glaucoma
Patients at Risk
Myriad
factors influence the way we study the development of glaucoma — and treat
it.
DANIEL
K. ROBERTS, O.D., M.S.
Primary open-angle
glaucoma (POAG) is an important public health problem in the United States. It's
characterized by a chronic, usually progressive, optic neuropathy in adults, which
produces an acquired atrophy of the optic nerve. This is associated with loss of
retinal ganglion cells and their axons. Patients with POAG have open anterior chamber
angles. There is no other known reason for the glaucomatous optic nerve changes.
Intra-ocular pressure (IOP) is often elevated, but may also be normal. Here, we'll
review the incidence and prevalence of POAG, as well as risk factors.
Epidemiological evaluations
An important issue in the study of POAG concerns the accuracy
and consistency of its diagnosis because it is not necessarily easy to establish
the presence of glaucomatous cupping of the optic nerve head. From a clinical perspective,
suspicious "cupping" may actually represent normal, physiologic variation, and seemingly
normal optic nerves may actually be abnormal. Also contributing to diagnostic difficulty
is the fact that the results of automated threshold perimetry, a mainstay of testing,
can be notoriously unreliable. Thus far, attempts to solve this problem with new
diagnostic testing strategies and methods have not produced the "magic bullet."
Certainly additional testing modalities can be helpful, but none is always a reliable
indicator to determine the presence or absence of POAG.
Another problem with epidemiologic investigations of POAG is that
elevated IOP is not considered as important for the diagnosis of disease as it once
was. Evolving and changing diagnostic criteria for POAG can impact the validity
of certain disease estimates and comparability among studies. In addition to over-
and under-estimation errors, non-homogenous study samples may lead to greater difficulty
in identifying factors with true association with POAG.
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The
prevalence of POAG in the U.S. begins to increase around age 40 in most racial and
ethnic groups.
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Prevalence
POAG is a worldwide disease with no known population completely
unaffected by it. It's estimated that more than 60 million people worldwide are
afflicted with the disease. Recently, the Eye Diseases Prevalence Research Group
(2004) calculated age-specific POAG prevalence estimates for white, black, and Hispanic
persons in the United States based on a meta-analysis of population-based studies
conducted in the United States, Australia and Europe. The data pooled from large
individual studies supports information relative to race and ethnicity and helped
project prevalence figures for the year 2020. (See charts on page 43.)
In whites, for example, about 1% of women age 55-59 have POAG.
For those 65-69 years old, prevalence increases to 1.58% and to 3.12% for patients
between 75-79 years old. For women, men and different ethnic groups, POAG prevalence
increases with age. In most age groups, African Americans consistently have the
highest POAG prevalence; whites the lowest. For example, for men age 65-69, the
prevalence is 1.64% in whites, 2.07% in Hispanics and 7.24% in African Americans.
Applying age, race, and gender prevalence estimates to 2000 U.S.
Census Bureau data, The Eye Diseases Prevalence Research Group estimates POAG currently
affects about 2.22 million people. With the aging of the U.S. population, this figure
is expected to soar, becoming an even greater public health concern. The number
of people in the United States with POAG is projected to increase to 3.36 million
by 2020.
Incidence
The incidence of POAG has not been studied as well as its prevalence.
In large part, this is due to greater difficulty in collecting relevant data. Whereas
prevalence indicates the number of existing cases of disease in a specified population
at a given time, incidence rates indicate the number of new cases developing in
a specified population during a particular interval of time. Incidence is the percentage
of an at-risk population that develops the disease. Alternatively, incidence is
commonly reported as the number of new cases per 100,000 people at risk for the
disease. Reliable incidence data, therefore, require good population data prior
to development of disease and adequate follow-up data to accurately determine if
and when the disease developed.
A
long-term, prospective investigation is the ideal study design to determine the
incidence of a disease. Because this is difficult and expensive, retrospective studies
are often substituted. These may suffer from unavailable and incomplete testing
data, the application of highly variable and outdated diagnostic data, and changing
disease definitions.
|
POAG
Prevalence in the U.S. |
Age
Group
(Years) |
White
(%)* |
African
American
(%) |
Hispanic
(%) |
WOMEN |
40-49
|
0.83
|
15.1
|
0.34
|
55-59
|
1.02
|
2.86
|
0.98
|
65-69
|
1.58
|
4.64
|
2.24
|
75-79
|
3.12
|
7.45
|
5.01
|
MEN |
40-49
|
0.36
|
0.55
|
0.39
|
55-59
|
0.85
|
3.06
|
1.00
|
65-69
|
1.64
|
7.24
|
2.07
|
75-79
|
3.14
|
11.65
|
4.23
|
| Estimated prevalence of open-angle glaucoma in the United States.
Data indicates percent of the age group affected, i.e., number of affected per 100
people in age group. |
The impact of technology
With newly available technology and treatment options, it's not
unusual to find a change in the incidence rate of a disease shortly after the introduction
of the new technology. Evaluating newly-diagnosed POAG residents between 1965 and
1980, the authors of a recent study calculated an age- and gender-adjusted annual
incidence of 14.5 new cases per 100,000 people, predominantly white. Prior to 1979,
the average annual incidence was 12.3 new cases per 100,000, but during the last
two years of the study (1979-1980), the incidence was 27.7 new cases per 100,000.
The authors propose the dramatic difference in incidence before and after 1979 is
most likely due to the advent of new medical therapy, namely beta-blockers, which
became available to area practitioners in the last two years of the study.
When a new, promising medical therapy becomes available, it should
not be surprising that practitioners may look more diligently for a disease. They
may even be more willing to make a particular diagnosis after a new therapy becomes
available.
Along with the age- and gender-adjusted incidence estimate for
POAG, researchers found POAG incidence rates increased significantly with age. Estimated
incidence in the fourth decade of life was 1.6 cases per 100,000 vs. 94.3 cases
in the eighth decade. The authors did not find a significant difference in the incidence
of POAG relative to gender.
Not much information exists on incidence relative to race and
ethnicity. The Barbados Eye Study, conducted from 1988 to 1992, consisted of a simple,
random sample of Barbadian-born citizens, between the ages of 40 and 84. In the
initial sample, 93% of 4,631 subjects reported their race as black. The observed
four-year incidence of POAG was 1.2% in 40-49 year-olds, 1.5% in 50-59 year-olds,
3.2% in 60-69 year-olds and 4.2% in those over 74. Obviously, these four-year estimates
are higher than annual incidence rates due to the longer period of study.
Risk factors
Several important risk factors for POAG have been identified (see
table below). Age is a well-established risk factor for POAG, with risk increasing
significantly after age 40. Although elevated IOP is clearly a risk factor for the
disease, it is not a requirement. Contemporary investigation of glaucoma reflects
this sentiment, with elevated IOP not required for its diagnosis. Rather, POAG diagnosis
depends on demonstrable cupping and visual field loss.
A greater awareness of corneal thickness' role in the accurate
measurement of IOP further diminishes the requirement for IOP elevation. Even with
adjustment for the influence of corneal thickness, adjustments based on population
data may be erroneous for individual patients.
African American race is an important and definitive risk factor
for POAG. Depending on the age group, POAG prevalence may be six times higher in
African Americans than in whites. In addition, POAG appears earlier and is likely
to be more severe in African Americans than whites. Corneal thickness is more likely
to be thinner in African Americans, a factor that may result in underestimation
of the IOP in some individuals. This may account for some, but not all, of the variation
between African Americans and whites relative to POAG. Hispanic and Latino ethnicity
is also now considered a risk factor for POAG. These patients are at greater risk
than whites, but lower than African Americans.
Family history is another important risk factor for POAG. Clearly,
the genetics are complex and not easily unraveled, but POAG in a close relative
is an important risk indicator. The Baltimore Eye Survey reported the relative risk
of developing POAG is more than three times higher for those who have a sibling
with POAG.
Although older age, elevated IOP, decreased central corneal thickness,
a family history of glaucoma, and African-American or Hispanic/Latino ethnicity
are generally the most recognized risk factors, a few other factors may also put
patients at risk. Systemic hypertension and hypotension also appear to be risk factors
for POAG. It's speculated that perfusion pressure within the microcirculation of
the optic nerve, as well as its auto-regulation, may be a factor in disease development
for some patients.
Diabetes is also a suspected risk factor for POAG. Although many
patients with diabetes have glaucoma, some believe the relationship disappears if
other variables are adequately controlled. Further study is necessary.
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Most
important risk factors for POAG |
|
PRIMARY RISK FACTORS
• Age
>40 years
• Elevated
intraocular pressure
� African
American race and Hispanic/Latino ethnicity
• Family
history of glaucoma
OTHER POSSIBLE
RISK FACTORS
• Myopia
• Systemic
hypertension and Hypotension
� Diabetes |
Myopia is another suspected risk factor. A two- to three-fold
increase in risk for glaucoma has been reported in people with at least low amounts
of myopia.
Management
Management goals for POAG include controlling IOP in order to
stabilize the optic nerve and nerve fiber layer, which will help stabilize the visual
field. The mainstay of treatment is topical medical therapy. Other options include
laser, filtering or cyclodestructive surgery. Initial therapy depends on a number
of patient-specific factors.
The amount of IOP reduction that will prevent further deterioration
of the optic nerve will vary from person to person. There is no way to know exactly
how much lowering will be required in a given patient. As a general rule, however,
lower IOP by 20% relative to pretreatment levels. Generally, more aggressive IOP
lowering is helpful when disease status is more severe. In all patients, continually
assess the adequacy of therapy by evaluating the optic nerve, nerve fiber layer
and visual field. Then, adjust target IOP based on this reassessment.
References will be furnished upon request.
Dr.
Roberts is a Professor of Optometry
at the Illinois College of Optometry in Chicago. He is active in clinical research
of glaucoma and spends time in clinical teaching at the Eye Institute.
Optometric Management, Issue: June 2006