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Central Corneal Thickness of Caucasians and African Americans in Glaucomatous and Nonglaucomatous Populations
Francis A. La Rosa, MD, PhD;
Ronald L. Gross, MD;
Silvia Orengo-Nania, MD
Arch Ophthalmol. 2001;119:23-27.
ABSTRACT
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Objective To determine whether there is a difference in central corneal thickness
between African American and Caucasian patients. If present, a difference
might alter the measurement of intraocular pressure and potentially the assessment
and management of glaucoma in these populations.
Methods Central corneal thickness was measured by means of ultrasound pachymetry
in African American (n = 56) and Caucasian (n = 32) patients with suspected
or confirmed glaucoma and control populations of African American (n = 26)
and Caucasian (n = 51) subjects in whom there was no evidence of elevated
intraocular pressure or glaucomatous optic nerve damage. Measurements of central
corneal thickness were then compared between different subpopulations by means
and population distribution analysis.
Results A statistically significant difference was noted between the mean (±SD)
central corneal thickness of all African American (including those with and
without glaucoma) (right eye, 531.0 ± 36.3 µm; left eye, 530.0
± 34.6 µm) and all Caucasian (including those with and without
glaucoma) (right eye, 558.0 ± 34.5 µm; left eye, 557.6 ±
34.5 µm) patients. Similar results were found when subpopulations were
tested. Distribution analysis of central corneal thickness measurements noted
the largest cluster of African American patients around 520 to 540 µm,
whereas the largest cluster of Caucasian patients was between 580 and 600
µm.
Conclusions African Americans were found to have thinner central cornea thickness
measurements than Caucasians. This finding in African Americans may lead to
lower applanation intraocular pressure readings compared with those of Caucasians,
potentially resulting in an underestimation of the actual level of intraocular
pressure.
INTRODUCTION
SUBSTANTIAL EVIDENCE supports the assertion that primary open-angle
glaucoma (POAG) has different clinical characteristics in African American
than in Caucasian patients.1-3
Indeed, the Baltimore Eye Survey, the largest American study to examine this
issue, demonstrated this disease to be the leading cause of irreversible blindness
in African Americans. Roughly 6 times as many African Americans as Caucasians
are diagnosed with glaucoma.1 They are about
10 years younger on initial diagnosis, and both disc and field damage are
noted to progress more rapidly.1 No single
cause has been attributed to this difference in initial diagnosis and progression
of this disease between the racial groups.
The literature on the intraocular pressure (IOP) measurement in various
racial groups at the time of initial examination is conflicting. The Baltimore
Eye Survey found a significantly lower IOP in untreated African Americans
(21.48 mm Hg) than in untreated Caucasians (24.15 mm Hg). Another large study
that examined this issue was the Barbados Eye Study.4
Their survey of an Afro-Caribbean population disclosed a significantly higher
IOP (25 mm Hg) in black patients than did the Baltimore Eye Survey.
Although IOP is an essential element in diagnosing and assessing the
management of glaucoma, there are several potential sources for error in measurement.
Goldmann applanation tonometry has long been the gold standard for the clinical
measurement of IOP. The Goldmann equation is based on a modification of the
Imbert-Fick principle, which describes the case of an ideal, dry, thin-walled
sphere.5 However, the corneal surface of the
eye exhibits properties that deviate from the ideal theoretical constructs.
Numerous studies have demonstrated the importance of corneal thickness
in determination of readings with the Goldmann applanation tonometer.6-9 Ehlers
et al8 calculated the instrument to be accurate
at a central corneal thickness (CCT) of 520 µm, with a cornea 70 µm
thicker resulting in intraocular pressure readings that were 5 mm Hg higher,
and a cornea 70 µm thinner resulting in a reading that was 5 mm Hg lower.
Whitacre et al9 showed that the extremes of
underestimation and overestimation span a range of almost 16 mm Hg, indicating
that measuring CCT may well have important implications with regard to IOP
measurement. Patients with a clinical diagnosis of normal-tension glaucoma
(NTG) have been shown to have significantly thinner corneas than either healthy
volunteers or patients with POAG.10 Argus11 showed that patients with a clinical diagnosis of
ocular hypertension (OHT) had significantly thicker corneas than a control
population of patients with POAG.11 Copt et
al12 were able to reclassify 56% of patients
previously followed up for OHT as being normotensive on the basis of CCTs
that were greater than average. Conversely, they were able to reclassify 31%
of patients followed up for NTG as having elevated IOP after correcting for
corneas that were thinner than normal. Specifically, actual IOP may be underestimated
in eyes with thinner CCT or overestimated in eyes with thicker CCT.
Another implication of the importance of corneal thickness on IOP measurement
is that IOP measurements are also modified by both photorefractive keratectomy
and laser in situ keratomileusis.13-14
This may be a confounding factor when patients with glaucoma who have undergone
one of these procedures are followed up.
The present study was performed to determine whether there was a difference
in CCT between African American and Caucasian patients that could lead to
misinterpretation and inaccurate measurement of IOP readings in African Americans
and, thus, potentially change the diagnosis and management of glaucoma in
that population.
PATIENTS AND METHODS
One hundred sixty-five consecutive male patients examined at the Houston
Veterans Affairs Hospital Eye Clinic, Houston, Tex, between March 1 and May
31, 1999, were included in the study. Eighty-three of these patients were
Caucasian and 82 were African American. Race was self-reported by the patient
and confirmed by demographic information found in the patient's medical record.
Exclusion criteria included any substantial corneal abnormality and intraocular
surgery within the past 3 months. All patients were informed of the study
and gave verbal consent to undergo corneal thickness measurements by pachymetry.
Primary open-angle glaucoma was defined as
gonioscopically open angles, typical visual field and/or optic nerve changes,
and a history of IOP greater than 21 mm Hg by Goldmann applanation tonometry.
Glaucoma suspects were patients with optic nerve
appearance suggestive of glaucoma without typical visual field changes, IOP
of 21 mm Hg or less, and open angles on gonioscopy. Ocular
hypertension was defined as IOP greater than 21 mm Hg by Goldmann applanation
tonometry, with normal-appearing optic nerves and visual fields, and open
angles on gonioscopy. Normal-tension glaucoma was
defined as typical optic nerve and visual field changes consistent with glaucoma,
a history of IOP that never exceeded 21 mm Hg, and open angles on gonioscopy.
Central corneal thickness was measured with an ultrasonic pachymeter
(DGH-2000 or DGH 500 "Pachette"; DGH Technology Inc, Frazer, Pa). The pachymetry
measurement recorded for each eye separately was the lowest of 5 measurements.
This value was believed to represent the most accurate measurement, as it
was likely to be closest to perpendicular to the corneal surface. Intraocular
pressure was measured twice in each eye by the same examiner before the pachymetry
measurement, and the mean was recorded. The patient's age, race, number of
glaucoma medicines, and any history of glaucoma surgery were also recorded.
Two examiners (F.A.L.R. or Lisa Hausler, COA) took all pachymetry and IOP
measurements. Cross-validation found the examiners to have consistent readings.
The CCT measurements were averaged after collection and are presented as mean
± SD. Two-tailed t test was then used to assess
statistical significance of the results. In addition, distribution analysis
is presented with the use of bin ranges (groups [bins]) of 20 µm (Excel,
version 5.0; Microsoft Corp, Redmond, Wash).
RESULTS
A total of 165 patients were enrolled. The demographic and baseline
characteristics of the patients are given in Table 1. The average age difference between the Caucasian population
and the African American population in this study was small, 2.5 years, and
not statistically significant (P = .17). In the patients
with glaucoma, IOP was managed with both surgery and medication, with no statistically
significant differences in IOP readings after treatment between all Caucasians
and all African Americans (P = .51). African American
patients were using approximately twice as many medications as Caucasians
(P<.05).
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Table 1. Patient Demographics*
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The average CCT of all subgroups is presented in Table 2. The average difference in CCT between Caucasians and African
Americans in all subgroups was statistically significant. In all subpopulations,
African American patients had smaller CCT values. In all patient subpopulations
together, the CCT difference was 27.3 µm (P<.001);
in the control subjects, 21.9 µm (P<.01);
in the glaucomatous populations taken as a whole (including POAG, suspects,
OHT, and NTG), 32.2 µm (P<.001); in the
POAG population, 30.65 µm (P<.05); in the
glaucoma suspect group, 28.0 µm (P<.05);
and in the NTG group, 42 µm (P<.05). With
distribution analysis, the largest cluster of CCT values in African American
patients was 60 µm lower than the peak frequency in Caucasian patients
when all groups were compared (Figure 1).
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Table 2. Central Corneal Thickness by Pachymetry and Intraocular Pressure
According to Race*
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Distribution analysis of central corneal thickness between African
American and Caucasian patients; 20-µm bin range.
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COMMENT
In this study of the racial difference in corneal thickness, we found
that there is a statistically significant difference, with thinner corneas
being present in African American patients. This difference may impact the
evaluation of IOP measurement, as it could lead to misdiagnosis and mismanagement
of glaucoma in the African American patients. To our knowledge, this is the
first attempt at examining racial differences in CCT. The literature has several
examples of race-specific corneal thickness measurements.12-13
These examples are a by-product of the location of the institutions where
the studies are conducted rather than a specific attempt at categorization
based on racial heritage. Thus, although the CCT results of these studies
in various racial groups are important, they only indirectly address, and
do not answer, the question of whether race is a contributing factor in differences
between CCT because they do not compare the groups. Studies such as that by
Copt et al,12 performed in Switzerland, would
seemingly represent a relatively homogeneous group of European Caucasians,
with an average CCT reported in their control population of 552 ± 35
µm, similar to our Caucasian control group average of 556 ± 32
µm. They did not study other racial groups. Mixed populations such as
that in the United States provide an important opportunity to address these
questions with the same investigators, in the same clinical setting, with
the same pachymeters.
The Goldmann applanation tonometer is theoretically only accurate for
CCT at 520 µm,6 and yet, as with our
study, virtually all studies using modern ultrasound pachymetry have noted
a mean CCT significantly thicker than this value. Indeed, the study by Morad
et al10 (conducted on an Israeli population),
detailing the significantly thinner corneas of patients with NTG, showed an
average CCT of 521 ± 37 µm, while their control population had
a CCT of 555 ± 32 µm. Almost all population surveys that have
defined the "normal" range of IOP have been based on Caucasian patients.15 If their average CCT matched (and likely did) the
values reported in this study and others in the literature, then the normal
range of IOP, if recalculated with CCT, might be lower.
Control populations were recruited for each category to ensure that
our data were not merely a reflection of the types of patients with glaucoma
we were able to recruit. Because we were able to include sufficient control
patients, this is extremely unlikely because of the nearly identical statistically
significant differences between African Americans and Caucasians in our control
population and all the glaucoma subpopulations examined.
Thinner corneas in African Americans may represent an important difference
that needs to be considered in the management of glaucoma. The adjustment
of IOP readings by corneal thickness may lead to more accurate assessment
of IOP control in patients with thinner or thicker corneas. In addition, in
patients with the thickest corneas, adjusted pressure reading may lead to
fewer medications or a less-frequent need for surgery. Although in this report
we have shown a statistically significant difference in CCT among different
racial groups, it certainly does not explain all differences between the natural
history of glaucoma in African Americans and Caucasians. For example, why
are there 6 times as many African Americans with the disease as Caucasians?
A thinner cornea alone does not address this issue. Why is the diagnosis of
glaucoma made at an average age of 10 years younger in African Americans?
Our data suggest that screening programs based on IOP alone would miss the
diagnosis in more African Americans, on the basis of thinner corneas, and
thus lead to an older age at detection. Issues that may be related to the
underestimation of IOP are the faster rates of nerve and field progression
and the higher incidence of irreversible blindness in African Americans. It
is also conceivable that clinicians are more comfortable with lower pressures
leading to progression of glaucoma in African Americans than in Caucasians,
and thus these patients may not be as aggressively treated.
Although all efforts were undertaken to control for variables that might
affect the data in this study, as in most clinical studies, some were beyond
our control. The fact that this study was limited to male patients is a potential
bias. There is no reason to believe that women would have a different distribution
or average value, and no study to date, to our knowledge, has found a sex-related
difference in CCT; however, this remains as a potential limitation of our
study. In addition, racial backgrounds in the United States are well known
to be of mixed heritage. We understand that purely phenotypic and demographic
categorization of patients has limitations, but we would expect this source
of error to blur any significant findings rather than accentuate the outcomes.
It is possible that if more precise mechanisms for assessing racial background
were to be devised, the difference in CCT might be even greater than was found
in this study.
Also, the Goldmann equation assumes not only corneal thickness but also
corneal rigidity or elasticity. These factors may also play a role in true
IOP measurement along with corneal thickness. Future studies should be devised
to examine this area.
This study raises the possibility that CCT may need to be taken into
account to accurately assess the actual IOP for the diagnosis and management
of glaucoma in African Americans. Furthermore, it highlights the dearth of
knowledge regarding CCT in virtually all populations throughout the world.
This includes racial and geographic variation as well as age differences and
the impact of corneal refractive procedures as they become more widely used.
Measurement of CCT may evolve to play an important role in the clinical measurement
of IOP and suggest that, just as one would perform baseline gonioscopy, a
baseline CCT may need to be obtained for all patients with glaucoma.
AUTHOR INFORMATION
Accepted for publication June 2, 2000.
We thank Lisa Hausler, COA, for her technical support in gathering all
the data for this study. For her help with the statistics, we thank Alice
Chuang, PhD, statistician at the University of Texas Health Science Center
in Houston.
Corresponding author: Silvia Orengo-Nania, MD, Cullen Eye Institute,
Department of Ophthalmology, Baylor College of Medicine, 6565 Fannin, Mail
Stop NC 205, Houston, TX 77030 (e-mail: silviao{at}bcm.tmc.edu).
From the Cullen Eye Institute, Department of Ophthalmology, Baylor
College of Medicine (Drs La Rosa, Gross, and Orengo-Nania), and Department
of Ophthalmology, Houston Veterans Affairs Hospital (Dr Orengo-Nania), Houston,
Tex.
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