Intraocular Pressure (IOP)

The eyes have many internal structures such as the iris, the lens, and the retina, but most of their interior is filled with fluid. This is not the external fluid we know as tears. They are Internal Eye Fluids -- two types of water-based gel that maintain the eyeball’s round shape (among other functions).

The front third of the eye contains two chambers holding a fluid called aqueous humor or just “aqueous”. They are:

The anterior chamber – between the cornea and the iris; and
The posterior chamber – between the iris and the lens.

They are connected so the aqueous can flow between them and bathe the cornea, iris and lens. The aqueous has a drainage system and is continually produced and drained out, keeping the front eye surface moist and clear of dust and debris.

The back two-thirds of the eye consist mostly of a larger area called the vitreous chamber, containing a second fluid called vitreous humor or just “vitreous”. The vitreous does not drain out, but is static and remains there for life. It touches against the aqueous behind the lens, but they do not mix, as the vitreous is more like a gel and the aqueous is more liquid.

Pressure Against the Eyeball Wall

Because the eye is fluid-filled, there is pressure exerted on its periphery. The fluids push against the entire spherical eyeball wall. The aqueous helps to maintain the cornea’s rounded shape and the vitreous maintains the eyeball’s shape within the eye socket. The aqueous also pushes to the rear against the vitreous.

In a healthy eye, this pressure, called the intraocular pressure (IOP), is stable and does not cause any damage. The range of normal IOP is 10 to 21 mmHg.

Faulty Drainage of Aqueous Fluid

The aqueous drainage channels enable used aqueous to leave the eye, enter the bloodstream and eventually be excreted from the body. If any blockage occurs in these channels, aqueous accumulates in the eye because production outpaces drainage. The increased amount of fluid raises the IOP.

Optic Nerve Damage and Glaucoma

Elevated IOP can damage the optic nerve where it leaves the retina and travels to the brain. The area where it exits is called the optic disc and its little crater-like center (where fibers are making a right-angle bend into the nerve sheath) is the “cup”. A normal cup is about one-third the size of the optic disc (with individual variations). The name “cup” comes from the fact that to the first physicians who saw them, the cup and optic disc looked like a teacup and saucer.

When IOP is too high, optic nerve fibers die off, making the cup look more hollowed or larger. Eye doctors refer to this as “cupping” of the nerve and it usually indicates glaucoma.

In past years it was thought that elevated IOP was the main cause of optic nerve damage. However, some people with normal IOP have glaucoma, and conversely, some with elevated IOP do not have glaucoma. That indicates that other factors are involved in causing glaucoma, and with ongoing research no doubt they will be discovered.

If you are interested in speaking with an ophthalmologist near where you live, please contact us today.