How did you get glaucoma?

In this section, we’ll talk about the causes of the two main types of primary glaucoma, open angle and angle closure. The things that are known to cause each of these two disorders are a bit different. But, the damage in both of them happens in the same general way after they get started, and many of the treatments are similar. For any complicated disease like glaucoma, there are no easy answers. It doesn’t come from only one thing or a single abnormality. To speak of one cause of a disease is simply not possible. The eye has many interconnecting parts, and each cell that makes up those parts is almost infinitely complex in the interweaving electrical and chemical pathways that help the cell to survive. And, as we will see, there are equally intricate pathways that tell the nerve cells to kill themselves under conditions where the disease is active (yes, they actually commit suicide).

One of the first questions we are often asked is: “what did I do that caused glaucoma?” To a large degree, the answer is “nothing”—there are almost no life-style choices that are known to be big factors in leading to either form of glaucoma. We will talk about personal behaviors that can help a bit with your glaucoma in the section How should you change your life? For example, we know that cigarette smoking and over-exposure to sunshine can be big contributors to two other eye diseases in older persons—cataract and age-related macular degeneration. But, smoking and sunlight are not at all related to glaucoma. Instead, it seems the most important things that determine glaucoma risk are how the eye was built and how it responds to changes in its environment.

The death of ganglion cells in both open angle and angle closure glaucoma results partly from weaknesses in the tissues around them and partly from defects in the ganglion cells themselves. It also results from under-responses or over-responses in our normal defense mechanisms. These factors can gang up to produce glaucoma by having several things go wrong at the same time. In fact, glaucoma probably happens only when more than one process is malfunctioning. Each of the single things that are part of the disease package is called a contributing risk factor. Together, all the contributing factors that wind up causing the glaucoma make up sufficient cause for it to occur. The mixed bag of contributors to the ‘sufficient cause’ can be different among people—even among members of the same family each of whom has glaucoma.

One major cause of the death of ganglion cells in both major types of primary glaucoma is how the wall of the eye responds to eye pressure. We discussed earlier how it can be worse in glaucoma to have a big, thin-walled eye than a smaller one, since the stress in the eye wall is higher (Figure 5). The fibers of ganglion cells are injured when the eye wall stress affects the optic nerve head and presses on them. Nerve fibers are supported as they go through the nerve head by other kinds of cells and tissues, including the small blood vessels that nourish them at this site. Eye wall stress also affects these other components badly. So, the behavior of the eye wall is translated into vision loss by physical stress at a critical place. Eyes that are bigger have more stress, explaining why very near-sighted people with longer eyes get more open angle glaucoma. Even normal-sized eyes whose walls responds badly to stress can kill fibers – in them it happens at eye pressures that most of us tolerate with no damage. This is one explanation for how people with normal eye pressure can get open angle glaucoma; their eye wall delivers more stress to the nerve fibers at normal pressure than most persons. The critical spot in the nerve head is called the lamina cribrosa, which stands up to eye wall stress better in some of us and worse in others. You would suffer more glaucoma damage if your lamina was weaker. In persons with angle closure glaucoma, the eye is smaller, which ought to be protective from wall stress, but their eye pressures are typically higher than normal and that produces enough stress to damage fibers and their supporting cells.

A lot of other factors are known to contribute to ganglion cell death and therefore to cause glaucoma’s vision loss. We know this by studying human eyes of persons who had glaucoma and donated their eyes after their death to an eye bank. We are often asked by glaucoma patients if it is worthwhile for them to be an eye donor. After all, their eyes are damaged, so surely, they aren’t useful. Most donated eyes are used to transplant the clear cornea to someone who needs a new one. Eyes from people with glaucoma are not useful for that kind of transplant, but donated glaucoma eyes are vitally important to study how glaucoma does its damage. Many studies have also been conducted with animals in whom glaucoma develops spontaneously or in whom a glaucoma-like condition is produced in one eye.

From the human glaucoma eyes and animal research, we have learned that glaucoma causes harmful chemicals to be released by cells at the optic nerve head and in the retina, where the ganglion cell body lives. The blood that nourishes ganglion cells and their fibers can fail to provide enough oxygen or enough energy-producing compounds that keep normal processes going. We will come back to more of these contributing features to ganglion cell death in the section Are there treatments other than lowering eye pressure?

We’ll now consider the contributing risk factors for open angle glaucoma and angle closure glaucoma separately from each other, since in many ways they are different.