glaucoma

Glaucoma has been called the "sneak thief of sight" because it is often without any symptoms until late in the course when damage has been severe. The glaucomas (there are really a number of types of glaucoma) represent a group of diseases of the eye, all of which have in common a damaging effect on the fibers of the optic nerve. As you will see below, this group of diseases which we call glaucoma represents a very  complex problem. Glaucoma is not simple, but it is treatable.

There may well be a number of causes or factors underlying the development of glaucoma in any given person. For many years, glaucoma was defined as a disease of elevated fluid pressure inside the eye. This is not the same as blood pressure. While many persons do have glaucoma based on elevation of this pressure, there are many who do not have it. Thus, we try to avoid defining the disease as one of elevated pressure. Rather, the definition is a descriptive one. This means that the disease is defined by the presence of these damaging changes that are observed in the optic nerve and NOT by what we think is causing that damage. Thus, the pressure in the eye is a risk factor in the disease rather than the disease itself. The higher the pressure, the greater the risk. However, elevated intraocular pressure is NOT necessary to have glaucoma. One may have severe glaucoma with relatively low intraocular pressure.  The reason the pressure is such an important risk factor, however, is that all glaucoma is TREATED by reducing the pressure (since we currently have no other way to treat it). This applies even to the so-called low pressure variety.

Let's take a look at  what it is that creates and controls the pressure inside the eye:

Inside every eye is a special gland known as the ciliary body which produces this watery fluid called the aqueous. This fluid fills the front compartment of the eye, a small space called the anterior chamber. (See the photo above, keeping in mind that the ciliary body is doughnut shaped, circling around the interior of the eye.) This aqueous fluid is constantly secreted by the ciliary body and released into the chamber while, at the same time, older fluid drains out through a special channel known as the trabecular meshwork. This is a like a strainer, and the aqueous "filters" through it, ultimately leading into a small circular duct known as the Canal of Schlemm. The trabecular meshwork and Canal of Schlemm together constitute the drainage channel of the eye. From there the fluid leaves the eye by being absorbed into the surrounding blood vessels.

Thus, the aqueous is in a constant state of turnover... new fluid is pumped into the eye while old fluid drains out.  At any given moment, the pressure inside the eye depends upon exactly how much fluid is present.  If more aqueous is secreted into the eye than drains out, the pressure will be elevated.  If the fluid drains out faster than new fluid is formed, then the pressure will be lower.

In the common type of glaucoma, known as open angle glaucoma, the drainage channel is widely accessible; that is, the aqueous can easily get to it. Notice the upper part of the photo above where it shows the angle as it appears open. Although the angle is open and the drainage channel is accessible, the aqueous somehow doesn't flow through it to get out of the eye.

Another type of glaucoma, known as narrow angle glaucoma, is associated with a restricted access to the drain. In the lower part of the photo it shows what a narrow angle looks like. See how the iris is much closer to the cornea, thereby making the depth of that recess much less. In narrow angle glaucoma, the drain itself would work just fine if the aqueous fluid could only get to it. If the drain is not accessible to the aqueous, even though the drain works well, the aqueous will not be able to get out of the eye. The pressure will rise, and this may damage to the sensitive nerve fiber layer of the retina. With a fully closed drain, then end-point stage known as angle closure glaucoma occurs.

Note that our photo above is strictly for illustration. In the real eye, the angle recess runs circularly all the way around the eye. The photo shows the two configurations, narrow and open, in the same eyeball only to illustrate what each looks like without needing two separate photos.

If enough damage occurs to the sensitive nerve fiber layer of the retina, it can be observed by the ophthalmologist while examining the optic nerve (see the photos below). The optic nerve is something like a cable consisting of millions of these individual nerve fibers coming from the retina. Loss of these fibers shows up as "cupping" or an erosion of the optic nerve surface because there are fewer fibers. With laser scanning technology, an image of the actual nerve fiber layer may disclose early loss before that loss shows up at the optic nerve during the eye examination.

It is ultimately the loss of the nerve fibers which causes loss of peripheral, or side, vision first and, later, central vision. This is the basis of glaucoma damage.

Using the traditional definition of glaucoma (high pressure in the eye), the ophthalmologist will measure the pressure level with an instrument called a tonometer. This painless measurement provides an actual numerical readout of the pressure. Most of the time, but not always, the pressure in a "normal" eye will vary from the low teens to about 21 or 22. Of absolute critical importance, however, is the health of the optic nerve and nerve fiber layer of the retina. By tradition, we usually consider a pressure of 21 or 22 as the upper limit of normal. However, about one third of all patients who have existing damage from glaucoma have never had pressures above 21.  We often examine an eye and observe damage to the nerve fiber layer and the optic nerve but find relatively low pressure. No one knows why these individuals have such damage. It may exist because of poor circulation to the optic nerve or because of some anatomic variation in the fibers of the optic nerve causing compression. Maybe, for them, a pressure of 17, for example, is abnormally high. In recent years, other eye features have been discovered to have a bearing on glaucoma.  

Just as some people have glaucoma with low pressure, we also know that many individuals run pressures that are well above 21 or 22, yet they have no disease. Our task, as ophthalmologists is to identify those who have glaucoma from those who do not and also to identify those persons who run pressures that might be potentially problematic if allowed to go untreated.  This is the only way we can treat those who need treatment while avoiding unnecessary treatment for those who do not have glaucoma. Clearly, then, we need more information than simple pressure measurements.  Following a recently published nationwide study of high pressure in the eye, we now have added a new measurement in the examination of many of our glaucoma patients.  It is called corneal pachymetry, the measurement of the thickness of the cornea.  At the time that the traditional pressure measuring instrument, known as the Goldman applanation tonometer, was developed, it was assumed that all corneas were about equally thick.  The tonometer works by flattening the cornea a measured amount and calibrating the internal eye pressure based on how much external pressure is applied to flatten it.  The calibration itself is based on a uniform corneal thickness concept.  (The assumption that all corneas were equally thick was made because there were no instruments available at that time to measure the thickness.)   However, now that we can measure this thickness with high accuracy, we have found that corneas may vary substantially from one person to another. Thus, the very thick cornea will require more effort to flatten it while the thin cornea will require less external pressure. As a result, the pressure reading on the tonometer may be higher than the real eye pressure if the cornea is thick, and the reading will be lower than the real pressure if the cornea is thin.  Now we may treat some patients whose pressure readings seem to be low but who have thin corneas; their real eye pressure may, in fact, be higher.

In evaluating your eyes for glaucoma, we must rely on clinical observation and expertise in knowing what a healthy optic nerve and nerve fiber layer look like. We need to know if there is damage to the peripheral vision. In a sense, we do not really test for glaucoma. We evaluate for it. Simple screening of the pressure without anything else is insufficient and may be very misleading. Ophthalmologists, physicians who have specialized in this training, are qualified to make this type of determination.

 The above diagrams show two optic nerves in cross section. The left one is normal.
The right one shows "cupping" from glaucoma.

Glaucoma does its damage to the nerve fiber layer of the retina. Although the pressure inside the eye is against every part of the eye (see the dark arrows in the photograph above...they represent the pressure pushing against the retina and optic nerve), it is the pressure on the nerve fibers that go into the optic nerve that are sensitive and are damaged. The goal of treatment is preservation of vision. At the present time, and for the indefinite future, the only way we know how to treat glaucoma or prevent vision loss is by reducing the pressure of the aqueous inside the eye. Our discussion now will center on diagnosis and treatment. Some special glaucoma situations will also be presented.

We begin our glaucoma evaluation by assessing the health of the various structures in your eye. Of course, we need to measure the pressure. As stated before, this is done with an instrument called a tonometer. The photo below shows an eye having its pressure checked with an "applanation" tonometer such as used in our office. This is the most accurate method of measuring the pressure.  A single eye drop is placed into the eye, and this allows us to measure the pressure with a high level of accuracy. There is no pain or discomfort, and it takes only a few seconds to perform. This instrument does NOT blow air against the eye and is basically without any feeling at all. There are still other instruments which can determine the pressure in the eye, one of which can even be used by the patient at home.

         
The Goldman applanation tonometer in use

A thorough microscopic examination of the eye including a detailed assessment of the health of the optic nerve is also performed. From there, we will continue with further diagnostic testing as indicated on an individualized basis. Corneal pachymetry (thickness measurement as described above) may be used in many circumstances to refine the accuracy of the pressure measurement.  We employ some of the most advanced computerized technology to aid in assessing the health of your optic nerves and nerve fiber layer. Remember, it is damage to these ocular structures ultimately damages your vision. The early changes may have no effect that you, yourself, can recognize initially. However, they may be measurable to us. Peripheral vision testing, will indicate the level of retinal sensitivity of the side vision, that area that glaucoma affects initially. Our visual field analyzer is the most sophisticated instruments available for this purpose. The addition of our laser scanning nerve fiber layer analyzer may permit us to observe very early changes long before they can be seen by direct observation or before they cause loss of side vision. This gives us a much better ability to detect early changes and treat the eye before severe damage occurs.

GLAUCOMA TREATMENT:

As stated, glaucoma is treated by reducing the pressure inside the eye, even if the pressure is not high to begin with. In the future, glaucoma may well be treated by changing one's genetic makeup in some fashion to make the eye resistant to whatever it is that really causes the problem in the first place.

In most cases of the common type of glaucoma (primary open angle), we reduce the pressure by the use of eye drops. There are many types, and the choice of which one, or ones, to use is different for different individuals.

In certain situations, laser treatment may be an option. While no one really knows how the laser works, it does have the capability of pressure reduction in many situations, particularly if eye drops are not working adequately well. In the type of glaucoma referred to as narrow angle glaucoma, laser treatment might even cure, not just control, the problem by creating a pathway for the aqueous fluid to leave the eye. Our practice has these lasers right in the office, and we have many years experience in using them effectively. Laser treatment for glaucoma usually takes only a short time, has a very high level of effectiveness, and it has a very low level of risk.

Finally, there are more difficult glaucoma situations that sometimes require actual surgery. This can be done as an outpatient, under local anesthesia, when indicated. Generally, surgery is not performed unless other means have not worked well. Every situation is different, and we will always discuss all of the options available to you so that, in the event surgery is needed, you will be fully informed. Please do not hesitate to ask us.

Glaucoma occasionally represents a complication of some other disease process. Here, we must first diagnose and treat the underlying condition, if known. In this sense, the treatment of the primary problem itself can reduce the pressure simply by eliminating the source of the elevated pressure. These so-called secondary glaucomas are not the typical type of glaucoma, and each must be treated individually.

A less common type of glaucoma called narrow angle glaucoma was noted above. It's end point of acute angle closure is a serious eye emergency. Click here to learn more about it.

We want you to realize that glaucoma can be treated, and vision loss can be prevented in most cases. Early diagnosis and treatment is important. The above description, we know, is complex. Glaucoma is complex. Only ophthalmologists have the experience and training to manage this disease. Have your eyes examined regularly to be checked for glaucoma. Don't wait until your vision has been impaired to detect this treatable condition.

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