Intraocular pressure (IOP) is the pressure of the fluid inside your eye. This fluid helps keep the eye’s shape and supports its normal function. The pressure depends partly on how much fluid the eye makes and how well it drains away. Eye-care professionals measure IOP with a test called tonometry. The result helps assess eye health, especially the risk of glaucoma, but it cannot diagnose glaucoma by itself.
What does intraocular pressure do inside the eye?
Your eyes contain different fluids and tissues that help them function properly. Intraocular pressure is closely related to the balance between the production and drainage of a clear liquid called aqueous humor. This fluid fills the front part of the eye, between the cornea (the clear outer surface) and the lens. It provides nutrients to eye tissues and helps maintain the eye's internal pressure.
The eye continually produces aqueous humor, and the fluid normally drains through a small drainage area where the iris (the colored part of the eye) meets the cornea. When fluid production and drainage remain balanced, eye pressure stays within a suitable range. If fluid cannot drain adequately, pressure may rise. The National Eye Institute explains that problems with this drainage system can contribute to increased eye pressure and a higher risk of glaucoma.
What is a normal intraocular pressure reading?
A commonly used reference range for intraocular pressure is approximately 10 to 21 millimeters of mercury (mmHg). Millimeters of mercury is the unit used to record pressure inside the eye. A reading above this range is generally considered higher than normal, although an eye-care professional interprets the result according to the person's overall eye health. MedlinePlus notes that corneal thickness can affect the reading, so the measured pressure does not always reflect the full clinical picture.
There is no single pressure value that determines whether an eye is healthy or whether glaucoma is present. Some people with elevated eye pressure never develop glaucoma, while others develop glaucoma despite readings within the usual range. According to the National Eye Institute, the level of pressure an optic nerve can tolerate differs from person to person. Eye specialists therefore consider pressure readings alongside other examination findings.
What can cause intraocular pressure to increase?
Eye pressure can rise when the eye produces more aqueous humor than it can drain or when fluid outflow becomes restricted. The change may be temporary or persistent, depending on the underlying cause. Cleveland Clinic identifies several factors that can contribute to elevated eye pressure, including problems with fluid drainage, eye injuries, and certain medications.
- Reduced fluid drainage: If the eye's drainage system does not allow aqueous humor to leave efficiently, fluid can build up and increase pressure.
- Changes in fluid production: Pressure may rise when the eye produces fluid faster than it can drain.
- Eye injury or inflammation: Damage or inflammation involving the eye may affect its internal pressure.
- Medication effects: Corticosteroid medicines can increase eye pressure in some people. Tell your eye-care professional about the medicines you use, including steroid eye drops.
An increased reading may be described as ocular hypertension when pressure is elevated without evidence of glaucoma-related damage to the optic nerve. Ocular hypertension is not the same as glaucoma, although it can increase the risk of developing the disease. An eye specialist may recommend monitoring or pressure-lowering treatment depending on the pressure level, the condition of the optic nerve, and other individual risk factors.
How is intraocular pressure measured?
Eye-care professionals measure intraocular pressure using a test called tonometry. The test is commonly performed during an eye examination and is used to help assess the risk of glaucoma and monitor people receiving glaucoma treatment. According to MedlinePlus, several methods are available, and the choice depends on the examination and the information the clinician needs.
Air-puff tonometry
For this method, you place your chin on a support and look into a measuring device. The machine releases a brief puff of air toward your eye and measures how the cornea responds. The test is quick and usually does not require numbing eye drops. You may notice the air puff, but it should not cause significant discomfort.
Applanation tonometry
In this method, the eye-care professional uses numbing drops before gently touching the surface of the cornea with a measuring instrument. The device assesses how much force is needed to flatten a small area of the cornea. This method is commonly used to obtain a pressure measurement during a detailed eye examination. The procedure is generally well tolerated because the eye is numbed first.
What happens after the measurement?
The pressure reading may be considered alongside other findings, such as corneal thickness, the appearance of the optic nerve, and tests of peripheral vision. If the initial result is unexpected or suggests elevated pressure, an eye-care professional may repeat the measurement or perform additional tests. A single reading is not enough to establish a diagnosis of glaucoma or determine the need for treatment.
Does high intraocular pressure cause symptoms?
Elevated intraocular pressure often causes no noticeable symptoms. A person may have increased pressure without pain, redness, or changes in vision, which is one reason routine eye examinations matter. Symptoms alone cannot reliably establish whether eye pressure is high. When glaucoma is suspected, an eye specialist assesses the pressure and examines the optic nerve and other aspects of eye health.
However, a sudden and substantial rise in pressure can occur in acute angle-closure glaucoma, a medical emergency. Symptoms may include:
- Sudden, severe eye pain
- Blurred vision or sudden changes in vision
- Redness of the eye
- Colored rings or halos around lights
- Severe headache, nausea, or vomiting accompanying eye symptoms
MedlinePlus identifies acute angle-closure glaucoma as an emergency because pressure can rise rapidly and threaten vision. These symptoms can have other causes, but sudden eye pain or vision changes, particularly when accompanied by redness, halos, headache, or nausea, require immediate medical assessment. Do not wait for a routine appointment if these symptoms develop suddenly.
How is an intraocular pressure result interpreted?
An eye-care professional considers the pressure reading together with the person's medical history, examination findings, and risk of optic nerve damage. Depending on the situation, further assessment may include measuring corneal thickness, examining the drainage angle, evaluating the optic nerve, or testing the visual field. These checks help distinguish elevated pressure without detectable nerve damage from glaucoma and other eye conditions.
Age and family history can also influence the need for glaucoma screening. The appropriate timing and frequency of eye examinations depend on individual risk factors and previous findings. If an eye specialist recommends follow-up pressure checks, attending these appointments helps track changes and determine whether further assessment or management is appropriate.
When pressure-lowering treatment is needed, the goal is to reduce the risk of damage to the optic nerve. Depending on the individual assessment, management may involve prescription eye drops, laser procedures, or surgery. The aim is not necessarily to reach one pressure number for everyone, but to achieve a level that is appropriate for the person's eyes and risk of vision loss.
Key points about intraocular pressure
Intraocular pressure is a normal part of eye function, maintained through the balance between fluid production and drainage. Measurements commonly fall between 10 and 21 mmHg, but a reading must be interpreted in context. High pressure can increase the risk of glaucoma, yet glaucoma can also occur when pressure is within the usual range. Tonometry helps clinicians assess eye health, while a comprehensive eye examination provides the broader information needed to interpret the result.