What is Eye Pressure?

Hand held tonometers (or intraocular pressure measurers)
can be used on people and animals too to find dangerous
eye pressure ranges!  via

We are in the last few days of Glaucoma Awareness Month, so I have dedicated today's post to one of the most commonly asked questions in my exam room: what is eye pressure?  Eye pressure or intraocular pressure (which means the pressure inside your eye) is something that is measured at every comprehensive eye examination, but in the general population it's very poorly understood.
Here's an easy guide to what eye pressure is and why your doctor measures it!

Think of your eye like a basketball.  The inside of the ball is "pumped up" to a certain pressure to achieve the shape and properties desirable for the ball's performance.  While we don't need our eye to bounce, we do need it to have enough of its own pressure to keep its shape.  The eye is a relatively soft tissue after all.  In addition to having enough pressure to keep the eye's shape and structure, the pressure in the eye also helps with the distribution and flow of nutrients to the intraocular tissue.  The eye needs a certain pressure to keep its structural channels open for nutrients to pass through and blood to flow to various areas.  So we do need a minimal eye pressure (typically 4 mmHg or more is required for the eye to function), but too much can result in structural damage too.  Too little pressure and your basketball deflates, too much and it bursts.  Your eye won't burst, but too much or too little can both be devastating to your ocular health and vision!

The front section of the eye is the anterior chamber.  Aqueous humor is made
in the ciliary body, and then flows around the lens, through the pupil, and bathes
the back surface of the corneal to deliver nutrients before draining out. via

Common question -- eye pressure is not blood pressure.  They are actually completely unrelated (but can interact to put you at higher risk for damage like glaucoma!). The pressure inside the eye comes not from blood flow, but from a clear fluid called the aqueous humor. The front of your eye (or anterior chamber as it is called scientifically) is the area of the eye from your lens forward.  This part of your eye needs nutrients and oxygen just like all other parts of your body, but there is a problem: anything between your iris and your cornea (the clear surface dome) has to be clear.  If blood vessels covered these

Aqueous humor fluid drains out at the anterior

chamber angle.  The angle needs to be open for fluid

to properly drain.  If the tissue is blocked here, your

eye pressure will acutely spike really highly! via

areas you would be looking through red lines all the time in your vision.  Instead the body uses a clear fluid called aqueous humor to deliver nutrients to the anterior chamber structures.  At any moment you have aqueous humor entering the front of the eye and exiting, passing nutrients through the system. The aqueous humor is made by the ciliary body (a structure that surrounds the iris), and is then drained through channels where the iris and cornea meet inside the eye (the anterior chamber angle).  If too much fluid is being made, or not enough is draining out, your eye pressure will rise.

This fluid is entirely inside of your eye, so even though we check your pressure by measuring externally, we are reading the internal eye pressure.  Just like the pressure inside of the baskteball.  If your eye is watering a lot people will often ask me if their pressure is too high.  Watering and eye pressure are almost always unrelated, since the fluid that causes high eye pressure is an internal eye fluid and not your tear film that is an external eye fluid.

When intraocular pressure is too high, the long term result is glaucoma, or death of the optic nerve.   If fluid is getting backlogged in the eye, it can change and shift the direction of that nutrient flow, even altering the flow of nutrients in the blood vessels in the back of the eye.  In time, this destroys the delicate nerve tissue that delivers information from the eye to the brain.  Glaucoma is typically a slow process -- you can have acute spikes in pressure that cause sudden damage (acute angle closure glaucoma is an example), but typically glaucoma takes years or decades to cause blinding vision loss.  And because in most people it's not the result of an acute high spike in pressure but years of chronic, mild elevation, you would not have any symptoms if you have glaucoma.  That's right, you won't feel different; you won't have red eyes; you won't have eye pain; and you won't notice vision problems.  Until you do and it is too late.  That's why glaucoma is something that we all need to be screened for yearly.  Unless your doctor is checking your eye pressure and optic nerve health, you won't know something is wrong until it is too late for us to restore your vision!

What's normal?
Normal eye pressure is typically under 21 mmHg.  Normal can vary from person to person however, so for some people even readings in the teens are too high and still damaging their eye.  Glaucoma is multifactorial, and there are so many different kind of glaucomas -- some have high pressure and some have perfectly normal pressure readings. As a general rule, if your eye pressure is in the high 20s or 30s, you probably have glaucoma or at least the strong risk of it.  Don't expect to feel a pressure sensation in your eyes unless your pressure is in the high 40s or 50s (at risk of acute and rapid vision loss!!).  Typically that pressure sensation you feel around the eyes towards the end of the day is actually just eye fatigue from tired eye muscles, but always best to get things checked.

For more information about glaucoma, check out What is Glaucoma? and How Does Your Doctor Check for Glaucoma?  for more patient-friendly reading!