NORMAL TENSION GLAUCOMAElevated intraocular pressure is a characteristic common to most types of glaucoma. But normal tension glaucoma strikes without any increase in eye pressure. As I have noted elsewhere on glaucoma-eye-info.com, a common symptom of glaucoma is elevated intraocular pressure. In fact, it is the primary factor identified for early-stage development of the disease (known as pre-glaucoma). But there also is a type of glaucoma called normal tension glaucoma in which intraocular pressure is not the main feature. Given the focus on intraocular pressure, people often make the mistake of assuming that if an ophthalmologist informs them that it is high, this automatically means that they either already have glaucoma or are destined to develop the disease. This is not true! Your ophthalmologist can explore treatment options with you that will effectively lower your intraocular pressure and prevent the irreversible damage to your optic nerve that is characteristic of glaucoma. Doing nothing or ignoring the advice of your ophthalmologist, which would leave your intraocular pressure uncontrolled, are the factors that can potentially make glaucoma your destiny. For this reason, it is important that you have your intraocular pressure checked regularly and follow the treatment advice of your ophthalmologist.
The element that is common to all types of glaucoma is damage to the optic nerve. The technical term for this is optic nerve atrophy.
 What does atrophy mean?Atrophy is a partial or complete wasting away of a tissue or body part. Have you ever noticed that the legs of people who are wheelchair bound tend to be thin compared to the rest of their bodies? The reason for this is because lack of use has caused a reduction in muscle mass. The leg muscle has atrophied.
In glaucoma, the problem is not a loss of muscle mass, but rather a loss of optic nerve function due to ongoing damage to nerve fibers. Think of it like this. Imagine that the cord for the Internet connection in a business office runs across a hallway. A rug is draped over the cord to make sure that no one trips over it. Despite some protection afforded by the rug, constant foot traffic causes small damages the to cord. Over time, these damages accumulate. The cord no longer is as efficient in transmitting electrical impulses from and to computers in the system. Eventually, a point is reached where the cord ceases to function and access to the Internet no longer is possible. In the above example, the cord can be replaced. However, in the human visual system, the optic nerve cannot be replaced. For this reason, it is crucial that we try to do everything that we can to prevent damage to it.
Also known as low tension or normal pressure glaucoma, normal tension glaucoma (NTG) is a form of the disease in which damage to the optic nerve occurs even though intraocular pressure is in the normal range (10-20 mmHg). What causes normal tension glaucoma?Causes of NTG remain unknown. What we do know is that in some people the optic nerve is susceptible to damage from even a normal amount of eye pressure. What remains elusive are those factors that are responsible for this susceptibility.
Researchers have observed that patients with normal tension glaucoma are more sensitive to slight elevations and fluctuations in eye pressure. Although everyone experiences changes in intraocular pressure throughout the day, these fluctuations normally are on average about 4 mmHg and never higher than 6 mmHg. For example, a person's intraocular pressure may be at 16 mmHg during the morning and 13 mmHg in the afternoon. While these minor fluctuations are not problematic in the normal eye, they apparently are sufficient to cause optic nerve damage in patients with normal tension glaucoma. Blood supply to the optic nerve also is another factor involved in normal tension glaucoma. When intraocular pressure increases, there are changes to the level of circulation in the blood vessels of the optic nerve. This adjustment helps to maintain irrigation of the optic nerve at optimal capacity. However, in normal tension glaucoma, this adjustment does not occur efficiently and the optic nerve is not irrigated properly. In any tissue in the body, lack of adequate blood supply can lead to atrophy.
If intraocular pressure is not high, how do you diagnose normal tension glaucoma?Good question! I examine the optic nerve for signs of damage using two diagnostic techniques: direct observation of the optic nerve and visual field evaluation. DIRECT OBSERVATION OF THE OPTIC NERVE For this procedure, I use an instrument called an ophthalmoscope, which I hold close to your eye. In a dimly lit room, light from the ophthalmoscope enables me to look through the pupil of your eye and examine the shape and color of the optic nerve.
A few years ago, I conducted a research study that evaluated the amount of healthy optic nerve fibers in patients with NTG compared to other types of glaucoma. I was able to confirm observations made by other scientists that patients with NTG tend to have greater reductions in the density of optic nerve fibers in the optic disc. I determined this by examining the something called the cup-to-disc ratio (C/D ratio).
When I use an ophthalmoscope to view your optic nerve, I also am looking for other specific differences in structures of the optic disc. In particular, I am looking for the presence of small hemorrhages (bleeding) on the disc. Although bleeding occasionally can be observed in all types of glaucoma, it occurs more frequently in normal tension glaucoma. For this reason, many scientists believe that vascular disease is a factor that should be considered as a cause of this type of glaucoma.
A second procedure that I use to assess normal tension glaucoma is called a visual field evaluation.
In other cases, such as scotomas, changes are noticeable. A scotoma is a localized defect in the visual field that is bordered by an area of normal vision. They are classified as relative and absolute.
Scotomas in patients with normal tension glaucoma may be deeper. This means that the patient cannot see anything but the brightest light in the area of the scotoma.
Before we discuss risk factors, let's first review what they mean. Risk factors are determined by examining data from large numbers of people. This allows us to look at the disease from a population perspective. When we do this, we are able to identify that certain groups or individual characteristics tend to be associated with the disease more often than others. These are classified as potential risk factors and then additional research is conducted to confirm them and to better understand them. Nevertheless, this does not mean that the risk factors cause the disease. Their association with it could be coincidental. It therefore is important that we place risk factors in their proper perspective. Being a member of one or more risk groups for glaucoma does not guarantee that you will develop the disease. However, it does mean that you should be more diligent about having regular eye examinations and following the advice of your ophthalmologist. Primary risk factors for normal tension glaucoma include:
Some studies have found that patients with NTG have more frequent headaches. This could be due to the vascular (blood circulation) problems described earlier. Elevated cholesterol levels also have been observed in patients with NTG. In North America, NTG is more prevalent in women than it is in men.
Because very little is known about the mechanisms by which normal eye pressure can damage some eyes, most ophthalmologists treat normal tension glaucoma by reducing the eye pressure as low as possible through the use medications, laser treatments and filtering surgery. If you've been diagnosed with NTG, I recommend that you do the following:
Remember, even though science is still trying to understand the causes of normal tension glaucoma, we are able to treat it. Make sure that you visit you eye-care professional regularly.
Return from Normal Tension Glaucoma to Types of Glaucoma
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I am particularly interested in examining a structure called the optic disc (also known as the optic nerve head). This is located in the back of the eye and represents the point at which the optic nerve fibers (there are about 1,200,000 of them) converge to transmit visual signals from the eye to the brain. 
The cup is a bright yellow area at the center of the optic disc. It is region of empty space; nerve fibers surround it, but none run through it. In the normal eye, the cup is restricted to a very small space. But in untreated glaucoma the cup occupies progressively more space in the optic disc as more and more optic nerve fibers are lost to damage. In NTG, the cup tends to be very large.
