside the eye.

Physiology and measurement
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.

Intraocular pressure is measured with a tonometer.

Influencing factors
Intraocular pressure measurement is also influenced by corneal thickness and rigidity.

Significance
Ocular hypertension (OHT) is defined by intraocular pressure being higher than normal, in the absence of optic nerve damage or visual field loss. Current consensus among optometrists and ophthalmologists 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. 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 usually decreases due to the slower production of aqueous humour. Intraocular pressure also varies with a number of other factors such as heart rate, respiration, exercise, fluid intake, systemic medication and topical drugs. Alcohol consumption leads to a transient decrease in intraocular pressure and caffeine may increase intraocular pressure.

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. 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).

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.

Because of the effect of corneal thickness and rigidity on measured value of intraocular pressure, 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.

Intraocular pressure may become elevated due to anatomical problems, inflammation of the eye, genetic factors, as a side-effect from medication, or during exercise. Intraocular pressure usually increases with age and is genetically influenced. 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.

Case History
One of the most extensive intraocular pressure case history records assembled to date is available online. As of December 29, 2006, this case history included over 12,755 intraocular pressure measurements over a period of 170 days under a wide variety of real life situations.

Additional intraocular pressure measurements are continuing to be recorded at a rate of up to 100 per day and this case history web site is updated regularly. The size of the data set and the frequency with which measurements are recorded may lead to additional insights into intraocular pressure changes in response to a diverse number of factors. One interesting recent discussion that arose directly from data mining the large data set involved the timing of using glaucoma eye drop medications.

Another question that is extensively discussed in the patient's commentary at the Case History site is the relationship between psychological stress and intraocular pressure. According to conventional wisdom, a patient's stress (e.g. nervousness or anxiousness) does not increase the patient's intraocular pressure the way it might increase their blood pressure. The data of the Case History challenges this convention wisdom, although a single case history will certainly not settle the question. What can be said at this point is that stress can seemingly increase intraocular pressure in at least the one individual being followed in this ambitious Case History project. Researchers will have to continue investigating the connection between stress and intraocular pressure.