During an eye exam, a tonometer will measure the pressure of the fluid behind the cornea. To be considered normal eye pressure, the measurement should be between 10 to 20 mmHg. The reading can vary by body position, time of day, and factors like medication or fluid intake. An increased pressure can be a warning sign of problems, most notably glaucoma.
The cornea of the eye is filled with a thick liquid called the aqueous humor. This fluid helps keep the cornea curved and inflated. The eye constantly produces new aqueous humor and drains the old. The pressure of the fluid is measured with a tonometer, which gauges the amount of force needed to flatten an area of the cornea. The thickness of the individual cornea can affect the results.
There are a variety of ways to measure eye pressure. Some tonometers involve direct contact with the eye, which usually requires a topical anesthetic, and are regarded as the more reliable method. Several non-contact methods involve a tonometer that pushes a puff of air at the cornea and measures the force of the air as it hits. This style is good for children or people who are nervous about having a machine touch their eyes.
The tonometer measures normal eye pressure in millimeters of mercury (Hg). The healthy range is from 10 to 20 mmHg, with the average being about 14 to 16 mmHg. The amount of pressure can vary throughout the day and can change depending on the position of the body. Factors that normally affect general health, such as exercise, medicine, or alcohol intake, can also affect eye pressure.
Although normal eye pressure usually increases with age, a marked increase or a difference in pressure in each eye can be warning signs for health problems. High pressure is often seen with glaucoma, although the disease can develop without it. Eye pressure above 21 mmHg can damage the optic nerve and can lead to blindness. If glaucoma is caught early, doctors may be able to slow the disease. Once the eye is damaged, however, it is difficult to repair. This is why it is important to have regular checkups to measure normal eye pressure and catch problems early.