The inside of the eye constantly produces a clear fluid called aqueous humor. This fluid nourishes the eye's structures and helps maintain its shape. Under normal circumstances, it flows in through one area and drains out through another, keeping the pressure inside the eye at a healthy level. When that drainage slows down or gets blocked, fluid builds up and pressure rises. That increased pressure is called elevated intraocular pressure, or IOP for short.

High eye pressure is one of the most well-known risk factors for glaucoma, and that's because pressure can press on and gradually damage the optic nerve. But here's something that surprises a lot of people: not everyone with high eye pressure develops glaucoma, and not everyone with glaucoma has high eye pressure. Some people develop glaucoma even when their pressure readings fall within the normal range, a form called normal-tension glaucoma. That means eye pressure is one important piece of the puzzle, but not the whole picture.

What ultimately matters is how the optic nerve responds to the conditions inside the eye. Some optic nerves are more vulnerable than others, and factors beyond pressure, including blood flow and the structure of the nerve itself, may also play a role.

Understanding the different types of glaucoma can help you make sense of what your eye care provider is looking for and why.

Open-angle glaucoma is by far the most common form. In this type, the drainage angle where fluid exits the eye appears open and unobstructed, but the drainage tissue itself isn't working efficiently. Fluid builds up slowly over time, and pressure gradually rises. This type almost never causes pain or obvious symptoms in its early stages, which is what makes it so hard to catch without regular eye exams.

Normal-tension glaucoma is a subtype of open-angle glaucoma where optic nerve damage occurs even though pressure readings are in the normal range. The exact reasons aren't fully understood, but it's an important reminder that eye pressure alone doesn't tell the whole story.

Angle-closure glaucoma happens when the iris, which is the colored part of your eye, physically blocks the drainage angle. It can develop gradually, or it can happen suddenly in what's called an acute angle-closure attack. An acute attack is a medical emergency that requires immediate attention.

Congenital glaucoma is a rare form that affects infants and young children when the eye's drainage system doesn't develop properly before birth.

Secondary glaucoma refers to glaucoma caused by another underlying condition, an injury, or a medication. Diabetes-related eye disease, inflammation inside the eye, and long-term use of corticosteroid medications are a few examples of things that can contribute to secondary glaucoma.

The most important thing to understand about glaucoma symptoms is that for the most common form, open-angle glaucoma, there often aren't any in the early stages. Vision can feel completely normal even as the optic nerve sustains damage. The first sign of vision loss is usually a gradual narrowing of peripheral vision, meaning what you can see at the edges of your visual field starts to shrink. Because this happens slowly, many people don't notice it until it's fairly significant.

As glaucoma progresses, the visual field continues to shrink. In advanced cases, it can feel like looking through a narrow tunnel.

Angle-closure glaucoma is very different. It can produce sudden, dramatic symptoms that come on without warning:

• Severe eye pain or headache
• Redness in the eye
• Blurred or hazy vision
• Seeing halos around lights
• Nausea or vomiting

If you experience any of these symptoms together, it's important to seek emergency eye care right away. An acute angle-closure attack can cause serious damage in a very short time.

Congenital glaucoma in infants may show up as cloudy-looking eyes, excessive tearing, sensitivity to light, or eyes that appear larger than expected.

Because glaucoma so often develops without symptoms, diagnosis depends on a thorough eye examination rather than how you feel day to day. An eye care provider uses a combination of tests to get a complete picture of your eye health and optic nerve status.

Here's what a glaucoma evaluation typically involves:

• Tonometry, which measures the pressure inside your eye. This is the "puff of air" test many people recognize, though there are other methods too.
• Ophthalmoscopy, which involves a close examination of the optic nerve, usually with a dilated pupil so the provider can see clearly to the back of the eye.
• Visual field testing, which maps your peripheral vision to check for any areas of loss. It involves responding to lights or targets that appear at different points in your visual field while you focus straight ahead.
• Optical coherence tomography (OCT), a non-invasive imaging test that creates detailed cross-sectional images of the optic nerve and the retinal nerve fiber layer to look for early signs of damage.
• Gonioscopy, a specialized test where a mirrored contact lens is placed gently on the eye to allow the provider to directly view the drainage angle and determine whether it's open or closed.
• Pachymetry, a quick test that measures the thickness of the cornea, which can affect how pressure readings are interpreted.

No single test is enough on its own. Glaucoma diagnosis involves looking at all of these findings together and often monitoring changes over time.

Glaucoma can affect anyone, but certain factors increase the likelihood that someone will develop it. Being aware of your risk factors is an important reason to keep up with regular eye exams, even when your vision seems fine.

Risk factors for glaucoma include:

• Age, as the risk increases significantly for adults over 60, though glaucoma can occur at any age
• Family history, since having a parent or sibling with glaucoma raises your own risk
• Elevated eye pressure, even without a glaucoma diagnosis yet
• Race and ethnicity, as people of African, Hispanic, or Asian descent face a higher risk for certain types of glaucoma at earlier ages
• Nearsightedness (myopia) for open-angle glaucoma, or farsightedness (hyperopia) for angle-closure glaucoma
• Previous eye injuries or surgeries
• Certain medical conditions, including diabetes and high blood pressure
• Long-term use of corticosteroid medications, including some eye drops

If any of these risk factors apply to you, it's worth talking with an eye care provider about how often you should have your eyes checked and what to watch for.

Glaucoma is widely misunderstood, and some of the most common misconceptions can lead people to underestimate how serious it is or to put off getting checked.

Myth: Glaucoma only affects older people. Fact: While the risk goes up with age, glaucoma can develop at any point in life, including in children and young adults. Congenital glaucoma affects infants, and some adults develop it in their 30s or 40s.

Myth: If my eye pressure is normal, I can't have glaucoma. Fact: Normal-tension glaucoma is a well-recognized form of the disease. Eye pressure is just one of several factors that matter.

Myth: I would know if I had glaucoma because my vision would be noticeably different. Fact: The most common form of glaucoma causes no pain and no obvious symptoms in its early stages. Many people don't notice peripheral vision loss until it has become fairly advanced, which is exactly why regular exams are so critical.

Myth: Once vision is lost from glaucoma, it can be restored. Fact: Damage to the optic nerve from glaucoma is permanent. This makes early detection the most powerful tool available for protecting your vision.

Because glaucoma doesn't always announce itself, regular comprehensive eye exams are your best defense. Even if your eyes feel completely healthy and your vision seems sharp, a routine exam can detect early changes that you'd never be able to notice on your own.

Adults with known risk factors are generally encouraged to be evaluated more frequently than those without them. It's worth having a conversation with an eye care provider about how often you should be screened based on your personal health history.

You should seek prompt or emergency care if you experience any sudden symptoms such as eye pain, severe headache, significant changes in vision, redness, or nausea alongside visual disturbances. These can indicate an acute angle-closure attack, which requires immediate attention.

Even outside of urgent symptoms, don't let "I feel fine" be the reason you skip an eye exam. With glaucoma, feeling fine is exactly the problem.