By George L. Spaeth

All physicians, including glaucoma specialists, seek to discover the fundamental causes of an individual's disease. For it is only when those causes have been identified that the most appropriate treatment can begin.

Persons with glaucoma have suffered some kind of tissue damage, frequently to the optic nerve. But knowing that the cause of glaucoma is optic nerve damage does not provide the physician nearly enough information. To initiate effective therapy requires knowing the reason for the tissue damage, so that it can be prevented or lessened.

Optic Nerve Damage

Let's consider optic nerve damage in a little more detail. The real questions that must be answered are, first, the general ones posed to researchers: What is the exact nature of the optic nerve damage characteristic of glaucoma? and What causes that damage? Furthermore, the glaucoma specialist must try to answer questions about individual patients: What is the exact nature of the optic nerve damage in this individual? and What specifically has caused that damage?

Of course, knowing the answers to the first questions is crucial for answering the second and ultimately most important ones. This means that research is vital for discovering the best treatment for individuals with glaucoma.

The story of glaucoma research began about 150 years ago, when Helmholtz invented an instrument that allowed looking inside the eye. For the first time, investigators could see the inside of eyes diagnosed with the newly-defined condition known as "glaucoma." What they saw was that the nerve that leads from the back of the eye into the brain, the optic nerve, was obviously damaged. Specifically, the surface of the nerve, the so-called "optic disc," had a bowl-like depression.

Two Theories of Optic Nerve Damage

Some suggested that the observed depression was related to the pressure of the aqueous humor, the fluid in the eye that keeps it firm so that it can serve its purpose as an optical instrument. It seemed reasonable that if the pressure of the aqueous on the inside of the eye was too high, it might directly kill optic nerve cells, leaving the bowl-like depression now often referred to as "cupping."

This theory continues to be the most popular. And most treatments for glaucoma aim in one way or another at lowering the pressure inside the eye.

Even long ago, however, other investigators proposed that it was not the direct pressure of the aqueous humor on the optic nerve that damaged it. Rather, they argued, this pressure is a problem mainly because it squeezes the blood vessels and thereby reduces the flow of blood to the optic nerve. With insufficient blood, the cells die, and when the cells die, they disappear, leaving the characteristic bowl-like depression or cupping.

The Role of Blood Flow in Optic Nerve Damage

Now, for the first time, technology is helping researchers, including those at the Glaucoma Service Foundation, understand the major mechanisms by which the optic nerve becomes damaged in patients with glaucoma. This is unquestionably one of the most exciting fields of investigation in all of ophthalmology.

Over the past 20 years, a variety of investigators have performed extensive studies, some of which have indicated that spasm of the blood vessels that provide blood to the optic nerve may be responsible for glaucoma in some patients. Some individuals, such as those with migraine, are predisposed to this type of spasm. It has been known for quite a few years that patients with migraine are predisposed to "low-tension" glaucoma, that is, glaucoma that has occurred even though the pressure inside the eye is normal or even lower than normal.

Others have found other specific indications of abnormality of blood flow in some patients with glaucoma. For example, it is becoming apparent that blood pressure is an important factor in determining whether or not optic nerve damage will progress in a glaucomatous eye.

More recently, Alon Harris, a physiologist at the University of Indiana specializing in blood-flow studies, working with one of our ex-Fellows, Louis Cantor, and with George Spaeth and Bob Sergott, who is in charge of the vascular laboratory at Wills Eye Hospital, has used a new technology that permits visualization of the blood vessels of the eye. Based on this technology, Dr. Harris has presented a number of studies describing changes in blood flow in the optic nerve in glaucoma and, for the first time, reported different patterns of blood-flow abnormality in different types of glaucoma.

Foundation researchers are actively studying this latter subject. At the annual meeting of the Association for Research in Vision and Ophthalmology, the most important annual meeting for presenting eye research, they showed that lowering the pressure inside the eye by surgery improved the blood flow in certain patients with glaucoma. In another paper, they related the amount of damage to the optic nerve in patients with glaucoma to the amount of abnormality of blood flow.

The Future

While we now know a fair amount about the relationship between blood flow in the eye and glaucoma, investigators have a long way to go. We and others are in an exciting race to come up with a comprehensive understanding of the various mechanisms by which the optic nerve becomes damaged in glaucoma.

Once these are known, physicians will be in a far better position to help each individual patient. As this new knowledge unfolds, we will see very exciting changes resulting in significant improvements in patient care.