Glaucoma (eBook)

Chapter 1

EPIDEMIOLOGY OF GLAUCOMA

Anne L. Coleman, M.D.

Glaucoma is the second leading cause of blindness in the world.1 As of the year 2000 66.8 million people are now estimated to have glaucoma, with 6.7 million bilaterally blind from this disease. Aside from its medical impact on the afflicted population, the total economic and social costs of glaucoma are virtually unknown.

In spite of many advances in the diagnosis and treatment of glaucoma, the fundamental causes of most glaucomas remain unknown. This hampers our ability to predict who will require treatment to prevent loss of vision. Generally, however, diseases do not occur at random; they instead present with specific patterns. The field of epidemiology (Table 1-1) seeks to identify these patterns and gain insight into why certain individuals succumb to disease and others do not. This knowledge, in turn, helps us understand the mechanism of disease and guides screening and treatment toward specific populations and subpopulations.

When glaucoma is broadly classified into primary open-angle glaucoma (POAG) and primary angle-closure glaucoma (PACG), our understanding of the epidemiology of glaucoma draws from both global and regional experiences. The distinct geographic tendencies of these entities throughout the world help guide public health resources dedicated to the diagnosis and treatment of glaucoma. This knowledge also helps individual practitioners as they evaluate and treat ever increasing numbers of patients from diverse ethnic backgrounds. These epidemiological evaluations rely on standardized definitions of both forms of glaucoma. This allows investigators to perform careful comparisons among different studies and draw meaningful conclusions.

Studies of the specific patterns of these diseases, or risk factors, are derived mostly from Europe, the United States, and Australia. Because PACG is less common in these regions, our understanding of its risk factors is limited, although some have been identified, such as age, gender, refractive error, and family history.

In contrast, POAG has been widely studied. Many population-based studies and clinical series now exist; they provide accurate information on the prevalence of this condition and its relationship to several risk factors, which, in turn, sheds light on the possible mechanisms of this disease. Among these risk factors, intraocular pressure (IOP), ethnicity, and age are well established. Diabetes mellitus, systemic hypertension, myopia, and migraine headaches seem more controversial.

PUBLIC HEALTH IMPACT OF GLAUCOMA

In the United States, were estimated to be 130,540 people blind from primary glaucoma in the year 2000.2 In Olmsted County, Minnesota, 9% of subjects aged 66 years when diagnosed with glaucomatous optic nerve damage or visual field loss were bilaterally blind at the 20-year follow-up, in spite of receiving treatment.3 Although it is easy to imagine the difficulties associated with living in today’s society with vision less than 20/400 or 3/60 in the better eye (World Health Organization’s criteria for blindness), many people do not understand the impact that loss of peripheral vision, depth perception, and contrast sensitivity can have on an individual’s life prior to blindness. Topical eye medications and glaucomatous visual field loss are associated with an increased risk of falls and fractures in the elderly.4 In addition, a diagnosis of glaucoma is an independent risk factor for injurious car crashes in subjects 55 to 87 years of age.5

Because the visual loss and medications associated with glaucoma affect the activities of daily living,6–10 the economic impact of glaucoma on society is essentially unknown. Although we can measure the cost of medically treating glaucoma,11 the costs of visual impairment and blindness are much more difficult to estimate.

Glaucoma is not a single disease but many. It is often categorized into PACG, secondary angle-closure glaucoma, POAG, secondary open-angle glaucoma, congenital glaucoma, and juvenile glaucoma.

As Figure 1-1 shows, the most common type of glaucoma in one region of the world may differ from that in another region. PACG is more common in Asia, whereas POAG is more evenly distributed throughout the world.1 In the United States, Europe, and Australia, 75 to 95% of glaucoma in Caucasians is POAG,1,12 whereas PACG accounts for 70 to 90% of glaucoma in China and India. 12–14 Most glaucoma in Japan is POAG, which includes normal-tension glaucoma.15 Unfortunately, very little is known about which type of glaucoma is most prevalent in Africa. However, this is most likely POAG because PACG is rare in African Americans.16 In South America and the Near East, clinical records suggest that POAG is the most common form of glaucoma, even though PACG is more prevalent there than in Caucasians in both the United States and Europe.1

This global perspective strongly suggests that people of certain ethnic groups are more likely to have either PACG or POAG. This helps the practitioner diagnose and treat the individual glaucoma patient. From a public health perspective, understanding the epidemiology of PACG and POAG helps us determine why certain individuals are more likely to develop these conditions, and can improve the screening, treatment, and prevention of blindness.

PRIMARY ANGLE-CLOSURE GLAUCOMA (SEE ALSO CHAPTER 16)

DEFINITION OF PRIMARY ANGLE-CLOSURE GLAUCOMA

The 1996 American Academy of Ophthalmology Preferred Practice Pattern defines PACG as “appositional or synechial closure of the anterior chamber angle caused by relative pupillary block in the absence of other causes of angle closure.”17 Because PACG is a relatively rare disease in Europe, the United States, and Australia, very little is known of its epidemiology. In addition, the diagnosis of PACG depends on gonioscopy. However, there is no universally accepted gonioscopic definition of angle closure, which complicates comparison of population-based studies on this condition.

RISK FACTORS FOR PRIMARY ANGLE-CLOSURE GLAUCOMA

Ethnicity

Several population-based studies have found that individuals of Eskimo or Chinese descent are at increased risk of PACG. The prevalence of PACG is 20 to 40 times higher in Eskimos than in Caucasians.12 In addition, a population-based survey from Singapore associated a history of Chinese descent with a greater risk for PACG.18 The increased risk in Eskimos probably results from a smaller eye size and more crowded anterior chamber.19 With the exception of a smaller corneal curvature, however, the axial length and anterior chamber depth of Taiwanese and Caucasian eyes are remarkably similar.20

FIGURE 1-1 Geographic distribution of primary angle-closure glaucoma (red) and primary open-angle glaucoma (POAG) (green) throughout the world. Although detailed studies on the epidemiology of glaucoma in South America and Africa are currently lacking, indirect evidence suggests that POAG is most common in these regions. Unshaded areas indicate insufficient study.

Age

Older patients are more likely to develop PACG. This increased risk probably results from steady growth of the crystalline lens throughout...