You may have heard your eye doctor mention measuring your eye pressure, but what does that mean, and what does it have to do with glaucoma?
Normal intraocular pressures average from 12-22 mm Hg. The “mm Hg” refers to millimeters of mercury, a scale for recording eye pressure. Anything that exceeds 21 mm Hg is considered hypertensive. A problem in the drainage of fluid produced in the eye causes increased pressure.
The Connection Between High Eye Pressure and Glaucoma
Eye pressure, also called intraocular pressure (IOP) or ocular hypertension, refers to the fluid pressure inside the eye. Studies show that high eye pressure can increase your risk for glaucoma, a sight-threatening ocular disease.
Left untreated, a consistently high IOP over an extended period causes the pressure to push on the optic nerve, causing damage and, consequently, permanent vision loss.
What Causes High IOP?
High IOP occurs when the aqueous humor, the clear fluid that fills the eye’s front chamber, can’t flow out of the eye normally. The purpose of the aqueous humor is to nourish specific structures in the front part of the eye, including the lens and parts of the iris and cornea, and to keep the eye pressurized so that it does not collapse.
The drainage system that allows the aqueous humor to drain from the eye must remain clear and intact to maintain normal IOP. When the fluid can no longer drain efficiently, a buildup of fluid can occur, consequently causing an increase in eye pressure.
How IOP Is Measured
Because changes in eye pressure do not cause any pain, they can go undetected for years, eventually causing vision loss. While home tonometry exists, the preferred way to accurately and consistently measure your eye pressure is through regular comprehensive eye exams. During an eye exam, your eye doctor will perform a tonometry test to measure your IOP and detect any changes in your eye pressure.
Types of Tonometry Tests
- Goldmann Applanation Tonometry Test
The Goldmann applanation tonometry test is the most commonly used and is considered the most reliable way to measure IOP.
To perform this painless test, your eye care team will place numbing drops with non-toxic dye into your eyes to ensure that the test does not cause any discomfort.
Next, with your head positioned at a slit lamp device for proper lighting and magnification of your eye, your eye doctor will gently touch the surface of your eye and apply a minimal amount of pressure to flatten a small area of your cornea. Finally, your eye doctor will measure your eye pressure based on the force needed to flatten your cornea.
- Non-Contact Tonometry/Air-Puff Tonometry
This test is similar to the Goldmann applanation test but uses a quick pulse of air to flatten a small section of your cornea.
- Rebound Tonometry
A rebound tonometry test involves placing a small probe against your cornea and moving it slightly to measure your eye’s response to the touch. Specific devices used for this type of test, such as the iCare device, are portable, do not require numbing drops, and can be used in the comfort of your home.
- Tonometer or Tonopen
The tomometer or tonopen is another portable device that measures your IOP by touching your cornea and evaluating its indentation. However, this device requires numbing drops before the test is performed.
Does Eye Pressure Remain Constant Throughout the Day?
No. Eye pressure can vary hourly, daily, and weekly. Therefore, measuring your eye pressure during an exam can only provide a small indication of your regular IOP. Large fluctuations in IOP levels throughout the day can be dangerous to your ocular health, especially if you have glaucoma.
While small eye pressure changes are normal, it’s essential to speak with your eye doctor to determine how to monitor your IOP — either at home or during more frequent eye exams. Most of the time, eye doctors recommend measuring eye pressure in the morning, so keep that in mind when you schedule your next exam.
Help Us Find a Cure
While there’s no cure for glaucoma, having regular comprehensive eye examinations to monitor your IOP is one of the ways glaucoma patients can manage their glaucoma. Advancements in glaucoma research continue to bring us closer to finding a cure and restoring vision lost from glaucoma. You can help find a cure with cash, stock, or a vehicle or boat donation. Your support will give hope to those living with glaucoma and accelerate our search for a cure and vision restoration.
Reviewed and updated on June 1, 2022; Reviewed for medical accuracy by Terri-Diann Pickering, MD
Terri-Diann Pickering, MD
Dr. Pickering completed her ophthalmology residency at the University of California San Francisco and a glaucoma fellowship at the USC-Doheny Eye Institute in Los Angeles after earning her degree from Harvard University Medical School. She is a clinical instructor at California Pacific Medical Center and a researcher with the Glaucoma Research and Education Group in San Francisco.