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Glaucoma: Definition and causes

illustration of a man with glaucoma

What is glaucoma?

Glaucoma is defined as a group of eye conditions that can damage your optic nerves, the information pathways between the eyes and brain.

Glaucoma is almost always caused by too much pressure inside one or both eyes. Once this pressure starts to affect the optic nerve, it can lead to irreversible levels of blindness.

People don’t typically experience symptoms until the optic nerve has already been damaged. Scheduling regular eye exams helps doctors catch eye pressure or optic nerve problems before vision loss occurs or worsens.

Glaucoma usually occurs later in life, but it can happen at any age. The Black and Hispanic population are at greater risk of glaucoma than other populations, but glaucoma affects all races.

If you’re diagnosed with elevated eye pressure or glaucoma, don’t panic. With routine check-ups and modern treatment, most patients have a good outlook toward keeping the vision they have.

Only an optometrist or ophthalmologist is trained to spot the early warning signs of glaucoma and to begin glaucoma treatment before vision loss occurs.

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What causes glaucoma?

Our eyes are filled with a fluid called aqueous humor that gives them their round shape. In a normal eye, the fluid filters in and out easily.

Sometimes fluid doesn’t flow out as easily as it flows in. When too much fluid builds up inside the eye, it causes internal eye pressure, called intraocular pressure or IOP,  to go up.

Eye pressure that’s higher than the normal range is known as ocular hypertension. People with ocular hypertension but no signs of glaucoma are called glaucoma suspects.

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 that connects in the back of the eye.

Eye pressure can be raised for several years before optic nerve damage starts to affect your eyesight. The early detection and treatment of high eye pressure are the keys to successful glaucoma prevention.

In most types of glaucoma, optic nerve damage and vision loss occurs because the pressure inside the eye (IOP) is too high.

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

Sometimes, glaucoma doesn’t have a direct cause.

There are four main types of glaucoma. The most common type is open-angle glaucoma, when untreated long-term pressure causes damage to the optic nerve.

Less often, glaucoma can occur in someone with normal eye pressure. This is called normal-tension glaucoma, but some doctors label it as a slower version of open-angle glaucoma.

Narrow-angle glaucoma (also called closed-angle or angle-closure glaucoma) comes on suddenly, either as a result of injury or illness, or without any reason at all. It requires immediate medical treatment to save a patient’s vision.

Secondary glaucoma is caused by a separate illness or condition. There are many forms of secondary glaucoma; each can come in either open-angle or narrow-angle form.

Developmental glaucoma (also called childhood or pediatric glaucoma) may be present from birth or caused by an injury or underlying condition and is usually diagnosed at a young age.

Glaucoma symptoms

While the more common open-angle glaucoma typically does not cause any symptoms until vision loss occurs, usually in your side (peripheral) vision.

However, narrow-angle glaucoma does cause symptoms. These can include:

  • Intense eye pain and/or headache that comes on quickly

  • Nausea

  • Vomiting

  • Blurry vision

If you think you have narrow-angle glaucoma, get emergency treatment right away.

SEE RELATED: Glaucoma symptoms

Types of glaucoma

The two main types of glaucoma are:

1. Primary open-angle glaucoma

This is the most common type of glaucoma and typically is the type that eye doctors describe to their patients. In primary open-angle glaucoma (POAG), the drainage angle that leads to the trabecular meshwork looks normal, but aqueous fluid exits the eye too slowly.

2. Angle-closure glaucoma

This is a less common type of glaucoma where the drainage is too narrow or begins to close, impeding the normal outflow of aqueous humor.

Angle-closure glaucoma can be either chronic (an anatomically narrow angle or temporary episodes of angle closures over time) or acute (the angle narrows or closes suddenly).

The term narrow-angle glaucoma often is used to describe glaucoma that occurs when the drainage angle is too small or is partially closed.

Other glaucoma types

Other types of glaucoma include:

Low-tension or normal-tension glaucoma

In these types of glaucoma, optic nerve damage and vision loss occurs even though IOP remains normal. Low blood pressure may be a risk factor.

Congenital glaucoma

Some children are born with a defect in the drainage angle that prevents the aqueous fluid from exiting the eye normally. Usually there are obvious symptoms, such as cloudy corneas, light sensitivity and watery eyes.

Secondary glaucoma

This type of glaucoma develops due to complications of certain medical conditions (including diabetes and high blood pressure) or because of other eye conditions (such as cataracts and uveitis), side effects of medications, or trauma to the eye.

Risk factors

Some people have a higher risk of developing open-angle glaucoma throughout the courses of their lives:

  • People over age 40.

  • People who have a family history of glaucoma.

  • People with certain medical conditions, such as diabetes and high blood pressure.

  • People with African, Asian or Hispanic heritage.

  • People who use certain long-term medications, specifically steroids.

  • People who have had eye injuries or other trauma, like eye surgery.

Detecting glaucoma

During routine eye exams, a small instrument is used to measure your intraocular pressure (IOP). During this test (called tonometry), eye drops may or may not be used to make your cornea less sensitive.

In Goldmann applanation tonometry (GAT), numbing eye drops are applied and a lightweight probe gently touches the eye to measure eye pressure.

In non-contact tonometry (NCT), a gentle puff of air flattens the center of the cornea briefly to measure eye pressure. No numbing eye drops are needed.

Normal IOP typically is considered to be readings below 21 mmHg (millimeters of mercury) — a unit of measurement based on how much force is exerted within a certain defined area.

Higher IOP readings indicate increased risk of glaucoma.

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.

Preventing glaucoma

Many times, glaucoma can’t be prevented. But there are things you can do to limit or eliminate potential vision loss.

The key to prevention is getting regular eye exams, even if your vision seems normal. People diagnosed with glaucoma or ocular hypertension often experience no symptoms at the time of the exam.

Exercise and physical activity sometimes lower eye pressure, but certain activities can actually raise eye pressure. A doctor can help you decide which exercises to focus on.

If you are diagnosed with elevated eye pressure or glaucoma and eye drops are prescribed, make sure to use them on the strict schedule provided by your doctor. This ensures that your eye pressure remains normal, limiting any vision loss.

Worried about glaucoma?

Visit your nearest eye care specialist to find out about glaucoma.

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