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Glaucoma: Definition and causes

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This article gives you the facts you need to know about glaucoma, including a definition of glaucoma, a discussion of what causes glaucoma, and what a "glaucoma test" is.

What is glaucoma?

Glaucoma is a serious eye condition that can cause blindness. It damages the optic nerve, which carries information from your eyes to the visual center in your brain. This damage can result in permanent vision loss.

Glaucoma is almost always caused by too much pressure inside one or both eyes. But it's also possible to have glaucoma even if the pressure inside your eyes is normal.

The most common type of glaucoma has no early warning signs and can only be detected during a comprehensive eye exam. If undetected and untreated, glaucoma first causes peripheral vision loss and eventually can lead to blindness. By the time you notice vision problems from glaucoma, it's too late. The lost vision cannot be restored, and it's likely you may experience additional vision loss, even after glaucoma treatment begins. The only way to protect yourself and your family from vision loss and even blindness from glaucoma is to visit an eye doctor for routine comprehensive eye exams. Only an optometrist or ophthalmologist is trained to spot the early warning signs of glaucoma and to begin glaucoma treatment before vision loss occurs.

Glaucoma usually occurs later in life, but it can happen at any age. Blacks and Hispanics are at greater risk of glaucoma than other populations, but glaucoma affects all races and everyone should have routine eye exams to have their eyes checked for this serious eye condition.

If you’re diagnosed with elevated eye pressure or early signs of glaucoma, don’t panic. With routine check-ups and prompt glaucoma treatment, vision loss often can be prevented or slowed, especially if glaucoma is detected early.

Unfortunately, vision loss that already has occurred from glaucoma typically cannot be reversed.

What causes glaucoma?

In most cases, glaucoma is caused by higher-than-normal pressure inside the eye — a condition called ocular hypertension. But sometimes, glaucoma can occur even when pressure inside the eye is normal.

A quick review of eye anatomy will help describe what causes glaucoma: The front portion of the eye is filled with a clear fluid called the aqueous humor that nourishes tissues inside the eye and is essential for maintaining the eye's normal shape.

The aqueous humor is constantly being produced within the eye in a structure called the ciliary body, which is located behind the iris. It drains from the eye through a mesh-like channel called the trabecular meshwork that's located in the angle formed inside the front of the eye, where the cornea and iris meet.

The balance between how much aqueous humor is being produced by the ciliary body and drains through the drainage angle is what determines the pressure inside the eye — called the intraocular pressure (IOP).

If something causes this drainage angle to close down or the trabecular meshwork to become clogged, the aqueous humor cannot drain from the eye fast enough, IOP increases beyond the normal range. This increases the risk of glaucoma.

The most common type of glaucoma occurs when intraocular pressure is too high for too long. This puts outward pressure on the inner walls of the eye, eventually damaging the optic nerve in the back of the eye that transmits information from the eye to the brain.

Eye pressure can slowly increase for several years before optic nerve damage starts to affect your eyesight. Or it can occur relatively quickly. Early detection and treatment of high eye pressure are usually key to successful glaucoma prevention.

SEE RELATED: Glaucoma FAQs

Several things can cause high eye pressure and glaucoma:

  • Family history of glaucoma

  • Hereditary predisposition to glaucoma

  • Underlying conditions like diabetes or high blood pressure

  • Regular use of certain medications, including steroids

  • Eye injury or trauma

  • Thin corneas (the clear layer in front of your pupils)

  • Extreme nearsightedness or farsightedness

  • Being over age 60

Sometimes, glaucoma doesn’t have an obvious or known cause.

Glaucoma symptoms

Most types of glaucoma typically cause no pain and produce no symptoms until noticeable vision loss occurs. This is why routine eye exams are so important — an eye exam is the only way to detect glaucoma before permanent vision loss occurs.

Read more about glaucoma signs and symptoms.

Types of glaucoma: Definitions

There are several types of glaucoma. These include:

Open-angle glaucoma. Also called primary open-angle glaucoma (POAG), this is the most common form of the disease. In POAG, the drainage angle of the eye remains open, but aqueous humor doesn't drain from the eye fast enough to keep pressure in the eye in the normal range. Left untreated, the elevated IOP eventually causes damage to the optic nerve and vision loss from glaucoma over time. Read more about open-angle glaucoma.

Angle-closure glaucoma. Also called closed-angle glaucoma, angle-closure glaucoma is a rare form of glaucoma where the iris bows forward, completely cutting off the drainage angle of the eye. It can occur suddenly (acute angle-closure glaucoma), and symptoms can include severe pain, red eye and nausea. Acute angle-closure glaucoma is a medical emergency and requires treatment as quickly as possible to prevent permanent vision loss.

Having narrow drainage angles in the eye increases the risk of angle-closure glaucoma. For this reason, angle-closure glaucoma sometimes is called narrow-angle glaucoma if it occurs gradually (chronic angle-closure glaucoma).

Secondary glaucoma. This is a type of glaucoma caused by another eye condition, illness or injury. Secondary glaucoma can occur in either open-angle or narrow-angle form. Read more about secondary glaucoma.

Developmental glaucoma. Also called congenital glaucoma, childhood glaucoma, or pediatric glaucoma, developmental glaucoma may be present from birth or caused by an injury or underlying condition that occurs and is detected early in life. Read more about developmental or congenital glaucoma.

Normal-tension glaucoma. In some cases, glaucoma damage occurs to the optic nerve and vision loss can occur even if your intraocular pressure is in the normal range. This less common form of the disease is called normal-tension glaucoma. The cause of normal-tension glaucoma is not fully known, but it may be related to reduced blood flow to the optic nerve due to conditions that may impair circulation.

What is a "glaucoma test"?

During a comprehensive eye exam, your eye doctor will perform a number of tests to determine if you have glaucoma or are at risk for the disease.

One of these tests checks the pressure inside your eyes, or IOP. This is called tonometry. Many people (even some eye care professionals) may call this the "glaucoma test," but in fact, your IOP measurement is just one factor in determining your risk of glaucoma.

Tonometry testing is a quick, painless, and important part of your routine eye exams.

There are two basic types of tonometry tests to check your IOP:

Applanation tonometry. For this test, a numbing drop is applied to your eyes and a small device (much smaller than a contact lens) briefly touches your eye to take the measurement. You feel nothing when this measurement is taken. Applanation tonometry is considered the most accurate form of IOP testing.

Non-contact tonometry. Also called NCT, this test does not require a numbing drop. You face an instrument that measures your IOP with a gentle puff of air to the surface of your eye. (Some people find the puff startling, but there is no discomfort.)

Tonometry measurements are a very important to help your determine of you have glaucoma or are at risk for the disease. But these "glaucoma tests" alone are insufficient to make that determination. Your eye doctor also will closely examine the appearance of the optic nerve inside your eye.

Other information gathered during a comprehensive eye exam to check for glaucoma may include one or more visual field tests, inspection of the drainage angle of your eyes (gonioscopy), and possibly a measurement of the thickness of your corneas.

Glaucoma treatment

The main treatments for glaucoma are topical medicine (prescription eye drops), laser treatments and/or surgical procedures. Most people who have glaucoma can have their eye pressure lowered by these therapies to prevent, stop, or slow the progression of glaucoma vision loss.

Read more about glaucoma treatment and glaucoma medications.

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