What Causes Glaucoma?

By Marilyn Haddrill; contributions and
review by Charles Slonim, MD

The cause of glaucoma generally is a failure of the eye to maintain an appropriate balance between the amount of internal (intraocular) fluid produced and the amount that drains away.

Underlying reasons for this imbalance usually relate to the type of glaucoma you have.

Just as a basketball or football requires air pressure to maintain its shape, the eyeball needs internal fluid pressure to retain its globe-like shape and ability to see.

But when something affects the ability of internal eye structures to regulate intraocular pressure (IOP), eye pressure can rise to dangerously high levels — causing glaucoma.

Unlike a ball or balloon, the eye can't relieve pressure by springing a leak and "deflating" when pressure is too high. Instead, high IOP just keeps building and pushing against the optic nerve until nerve fibers are permanently damaged and vision is lost.

Glaucoma and Eye Anatomy

When glaucoma progresses, injury to the neurons ultimately leads to eye damage in the form of peripheral vision loss. However, eye damage appears to begin first in the brain as connectivity is lost. This discovery reported in early 2010 is a major breakthrough that could lead to improved early diagnosis and treatments of the eye disease, according to Vanderbilt Eye Institute (VEI) researchers in Nashville.

"If you followed the disease long enough, eventually the optic nerve, then the retina, show signs of degeneration," said VEI research director David Calkins, PhD. "So the degeneration works in reverse order. It starts in the brain and works its way back to the retina..."

To understand what causes glaucoma, you first must know something about the human eye's anatomy and how intraocular fluid moves through the eye:

1. The clear fluid inside the eye is produced by the ciliary body, which is a small, circular structure found behind the iris or colored portion of the eye.
2. This fluid, known as the aqueous humor, flows behind the iris and through the pupil or central opening in the middle of the iris. The aqueous humor (or "aqueous") then fills the anterior chamber, a space between the back of the clear cornea and the front of the iris.
3. The aqueous escapes from the eye through a drainage structure known as the filtration angle, which is the angle formed inside the anterior chamber between the iris and the peripheral cornea.
4. The aqueous filters through this angle and through the sclera or white part of the eye and then joins with the network of veins outside the eye.
5. Any disruption of this outflow of aqueous — including from certain eye injuries — can result in an increase in IOP.

Anatomically, the eye's filtration angle is referred to as being either "open" or "closed" (narrow). The narrower the angle, the more difficult it is for the aqueous to flow through it.

An open angle also can hinder the outflow of aqueous, if structural damage exists within the ocular tissues of the angle itself.

[Read more about open angle glaucoma and narrow angle glaucoma.]

Glaucoma, Blood Flow and Optic Nerve Damage

While high IOP often is associated with glaucoma, this eye disease also can occur when internal eye pressure is normal (normal-tension glaucoma). People with this condition have highly pressure-sensitive optic nerves that are susceptible to irreversible damage from what ordinarily would be considered "normal" IOP.

Conversely, certain people with elevated intraocular pressure known as ocular hypertension may never develop glaucoma.

Most conventional methods of screening for glaucoma involve testing eyes for the presence of high IOP. But because glaucoma can occur even without high IOP, direct examination of the optic nerve and visual field testing are essential in making (or ruling out) the diagnosis of glaucoma.

In normal-tension glaucoma, some theories suggest that decreased blood flow to the optic nerve might be a factor. Inadequate blood flow also may help create high IOP contributing to optic nerve damage.

Some studies also indicate that poor blood flow within the eye can be directly correlated with blind spots (scotomas) that develop within the visual field, as occurs with glaucoma.*

An intriguing study reported in August 2007 indicates a potential common cause of both glaucoma and Alzheimer's, which creates brain lesions and accompanying memory loss. London researchers who conducted the study found that buildup of a protein known as beta-amyloid in the eye's retina and brain tissue appears related to development of both glaucoma and Alzheimer's.

However, abnormal accumulation of beta-amyloid proteins does not mean that someone with Alzheimer's will have glaucoma or vice versa. Researchers said similarities between certain eye and brain tissue could explain why the buildup of beta-amyloid proteins can affect both the eye and the brain.

Increasingly, glaucoma treatments now are being investigated for their ability to protect nerve cells in the eye from damage.

*Ophthalmology Clinics of North America, September 2005

Judith Lee, Gretchyn Bailey and Dr. Vance Thompson also contributed to this article.

[Page updated March 2010]