Intraocular pressure (IOP) is the measurement of fluid pressure within the eye. This pressure is generated by a clear fluid called the aqueous humor, which fills the front part of the eye. A stable internal pressure is necessary for the eye to maintain its shape and function correctly. The balance between the production and drainage of this fluid determines the eye’s internal pressure at any given moment. Measuring this pressure is a standard component of a comprehensive eye examination.
The Purpose of Measuring Intraocular Pressure
Measuring IOP is a primary method for assessing the risk of glaucoma, a condition that can cause irreversible vision loss. The eye continuously produces aqueous humor, which normally drains away through a structure called the trabecular meshwork. If this drainage system becomes inefficient or blocked, the fluid builds up, causing the pressure inside the eye to increase. This elevated pressure is known as ocular hypertension.
Sustained high pressure can exert force on the optic nerve, the bundle of nerve fibers at the back of the eye that transmits visual information to the brain. Over time, this force can damage the delicate nerve fibers, leading to the gradual loss of peripheral vision. Because this damage is often painless and progresses slowly, individuals may not notice changes in their vision until significant loss has occurred.
Tonometry Techniques
The procedure for measuring intraocular pressure is called tonometry. The “gold standard” method, trusted for its accuracy, is Goldmann Applanation Tonometry (GAT). This technique involves administering anesthetic eye drops to numb the eye’s surface and a small amount of a yellow dye called fluorescein. The patient rests their chin and forehead on a machine called a slit lamp, and a small, flat-tipped cone gently makes contact with the cornea to measure the pressure.
A common screening method is Non-Contact Tonometry (NCT), often referred to as the “air puff” test. This technique does not require eye drops or direct contact with a probe. A machine delivers a brief, gentle puff of air at the cornea, causing it to flatten momentarily. The instrument measures the cornea’s resistance to the air puff, which corresponds to the eye’s pressure.
Other portable, handheld devices are also used in various clinical situations. Instruments like the Tono-Pen are electronic and use a gentle tapping motion against the anesthetized cornea to obtain a reading. Rebound tonometers, such as the iCare device, propel a tiny, lightweight probe against the cornea and measure its rebound speed. These portable methods are useful for patients who cannot be positioned at a slit lamp, including children or individuals with limited mobility.
Interpreting Measurement Results
Intraocular pressure is measured in units of millimeters of mercury (mm Hg), the same unit used for measuring blood pressure. A reading in the range of 12 to 22 mm Hg is considered within the normal limits for most people. However, what is considered a “normal” pressure can vary from one individual to another.
A single pressure reading that falls outside of this range is not an automatic cause for alarm. An eye care professional interprets the measurement in the context of the patient’s complete eye health profile. This includes an assessment of the optic nerve’s appearance, the thickness of the cornea, and the results of any visual field tests that map peripheral vision.
Conversely, some people may develop glaucoma damage even with pressures in the normal range, a condition called normal-tension glaucoma. An unusually low IOP, or hypotony, can also be a concern as it may indicate other issues, such as a leak in the eyeball or inflammation.
Factors Influencing IOP Readings
Several factors can cause temporary fluctuations in intraocular pressure or affect the accuracy of a measurement. The thickness of the cornea is a significant variable; a thicker-than-average cornea can lead to an artificially high reading, while a thinner cornea may result in a deceptively low one. For this reason, a practitioner may measure corneal thickness using a procedure called pachymetry to better interpret the IOP results.
IOP is not static and can change throughout the day, following a natural diurnal rhythm. For many people, the pressure is highest in the morning hours shortly after waking up and gradually decreases as the day progresses. An eye doctor might recommend measuring pressure at different times of the day to understand a patient’s specific IOP pattern.
Patient actions and external factors can also have a temporary effect on the measurement. Activities such as holding one’s breath, straining, or even wearing a tight necktie can momentarily increase IOP. Consuming a large amount of caffeine shortly before a test may also cause a temporary spike in pressure.