Download Intraocular pressure in an American community. The Beaver

Investigative Ophthalmology & Visual Science, Vol. 33, No. 7, June 1992
Copyright © Association for Research in Vision and Ophthalmology
Intraocular Pressure in an American Community
The Deover Dom Eye Study
Barbara E. K. Klein, Ronald Klein, and Karhryn L. P. Linron
The Beaver Dam Eye Study is a population-based study of age-related eye diseases in persons 43-86 yr
of age. Applanation tonometry was done on all study subjects. Mean intraocular pressure (IOP) increased significantly with age. Mean IOP differed little between the sexes and was not significantly
different after age adjustment (in right eyes of 2721 women, it was 15.5 mm Hg, and in right eyes of
2135 men, it was 15.3 mm Hg). There was an association of IOP with systolic and diastolic blood
pressures, body mass index, hematocrit, serum glucose, glycohemoglobin, cholesterol level, pulse,
nuclear sclerosis, season, and time of day of measurement. These data confirm that, in a general
population, IOP is associated with important systemic and ocular characteristics. Those characteristics should be considered in further research on determinants of IOP. Invest Ophthalmol Vis Sci
33:2224-2228, 1992
Intraocular pressure (IOP) is an inherent physiologic characteristic of importance in maintaining
structure and function of the eye. Correlates of this
measurement include other important physiologic parameters that may need to be considered in investigating determinants of IOP. Because IOP is the ocular
parameter that is associated most commonly with
glaucoma,1"4 it may be important to evaluate these
physiologic correlates of IOP; they may confound relationships between IOP and glaucoma. We briefly
describe the distribution of IOP and investigate its correlates in the population participating in the Beaver
Dam Eye Study.
persons in the target age range, 4926 were evaluated
(83.14%).
Relevant parts of the study evaluation included the
date of birth and a history of ocular trauma, surgery,
medications, glaucoma, and diabetes. Most of the participants were white (99.4%). Blood pressure was
measured according to the Hypertension Detection
and Follow-up Program protocol.6 Height and weight
were measured with a Health-o-Meter scale (Continental Scale Corp., Bridgewater, IL). The IOP was
measured with a Goldmann applanation tonometer.
A drop of Fluress (Armour, Kankakee, IL) was instilled in each eye. The tonometer was set at 10. The
measurement was taken as the examiner viewed the
mires through the prism. When the end point was
reached, the examiner moved the slit lamp away from
the eye and recorded the reading. The procedure was
repeated for the other eye. The time of measurement
was recorded. Iris pigmentation was evaluated and
compared with three standard color 35-mm slides.
The protocol specifies categories of pigmentation
from a lightly pigmented iris (blue) through a heavily
pigmented one (brown).7 After assessing anterior
chamber depth, the pupils were dilated with one drop
each of tropicamide 1% and phenylephrine 2.5%.
When the pupils were dilated, a clinical assessment of
the presence and severity of cataract was made, and
photographs were taken with the illuminating beam
at 45° to the viewing system.7 These photographs subsequently were graded by comparison with standard
photographs according to the protocol.7 A blood specimen was obtained, and glucose and glycosylated hemoglobin were measured. The subjects were classified
as having no diabetes if there was a negative history of
Materials and Methods
A private census of Beaver Dam was done, and the
procedures have been published elsewhere.5 In brief,
6612 households were identified, of which 3715 had
at least one occupant 43-84 yr of age. Thereafter,
each individual in the target age range was contacted
for a study appointment. Three subjects were examined whose age was 86 yr at the time of testing. Their
data are included in this report. Of the total of 5925
From the Department of Ophthalmology, University of Wisconsin Madison, Madison, Wisconsin.
Supported by National Institutes of Health (Bethesda, Maryland)
grant 5U10 EY 10 6594 (RK and BEKK).
Presented in part at the Annual Meeting of the Association for
Research in Vision and Ophthalmology, Sarasota, Florida, April 28
to May 3, 1991.
Submitted for publication: October 2, 1991; accepted December
21, 1991.
Reprint requests: Barbara E. K. Klein, MD, MPH, Department
of Ophthalmology, 600 Highland Avenue, Madison, WI 53792.
2224
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INTRAOCULAR PRESSURES IN DEAVER DAM / Klein er ol
No. 7
2225
We investigated the relationship between nuclear
sclerosis and IOP (Table 2). There was a significant
trend of increasing IOP with increasing severity of nuclear sclerosis. In those eyes that were either aphakic
or had intraocular lens implants, the mean IOP was
between the mean for those eyes in thefirsttwo categories of severity of nuclear sclerosis (Table 2). A history
of other eye surgery or trauma was not associated with
increased mean IOP.
The participants were categorized as to definite, possible, or no history of diabetes. There was a significant
difference between the IOP means among these
groups (Table 2). There also was a seasonal effect on
IOP; measurements during the summer months
(July, August, and September) were significantly
lower (15.2 mm Hg) than those during the winter
months (January, March, and April; 15.7 mm Hg).
There was no significant effect of iris color, refractive
error, cigarette smoking, or alcohol consumption on
IOP. A history of cardiovascular disease was not related to IOP.
Table 3 describes the multiple linear-regression
analysis for the variables significantly associated with
IOP. Data for persons who reported receiving medicine for glaucoma or who had had surgery for glaucoma were excluded from this analysis. Systolic blood
pressure, time of day of examination, body mass index, glaucoma history, refractive error, cholesterol
level, hematocrit, female sex, month of examination,
pulse rate, severity of nuclear sclerosis, diastolic blood
pressure, and glycosylated hemoglobin were all selected in the stepwise analysis. Age did not enter as a
significant factor when considered with the other variables.
diabetes, blood glucose was less than 200 mg/dl, and
glycosylated hemoglobin was within two standard deviations of the mean for the particular age-sex subgroup. Informed consent was obtained for each subject.
We used the Statistical Analysis System (SAS, Cary,
NC) to analyze the data.8 Because the distribution of
IOP does not differ from normality, parametric tests
were chosen where applicable. Univariate associations with IOP were detected by Student's t-test, and
analysis of variance techniques were used for categoric factors, with Pearson correlation coefficients for
continuous factors. Confidence intervals for the
correlation coefficients were calculated using the
method developed by Fisher.9
Results
There was little difference between the mean IOP of
right and left eyes (0.14 ± 2.11 m m Hg) or their association s with other variables. Therefore, data for right
eyes only are presented. The distribution of IOP in the
right eye by sex and age is given in Table 1. There was
a small but significant change in mean IOP with increasing age (P < 0.05). We found IOP greater than 21
mm Hg to be more frequent in older age groups (P
< 0.01, by test for trends). Women had higher mean
IOP than did men, but the difference was not great
and was only borderline significant (P = 0.06) when
adjusted for age (by multiple-linear regression).
During the interview, the participants were asked
whether they had glaucoma or were receiving medication for this disease. The mean IOP was significantly
higher (20.1 mm Hg) in those with a positive compared with a negative history (15.3 mm Hg).
Table 1. Percent distribution of intraocular pressure in right eyes by age and sex
IOP (mm Hg)
Age (years)
Sex
N
0-12
13-15
16-18
19-21
22-24
25+
Mean
SD
43-49
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
448
385
340
327
347
287
348
325
391
299
340
233
276
168
231
111
2721
2135
21.9
22.9
21.8
24.2
23.6
16.4
17.8
15.1
12.5
20.1
15.3
15.9
16.3
19.6
18.2
22.5
18.5
19.6
35.5
37.4
33.2
39.1
29.4
40.8
28.5
32.9
29.4
32.4
32.7
36.1
31.2
30.4
36.8
31.5
32.0
35.4
32.6
31.2
32.4
24.2
34.9
27.2
38.2
36.0
39.4
30.8
32.9
31.8
31.5
32.7
29.4
33.3
34.2
30.5
6.9
6.2
2.7
1.3
2.7
2.1
4.3
2.4
3.2
4.0
4.9
4.7
3.5
5.6
6.2
6.0
3.5
2.7
3.8
3.4
0.5
1.0
0.3
0.9
0.3
15.0
14.7
15.1
14.9
15.1
15.4
15.6
15.8
16.0
15.4
15.9
15.6
15.9
15.8
15.5
14.9
15.5
15.3
2.9
3.1
3.0
3.3
3.2
3.4
3.1
3.3
3.4
3.4
3.3
3.3
3.6
4.2
3.6
3.9
3.3
3.4
50-54
55-59
60-64
65-69
70-74
75-79
80-86
TOTAL
9.7
9.5
7.5
12.2
11.8
10.5
12.3
11.4
14.7
10.3
13.4
9.5
9.5
9.0
10.6
9.7
Twenty-seven "unreliable" and 43 "not obtainable" excluded from the analyses.
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1.1
0.6
1.5
1.5
0.7
0.9
0.4
1.5
1.8
2.6
0.9
0.9
1.0
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Vol. 33
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / June 1992
Table 2. Mean intraocular pressure in right eyes by subject characteristics
Variable
Nuclear sclerosis
<Stdl
<Std2
<Std3
<Std4
>Std4
GC
Aphakic or IOL
Glaucoma Hx
No
Yes
Diabetes status
No
Hx, but no meds/symptom
Yes
N
Mean (mm Hg)
SD
847
1790
1251
625
25
64
258
15.0
15.2
15.7
15.9
16.0
16.5
15.1
3.0
3.2
3.3
3.7
3.3
3.8
4743
113
15.3
20.1
3.2 1
4.4
4418
55
383
15.3
16.1
16.0
3.3 1
4.0
3.9
P value*
<0.0001
J
<0.0001
0.0003
3.6 J
Trends test.
Discussion
The Beaver Dam Eye Study is a current large population-based study in American adults. It provides information about ocular and systemic parameters.
Thus, it is possible to evaluate independent relationships of other physiologic features with IOP. In the
future, such data will provide the opportunity to determine whether the other characteristics also influence or confound the relationship of IOP to disease
(eg, glaucoma, altered retinal sensitivity, and decreased contrast sensitivity).
There was a small positive relationship of age and
IOP in the adults in Beaver Dam, Wisconsin. Positive
correlations of age and IOP in America and Europe
have been reported in most studies of this relationship.10"16 Both black and white Americans participated in the Health and Nutrition Examination Survey. A positive relationship was found in both groups.
A negative association between these variables was
seen in Japanese studies.1718 Whether this difference
is related directly to specific ocular characteristics that
differ between ethnic groups o
mental effect is not known.
Eye color was defined according to a scale of increasing pigmentation in the Beaver Dam Eye Study.
Standard 35-mm slide photographs were used by the
examiner for comparison with the subject's iris. No
relationship was seen between the degree of pigmentation and IOP. Although pigmentary glaucoma may be
related to mechanical obstruction from pigment in
the trabeculum, there is no evidence in these analyses
to suggest that iris pigment plays a role in the height of
the IOP level in the absence of characteristics of pigmentary glaucoma. Others reported an effect of iris
color on IOP.19 The difference between our findings
and theirs may reflect differences in ethnicity between
the study populations or chance.
The severity of nuclear sclerosis was based on gradings of slit-lamp photographs according to the study
protocol.7'20 It was correlated with a higher mean IOP.
The finding that mean IOP in aphakic eyes or those
with lens implants was similar to that found in those
with less severe nuclear sclerosis is compatible with
Table 3. Multiple regression analysis for intraocular pressure in right eyes
Variable •
Coefficient
P value
Partial R2
Total R2
Systolic blood pressure (mm Hg)
Time of day of exam (hr)
Body mass index
Glaucoma history (yes/no)
Refractive error (diopters)
Cholesterol (mg/dl)
Hematocrit (mm)
Sex (F)
Month of exam (1-12)
Pulse (beats/min)
Nuclear sclerosis
Diastolic blood pressure (mm Hg)
Glycosylated hemoglobin (%)
0.019
-0.001
3.883
8.791
-0.107
0.004
0.075
0.473
-0.053
0.019
0.172
0.018
0.074
0.0001
0.0001
0.0001
0.0001
0.0001
0.0007
0.0001
0.0001
0.0002
0.0157
0.0016
0.0014
0.0162
0.039
0.024
0.007
0.006
0.006
0.005
0.004
0.005
0.003
0.002
0.002
0.002
0.001
0.104
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No. 7
INTRAOCULAR PRESSURES IN DEAVER DAM / Klein er ol
the hypothesis that there is a mechanical effect if a
larger lens compromises aqueous outflow; this would
be relieved by lens removal. Such an interpretation
was suggested by some authors who observed amelioration of phacomorphic glaucoma after cataract surgery.21 An alternative explanation may be that cataract surgery influences the aqueous physiology and
leads to lower IOP.
A history of diabetes and medications to lower
blood sugar was elicited during the study interview.
Those who said they had diabetes or were suspected of
having it had a significantly higher mean IOP than
those without diabetes. The IOP among the study participants in the Wisconsin Epidemiologic Study of Diabetic Retinopathy were slightly higher than the mean
of a nondiabetic comparison group evaluated at the
same time.13 Other investigators have found an increased frequency of elevated IOP in people with diabetes.2223 Although a physiologic explanation is unclear, the health care implication is that persons with
diabetes may be at increased risk of glaucoma, and
ophthalmologists who follow such persons should be
evaluating the patient for the possible development of
glaucoma.
Other systemic factors may influence IOP. Blood
pressure has been found in several studies to be associated significantly with 1OP.121319-31 There may be a
direct effect of systolic blood pressure on ultrafiltration and, through this mechanism, on IOP.1827
Body mass index, a measure of obesity, was correlated positively with IOP. Others found a similar relationship of obesity with IOP.25 Because corticosteroid
secretion is increased in obese persons, this may explain this relationship.30 Mechanically, it has been
suggested that orbital pressure from excess fat may
cause a rise in episcleral venous pressure and decreased outflow facility.18*27
Diurnal IOP variation could not be assessed in our
study; the participants only underwent one study examination. The time of the appointment depended
only on scheduling concerns. Nevertheless, a pattern
emerged of mean IOP tending to be higher earlier in
the day. This finding was compatible with the report
on the Bedford Glaucoma Survey;32 positive screenings for glaucoma were more likely to occur in the
morning. This relationship has important implications for those who treat people with glaucoma and
who tailor this treatment to the IOP level.
The pulse rate was found to be correlated to IOP in
earlier studies.12-27 An association of IOP with cholesterol level also was reported previously.12-30 In addition, we found hematocrit was significantly related to
IOP; similarly, hemoglobin and erythrocyte count
were associated with IOP in another study.25 The biologic importance of the association of these cardiovas-
2227
cular risk variables with IOP is unclear, but they appear to be more universal than just an ethnic variation.
Seasonal variations were reported by others,28'33
with slightly higher pressures found in the winter
months. We found a similar pattern in Beaver Dam.
From these earlier studies, we might question whether
this represents a response of people from a northern
European ethnic background or is related to the
northern latitudes. However, one of these studies was
conducted in Israel.34 The seasonality may be related
to the light, temperature, and humidity changes.
There are other annual rhythms affecting physiologic
systems.33 The variation in IOP may be a result of
these other physiologic changes or may be intrinsic to
the regulatory systems in the eye.
The relationships reported emphasize the physiologic interdependence of the eye with other organ systems. These relationships and the influence of external environmental conditions indicate that these too
must be considered when studying IOP and its relationship to eye disease.
Key words: intraocular pressure, epidemiology, Beaver
Dam Eye Study, age-related eye diseases
Acknowledgments
The authors thank the community of Beaver Dam, Wisconsin, and their health care providers for their enthusiastic
cooperation throughout all phases of the study; Moneen
Meuer and Sarah Baumgart for project coordination; Stacy
Meuer for photograph management; Barbara Houser for
data management; Ann Varda, Anik Ganguly, and Karl
Jensen for programming and computing; Yvonne M. Bellay, Dayna S. Dalton, Norma Dorn, Kathy Peterson, and
Kathryn Burke for examining all subjects; Michael Neider
for photography training and equipment design; Carol
Hoyer, Deborah Reiderer, Maria Swift, and A. Jeffrey
Whitehead for photograph grading; Helen Soldner for reception; David L. DeMets for statistical advice; and Julie K.
Olson, Kristine A. Tway, and Luann Soule for manuscript
preparation.
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