What are the types of glaucoma?

Take Home Points

There are now 80 million persons in the world with some form of glaucoma based on surveys that examined thousands of randomly selected adults in nearly every continent. Research done at our Glaucoma Center of Excellence predicts that the number with glaucoma will continue to increase due to the fact that the proportion of older people in the world population is increasing and glaucoma is more likely to occur in older people. Glaucoma is the second leading cause of blindness worldwide.

There are several forms of glaucoma, and the two most common forms increase with older age. At 40 years old, less than one in 100 persons have glaucoma, while over age 80, nearly one in ten is affected. These rates are different depending upon the region of the world your ancestors came from, as well as other factors. For example, African-derived people have four times more open angle glaucoma than persons who are European-derived. For angle closure glaucoma, Asians have four times more than either European- or African-derived persons.

Of those with glaucoma, the majority (about two-thirds) have the form called open angle glaucoma. In the United States alone, there are estimated to be 3 million adults with open angle glaucoma, about 500,000 with angle closure glaucoma, and another 5-10 million persons who are potential glaucoma patients (glaucoma suspects) due to various kinds of risk factors. Risk factors are features of their eyes or other attributes that make glaucoma more likely to develop.

Both major types of glaucoma have the word "angle" in their name. The angle is a circular zone on the inside part of the eye where the cornea (the clear front wall of the eye) meets the iris (the colored part of the eye). This angle area runs all around the part of the eye where the white sclera and the colored iris meet. In general, an individual either has an open angle or a narrow to closed angle (Figure 6). This is determined by examining the eye with a special lens under magnification called gonioscopy (see section What tests are needed to diagnose glaucoma?).

Imaging the angle
Figure 6: Imaging the angle. A slice through the front of two eyes taken with a special camera shows an open angle above and a closed angle below (images made with anterior segment optical coherence tomography). The arrow points to where the iris is separate from the cornea in the open angle (upper picture). In the lower picture, the iris is so close to the cornea that it nearly touches the trabecular meshwork (in front of arrowhead).

The aqueous humor is the fluid that nourishes the eye, circulates from back to front, and maintains the eye pressure. Fluid normally moves from where it is produced in the ciliary body, between the lens and the back of the iris, through the round, dark opening in the iris called the pupil and leaves the eye at the trabecular meshwork in the angle. Just at the base of the meshwork, fluid can also move out through a second pathway called the uveoscleral outflow. In most eyes, fluid easily gets to the angle from the back of the eye, and these are open angle eyes. In a few eyes, the front parts of the eye are too crowded together, and the colored iris can get close enough to block the angle at the meshwork. These are angle closure eyes.

By numbers alone, the majority of those with glaucoma have the open angle type. In the eyes with open angle glaucoma, the structures that make aqueous humor and the channels out of which aqueous leaves the eye are normal in appearance when the doctor examines them. The problems that lead to poor aqueous outflow are microscopic—too small to detect in a clinical exam. Open angle glaucoma can happen at any level of eye pressure, and half of those with open angle glaucoma have normal levels of eye pressure. This often surprises those who think of glaucoma as always due to high eye pressure. Glaucoma can occur at normal levels of eye pressure due to weaknesses in the eye and its structures that are enough to kill ganglion cells. Some of these weaknesses have been identified at the microscopic, genetic and biochemical levels. We will deal with these in more detail in the next section How did you get glaucoma?

For the other half of those with open angle glaucoma, the pressure that damages their vision is actually above normal. In these persons, various abnormalities lead the eye pressure to be high, and, if the eye and ganglion cells are susceptible to the effects of such higher pressure, glaucoma ensues. There are different theories for why the pressure is above normal in open angle glaucoma eyes with higher pressure. The best explanation is that there is a problem that prevents fluid from leaving the angle rapidly enough. The most likely reason for this is premature death of the cells in the aqueous outflow area of the angle. Another probable factor is blockage of the outflow channels by very tiny collections of material that shouldn't be there. Because the primary cause of disease in this group of open angle glaucoma patients is poor flow through the meshwork, some new potential treatments for open angle glaucoma try to improve this flow. Since lowering eye pressure helps all persons with glaucoma, this could help both those with the higher and the lower pressure types of open angle glaucoma.

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