Study Proves Lowering IOP Preserves Vision

Before this study was done, there was a difference of opinion on whether intraocular pressure (IOP) was involved in optic nerve damage even when IOP was in the normal range.

The collaborative Normal Tension Glaucoma (NTG) Study completed in 1998 was a ground-breaking project, the first scientifically structured study to compare treatment versus no treatment for any kind of glaucoma.

Involving hundreds of volunteer subjects for over a decade, this international collaborative effort addresses the critical questions of:

1. Whether intraocular pressure (IOP or eye pressure), even at normal levels, plays a role in NTG,
2. Whether it is clinically feasible and worthwhile to lower the eye pressure in persons with NTG, 
3. Whether the benefits of lowering eye pressure outweigh any risks and side effects.

Some of these major study findings were published in the October, 1998 issue of the American Journal of Ophthalmology.

Study Background

Normal-tension glaucoma, also known as normal pressure or low tension glaucoma, is unique among the glaucomas in that damage to the optic nerve can happen without any rise in eye pressure. For many years, ophthalmologists have been uncertain about how to best treat normal tension glaucoma, as there was no solid evidence to show that lowering eye pressure prevents continued visual field loss.

In 1984, the Glaucoma Research Foundation (GRF) sponsored an interdisciplinary seminar on NTG, which resulted in a multi-center collaborative study to find out whether a substantial drop in pressure would halt or slow the progression of the disease. The GRF-sponsored study began in 1986 and involved 24 study centers around the world.

The study randomly divided the volunteer subjects with NTG into two groups. Both groups were examined for any evidence of NTG progression, which included a change in the visual field or a change in the optic nerve appearance. One group, although carefully monitored for any NTG progression, did not have any treatment to reduce eye pressure. The other group, once they started to show NTG progression, had their eye pressure reduced by 30%. The treatments available to reduce the eye pressure included various eye drops (the medications to be used were set at the start of the study), laser and/or filtering surgery.

Results: Lowering Eye Pressure Makes a Difference

The scientific part of the study clearly shows that lowering eye pressure slows progression of normal tension glaucoma. These important results finally establish that even normal eye pressure plays a role in NTG.

Not all volunteer subjects responded in the same way, however. About 2/3 of the untreated group appeared not to show any progression for the first three years of follow up. In addition, about 1/6 of the treated subjects experienced NTG progression even though their eye pressure was reduced. There is clearly still a lot to learn about the causes and progression of normal tension glaucoma. There may be some factors, other than eye pressure, that can damage the optic nerve or make the nerve more susceptible to eye pressure, and these factors remain to be determined.

Data studied to improve patient treatment

The data from the study was also examined from a clinician’s point of view, that is, how to best treat a patient with NTG. Investigators found that although lowering the eye pressure helped to prevent progression of NTG, the surgically treated volunteer subjects had a greater chance of developing cataracts. Increased cataract formation is a possible complication of glaucoma filtering surgery. It is important, therefore, to weigh the benefits of lowering the eye pressure to the risk of developing cataracts (which can usually be successfully treated using modern cataract surgery).

Another consideration in the treatment of normal tension glaucoma is the study’s evidence of how slowly NTG may progress. About 2/3 of volunteers subjects in the untreated group showed little or no progression in the first three years, although the majority of the subject group did eventually show some progression. If someone with NTG is progressing very slowly or has no measurable progression, the benefits of lowering their eye pressure will have to be considered very carefully, since the patient may be exposed to the risks and side effects of the various treatment options.

There is still more to be learned from this study. In particular, investigators plan to look at those patients whose visual field loss progressed, how rapidly and in what ways vision changes, and whether there are any advance clues to determine who is at risk for vision loss. This will help doctors better judge which patients are in greatest need of treatment versus those who might not need immediate treatment.

Thanks to Stephen M. Drance, OC, MD, Emeritus Professor of Ophthalmology at the University of British Columbia, Vancouver, Canada and Douglas R. Anderson, MD, Professor of Ophthalmology, Bascom Palmer Eye Institute, University of Miami School of Medicine, Miami, Florida for contributing to this article. Both were principal investigators of the Collaborative Normal Tension Glaucoma Study.

First posted May 31, 2005; Last reviewed August 19, 2022