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!

Glaucoma is a spectrum of ocular diseases that cause optic nerve destruction

There are so many different types of glaucoma, and all of them work in different ways.  One thing unites them all: the optic nerve that delivers the message from the eye to the brain is destroyed.  The destruction is most commonly slow and painless, but in some types of glaucoma can be very quick with acute headaches and halos around lights.  Damage to the optic nerve causes PERMANENT vision loss, and that is why glaucoma is such a devastating ocular disease.

To simplify this discussion, let's talk about the most common form of glaucoma: primary open angle glaucoma.

Glaucoma typically is caused by high pressure building up inside the eye.  There is a fluid called the aqueous humor that is made to deliver nutrients to the internal components of the eye.  If too much fluid gets made, or if too much is retained inside the eye instead of draining out into surrounding tissue, then the fluid builds up in the eye.  This creates increasing pressure pushing against the back of the eye and the delicate optic nerve.

Eye Pressure or Intraocular Pressure

Goldmann Tonometry, the "gold standard" pressure reading Photo via http://www.sco.edu/

Every time you visit your eye doctor, a pressure reading is taken.  Most commonly an "air puff" is used, but there are also sophisticated hand-held and microscope attached instruments called tonometers that can be used.  Pressure is measured in mm of mercury (mmHg).  Average pressure is 10-21 mmHg.  Typically numbers over 21 are considered elevated, and thus suspect for glaucoma, but there are varying factors that may affect these eye pressure readings.  Your doctor will investigate further if your pressure is reading high.


What Glaucoma Looks Like

Most offices use retinal photographs to show patients what an eye disease looks like, but glaucoma is a disease that is not easy to "see."  What your doctor looks for to detect glaucoma is atypical appearance of the optic nerve.

Photo via http://www.glaucoma-eye-info.com/

Everyone has a hole in their optic nerve where blood vessels pass through.  The average person has a hole that is smaller than half the diameter of the nerve (is that complicated enough??).  The photo above shows a normal, small hole. The larger the hole, the generally more suspicious it is for glaucoma.  Here's the thing, everyone has a different size optic nerve and a different size hole that can be entirely normal.  In the average eye, the size of that hole is constant through life.  In glaucoma, the size of the hole in the nerve will get LARGER with time, because the nerve tissue is getting destroyed.  This is why taking routine retinal photography is such a great option, and why everyone should be routinely examined by their eye care professional even if they don't have vision trouble or had no eye diseases several years prior.  Glaucoma is PROGRESSIVE, which means you might not have it today, but you may have it in a few years.  The only true way to detect it is by monitoring for changes.

2003

2011: Note hole or cup is getting BIGGER near the bottom edge. If you look close, you can see a hemorrhage near the area of inferior damage (a tell-tale Drance heme)

2012:  Even thinner at the inferior edge of the nerve!!