Primary Open-Angle Glaucoma
George L. Spaeth
Daniel Lee
INTRODUCTION
Glaucoma, in its many forms, is the leading cause of irreversible blindness worldwide. It is a complex family of progressive optic neuropathies where intraocular pressure (IOP) and other contributing factors lead to a characteristic pattern of optic nerve atrophy and visual field loss. Glaucomas are often classified into primary and secondary glaucomas. Primary open-angle glaucoma (POAG) is essentially a diagnosis of exclusion in which no recognizable cause of damaging IOP is identified despite meticulous clinical examination. In many cases, a cause for the pressure that produces damage can be determined, such as inflammation, trauma, or neovascularization. These are often classified as secondary glaucomas and treatment can be directed toward the specific cause. The glaucomas can also be divided on the basis of the nature of the anterior chamber angle. In angle-closure glaucoma, the pressure becomes elevated as a consequence of the resistance to aqueous outflow due to contact or adhesions between the iris and the trabecular meshwork. In contrast, in the open-angle glaucomas, aqueous humor has clear access to the trabecular meshwork. The primary glaucomas are often further characterized according to the age of onset of the condition. Those occurring at or shortly after birth are termed congenital, those occurring after infancy and before age 40 years are termed juvenile glaucomas, and those first becoming apparent after the age of 40 are termed adult-onset open-angle glaucoma. The glaucomas occurring in infants represent a separate group and are discussed elsewhere in this book (see Chapter 11).
DEFINITION
The definition of glaucoma has changed dramatically since it was first used in ancient Greece; the word has come to mean different things to different people. The definition is still evolving and, as a consequence, there can be unfortunate and disturbing confusions during communication. Figure 12-1 illustrates a rough timeline of these changes.
Until the late 19th century, the definition of glaucoma was based largely on the presence of symptoms such as blindness or pain. The development of statistical theory, the availability of the tonometer, and the concept of disease as
a “deviation from normal,” all led to glaucoma being defined solely in terms of elevated IOP above 21 or 24 mm Hg, pressures greater than two and three standard deviations above the mean, respectively.
a “deviation from normal,” all led to glaucoma being defined solely in terms of elevated IOP above 21 or 24 mm Hg, pressures greater than two and three standard deviations above the mean, respectively.
Studies conducted largely in the 1960s showed that only around 5% of individuals with an IOP above 21 mm Hg actually developed optic nerve damage and visual field loss. Other studies showed that around one-third of the patients who had characteristic optic nerve and visual field changes of glaucoma had IOPs in the range of normal. These two observations forced a total rethinking of the definition of glaucoma. Many authors began using the terms “low-tension glaucoma” or “normal-pressure glaucoma” or “high-tension glaucoma.” As more attention became focused on the idea of glaucoma as an “optic neuropathy,” the idea that glaucoma could be related solely to elevated pressure, as occurs in the angle-closure glaucomas, became less recognized. This led to the strange situation of a person with an IOP of 80 mm Hg in exquisite pain being considered to have “angle closure,” but not being considered to have “angle-closure glaucoma.” Recently, some authors have subdivided glaucoma into pressure-dependent and pressure-independent types. Although it is certain that there are factors that increase or decrease the susceptibility of the ocular tissues to damage by IOP, it is not clear that these factors ever act alone and without some contribution from the effects of IOP, even when the IOP is in the normal or subnormal range.
In Figure 12-1, glaucoma is defined as the process leading to characteristic progressive ocular tissue damage, at least partially caused by IOP, regardless of the level of IOP. Findings and symptoms of early or moderate glaucoma are found in almost all people, even those who do not have glaucoma. There is nothing diagnostic, for example, about a cup-to-disc ratio of 0.5. Although this could represent a significant glaucomatous change, it may also occur in a person without glaucoma. Furthermore, there is nothing diagnostic about an IOP of 25 mm Hg. This is a common finding in people who do not have glaucoma. Thus, identification of characteristics that occur only (or almost only) in glaucoma is important (Fig. 12-2).
 FIGURE 12-1. Definition of glaucoma. The changing definition of glaucoma over the years. |
 FIGURE 12-2. Glaucoma characteristics. Overlap of characteristics that occur in patients with and without glaucoma. |
EPIDEMIOLOGY
Glaucoma affects individuals of all ages, all races, and in all geographic areas. It is not surprising that estimates of the prevalence of glaucoma vary widely. This variation is related to the differing definitions of glaucoma, differing methods of examination, and different expressions of the constellation of loosely related conditions called POAG.
Congenital glaucoma represents a distinct entity that is extremely rare. Most juvenile glaucoma is genetically determined, and although much more common than the congenital types of open-angle glaucoma, it is still relatively uncommon.
The majority of patients with glaucoma are older than 60 years of age and the prevalence of glaucoma increases with age.
It is difficult to generalize the amount of blindness caused by glaucoma. Again, glaucoma is a variable condition, with variable definitions. However, the incidence clearly increases markedly with age and especially in African Americans. The prevalence of glaucoma in African Americans over the age of 80 may be more than 20%.
Worldwide, the incidence of glaucoma is estimated at around 2.5 million per year. The prevalence of blindness due to POAG is probably around 3 million. In the United States, around 100,000 individuals are blind in both eyes as a result of glaucoma.
PATHOPHYSIOLOGY
The hallmark of glaucoma is ocular tissue damage, especially to the optic nerve and retinal nerve fiber layer (RNFL).
Figure 12-3 illustrates possible ways in which the tissue damage can develop. An initiating factor is IOP within the normal or the elevated range, causing mechanical deformation and leading to subsequent damage. Also illustrated are additional factors related to the structural deformation caused by IOP.
Toxic agents or autoimmune mechanisms may cause damage and eventual death of the retinal ganglion cells, leading to loss of tissue and structural damage, which itself may facilitate the IOP-related damage. Abnormal autoregulation of blood flow to the optic nerve has also been postulated to play a role in glaucomatous optic neuropathy.
 FIGURE 12-3. Pathogenesis of glaucoma. Pathogenesis of ocular tissue damage in glaucoma. IOP, intraocular pressure. |
The final common pathway in all the POAGs is the death, sometimes by necrosis, but usually by apoptosis, of the retinal ganglion cells. This may lead to further damage in the retina, optic nerve, and brain. Feedback loops make this oversimplified schema far more complex. For example, “structural change” itself predisposes to cell damage. Some of the factors that may be involved in this cascade of events are shown in Table 12-1.
TABLE 12-1. Some Factors Involved in the Development of Tissue Damage in Glaucoma | |
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HISTORY
A thorough history is the single most important part of the examination of a patient with POAG, even though the disease is relatively free of symptoms in its early stages. The history is the part of the examination that develops the relationship between the patient and the physician that is essential to successful diagnosis and management of glaucoma. In addition, a meticulous history will frequently uncover symptoms of the disease, even in relatively early or moderate stages. For example, patients may note difficulty seeing in the dark, mild eye ache, or a sense that vision simply is not as good as it was in the past. Furthermore, taking a history leads to eliciting the risk factors for glaucoma and, even more importantly, the risk factors for becoming blind from glaucoma (Tables 12-2 and 12-3).
TABLE 12-2. Risk Factors for the Development of Glaucoma | |||||||||||||||||||||||||||||||||||||||||
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Average-pressure glaucoma has been associated with various vascular risk factors (i.e., migraine, low blood pressure, sleep apnea). Particular attention should be placed on these issues in patients suspected of having this type of glaucoma. The essential question in history taking is “How are you?” The physician must stress that the most important part of the question is the word “you.” Glaucoma is a frightening condition that frequently decreases the patient’s quality of life simply as a result of the patient knowing he or she has glaucoma. There is a long history of associating glaucoma with IOP. The physician at every appropriate opportunity must indicate to the patient that both the patient’s and physician’s concern relates to the patient’s health, not primarily to the patient’s IOP. An evaluation of health requires taking a competent, compassionate history.
TABLE 12-3. Risk Factors for Becoming Blind from Glaucoma | ||||||||||
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CLINICAL EXAMINATION
The clinical examination of the patient suspected of having POAG differs only from the standard examination in emphasis. An essential part of the examination is a meticulous search for the presence or absence of a relative afferent pupillary defect (APD). An APD can be present before detectable field loss. In addition, the presence of APD indicates that there is definite optic nerve damage, which triggers a necessary search for the cause of that damage. Checking the patient for an APD is a part of every complete examination of the patient with glaucoma.
External and Biomicroscopic Examination
Biomicroscopic examinations of the patient with glaucoma differ from the standard biomicroscopic examination only in that the physician searches for the topical side effects of the medications that the patient may be using (Fig. 12-4) and findings that may be associated with glaucoma, such as a Krukenberg spindle.
Gonioscopy
Gonioscopy is an essential part of the evaluation of a patient with glaucoma. The examiner needs to search for the signs of pigment dispersion syndrome, exfoliation syndrome, and angle recession. Gonioscopy needs to be repeated at about yearly intervals, because anterior chamber angles that are initially deep may become increasingly narrow with age, leading eventually to chronic or, in rare cases, acute angle closure. Because miotics can cause marked shallowing of the anterior chamber angle, gonioscopy should be performed after any miotic is started or after a concentration of a miotic is changed. The Spaeth gonioscopic grading system provides a rapid, quantitative, and clinically useful method of describing and recording the anterior chamber angle (see Chapter 3).
Posterior Pole
POAG is primarily a disease of the optic disc. Consequently, knowledgeable, extensive evaluation of the optic nerve is a mandatory part of the evaluation of the patient suspected of having glaucoma and of the continuing evaluation of the patient. In the diagnosis of POAG, evaluation of the optic nerve is the single most important aspect of the assessment. In the management of glaucoma, the nature of the optic disc is the second most important aspect of the examination, a meticulous history being the first.
The optic disc is best examined with the pupil in a dilated state. After dilation the optic nerve is examined stereoscopically at the biomicroscope using a strong plus lens such as a 60- or 66-diopter lens. This is best done with the beam narrowed to a thin slit and the magnification on high (1.6 or 16×) using a Haag-Streit 900 series slit-lamp biomicroscope. The examiner gains an idea of the disc topography by this method. Also, the size of the disc is measured. To measure the vertical height of the disc, the beam is widened so that the horizontal extent of the beam is the same width as the width of the optic nerve (Fig. 12-5). The beam is then narrowed vertically until the vertical diameter of the optic disc exactly matches the vertical extent of the beam. The vertical diameter of the disc is determined by reading the reticule on the slit lamp and applying the appropriate correction factor. Although these factors vary slightly with the Volk and the Nikon lenses, a rough approximation is that for the 60-diopter lens, one multiplies the reading on the reticule by 0.9; for the 66-diopter lens, no correction factor is needed; and for the 90-diopter lens, the measurement on the reticule is multiplied by 1.3. The normal disc has a diameter between 1.5 and 1.9 mm vertically. The next step employs the direct ophthalmoscope.
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