Understanding Glaucoma
Glaucoma is a term that encompasses a broad spectrum of diseases from chronic to acute, from quickly debilitating to slowly progressive, from terribly painful to so completely silent that one might become legally blind before receiving a diagnosis. Chronic "silent" glaucoma is one of the most common forms of the disease, affecting millions of people in the United States and costing multi-millions to treat. It is estimated that half of the sufferers of chronic glaucoma have not yet been diagnosed, and the population at risk is growing dramatically because the disease incidence rapidly rises in patients above age forty. Chronic glaucoma is most often associated with elevations in intra-ocular pressure, that is, the pressure of the liquid contents of the eye against the inner walls of the eye. When this pressure is too high, the sensitive blood vessels and neural tissues of the eye are compressed and ultimately damaged. An ophthalmologist is specially trained in measuring the pressure in the eye, a simple, painless and quick procedure called tonometry. Ophthalmologists also treat abnormal pressures using medications, laser- and surgical-procedures. Ophthalmologists understand when a pressure is abnormal relative to the health of the patient in general and the eye in particular, and know what other signs to examine for evidence of glaucoma. Notably, not all higher-than-average pressures mean a patient has glaucoma, and patients with progressive glaucoma can have apparently normal pressures. The elusive character of glaucoma necessitates that everyone reaching the age of forty undergo a thorough eye exam by a trained professional.
What Factors are Involved in Eye Pressure?
Behind the colored part of the eye (the iris) is another circularly shaped muscle called the ciliary body. This muscle not only controls the shape of the lens, but also produces the aqueous humor, a watery substance that nourishes the anterior portion of the eye and keeps its healthy. This fluid circulates from behind the iris, forward through the pupil and around the front of the iris to drain where the iris meets the white of the eye (sclera), a place known as the anterior chamber angle. Encircling this angle is a structure called the trabecular meshwork, microscopically organized like a sieve. The aqueous humor flows into the trabecular meshwork and from there into collector channels that feed into small veins in the eye. These veins ultimately feed into the veins draining the head. The aqueous humor in the eye is under pressure. If it were not, the eye would deflate! The normal pressure in the eye is between 7 and 21 mm of mercury.
Who is at Risk for Primary Open Angle Glaucoma?
Primary open angle glaucoma, or POAG, is a condition in which the intra-ocular pressure is elevated and the eye looks macroscopically normal and causes no complaints to the patient. This is why glaucoma has been termed the "silent blinder". As people get older, their production of aqueous humor decreases, but so does their ability to drain off the aqueous humor. This imbalance leads to a condition of elevated eye pressure. Unfortunately, the aging delicate tissues inside the eye are probably more susceptible to this elevated pressure and so the risk for glaucoma increase dramatically with age. By some estimates, 2 million people in the United States suffer from glaucoma, and the majority of these patients are older individuals. Research has shown that African-Americans stand a fourfold greater risk for developing glaucoma and some studies suggest that women may be somewhat more susceptible than men. Certainly, it is known that women live approximately 8 years longer than men and one of the costs of that added longevity is increased likelihood to develop illnesses associated with old age.
How does Glaucoma Blind the Eye?
Glaucoma causes blindness by damaging the optic nerve, the neural structure that transmits visual information from the eye to the brain. It is not yet known whether the elevated eye pressure crushes the optic nerve directly or whether it crushes the blood vessels supplying the optic nerve. In any case, however, death comes to the delicate fibers (or axons) comprising the optic nerve and as they die, so goes the ability of the retina to transmit visual information. With Primary Open Angle Glaucoma, the cell loss may take years or even decades. With other acute forms of glaucoma, associated with marked elevations in intraocular pressure, the larger blood vessels of the eye can be constricted and the eye can suffer a stroke, after which vision may be lost completely within minutes.
What Findings Determine the Presence of Primary Open Angle Glaucoma?
Primary open angle glaucoma constitutes a triad of findings. First, there is elevated pressure within the eye. Second, there are changes in the optic nerve shape that can be detected by an eye doctor using a number of different methods, from direct visualization to scanning laser microscopes. Finally, there are changes in the patient's vision. These visual changes initially do not affect the patient's ability to see clearly (or visual acuity), and patients with advanced glaucoma can have normal acuity! Instead, these changes are usually associated with losses in the peripheral vision, losses that might only be detectable with measurements called visual field testing. Part of the reason that testing for glaucoma is so difficult is that patients may only get their eyes checked once a year or less, and that checkup might omit either a pressure check or examination of the optic nerve. Because the intraocular pressure varies throughout the day (especially in patients with glaucoma), it would be difficult for most screening regimens to detect many cases of the disease. Glaucoma specialists are ophthalmologists with special training in the diagnosis and management of glaucoma. They not only understand who might be at risk but also understand where to look for evidence of the many types of disease that can cause intermittent, chronic or acute elevations in eye pressure. Notably, they are particularly sensitive in identifying the growing number of patients who have visual field changes and optic nerve damage consistent with glaucoma but have apparently normal intraocular pressures, a condition called Low Tension Glaucoma.
Medical Therapies
At present, the mainstay of treatment for glaucoma is to lower the eye pressure. Although we know that this certainly cannot be the only factor that causes disease, it is the only one medical science currently can manipulate. The medications available for treating glaucoma come from a number of classes of drugs, most of which are administered as eyedrops. The majority of standard medications reduce the production of aqueous humor so that it is more even with rate of drainage. The problem with these drugs is that, in reducing aqueous humor, they are also reducing an important nutritive supply to the anterior segment of the eye. A second class of drugs increases the outflow of aqueous, either by opening up the trabecular meshwork or by expanding auxiliary outflow channels known as the uveo-scleral outflow. Finally, some drugs seem to have both actions, i.e., reducing aqueous production and increasing outflow. In any case, every one of these drugs has a number of side effects. Some of these medications should be avoided in patients with certain pre-existing health problems. Ophthalmologists, as physicians, are trained to understand the possible interactions of eye medications with other systemic diseases and medications. They will evaluate a patient's entire health status as well as their ocular status to determine which therapeutic regimen is safest and most effective.
Laser Therapies
Certain types of glaucoma may respond as well or better to laser therapy than to medications. Like medications, laser therapy for glaucoma attempts to lower pressure, either by reducing the production of aqueous or by easing the escape of aqueous from the eye. Almost all laser procedures for glaucoma can be performed in the outpatient setting with only topical anesthesia. Lasers can move or remove parts of the iris, if it is found that the iris is preventing adequate outflow of aqueous (fluid) by blocking the fluid path. Lasers can destroy parts of the ciliary body without the need to open the eye by transmitting energy directly through the eye. Lasers can widen the pores of the trabecular meshwork and assist outflow in that way. The glaucoma specialist will discuss to each patients specific type of glaucoma and then help the patient decide what therapy would best be suited. Lasers have the advantage of causing very few long-term side effects, and no systemic side effects.
Selective Laser Trabeculoplasty (SLT)
The SLT is a specific type of laser treatment for glaucoma and represents a significant improvement in the treatment of glaucoma. A microscopic laser light is used to help speed the flow through the drainage channels out of the eye. A major advantage of this laser over previous lasers is that the SLT is repeatable when medically indicated while previous treatments could not be repeated multiple times and still have a good response.
