Harvard Health Letter

The glaucoma you may be missing

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.

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