By George L. Spaeth
Both patients and doctors tend to underestimate the value of diagnosis. Foremost in their minds is getting and providing effective treatment. In fact, however, the key to effective treatment of glaucoma or any disease lies in an accurate diagnosis.
“Diagnosis” sounds simple and, indeed, the meaning of the word is simple. Diagnosis means putting a descriptive label on a problem so that appropriate measures can be taken to solve it.
Unfortunately, putting the label “glaucoma” on a set of signs (things a doctor looks for) and symptoms (things a patient notices) is difficult because our understanding of what causes glaucoma is still relatively meager. The current definition of the disease is the presence of ocular tissue damage apparently related at least partially to the pressure of the fluid in the eye (intraocular pressure).
When the definition of glaucoma was “a condition in which the intraocular pressure is above 21 mm Hg (millimeters of mercury, units in which pressure is measured),” the diagnosis of glaucoma was easy. One simply measured the pressure and that determined whether or not glaucoma was present. But that method of defining glaucoma was wrong. It was seriously wrong!
In order to get an idea of the likelihood of whether a particular sign, for example, “intraocular pressure above 21 mm Hg” or symptom, for example, “decreased ability to see objects that are not straight ahead” truly supports a diagnosis of glaucoma, that is, the presence of ocular tissue damage apparently related at least partially to intraocular pressure, two things about the test must be determined. First, how sensitive is it? That is, how likely is it that the test will “find” among those tested every person who actually has glaucoma. Second, how specific is the test? That is, how likely is it that everyone whom the test suggests has glaucoma, actually has glaucoma?
Since using the sign “intraocular pressure over 21 mm Hg” to determine the presence of glaucoma misses one third of those who actually have glaucoma, the test is not very sensitive. Its specificity is even poorer, since over 90% of people with this level of pressure do not have glaucoma.
If the level of intraocular pressure is neither a very sensitive nor specific sign of glaucoma, how can an accurate diagnosis be made? The short, but important, answer is: “With difficulty.” There is no pregnancy test for glaucoma. There is no dipstick indicator. There is no one test that is either encouragingly sensitive or specific. There is no easy answer.
With this point in mind let us consider (1) the three major types of signs the glaucoma specialist considers in making a diagnosis: intraocular pressure, visual field, and the condition of the optic nerve; and (2) the variety of possible symptoms, such as decreased ability to see objects that are not straight ahead, decreased ability to perceive motion and to recognize color, etc.
As just pointed out, intraocular pressure alone is virtually useless as a sign indicating a certain individual has glaucoma. Still, it is one essential factor, since glaucoma is a condition at least partially related to intraocular pressure.
Another such critical but not definitive factor is the patient’s visual field. Reduced visual field is a less sensitive but more specific indicator of glaucoma than intraocular pressure above 21 mm Hg. However, if doctors relied only on reduced visual field to detect glaucoma, they would miss almost everybody who has early glaucoma. But when there is a visual field defect, that defect is a sign that almost always indicates that something is wrong, though that something is not necessarily glaucoma. The problem could be due to a retinal detachment, multiple sclerosis, an optic nerve that was made strangely at birth, or a variety of other conditions. Nevertheless, a reduced visual field is more likely to be a sign of glaucoma than an intraocular pressure above 21 mm Hg. Figure 1 shows a plot of a visual field that indicates a fairly typical defect due to glaucoma.
Optic Nerve Damage
The tissues that can be damaged in glaucoma include the cornea, the iris, the lens, the retina, and, most importantly, the optic nerve. Abnormality of the optic nerve takes many forms in patients with glaucoma. One of the most important of these is a bowl-shaped depression of the optic nerve called “cupping,” which can be detected by looking into the eye.
Cupping of the optic nerve is one of the very important signs of the presence of glaucoma. Simply because a person has a cup-shaped optic nerve does not necessarily mean that he has glaucoma. However, generally, the larger the cup, the greater the likelihood of glaucoma. Figure 2 shows an optic nerve without any signs of glaucoma. Figure 3A shows the optic nerve of a patient with a high intraocular pressure but an optic nerve that still looks healthy; Figure 3B shows the same nerve after it has been damaged by pressure and the nerve has become “cupped.” Figure 4 shows a nerve that is both cupped and has a hemorrhage.
Signs, such as elevated intraocular pressure, reduced visual field, and optic nerve cupping, are flags that alert the physician that something may be wrong. Symptoms are flags for the patient and the physician alike. They are, in many ways, more important than signs because they show that whatever is happening is affecting the patient’s health. And preserving or enhancing the patient’s health is the primary task of the physician. Thus, symptoms are especially important. Even though they may not always be sensitive indicators of glaucoma, especially early glaucoma, they are often highly specific, and always need to be carefully considered.
Headaches that occur while reading or when going into dark places, especially when associated with misty vision or haloes, are strongly suggestive of one type of glaucoma. Blurred vision and eye aches after vigorous exercise suggest another type. Poor color perception and trouble seeing at night indicate the possibility of yet another variety. Awareness that parts of visual objects vision are missing, for example, the first letters of words, is an indicator that something is wrong — perhaps macular degeneration, perhaps glaucoma, perhaps something else. In any case, all of these symptoms require explanations.
One of the most important symptoms of glaucoma is a general feeling that vision has deteriorated. This general sense may merely indicate that the person needs new glasses or that he/she has a cataract; however, it can also be a sign of something more serious, such as glaucoma.
The Importance of Change
Perhaps the most important thing to look for in diagnosing glaucoma is the occurrence of change. If the intraocular pressure has risen from where it used to be, that’s reason for concern. If the visual field is less than it used to be, something is usually wrong. If the cup of the optic nerve has become larger, that is almost certainly a sign that the person has glaucoma. Finally, if a patient knows he cannot see as well as he used to, glaucoma could be the reason.
The likelihood that glaucoma is present increases in relationship to the prominence of these various signs and symptoms of the various types of glaucoma. Such signs and symptoms alert the patient and the physician to the possibility that glaucoma may be present; they indicate probabilities.
Knowing that the probability of glaucoma is high is obviously a crucial step toward appropriate treatment. Yet, a diagnosis simply of “glaucoma,” which says nothing about the cause or the likely clinical course of the disease in a particular individual, is of almost no help in deciding on appropriate treatment.