What is Eye Pressure?

Eye pressure is part of eye health, but many people don’t fully understand what it is or why it matters. That's why, here we will explain in simple and clear terms what eye pressure is, how it works, and what happens if it is too high or too low.

Eye Pressure in a Nutshell

Eye pressure, or intraocular pressure (IOP), is the fluid pressure inside your eye. Just like blood pressure helps move blood through your body, eye pressure keeps your eye in the right shape and helps it function properly.

Normal eye pressure is usually between 10 and 21 mmHg (millimeters of mercury). If the pressure goes too high or too low, it can cause serious eye problems, including vision loss.

How Does Eye Pressure Work?

1. What Creates Eye Pressure?

The eye is filled with a special fluid called aqueous humor. This fluid provides nutrients to the eye, removes waste, and keeps the eye inflated so it doesn’t collapse.

The aqueous humor is made inside the eye and constantly flows out through a drainage system called the trabecular meshwork (a small filter-like structure at the front of the eye). This balance between fluid production and drainage controls eye pressure.

Your eye must keep the right amount of pressure for clear vision and healthy function.

2. What Controls Eye Pressure?

Eye pressure is affected by several factors:

Fluid production
Fluid drainage
Genetics
Age
Medications

If your eye makes too much aqueous humor, pressure rises. If the drainage system is blocked or slow, pressure increases. Some medicines (like steroids) can raise eye pressure as well.

Some people naturally have higher or lower eye pressure. As you get older, the drainage system may not work as well and cause IOP problems.

What Happens if Eye Pressure is Too High?

If eye pressure rises above 21 mmHg, it is called ocular hypertension.

When left untreated, this can damage the optic nerve, the part of your eye that sends images to the brain. This damage of the optic nerve is called glaucoma, a disease that can lead to permanent vision loss.

Symptoms of High Eye Pressure

The worrying part of high eye pressure is that it doesn't cause symptoms until serious damage occurs. This is why regular eye check-ups can save vision!

In some cases, very high intraocular pressure causes:

Blurry vision
Eye pain
Seeing halos around lights
Headaches
Eye bulging
Tunnel vision (loss of peripheral vision)

There are two types of glaucoma, related to having different mechanisms that lead to increased pressure inside the eye - open-angle glaucoma and angle-closure glaucoma.

Open-angle glaucoma is the most common. It happens slowly over time, causing gradual vision loss.

Angle-closure glaucoma, on the other hand, is a medical emergency. Pressure rises suddenly, causing severe pain, nausea, and rapid vision loss.

What Can Cause High Eye Pressure?

Aging (drainage system slows down with age)
Family History (if close relatives have glaucoma, you may be at higher risk)
Steroid Medications (long-term use of steroids in inhalers for asthma, skin creams, eye drops)
Eye Injuries (trauma can block fluid drainage)

Treatment for High Eye Pressure

Eye Drops (reduce fluid production or improve drainage)
Oral Medications (in severe cases, pills can help lower pressure)
Laser Therapy (opens the drainage system to lower pressure)
Surgery (creates a new drainage path when other treatments fail)

What Happens if Eye Pressure is Too Low?

If eye pressure drops below 5 mmHg, it is called hypotony. This is rare but can be dangerous, especially after eye surgery or trauma.

Symptoms of Low Eye Pressure

The difference between high and low pressure is, lower IOP does have noticeable symptoms, for example:

Blurred vision
Distorted vision
Eye discomfort
Eye pain
A sunken (soft) eye

Low IOP is extremely dangerous and should be treated like an emergency.

The eyeball can lose its shape, affecting vision and leading to severe complications - retinal detachment or macular swelling. Retina detachment is when the retina peels away from the eye wall. Macular swelling is when fluid builds up in the central vision area.

What Can Cause Low Eye Pressure?

Eye Injury or Surgery (fluid may leak out of the eye, lowering pressure)
Inflammation (conditions like uveitis can reduce fluid production)
Medications (some drugs used to lower high eye pressure can make it too low)

Treatment for Low Eye Pressure

Stopping or adjusting medications (if eye drops are causing low pressure, the doctor may reduce the dose)
Medications to increase fluid production (some drugs help restore normal pressure)
Surgery (if fluid is leaking, surgery may be needed)

How to Measure Eye Pressure?

Eye doctors check ocular pressure using special instruments.

Tonometry is the most common method of measuring eye pressure. Several tonometry devices are widely used - Goldmann Applanation Tonometry, rebound tonometry and "Air Puff" Tonometry.

Goldmann Applanation Tonometry is a done at the slit lamp and is currently the gold standard when it comes to measuring the intraocular pressure. The eye is numbed with eyedrops and colored with a special dye called fluorescein. The eyecare physician proceeds to touch your eye with the special cone while shining blue light at it. The configuration of fluid displacement gives us a fery precise reading of the pressure inside your eye.

Rebound Tonometry is a small handheld device, which on activation has a movable tip that touches the eye’s surface to measure pressure (shown on the photo above). It's slightly uncomfortable to have it in front of your face and resist the urge to blink, but it is over quickly and is not painful at all.

A non-contact tonometry (NCT), or "Air Puff" tonometry device is an alternative. It's a puff of air blown onto the eye to estimate pressure. It feels like a quick, gentle burst of air in your eye, which might make you blink or flinch, but it’s completely painless and over in a second.

Additional tests may be recommended depending on the case and the eye damage from low or high IOP. For example, an OCT Scan (Optical Coherence Tomography) checks for optic nerve damage; and a visual field test measures side (peripheral) vision to detect glaucoma.

How Often to Check Eye Pressure?

Demand a check of your eye pressure during every eye doctor visit. It takes a second, doesn't cause any pain, and will put any concerns to rest.

Here is an example eye pressure screening calendar:

No risk factors – Every 1-2 years after age 40
At risk for glaucoma – Every year or as recommended by your doctor
If you have glaucoma – Every 2-3 months, to ensure treatment is working

Conclusion: Why Eye Pressure Matters

Maintaining healthy eye pressure is key for good vision. High eye pressure can lead to glaucoma and blindness if untreated. Low eye pressure can cause vision problems and eye damage. Regular eye exams can help detect problems on time before vision is lost.

External resources:

Eye Pressure, American Academy of Ophthalmology
Ocular Hypertension, WebMD
What Is a Normal Eye Pressure Range? Healthline

✅ Medically reviewed

Checked by Atanas Bogoev, MD.