pen-angle glaucomas are those in which there are the classic signs of glaucoma in the absence of a compromised angle. That is — the angle is not narrowed to any significant degree.

Primary open-angle glaucoma

This is the most common glaucoma affecting Caucasians and persons of African ancestry. Its incidence increases with age. POAG has no symptoms — IOP slowly rises and the disease often goes undetected — for which reason it has been termed the “sneak thief of sight”. It is painless and the patient often does not realize that he or she is slowly losing vision until the later stages of the disease. However, by the time the vision is impaired, the damage is irreversible.

In POAG, there is no visible abnormality of the trabecular meshwork. It is believed that something is wrong with the ability of the cells in the trabecular meshwork to carry out their normal function, or there may be fewer cells present, as a natural result of aging. Some believe it is due to a structural defect of the eye's drainage system. Others believe it is caused by an enzymatic problem.

Glaucoma is really about the problems which occur as a result of increased IOP. The average IOP ranges between 14 and 20 millimeters of mercury (mm Hg). A pressure of 22 is considered to be suspicious and possibly abnormal. However, not all patients with elevated IOP develop glaucoma-related eye damage.

Once a sufficient number of nerve cells are destroyed, “blind spots”, or scotomas, begin to form in the field of vision. These scotomas usually develop first in the peripheral field. Later, the central vision, which we experience as “seeing”, is affected. Once visual loss occurs, it is irreversible because once the nerve cells are dead, nothing can restore them at the present time.

POAG is a chronic disease which is presently incurable. However, it can be slowed or arrested by treatment. Since there are no symptoms, many patients find it difficult to understand why lifelong treatment with expensive drugs is necessary, especially when these drugs are often bothersome to take and have a variety of side effects.

When POAG occurs under the age of 35 years, it is by convention called juvenile primary open-angle glaucoma (JOAG). This is often familial. The gene is presently being actively sought. At this time, it has been localized in several large families to the short arm of chromosome 1.

Pigment Dispersion Syndrome
(Pigmentary Glaucoma — see Glaucoma in Young People)

Exfoliation Syndrome

This is the most common identifiable cause of glaucoma worldwide. Like pigmentary glaucoma, it has been often under diagnosed. It is found everywhere in the world, but is most common among people of European descent. In about 10% of the population over age 50, a whitish material, which looks on slit-lamp examination somewhat like tiny flakes of dandruff, builds up on the lens of the eye. This exfoliation material is rubbed off the lens by movement of the iris and at the same time, pigment is rubbed off the iris. Both pigment and exfoliation material clog the trabecular meshwork, leading to IOP elevation, sometimes to very high levels.

Exfoliation syndrome can lead to both open-angle glaucoma and angle-closure glaucoma, often producing both kinds of glaucoma in the same individual. Not all persons with exfoliation syndrome develop glaucoma. However, if you have exfoliation syndrome, your chances of developing glaucoma are about six times as high as if you don't. It often appears in one eye long before the other, for unknown reasons. If you have glaucoma in one eye only, this is the most likely cause. It can be detected before the glaucoma develops, so that you can be more carefully observed and minimize your chances of vision loss.

Low-Tension Glaucoma

Is it possible to have been diagnosed with glaucoma and have “normal” intraocular pressure (IOP)? In a nutshell, the answer is yes. Interestingly, this is not a rare disorder as it may be identified in approximately one-third of all individuals diagnosed with glaucoma. In fact, as many as 50% of the patients with “high” tension glaucoma, or glaucoma associated with elevated intraocular pressure, will have presented without increased IOP at the time of initial diagnosis.

Low-tension glaucoma or, as ophthalmologists now call it, normal-tension glaucoma, has been classically defined as open-angle glaucoma developing in a person in whom the IOP never goes above 22 mm Hg. For a long time, this was thought to be a rare disease. It is now being realized that the number of persons with low-tension glaucoma has been vastly underestimated. In Japan, for instance, twice as many people have low-tension glaucoma as high-tension glaucoma. It is this disease (or really, group of diseases waiting to be elucidated) in which risk factors other than IOP account for damage.

While the underlying mechanism of damage to the optic nerve in normal pressure glaucoma is currently unknown, doctors currently believe that several processes may be involved. In some individuals, spasm of the arterial blood supply to the optic nerve is thought to contribute to the loss of visual field. These patients typically complain of very cold fingers and toes when outdoors and may in fact, notice a change in hand color as well. Additionally, migraine headaches are common in this patient population. While controversial, certain medications known as calcium channel blockers are being investigated in order to determine their risks and benefits in these patients.

Another hypothesis is that the intraocular pressure, while within the “normal” range for 95% of the general population, is too high for these patients’ individual eyes. Medical or surgical reduction in IOP remains the mainstay of therapy. This, however, presents a challenge to both physician and patient alike since the pressure is often quite low even before intervention.

Interestingly, in addition to having low intraocular pressure, individuals with this disorder typically exhibit low systemic blood pressure. This may be seen both in healthy individuals as well as in patients who experience a side-effect of antihypertensive therapy. In fact, 24-hour ambulatory blood pressure studies have revealed that patients with normal tension glaucoma may develop significant “dips” in systolic blood pressure during sleep that may interfere with the blood supply to the optic nerve. Since these “dips” in blood pressure occur more commonly in patients taking antihypertensive medication, it is important to inform your ophthalmologist of any change in medication or new medical problems that have developed since the last examination.

As with other forms of glaucoma, normal tension glaucoma typically produces no visual symptoms until a moderate amount of optic nerve damage has occurred. Some patients may notice a sense of overall darkening in their visual environment. Others may become aware of “blind spots” in their vision.

The diagnosis of normal tension glaucoma is often a more challenging task than with other forms of glaucoma since the intraocular is normal. Indeed, a number of non-glaucomatous processes can produce similar changes in the optic nerve appearance, referred to as “cupping”. Optic nerve stokes, commonly associated with a systemic vasculitis known as giant cell arteritis, may cause visual field disturbances and changes in the morphology of the optic nerve that resemble normal tension glaucoma. In addition, mass lesions inside the brain, previous episodes of optic neuritis (inflammation of the optic nerve), hereditary diseases of the optic nerve, toxic effects of heavy tobacco and alcohol abuse, and trauma to the optic nerve may produce similar changes in the optic nerve that resemble normal tension glaucoma. With a careful history and complete ocular examination, arriving at the correct diagnosis is made less difficult.

Much remains to be learned about the cause, diagnosis, and treatment for this not uncommon variety of glaucoma. With modern advances in optic nerve imaging technology, precise instrumentation using ultrasound and laser to measure ocular blood flow, and new topical medications shortly on the horizon, we are optimistic that our increased understanding of this disorder will provide patients with more effective therapy and better overall visual prognoses.

In summary, the terms high-tension and low-tension glaucoma are misleading. Glaucomatous damage can be thought of as consisting of two basic forms — mechanical and nonmechanical (vascular and other). The higher the pressure, the greater the component of mechanical damage. The lower the pressure at which damage occurs or progresses, the greater the nonmechanical component. In the coming decade, these factors will hopefully become much more understood. In the meantime, there is no magic number cutting off one disease from another. It is merely a statistician's reference point.

© Leo D. Bores, MD, 1996 — 2002