Intraocular pressure (IOP) is the fluid pressure inside the eye. This pressure is maintained by a clear fluid called aqueous humor, which is continuously produced and drained from the front of the eye. This regulation keeps the eye firm and maintains its spherical shape so it can function correctly. A balanced IOP is an indicator of eye health that ophthalmologists monitor during routine examinations.
The Normal Range for Eye Pressure
The accepted range for normal intraocular pressure is between 10 and 21 millimeters of mercury (mmHg). This range is a statistical average, with the mean value being around 15.5 mmHg. What is considered normal for one person may not be for another, as some individuals tolerate higher pressures, while others may experience damage even within the normal range.
An individual’s “normal” pressure is determined by an eye doctor who considers the unique structure of their eyes. A primary factor is the thickness of the cornea, the clear front surface of the eye. A thicker cornea can lead to an artificially high pressure reading, while a thinner one can result in a deceptively low reading. For this reason, eye exams often include measuring corneal thickness to accurately interpret IOP.
How Eye Pressure Is Measured
The clinical procedure for measuring intraocular pressure is called tonometry. There are several methods, but two are most commonly experienced by patients. The choice of test can depend on the patient and the specific clinical situation.
A frequent screening method is non-contact tonometry, often called the “air-puff” test. This technique uses a gentle puff of air to momentarily flatten the cornea. The instrument calculates the eye’s internal pressure by analyzing how the cornea resists this puff of air. It is a quick, non-invasive procedure that does not require numbing eye drops.
The most accurate method is Goldmann applanation tonometry, sometimes known as the “blue light” test. This test requires numbing eye drops and a yellow dye placed in the eye. The patient rests their chin and forehead on a slit lamp, and a small cone gently touches the cornea to measure the force required to flatten it.
Factors That Influence Intraocular Pressure
Intraocular pressure is not a static number; it fluctuates throughout the day and in response to various activities. One influence is the time of day, with pressure often reaching its highest point in the morning. This pattern is known as diurnal variation and is related to the body’s natural circadian rhythms.
Body posture also affects IOP, as lying down can increase pressure compared to sitting or standing. Certain physical activities and exercises can have a temporary effect as well. For instance, some aerobic exercises may lead to a transient reduction in IOP, while heavy lifting can cause a brief spike. Caffeine intake can also cause a temporary and slight increase in eye pressure.
What Abnormal Eye Pressure Can Indicate
A consistently high intraocular pressure reading above 21 mmHg, without any signs of optic nerve damage, is diagnosed as ocular hypertension. This condition itself does not mean a person has glaucoma, but it is the primary risk factor for developing it. Ocular hypertension is a “silent” condition because it has no symptoms, making regular eye exams necessary for detection.
Glaucoma is a disease characterized by damage to the optic nerve, which transmits visual information from the eye to the brain. Sustained high IOP is a primary cause of this damage, as the pressure can compress and harm the delicate nerve fibers over time, leading to irreversible vision loss. An ophthalmologist may recommend treatment, such as medicated eye drops, to lower the pressure and reduce the risk.
Less common is abnormally low eye pressure, or hypotony. This condition is defined as an IOP of 5 mmHg or less and can occur after eye surgery due to a fluid leak or chronic inflammation. When pressure is too low, the eye can lose its structural integrity, potentially causing distortions in the cornea and retina that can impair vision.