Normal-tension glaucoma (NTG) is very similar to primary open-angle glaucoma except that damage to the optic nerve occurs even though eye pressure is not elevated. Learn about the symptoms, diagnosis, and treatment of NTG.
There is still debate among experts as to whether NTG is truly a separate and distinct disease from primary open-angle glaucoma. There have been alternative names for NTG including “low-tension glaucoma” or “normal-pressure glaucoma.” For the purposes of this article, we will continue to use “normal-tension glaucoma” or NTG.
How is Normal-Tension Glaucoma Diagnosed?
Just as in primary open-angle glaucoma, NTG is diagnosed by observing characteristic changes of the optic nerve and/or the visual field that are consistent with glaucoma. One of the distinguishing features, however, is the absence of a measured elevated eye pressure greater than 21 millimeters of mercury (mmHg). Why 21 mmHg? Assessments of the distribution of eye pressures in various populations demonstrate that statistically speaking, a vast majority of patients will have eye pressures less than 21 mmHg. This is partially what makes the diagnosis of NTG difficult because it is possible that although one’s eye pressure may be less than 21 mmHg when the measurement is taken, it may be more elevated at other times of the day. It is also important to note that 21 mmHg is not a “magic” number and that glaucoma can cause damage at lower eye pressures.
However, there are also some features that may help your eye doctor diagnose NTG. Many patients with NTG may have signs or symptoms of impairment in the vascular system (blood vessels), such as migraine headaches, cold hands and feet, or low blood pressure. In addition, NTG patients have a greater tendency to have optic nerve hemorrhages in the small blood vessels that are on or close to the optic nerve. When your eye doctor observes this, it will alert him or her to initiate or advance your treatment, as it can be a sign that the disease is worsening.
Another distinguishing feature of NTG is that sometimes the visual field changes that are seen on formal testing are closer to the central region of vision than is typically seen in primary open-angle glaucoma. When this occurs, most eye doctors will initiate more aggressive treatment. On the other hand, if your disease is mild and there is very little or no visual field change, your eye doctor may elect to “watch and wait,” carefully observing your optic nerve and visual field during follow-up visits before initiating any treatment. NTG typically progresses slowly, so the good news is that there is time to make adjustments to your treatment. However, it is still important to follow-up regularly with your eye doctor even if treatment is not started.
What Medical Tests are Required?
Most of the time, the characteristic appearance of your optic nerve will clue your eye doctor into the correct diagnosis, but sometimes the picture is more confusing. In these more unusual cases, your eye doctor may elect to have brain imaging performed to ensure that there is not a different cause for the appearance of the optic nerves. Otherwise, the tests your eye doctor will obtain are the same as those for primary open-angle glaucoma. In addition to a comprehensive eye exam, you will typically undergo optic nerve imaging to make quantitative measurements of your optic nerve, as well as formal visual field testing to check your peripheral field of vision.
What are the Treatment Options?
That treatment of NTG is similar to that of primary open-angle glaucoma—the only proven therapy is to lower eye pressure. A few decades earlier, it was not clear that lowering eye pressure was a useful treatment for a disease in which the eye pressure was already “normal,” but the well-designed Collaborative Normal-Tension Glaucoma Study proved that lowering eye pressure in patients with NTG slowed the progression of disease. Indeed, this study showed that reducing the eye pressure by 30 percent could prevent visual field loss in half of patients who were shown to be worsening or had a visual field change that was close to affecting the central vision.
Which eye medications are the best for treating NTG? Again, the treatment is similar to primary open-angle glaucoma, so many eye doctors would elect to start a prostaglandin analogue, such as latanoprost. However, because brimonidine (trade name Alphagan) has been shown to have optic nerve protection (neuroprotective) effects in animal models and human studies, many eye doctors will elect to use brimonidine as well. In the case of medication failure, the eye doctor may also consider laser treatment or surgical interventions such as trabeculectomy to achieve very low eye pressures.
Patients often ask if there are other lifestyle changes that can help slow the progression of NTG. Certainly, one issue to be aware of is overtreatment of high blood pressure. Since we suspect that blood flow to the optic nerve may play a larger role in NTG, we want to optimize that blood flow. Of course, managing high blood pressure is important for lowering the risk of heart disease and stroke, so be sure to speak with both your ophthalmologist and your primary care doctor about whether adjustments to your blood pressure medications should be made. Finally, many NTG patients actually report that their blood pressure is “normal” or “low.” In some cases it may helpful to consider low risk interventions to improve blood pressure, such as not cutting out salt and avoiding dehydration.
In summary, NTG glaucoma can be a difficult disease to treat because options are limited to lowering eye pressure. There are significant research efforts to identify treatments that will protect the optic nerve or have an influence on ocular blood flow. Your eye doctor may ask you about your blood pressure, to ensure that you are not being over-treated for any high blood pressure since very low blood pressure can result in inadequate blood flow to the optic nerves. The good news is that many NTG patients have mild or moderate disease that progresses very slowly and may not have an impact on a person’s vision or quality of life.