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Normal eye pressure varies throughout the day, just like blood pressure. It’s internal eye pressure, medically called intraocular pressure, or IOP. And again, like blood pressure, normal eye pressure is defined by a range of numbers. In this case, 12 to 22 millimeters of mercury (mm Hg), the scientific scale for measuring pressure. It’s measured by that “puff of air” eye test you get in the eye doctor’s office.
Normal eye pressure is monitored because of the risk of glaucoma. Elevated (above normal) internal eye pressure puts you at risk for glaucoma and permanent damage to the optic nerve. But normal eye pressure doesn’t guarantee you won’t get glaucoma.
How Glaucoma Can Affect Normal Eye Pressure
Some studies have found that as many as 40 percent of those who develop glaucoma have normal eye pressure. This is called low-tension or normal-pressure glaucoma. What that shows is that glaucoma is not one disease, but multiple diseases. People diagnosed with low-tension glaucoma get the same treatment as those with high eye pressures because it prevents further loss of vision.
What the different kinds of glaucoma have in common is that the optic nerve is damaged and the person shows signs of changes in their vision. High eye pressure is a risk factor that just makes you more likely to progress to glaucoma. On the flip side of that, having an eye pressure below normal does not protect you from ever having glaucoma.
People with above-average eye pressure but no signs of optic-nerve damage have “ocular hypertension,” and this puts them at slightly higher risk of full-blown glaucoma. Depending on the person’s other risk factors for glaucoma, a doctor may recommend starting pressure-lowering drug therapy.
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Everyone over the age of 60 is considered at increased risk for glaucoma, but each of the following factors further increases risk.
- African-American, over age 40
- Family history of glaucoma
- History of previous eye injury
- History of steroid use
- Elevated internal eye pressure (usually detected during an eye exam)
- Other related health problems, including high blood pressure, high cholesterol, lupus, diabetes, and migraine headaches
Secondary glaucoma can be caused by an eye injury, inflammation or infection, a very mature cataract, diabetes, lupus, or certain medications (particularly steroids). Sometimes children are born with congenital glaucoma, a rare and usually inherited birth defect that can be treated with surgery.
Heart disease is linked to glaucoma. People diagnosed with, or treated for open-angle glaucoma have a higher risk of death from heart disease, particularly if they have been treated with beta-blocker medication. More research is needed on this issue, but you should talk to your eye doctor if you have glaucoma and are taking beta-blocker medications.
Thyroid abnormalities also may be risk factors for glaucoma. Excess mucopolysaccharides, a type of sugar molecule that can occur with underactive thyroid (hypothyroidism), can build up in the trabecular meshwork of the eye and raise intraocular pressure.
In the end, it’s important to keep an eye on your eye pressure, but it’s not the whole story. Although optometrists routinely do that “air puff” test as a simple screening test for glaucoma, the gold standard is a full dilated eye exam by a doctor who can examine your retina and optic nerve end carefully for signs of trouble.
The earlier treatment starts for glaucoma—even if eye pressures are still in the normal range—the more likely you are to prevent damage to your vision.
Originally published in 2017, this post is regularly updated.