Open-Angle Glaucoma

Primary open-angle glaucoma is the most common type, accounting for 70-80 percent of glaucoma cases worldwide. The designation primary means that there is no identifiable injury or disease process, such as diabetes or high blood pressure, that is causing the glaucoma. Open angle signifies that the angle formed by the iris and cornea is open rather than closed, as in primary angle-closure glaucoma.

Primary open-angle glaucoma typically occurs in patients forty years of age and older. It is a leading cause of blindness in the USA and in many other countries. POAG usually affects both eyes and is an especially dangerous form of the disease because it can progress gradually and go unnoticed for years. The only way to detect primary open-angle glaucoma before it becomes a serious problem is through regular eye examinations that check for glaucoma specifically

The main problem behind primary open-angle glaucoma is elevated intraocular pressure. Intraocular pressure refers to the pressure that the fluid inside the eye exerts on the inner surface and structures of the eyeball. However, it is important to understand that if your ophthalmologist informs you that your pressure is high, this alone does not mean that you have glaucoma. Intraocular pressure that is at or above 21 mmHg is a risk factor for glaucoma, but it is not the disease itself. The unifying factor that links all types of glaucoma is damage to the optic nerve that progresses over time and that results in a loss of optic nerve fibers.

You might be wondering what can cause the pressure inside your eye to become elevated. It occurs because there is too much of a fluid known as aqueous humor in the front part of the eye (anterior chamber). Aqueous humor is produced in a structure called the ciliary body. It circulates throughout the anterior chamber and then is filtered through a structure called the trabecular meshwork before it is returned to the bloodstream through Schlemm's canal (see the figure below).

Overproduction of aqueous humor is one way that intraocular pressure potentially can be raised. Although this does occur in some people, it is relatively rare and does not account for most cases of POAG. The main mechanism by which intraocular pressure becomes elevated is through the obstruction of aqueous humor drainage. This occurs through the trabecular meshwork, which acts much like a drain strainer in your kitchen sink to filter out debris before the aqueous humor returns to the blood stream.

It might be tempting to assume that the trabecular meshwork simply becomes clogged with debris. This would impede aqueous humor outflow and raise intraocular pressure. Blockage of this type does occur in some cases, such as in pigmentary glaucoma and pseudoexfoliative glaucoma. However, in a majority of primary open-angle glaucoma cases, the fundamental problem appears to be changes in the structural components and integrity of the trabecular network itself rather than an overproduction of particles that become trapped in the network.

The simple answer is NO. This is why POAG often is referred to as a silent thief of vision and why having regular eye examinations that check for glaucoma is essential. In POAG intraocular pressure usually increases gradually over a period ranging from several months to several years. The body adapts to these changes in eye pressure and in most cases the individual does not perceive the differences. In primary angle-closure glaucoma, another form of the disease, intraocular pressure can increase suddenly and rapidly. This represents a medical emergency that must be addressed immediately. Affected individuals not only perceive the increase in eye pressure, but often describe it as the worst pain that they ever have experienced. However, it is important to understand that these sudden increases in intraocular pressure represent a minority of glaucoma cases. Most of the time, pressure increases are so gradual that they are not felt at all.

Although progression of primary open-angle glaucoma usually cannot be detected by individuals until significant problems with vision have occurred, a characteristic sign that your ophthalmologist can observe is increased cupping in a structure in the back of the eye called the optic disc. The optic disc is the location where nerve fibers called ganglion cell axons exit the eye to form the optic nerve.

Think about a teacup. If you look down at it from above, you will see the outer rim of the cup surrounding a smaller circle that represents its bottom. Your ophthalmologist sees a similar image when she or he looks through the pupil of your eye to see the retina. The optic disc, which is packed with nerve fibers extending from retinal ganglion cells is analogous to the outer rim of the teacup. At the center of the optic disc there is a smaller circle in which the color changes to a brighter yellow. This represents the bottom of the teacup in our analogy.

The area representing the bottom of the teacup also is called the disc excavation and is empty space. This is the space where blood vessels enter and exit the eye. Nerve fibers surround it, but none run through it. The disc excavation should not occupy an area greater than 35% of the total optic disc. Excavations above 35% indicate a loss of optic nerve fibers and the onset of glaucoma. If left untreated, there will be a progressive loss of vision that eventually results in blindness. While damage to the optic nerve is irreversible, there are a number of treatments available (medications, eye drops, and surgery) that can slow or stop further damage from occurring.

A number of factors alone or in combination can place a person at risk for primary open-angle glaucoma. However, before I discuss them, I want to emphasize that the term risk factor is a population statistic. This means that when population samples of people diagnosed with primary open-angle glaucoma are compared to population samples that do not have it, certain things emerge as being associated with the disease. This does NOT mean that if you are a member of one or more risks groups, it guarantees that you will develop primary open-angle glaucoma. However, it does mean that you should have more frequent eye examinations that monitor the pressure inside your eye and the condition of your optic nerve for signs that you might be developing glaucoma. We only get one pair of eyes. It is important to take care of them.

Risk factors for primary open-angle glaucoma can be divided into two important groups: major risk factors and associated risk factors.

Major Risk Factors: These have been linked directly to POAG.

• Age: 40 years and older
• History of elevated intraocular pressure
• Family history of glaucoma
• Ethnicity: African American and Hispanic
• Myopia (nearsightedness)

Associated Risk Factors: These are health conditions that are related to glaucoma.

• Diabetes
• Hypertension (high blood pressure)
• Hypercholesterolemia (high cholesterol)
• Heart Disease (especially with associated atherosclerosis)
• Vasospasm (spasm or constriction of blood vessels)

Because primary open-angle glaucoma is a stealth destroyer of eyesight, it is important that everyone have regular eye examinations that specifically check for it. How often you have an examination depends upon your history and risk-factor status. Generally, if you are less than forty years of age and have none of the risk factors above, you should be screened every 3-5 years. If you are forth years of age or older and have none of the other risk factors above, you should be screened every 2-3 years. If you have one or more of the risk factors above, you should be screened more often. Check with your ophthalmologist to determine the frequency in which you should be screened.