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Am J Ophthalmol. Author manuscript; available in PMC 2009 November 1.
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Published in final edited form as:
Am J Ophthalmol. 2008 November ; 146(5): 741–746. doi:10.1016/j.ajo.2008.05.048.
Intraocular Pressure, Central Corneal Thickness, and Prevalence
of Open-Angle Glaucoma: The Los Angeles Latino Eye Study
Brian A. Francis, MD, MS1, Rohit Varma, MD, MPH1,2, Vikas Chopra, MD1, Mei-Ying Lai,
MS2, Corina Shtir, MS2, Stanley P. Azen, PhD2, and Los Angeles Latino Eye Study Group3
1Doheny Eye Institute, Keck School of Medicine, University of Southern California, Los Angeles, California
2Department of Preventive Medicine, Keck School of Medicine, University of Southern California, Los
Angeles, California
3See Acknowledgments for members of the Los Angeles Latino Eye Study Group
Abstract
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Purpose—To examine the relationship between the prevalence of open-angle glaucoma (OAG)
and intraocular pressure (IOP) and the impact of central corneal thickness (CCT) on this relationship.
Design—Population based cross-sectional study.
Methods—The study cohort consisted of 5970 participants from the Los Angeles Latino Eye Study
(LALES) with no history of glaucoma treatment and with complete ophthalmic examination data.
The relationship between the prevalence of OAG and IOP was contrasted across persons with CCT
designated as thin, normal or thick.
Results—Prevalence of OAG was exponentially related to IOP. When stratified by CCT, persons
with thin CCT had a significantly higher prevalence of OAG than did those with normal or thick
CCT’s at all levels of IOP. Adjusting each IOP individually for CCT did not impact significantly the
relationship between the prevalence of OAG and IOP.
Conclusions—These findings suggest that adjusting for the impact of CCT on IOP by correction
algorithms is not necessary in a population analysis of glaucoma prevalence; CCT and other
associated corneal properties, however, are important independent risk factors for the prevalence of
OAG.
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INTRODUCTION
Elevation of intraocular pressure (IOP) is no longer considered a key element in the definition
and diagnosis of open-angle glaucoma1 (OAG), yet it remains the only treatable risk factor
Correspondence and reprint requests to Rohit Varma, MD, MPH, Doheny Eye Institute, Suite 4900, 1450 San Pablo Street, Los Angeles,
CA 90033. Phone: (323) 442-6411; fax: (323) 446-6412; E-mail: rvarma@usc.edu.
Contributions: Design and conduct of the study (RV, SA); collection, management, analysis, and interpretation of the data (BAF, VC,
ML, CS, SPA, RV); and preparation, review, or approval of the manuscript (BAF, VC, ML, CS, SPA, RV).
Financial Disclosure: The authors have no proprietary or commercial interest in any materials discussed in the manuscript.
Statement about Conformity: The study protocol was approved by the Institutional Review Board (IRB)/Ethics Committee at the
University of Southern California and all study procedures adhered to the recommendations of the Declaration of Helsinki. Written
consent was obtained from all participants.
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and is known to be associated with presence and progression of the disease.2-5 Although IOP
can be misleading on an individual case basis, large population-based surveys of eye disease
have maintained the link between IOP and OAG. Specifically, the overall prevalence of OAG
in a population increases with higher IOP; in fact, some populations have shown an IOP level
above which the prevalence of OAG increases exponentially.6-9 This has led to the clinical
practice of treating ocular hypertension when it exceeds a certain level.
Recent studies on central corneal thickness (CCT) and its impact on applanation tonometry
have shown that CCT does affect the accuracy of the IOP reading, with thinner corneas giving
a falsely low reading while thicker corneas yield a falsely high reading.10,11 This has prompted
the development of “correction factors” and algorithms that attempt to adjust the applanation
IOP based on deviation from a mean or normal CCT.
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In the Los Angeles Latino Eye Study (LALES), we have reported the prevalence of OAG in
Latinos to be 4.74% (95% CI: 4.22%-5.30%).12 In the study described herein, we examined
the relationship between IOP and the prevalence of OAG in Latinos and the impact of CCT on
this relationship. Our intent was not to analyze CCT as a screening tool for OAG, but, rather,
to determine if stratifying or adjusting for CCT had an independent impact on the relationship
between the prevalence of OAG and the measured IOP. Specifically, we explored two
hypotheses: 1) compared to the normal CCT group, the thin CCT group would have a steeper
curve on the prevalence of OAG-IOP graphs, with OAG prevalence rising more sharply at
lower IOP: 2) the thick CCT group would have the flattest OAG prevalence-IOP curve, and
corrected IOP curves (using existing correction factors based on CCT) would begin their
exponential rise later (at higher IOPs) and the rise of this curve would be steeper than the
uncorrected curve. These analyses would allow us to further elucidate the nature of the
relationship between IOP, CCT and the prevalence of OAG in a population-based sample,
highlighting in particular the role of CCT as an independent risk factor for the presence of
OAG.
MATERIALS AND METHODS
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The study population consisted of subjects from the Los Angeles Latino Eye Study (LALES),
a population-based prevalence study of eye disease among Latinos aged 40 years and older in
Los Angeles (LA) County. Demographic and socioeconomic characteristics of Latinos in the
six census tracts of La Puente, California, were shown to be representative of the Latino
population in LA County and in the United States as a whole.13 The study received Institutional
Review Board approval, and all study procedures adhered to the principles for research on
human subjects as stipulated by the Declaration of Helsinki. All eligible residents (Latino, age
>40) underwent a detailed, standardized eye examination, including visual acuity testing, IOP
measurement with Goldmann applanation tonometry, visual field testing (Humphrey Visual
Field Analyzer II, SITA standard 24-2 [Carl Zeiss, Dublin, CA]), simultaneous stereoscopic
optic disc photography, optical coherence tomography (OCT) imaging and frequency doubling
technology (FDT) perimetry.
A two-step process was used to diagnose OAG, and has been described previously in greater
detail.12 First, the clinical history was obtained; this included a history of or treatment for
glaucoma, family history of glaucoma, and treatment for other ocular diseases such as cataract,
diabetic retinopathy and age-related maculopathy. In addition, a detailed clinical evaluation of
visual acuity, IOP, and CCT was done, as were gonioscopy and examination of the anterior
and posterior segments of the eye—all of which were performed on a single clinic visit. The
second step involved two glaucoma specialists, who reviewed all of the data as well as the
optic disc photographs and visual field examination results before making a diagnosis. The
specialists independently graded the optic disc and visual field information for each eye, then
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arrived at a diagnosis of normal, glaucoma suspect, or OAG based on standardized criteria,
independent of IOP data. The latter were used only after a diagnosis of OAG had been made,
and then only to differentiate between normal and ocular hypertensive individuals. If the two
specialists were in agreement, their diagnosis was assigned to that specific eye. If there was a
disagreement, a third glaucoma specialist reviewed the data, and agreement of 2 of the 3
specialists was used to assign the diagnosis. The OCT and FDT data were not used in the
diagnosis of OAG.
IOP was measured with the Goldmann applanation tonometer using the average of three
readings obtained by a certified ophthalmic technician. The CCT was measured with an
ultrasound pachymeter (DGH, Exton, Pennsylvania), and was based on the average of three
consecutive measurements. Subjects were excluded from this analysis if measurements of IOP,
CCT, visual fields or optic nerve photography were inadequate. Individuals with significant
corneal pathology such as dystrophy, edema or scar were excluded, as were those currently on
IOP lowering therapy or with a history of glaucoma surgery. A total of 6130 participants
completed the clinical eye examination and glaucoma evaluation, of whom 158 were excluded
for one or more of the above-mentioned reasons.
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One eye of each participant was selected based on the following criteria. If the participant had
only one eye diagnosed with OAG, then that eye was selected. If both eyes were glaucomatous
or non-glaucomatous, the eye with the worse mean deviation on Humphrey visual field testing
was selected.
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The correction algorithms were obtained from previous studies on CCT and its impact on
Goldmann applanation tonometry. The most conservative impact was reported by Whitacre
and Hassanein10, who found a 1 mmHg difference in IOP per 50 micron change in CCT. An
intermediate correction factor of 2.5 mmHg per 50 microns was proposed by Pillunat and
colleagues as a result of a cannulation study of 125 patients who underwent cataract surgery
with manometric water column and applanation tonometry measurements. (Pillunat LE, et al.
IOVS 2003;44:ARVO E-Abstract 4237) The same factor was adopted by Shih and
coworkers14 in their assessment of the impact of IOP adjustment for CCT on the clinical
management of glaucoma patients. The final algorithm carries a correction factor of 3.5 mmHg
per 50 microns of CCT, and is based on the work by Ehlers and colleagues,15,16 who found
a value of 3.57 mmHg per 50 microns, and the meta-analysis performed by Doughty and
Zaman,17 which showed a deviation of 3.33 mmHg per 50 microns in linear regression
analysis. Of note, the starting points of CCT used as the basis for correction calculations differs
from study to study. For example, in Ehler’s study and Orssengo and Pye’s model of the cornea,
520 microns was used, while in the cannulation study by Pillunat the figure was calculated as
550 microns. We therefore used the value determined by the meta-analysis by Doughty and
Zaman (545), as well as our population mean (550). These correction factors add or subtract
the specified amount from the IOP value according to the linear formula:
Accordingly, a plot of unadjusted IOP and its relationship to OAG prevalence was calculated
for the LALES population. The curve was analyzed in the following manner to determine the
IOP level at which occurred the greatest turning point in increasing prevalence for OAG. First,
the un-weighted Lowess estimation curve for raw data was used, as it best captures local trends
and allows for identification of the turning point or points at which the slope value increases
sharply. Next, the tangent slope to the LOWESS curve at each of its points was calculated [m
= (yi+1 − yi)/(xi+1 − xi)]. The slope measure accounts for the variability in both dependent and
independent variables, and thus most accurately captures the amount by which a difference
exists in OAG prevalence rates for consecutive IOP values. Finally, changes in the slope values
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were compared for each pair of consecutive points by measuring the increase in consecutive
slope values and the difference between consecutive slope values. The IOP point at which these
2 values were greatest was considered to be the turning point in the increasing prevalence of
OAG.
The first analysis involved plotting unadjusted Goldmann IOP and OAG prevalence while
stratifying patients into sub-groups based on CCT. The CCT values that corresponded to thin,
normal and thick were: ≤510 microns, 511-580 microns, and >580 microns. The curves for the
three different CCT groups were compared statistically by a test for difference between slopes
following log transformation. Next, to determine how much of this difference could be
explained by the impact of the CCT, the IOPs for the thin and thick CCT groups were
recalculated using the most conservative correction algorithm, and the prevalence curves were
replotted. Odds ratio estimates for OAG prevalence as a function of IOP were then calculated
in the three CCT tertiles (CCT ≤534 microns, 534-564 microns, >564 microns). Tertiles were
used due to the small numbers in the various subgroups. For example, there were very few
subjects with a CCT <510 in the highest IOP group, and, similarly, there were few subjects
with a CCT >580 who were in the lowest IOP group.
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In the second analysis there was no CCT stratification, but each IOP was individually adjusted
according to the three correction algorithms, and the population curve was recalculated. Since
the data plots approximated exponential curves, they were transformed into linear data by log
transformation in order to facilitate comparison. These curves were then compared using the
test for differences between slopes following log transformation.
RESULTS
A total of 6130 participants completed the glaucoma evaluation. After exclusion of those with
missing data (n=73) and those with a history of glaucoma treatment (n=89; note, 2 had both),
a total of 5970 individuals were included in the present analysis.
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The age and gender, as well as IOP and CCT measurements of the study population are shown
in Table 1. Figure 1 shows the relationship of the prevalence of OAG to IOP as stratified by
the three CCT groups. The thin CCT group (≤510 microns) showed the greatest increase in
OAG prevalence as a function of IOP. The normal group (511-580 microns) showed an
intermediate increase, whereas the group with the thickest CCT (>580 microns) showed the
least increase. When the thick and thin groups were adjusted for the impact of CCT on IOP,
the prevalence curves shifted towards the normal CCT curve (data not shown). A comparison
of slopes after log transformation showed the following differences: thin CCT compared to
normal CCT, P <.05; thick CCT compared to normal CCT, P =.07; and thin CCT compared to
thick CCT, P =.005.
A calculation of odds ratios of OAG prevalence for IOP groups (<15, 15-20, 21-25, ≥26 mmHg)
in each of the CCT groups is shown in Table 2. Although there is overlap between the 95%
confidence intervals for the odds ratios, the largest difference is seen in the highest IOP group,
with the thin CCT group having the highest, normal CCT having intermediate, and thick CCT
showing the lowest odds ratios for prevalence of OAG. An additional analysis of CCT as a
continuous variable (instead of stratified) included in the model found no statistical
significance. Similarly, an analysis of an interaction term between CCT and IOP as continuous
variables was not significant.
Figure 2 shows the Lowess curve for OAG prevalence-IOP relationship determined by
unadjusted Goldmann tonometry readings. Using the tangent slope strategy to identify turning
points, an IOP range of 19 to 20 mmHg was identified as the turning point where there was
the greatest increase in OAG prevalence.
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Table 3 shows the prevalence of OAG for uncorrected and corrected IOP levels, using the three
correction algorithms. There is a statistically significant trend for an increase in the prevalence
of OAG with increase of IOP for all groups (P < .0001). All four curves begin an exponential
increase in OAG prevalence at an IOP of approximately 19 to 20 mmHg, as determined by
slope calculations. At this point, all of the adjusted IOP curves rise more steeply than does the
unadjusted IOP curve, but there is no discernible difference between the three adjusted curves.
The R-square value for the uncorrected IOP curve was 0.89, while values for the three curves
with adjusted IOP were nearly identical, with R-square = 0.94. There was no statistically
significant difference between the uncorrected or corrected curves by slope comparison after
log transformation. Using the population mean of CCT = 550 microns, there was a slight shift
in the curves, but the outcome was not significantly different.
DISCUSSION
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Although an elevated IOP is not essential for the diagnosis of glaucoma, its importance in the
pathogenesis of the disease is supported by evidence from both interventional and
epidemiologic studies. Specifically, lowering of the IOP has been shown to reduce the risk of
development or progression of OAG in patients with ocular hypertension, early glaucoma, and
normal tension glaucoma,2-4 and lower IOP is associated with less progression in advanced
glaucoma.5 Population-based studies have also shown a clear relationship between the
prevalence of OAG and the level of IOP.6-9
The “gold standard” for IOP measurement is Goldmann applanation tonometry, in which a
measured force is used to indent the cornea a standardized surface area. Based on the ImbertFick principle, pressure inside of the eye is proportional to the force required for indentation.
One source of error, however, is resistance of the cornea itself to indentation. Accordingly,
applanation is actually a measurement of the true IOP plus resistance by the cornea, and the
corneal resistance can be affected by thickness, hydration, elasticity and, perhaps, as yet
unknown factors. Corneal thickness has been shown to affect IOP readings, with thin corneas
resulting in a falsely low IOP, and thick corneas resulting in a falsely high IOP.18,19
The importance of CCT was highlighted by the Ocular Hypertension Treatment Study, in which
it was found to be a significant risk factor, independent of IOP, for the development of
glaucomatous damage.3 Although CCT has been associated with the severity of glaucomatous
damage,20 its impact as a risk for OAG is not known.
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To our knowledge, our study is the first population-based analysis of OAG prevalence and its
relationship to IOP with respect to CCT. We analyzed the impact of CCT in two ways: first,
by dividing the population based on CCT and observing the difference between OAG
prevalence and IOP before and after adjusting for the impact of CCT on IOP within these
groups, and, second, by examining the impact of adjusting IOP for the entire population based
on various CCT “correction” algorithms.
One possible bias in our study was the exclusion of participants currently receiving medical
treatment or with a history of laser or surgical therapy for glaucoma. This may have eliminated
some individuals with a high baseline IOP that was noted prior to the study and may have
skewed the prevalence towards lower IOP. However, the aim of the study was to examine the
relationship between baseline, untreated IOP and OAG prevalence. Inclusion of these patients
would quite likely have skewed the prevalence of OAG towards a lower IOP by including
glaucoma patients with medically or surgically lowered IOP. However, without reliable data
on the pretreatment baseline IOP of these individuals, their exclusion was considered to be
necessary.
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After the population was divided into thin, normal and thick CCT measurements, a clear
difference was seen in the relationship between OAG prevalence and IOP. The group with the
thin CCT had the greatest risk of having OAG at a given IOP, risk of those with normal CCT
was intermediate, and risk for those with a thick CCT was lowest. Furthermore, when IOP was
adjusted for the thin and thick CCT groups using the most conservative algorithm, the curves
shifted towards the normal, albeit not completely. This suggests that the impact of IOP
measurement is partially responsible for the difference seen in OAG prevalence and IOP
between thin, normal and thick CCT, but that there is an independent risk related to CCT itself.
It is possible, however, that this independent risk is due to other corneal properties associated
with CCT.
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We found that adjusting each IOP value within the population had a small but measurable
impact on the prevalence of the OAG-IOP curve, with each correction algorithm shifting the
curve more steeply towards the exponential. Interestingly, both adjusted and unadjusted curves
began their exponential increase in OAG prevalence at an IOP of approximately 19-20 mmHg,
which coincided with two standard deviations above the mean IOP. However, this difference
is not sufficient to warrant an adjustment in IOP in epidemiologic studies. It should be noted
also that none of the correction algorithms used have been independently validated, and they
likely represent an oversimplification of a complex relationship between corneal properties
and applanation tonometry. The relationship between IOP and CCT is not likely to be linear,
and CCT may not be as important as are other corneal factors, such as hydration or rigidity.
The results of the study reported herein support the relationship between IOP, CCT and OAG
prevalence in the Latino population. Further studies validating this relationship in other
populations should be determined.
Acknowledgements
Support: National Institutes of Health Grants: NEI U10-EY-11753 and EY-03040 and an unrestricted grant from the
Research to Prevent Blindness, New York, NY. Rohit Varma is a Research to Prevent Blindness Sybil B. Harrington
Scholar.
The Los Angeles Latino Eye Study Group, University of Southern California, Los Angeles, CA: Rohit Varma, MD,
MPH; Sylvia H. Paz, MS; Stanley P. Azen, PhD; Lupe Cisneros, COA; Elizabeth Corona; Carolina Cuestas, OD;
Denise R. Globe, PhD; Sora Hahn, MD; Mei-Ying Lai, MS; George Martinez; Susan Preston-Martin, PhD; Ronald
E. Smith, MD; LaVina Tetrow, Mina Torres, MS; Natalia Uribe, OD; Jennifer Wong, MPH; Joanne Wu, MPH; Myrna
Zuniga.
Battelle Survey Research Center, St. Louis, MO: Sonia Chico, BS; Lisa John, MSW; Michael Preciado, BA; Karen
Tucker, MA.
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Ocular Epidemiology Grading Center, University of Wisconsin, Madison, WI: Ronald Klein, MD, MPH.
References
1. American Academy of Ophthalmology. Preferred Practice Patterns in the Management of Glaucoma
and Ocular Hypertension. San Francisco, CA: AAO Press; 2003.
2. Kass MA, Heuer DK, Higginbotham EJ, et al. The Ocular Hypertension Treatment Study: a randomized
trial determines that topical ocular hypotensive medication delays or prevents the onset of primary
open-angle glaucoma. Arch Ophthalmol 2002;120:701–713. [PubMed: 12049574]
3. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors
that predict the onset of primary open-angle glaucoma. Arch Ophthalmol 2002;120:714–20. [PubMed:
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between untreated patients with normal-tension glaucoma and patients with therapeutically reduced
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5. The AGIS Investigators. The Advanced Glaucoma Intervention Study (AGIS): 7. The relationship
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10. Whitacre M, Stein RA, Hassanein K. The effect of corneal thickness on applanation tonometry. Am
J Ophthalmol 1993;115:592–596. [PubMed: 8488910]
11. Feltgen N, Leifert D, Funk J. Correlation between central corneal thickness, applanation tonometry,
and direct intracameral IOP readings. Br J Ophthalmol 2001;85:85–87. [PubMed: 11133718]
12. Varma R, Ying-Lai M, Francis BA, et al. Prevalence of open-angle glaucoma and ocular hypertension
in Latinos: the Los Angeles Latino Eye Study. Ophthalmology 2004;111:1439–1448. [PubMed:
15288969]
13. Varma R, Paz SH, Azen SP, et al. The Los Angeles Latino Eye Study: design, methods, and baseline
data. Ophthalmology 2004;111:1121–1131. [PubMed: 15177962]
14. Shih CY, Graff Zivin JS, Trokel SL, Tsai JC. Clinical significance of central corneal thickness in the
management of glaucoma. Arch Ophthalmol 2004;122:1270–1275. [PubMed: 15364705]
15. Ehlers N, Hansen FK. Central corneal thickness in low-tension glaucoma. Acta Ophthalmol (Copenh)
1974;52:740–746. [PubMed: 4479566]
16. Ehlers N, Hansen FK, Aasved H. Biometric correlations of corneal thickness. Acta Ophthalmol
(Copenh) 1975;53:652–659. [PubMed: 1242286]
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a review and meta-analysis approach. Surv Ophthalmol 2000;44:367–408. [PubMed: 10734239]
18. Whitacre MM, Stein R. Sources of error with use of Goldmann-type tonometers. Surv Opthalmol
1993;38:1–30.
19. Francis BA, Hsieh A, Lai MY, et al. Effects of corneal thickness, corneal curvature, and intraocular
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20. Herndon LW, Weizer JS, Stinnett SS. Central corneal thickness as a risk factor for advanced glaucoma
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Figure 1.
Relationship between the prevalence of open-angle glaucoma and uncorrected intraocular
pressure (IOP) stratified by central corneal thickness (CCT) in microns in the Los Angeles
Latino Eye Study.
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Figure 2.
Lowess curve for relationship between prevalence of open-angle glaucoma and intraocular
pressure (IOP) using unadjusted Goldmann tonometry data in the Los Angeles Latino Eye
Study. Since we are interested in the IOP levels for which we observe an increase in prevalence
of OAG, we take into account positive slope values for determining turning points.
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Table 1
Demographics and Clinical Data of Participants in the Los Angeles Latino Eye Study (n = 5970)
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Age (years)
2323 (38.9%)
1814 (30.4%)
1146 (19.2%)
553 (9.3%)
134 (2.2%)
Gender
Male
2489 (41.7 %)
Female
3481 (58.3 %)
Intraocular Pressure (mmHg)
Mean (SD)
14.5 (3.2)
<15 mmHg
3232 (54.1%)
15 - 20
2527 (42.3%)
21 – 25
181 (3.0%)
>25
30 (0.5%)
≥21
211 (3.5 %)
Central Corneal Thickness (microns)
Mean (SD)
550.0 (3.5)
≤ 510
719 (12.0 %)
511 – 580
4196 (70.3 %)
> 580
1055 (17.7 %)
40 – 49
50 – 59
60 – 69
70 – 79
80+
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Table 2
Odds Ratio Estimates of the Prevalence of Glaucoma for Intraocular Pressure
(IOP) Groups Stratified by Central Corneal Thickness (CCT) Groups in the Los
Angeles Latino Eye Study
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Intraocular Pressure (mmHg)
Odds Ratio (95% CI)
Central Corneal Thickness (≤ 534 microns)
<15
1.00
15 – 20
2.02 (1.24 – 3.29)
21 – 25
7.17 (3.30 – 15.60)
≥ 26
74.59 (17.71 – 314.08)
Central Corneal Thickness (534 to 564 microns)
<15
1.00
15 – 20
2.22 (1.33 – 3.71)
21 – 25
10.88 (4.73 – 25.02)
≥26
44.71 (10.55 – 189.41)
Central Corneal Thickness ( > 564 microns)
<15
1.00
15 – 20
2.84 (1.63 – 4.95)
21 – 25
8.01 (3.24 – 19.82)
≥26
38.13 (9.99 – 145.66)
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Table 3
Prevalence of Open-angle Glaucoma Stratified by Intraocular Pressure (IOP) Levels and Different CCT-related Correction Algorithms
for IOP in the Los Angeles Latino Eye Study
Prevalence of Open-Angle Glaucoma at various IOP levels
15 mmHg
18 mmHg
21 mmHg
25 mmHg
30 mmHg
Uncorrected
6.5 %
13.5 %
20.5 %
29.8 %
41.5 %
C1
7.0 %
14.1 %
21.2 %
30.7 %
42.5 %
C2
6.7 %
13.9 %
21.2 %
31.0 %
43.1 %
C3
6.7 %
14.3 %
21.9 %
32.0 %
44.6 %
C1 = Correction algorithm #1; 1 mmHg per 50 micron change in CCT (central corneal thickness)
Trend Analysis
p<0.0001
p<0.0001
p<0.0001
p<0.0001
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Correction Algorithm
C2 = Correction algorithm #2; 2.5 mmHg per 50 microns change in CCT
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C3 = Correction algorithm #3; 3.5 mmHg per 50 microns change in CCT
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