Sometimes normal eye pressures mask the condition.
Ever get an eye pressure measurement at the eye doctor’s office? The doctor directs a probe or a puff of air at your cornea to find out if the pressure inside the eye is elevated, often a major sign of glaucoma. But increased eye pressure isn’t always an accurate way to detect the condition. You can sometimes have normal eye pressure and still have glaucoma.
Types of glaucoma
The vision loss of glaucoma is caused by damage to the optic nerve. That’s the nerve that sends electrical signals to the brain, which then interprets the signals as images. Nerve damage often results when pressure gets too high because of fluid buildup inside the eye. With the most common type of glaucoma, open-angle glaucoma, damage to the nerve is usually painless and occurs gradually. (With the less common closed-angle glaucoma, people can experience sudden pain and nausea.)
But sometimes nerve damage occurs without high pressure or fluid buildup. That’s called normal-tension glaucoma (NTG). “In some patients, pressure in the normal range may cause damage. It may have something to do with intracranial pressure in the brain or compromised blood flow, depriving the nerve of oxygen and causing damage. But the consensus is that the optic nerve is just more fragile in some patients,” says ophthalmologist Dr. Lucy Shen, a glaucoma specialist at Harvard-affiliated Massachusetts Eye and Ear Infirmary.
Laser trabeculoplasty for glaucoma
Illustration by Scott Leighton
In this procedure, the surgeon uses a high-energy beam of light to improve the flow of fluid in and out of the eye.
Diagnosis and treatment
Glaucoma is usually painless, with no symptoms until you begin losing peripheral vision. That’s why comprehensive eye exams are important. But sometimes NTG is missed despite an exam. “We see that happen in cases where the optic nerve was not carefully evaluated,” says Dr. Shen.
Diagnosing NTG involves measuring the pressure in your eye and the thickness of the cornea, inspecting the optic nerve for damage, testing the visual field to determine the function of the nerve, and taking pictures of the optic nerve and the back of the eye. NTG is treated the same way as regular glaucoma: by trying to lower eye pressure—even if the pressure is “normal.” This is done using either eye drops or laser therapy (trabeculoplasty) to improve fluid drainage in the eye. If the glaucoma continues to worsen with these treatments, surgery to enlarge the eye’s drain may be necessary.
What you can do
In order to prevent NTG and any other glaucoma, avoid smoking and excessive caffeine consumption, which may raise eye pressure. Dr. Shen says no particular nutrients are known to help prevent glaucoma. The best way to catch and stop glaucoma may be with comprehensive eye exams. People at risk for glaucoma should get an annual dilated eye exam. People over 65, African Americans, people with a family history of glaucoma, and people with heart disease, high blood pressure, diabetes, or underactive thyroid are at increased risk.