Glaucoma is a group of diseases that can lead to damage to the eye’s optic nerve and result in blindness. Open-angle glaucoma, the most common form of glaucoma, is one of the leading causes of blindness in the United States and the number one cause of blindness among African Americans. Glaucoma usually has no early symptoms, and by the time people experience problems with their vision, they usually have lost a significant amount of their sight.
How Open-Angle Glaucoma Develops
Increased pressure inside the eye is an important cause of open-angle glaucoma. In the front of the eye is a space called the anterior chamber. A clear fluid flows continuously in and out of this space and nourishes nearby tissues. The fluid leaves the anterior chamber at the angle where the cornea and iris meet. When the fluid reaches the angle, it flows through a spongy meshwork, like a drain, and leaves the eye.
Open-angle glaucoma gets its name because the angle that allows fluid to drain out of the anterior chamber is open. However, for unknown reasons, the fluid passes too slowly through the meshwork drain. As the fluid builds up, the pressure inside the eye rises. Elevated eye pressure can damage the optic nerve; a healthy optic nerve is necessary for good vision. When the optic nerve is damaged from increased pressure, glaucoma—and vision loss—are the result.
At first, open-angle glaucoma has no symptoms. People are not aware that glaucoma is affecting their vision, and there is no pain. When glaucoma remains untreated, people may notice that although they see things clearly in front of them, they miss objects to the side and out of the corner of their eye. Without treatment, people with glaucoma may find that they have no side vision. Over time, the remaining vision may decrease until there is no vision left.
The Ocular Hypertension Treatment Study
Prior to the Ocular Hypertension Treatment Study, there was no clear evidence as to whether reducing elevated pressure in the eye would delay or prevent the onset of glaucoma. Elevated pressure in the eye, a common condition affecting three to six million people in the United States, is thought to be the leading risk factor for development of open-angle glaucoma. For the purposes of this study, ocular hypertension—which can be diagnosed by an eye care professional—was defined as pressure of 24 mm Hg or greater in at least one eye.
Despite the lack of convincing evidence, approximately 1.5 million people in the U.S. with elevated eye pressure and no glaucoma damage are being treated with medications that lower this pressure. There was a need for a well-controlled clinical trial to determine whether medical reduction of elevated intraocular pressure could delay or prevent the onset of glaucoma. The resulting data would enable clinicians and patients to make rational choices and health care planners ensure that medical resources were being allocated in a safe and cost-effective manner.
The primary goal of the Ocular Hypertension Treatment Study was to determine whether reducing elevated eye pressure delayed or prevented the onset of glaucoma and subsequent vision loss in people at risk of developing the disease. Patient recruitment took place between February 28, 1994 and October 31, 1996. A total of 1636 individuals were selected to participate; 817 were assigned to receive topical ocular medication (eye drops), and 819 were assigned to observation. All of the medications used in the study were commercially available.
Scientists found that eye drops used to reduce pressure inside the eye were effective in delaying the onset of primary open-angle glaucoma. After five years, researchers found that treatment reduced the onset of primary open angle glaucoma by more than 50 percent.
The Ocular Hypertension Treatment Study is the first large-scale study to demonstrate that lowering eye pressure—a risk factor for the development of primary open-angle glaucoma—can safely and effectively delay and possibly prevent the disease.
High Risk Factors for Glaucoma
In a companion paper, also published in the June 2002 issue of Archives of Ophthalmology, the authors report finding several factors predictive of those who developed primary open angle glaucoma. These included personal risk factors, such as older age and African descent, as well as ocular risk factors, such as higher eye pressure, certain characteristics in the anatomy of the optic nerve, and thinness of the cornea. The authors note that “corneal thickness provides new information about the risk of developing primary open-angle glaucoma and we recommend its measurement in the clinical evaluation of patients with ocular hypertension.”
The authors also point out that the predictive factors for glaucoma, as identified in the Ocular Hypertension Treatment Study, “are most likely to be helpful for assessing the risk of patients who resemble the study participants, i.e. ocular hypertensive individuals with eye pressure between 24 and 32 mm Hg and no evidence of glaucoma damage.” Only about two percent of Americans have these high levels of pressure in at least one eye.
The authors conclude by saying that the study results “suggest that a clinician caring for an ocular hypertensive patient can assess that individual’s risk” for developing glaucoma by considering age, race, eye pressure, optic nerve anatomy, and central corneal thickness. By considering these factors, “the clinician can identify patients who are at moderate to high risk for developing [glaucoma] and who are more likely to benefit from early medical treatment,” the authors state.