BMC Ophthalmology

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BMC Ophthalmology
Research article Open Access
Comparison of age-specific cataract prevalence in two
population-based surveys 6 years apart
Ava Grace Tan†, Jie Jin Wang*†, Elena Rochtchina† and Paul Mitchell†
Address: Centre for Vision Research, Westmead Millennium Institute, Department of Ophthalmology, University of Sydney, Westmead Hospital,
Westmead, NSW, Australia
Email: Ava Grace Tan - ava_tan@wmi.usyd.edu.au; Jie Jin Wang* - jiejin_wang@wmi.usyd.edu.au;
Elena Rochtchina - elena_rochtchina@wmi.usyd.edu.au; Paul Mitchell - paul_mitchell@wmi.usyd.edu.au
* Corresponding author †Equal contributors
Abstract
Background: In this study, we aimed to compare age-specific cortical, nuclear and posterior
subcapsular (PSC) cataract prevalence in two surveys 6 years apart.
Methods: The Blue Mountains Eye Study examined 3654 participants (82.4% of those eligible) in
cross-section I (1992–4) and 3509 participants (75.1% of survivors and 85.2% of newly eligible) in
cross-section II (1997–2000, 66.5% overlap with cross-section I). Cataract was assessed from lens
photographs following the Wisconsin Cataract Grading System. Cortical cataract was defined if
cortical opacity comprised ≥ 5% of lens area. Nuclear cataract was defined if nuclear opacity ≥
Wisconsin standard 4. PSC was defined if any present. Any cataract was defined to include persons
who had previous cataract surgery. Weighted kappa for inter-grader reliability was 0.82, 0.55 and
0.82 for cortical, nuclear and PSC cataract, respectively. We assessed age-specific prevalence using
an interval of 5 years, so that participants within each age group were independent between the
two surveys.
Results: Age and gender distributions were similar between the two populations. The age-specific
prevalence of cortical (23.8% in 1st, 23.7% in 2nd) and PSC cataract (6.3%, 6.0%) was similar. The
prevalence of nuclear cataract increased slightly from 18.7% to 23.9%. After age standardization,
the similar prevalence of cortical (23.8%, 23.5%) and PSC cataract (6.3%, 5.9%), and the increased
prevalence of nuclear cataract (18.7%, 24.2%) remained.
Conclusion: In two surveys of two population-based samples with similar age and gender
distributions, we found a relatively stable cortical and PSC cataract prevalence over a 6-year period.
The increased prevalence of nuclear cataract deserves further study.
Background
Age-related cataract is the leading cause of reversible visual
impairment in older persons [1-6]. In Australia, it is
estimated that by the year 2021, the number of people
affected by cataract will increase by 63%, due to population
aging [7]. Surgical intervention is an effective treatment
for cataract and normal vision (> 20/40) can usually
be restored with intraocular lens (IOL) implantation.
Cataract surgery with IOL implantation is currently the
most commonly performed, and is, arguably, the most
cost effective surgical procedure worldwide. Performance
Published: 20 April 2006
BMC Ophthalmology 2006, 6:17 doi:10.1186/1471-2415-6-17
Received: 14 December 2005
Accepted: 20 April 2006
This article is available from: http://www.biomedcentral.com/1471-2415/6/17
© 2006 Tan et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
BMC Ophthalmology 2006, 6:17 http://www.biomedcentral.com/1471-2415/6/17
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of this surgical procedure has been continuously increasing
in the last two decades. Data from the Australian
Health Insurance Commission has shown a steady
increase in Medicare claims for cataract surgery [8]. A 2.6-
fold increase in the total number of cataract procedures
from 1985 to 1994 has been documented in Australia [9].
The rate of cataract surgery per thousand persons aged 65
years or older has doubled in the last 20 years [8,9]. In the
Blue Mountains Eye Study population, we observed a onethird
increase in cataract surgery prevalence over a mean
6-year interval, from 6% to nearly 8% in two cross-sectional
population-based samples with a similar age range
[10]. Further increases in cataract surgery performance
would be expected as a result of improved surgical skills
and technique, together with extending cataract surgical
benefits to a greater number of older people and an
increased number of persons with surgery performed on
both eyes.
Both the prevalence and incidence of age-related cataract
link directly to the demand for, and the outcome of, cataract
surgery and eye health care provision. This report
aimed to assess temporal changes in the prevalence of cortical
and nuclear cataract and posterior subcapsular cataract
(PSC) in two cross-sectional population-based
surveys 6 years apart.
Methods
The Blue Mountains Eye Study (BMES) is a populationbased
cohort study of common eye diseases and other
health outcomes. The study involved eligible permanent
residents aged 49 years and older, living in two postcode
areas in the Blue Mountains, west of Sydney, Australia.
Participants were identified through a census and were
invited to participate. The study was approved at each
stage of the data collection by the Human Ethics Committees
of the University of Sydney and the Western Sydney
Area Health Service and adhered to the recommendations
of the Declaration of Helsinki. Written informed consent
was obtained from each participant.
Details of the methods used in this study have been
described previously [11]. The baseline examinations
(BMES cross-section I) were conducted during 1992–
1994 and included 3654 (82.4%) of 4433 eligible residents.
Follow-up examinations (BMES IIA) were conducted
during 1997–1999, with 2335 (75.0% of BMES
cross section I survivors) participating. A repeat census of
the same area was performed in 1999 and identified 1378
newly eligible residents who moved into the area or the
eligible age group. During 1999–2000, 1174 (85.2%) of
this group participated in an extension study (BMES IIB).
BMES cross-section II thus includes BMES IIA (66.5%)
and BMES IIB (33.5%) participants (n = 3509).
Similar procedures were used for all stages of data collection
at both surveys. A questionnaire was administered
including demographic, family and medical history. A
detailed eye examination included subjective refraction,
slit-lamp (Topcon SL-7e camera, Topcon Optical Co,
Tokyo, Japan) and retroillumination (Neitz CT-R camera,
Neitz Instrument Co, Tokyo, Japan) photography of the
lens. Grading of lens photographs in the BMES has been
previously described [12]. Briefly, masked grading was
performed on the lens photographs using the Wisconsin
Cataract Grading System [13]. Cortical cataract and PSC
were assessed from the retroillumination photographs by
estimating the percentage of the circular grid involved.
Cortical cataract was defined when cortical opacity
involved at least 5% of the total lens area. PSC was defined
when opacity comprised at least 1% of the total lens area.
Slit-lamp photographs were used to assess nuclear cataract
using the Wisconsin standard set of four lens photographs
[13]. Nuclear cataract was defined when nuclear opacity
was at least as great as the standard 4 photograph. Any cataract
was defined to include persons who had previous
cataract surgery as well as those with any of three cataract
types. Inter-grader reliability was high, with weighted
kappa 0.82 for cortical cataract, 0.55 (simple kappa 0.75)
for nuclear cataract and 0.82 for PSC grading. The intragrader
reliability for nuclear cataract was assessed with
simple kappa 0.83 for the senior grader who graded
nuclear cataract at both surveys. All PSC cases were confirmed
by an ophthalmologist (PM).
In cross-section I, 219 persons (6.0%) had missing or
ungradable Neitz photographs, leaving 3435 with photographs
available for cortical cataract and PSC assessment,
while 1153 (31.6%) had randomly missing or ungradable
Topcon photographs due to a camera malfunction, leaving
2501 with photographs available for nuclear cataract
assessment. Comparison of characteristics between participants
with and without Neitz or Topcon photographs in
cross-section I showed no statistically significant differences
between the two groups, as reported previously
[12]. In cross-section II, 441 persons (12.5%) had missing
or ungradable Neitz photographs, leaving 3068 for cortical
cataract and PSC assessment, and 648 (18.5%) had
missing or ungradable Topcon photographs, leaving 2860
for nuclear cataract assessment.
Data analysis was performed using the Statistical Analysis
System (SAS, SAS Institute, Cary, NC, USA). Age-adjusted
prevalence was calculated using direct standardization of
the cross-section II population to the cross-section I population.
We assessed age-specific prevalence using an
interval of 5 years, so that participants within each age
group were independent between the two cross-sectional
surveys.
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Results
Characteristics of the two survey populations have been
previously compared [14] and showed that age and sex
distributions were similar. Table 1 compares participant
characteristics between the two cross-sections. Cross-section
II participants generally had higher rates of diabetes,
hypertension, myopia and more users of inhaled steroids.
Cataract prevalence rates in cross-sections I and II are
shown in Figure 1. The overall prevalence of cortical cataract
was 23.8% and 23.7% in cross-sections I and II,
respectively (age-sex adjusted P = 0.81). Corresponding
prevalence of PSC was 6.3% and 6.0% for the two crosssections
(age-sex adjusted P = 0.60). There was an
increased prevalence of nuclear cataract, from 18.7% in
cross-section I to 23.9% in cross-section II over the 6-year
period (age-sex adjusted P < 0.001). Prevalence of any cataract
(including persons who had cataract surgery), however,
was relatively stable (46.9% and 46.8% in crosssections
I and II, respectively).
After age-standardization, these prevalence rates remained
stable for cortical cataract (23.8% and 23.5% in the two
surveys) and PSC (6.3% and 5.9%). The slightly increased
prevalence of nuclear cataract (from 18.7% to 24.2%) was
not altered.
Table 2 shows the age-specific prevalence rates for cortical
cataract, PSC and nuclear cataract in cross-sections I and
II. A similar trend of increasing cataract prevalence with
increasing age was evident for all three types of cataract in
both surveys. Comparing the age-specific prevalence
between the two surveys, a reduction in PSC prevalence in
cross-section II was observed in the older age groups (≥ 75
years). In contrast, increased nuclear cataract prevalence
in cross-section II was observed in the older age groups (≥
70 years). Age-specific cortical cataract prevalence was relatively
consistent between the two surveys, except for a
reduction in prevalence observed in the 80–84 age group
and an increasing prevalence in the older age groups (≥ 85
years).
Similar gender differences in cataract prevalence were
observed in both surveys (Table 3). Higher prevalence of
cortical and nuclear cataract in women than men was evident
but the difference was only significant for cortical
cataract (age-adjusted odds ratio, OR, for women 1.3,
95% confidence intervals, CI, 1.1–1.5 in cross-section I
and OR 1.4, 95% CI 1.1–1.6 in cross-section II). In con-
Table 1: Participant characteristics.
Characteristics Cross-section I Cross-section II
n % n %
Age (mean) (66.2) (66.7)
50–54 485 13.3 350 10.0
55–59 534 14.6 580 16.5
60–64 638 17.5 600 17.1
65–69 671 18.4 639 18.2
70–74 538 14.7 572 16.3
75–79 422 11.6 407 11.6
80–84 230 6.3 226 6.4
85–89 100 2.7 110 3.1
90+ 36 1.0 24 0.7
Female 2072 56.7 1998 57.0
Ever Smokers 1784 51.2 1789 51.2
Use of inhaled steroids 370 10.94 478 13.8^
History of:
Diabetes 284 7.8 347 9.9^
Hypertension 1669 46.0 1825 52.2^
Emmetropia* 1558 42.9 1478 42.2
Myopia* 442 12.2 495 14.1^
Hyperopia* 1633 45.0 1532 43.7
n = number of persons affected
* best spherical equivalent refraction correction
^ P < 0.01
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t
rast, men had slightly higher PSC prevalence than women
in both cross-sections but the difference was not significant
(OR 1.1, 95% CI 0.8–1.4 for men in cross-section I
and OR 1.2, 95% 0.9–1.6 in cross-section II).
Discussion
Findings from two surveys of BMES cross-sectional populations
with similar age and gender distribution showed
that the prevalence of cortical cataract and PSC remained
stable, while the prevalence of nuclear cataract appeared
to have increased. Comparison of age-specific prevalence,
with totally independent samples within each age group,
confirmed the robustness of our findings from the two
survey samples. Although lens photographs taken from
the two surveys were graded for nuclear cataract by the
same graders, who documented a high inter- and intragrader
reliability, we cannot exclude the possibility that
variations in photography, performed by different photographers,
may have contributed to the observed difference
in nuclear cataract prevalence. However, the overall
Table 2: Age-specific prevalence of cataract types in cross sections I and II.
Cataract type Age (years) Cross-section I Cross-section II
n % (95% CL)* n % (95% CL)*
Cortical 50–54 473 4.4 (2.6–6.3) 338 7.4 (4.6–10.2)
55–59 522 9.2 (6.7–11.7) 542 9.0 (6.6–11.5)
60–64 615 16.4 (13.5–19.4) 556 16.7 (13.6–19.8)
65–69 653 26.2 (22.8–29.6) 581 23.6 (20.1–27.0)
70–74 516 31.2 (27.2–35.2) 514 35.4 (31.3–39.6)
75–79 366 40.2 (35.1–45.2) 332 39.8 (34.5–45.1)
80–84 194 58.8 (51.8–65.8) 163 42.9 (35.3–50.6)
85–89 74 52.7 (41.1–64.4) 73 54.8 (43.1–66.5)
90+ 22 68.2 (47.0–89.3) 14 78.6 (54.0–103.2)
PSC 50–54 474 2.7 (1.3–4.2) 338 2.4 (0.7–4.0)
55–59 522 2.9 (1.4–4.3) 541 2.6 (1.3–3.9)
60–64 616 4.6 (2.9–6.2) 548 5.7 (3.7–7.6)
65–69 655 6.3 (4.4–8.1) 573 4.5 (2.8–6.3)
70–74 517 6.8 (4.6–8.9) 505 9.7 (7.1–12.3)
75–79 367 11.4 (8.2–14.7) 327 9.5 (6.3–12.7)
80–84 196 12.2 (7.6–16.9) 155 10.3 (5.5–15.2)
85–89 74 18.9 (9.8–28.1) 69 11.6 (3.9–19.4)
90+ 23 21.7 (3.5–40.0) 11 0.0
Nuclear 50–54 323 1.6 (0.2–2.9) 331 0.9 (–0.2–1.9)
55–59 386 2.3 (0.8–3.8) 507 3.6 (1.9–5.2)
60–64 453 5.3 (3.2–7.4) 501 11.6 (8.8–14.4)
65–69 478 17.2 (13.8–20.1) 534 18.5 (15.2–21.9)
70–74 392 27.6 (23.1–32.0) 453 36.0 (31.6–40.4)
75–79 255 45.1 (39.0–51.3) 302 55.6 (50.0–61.3)
80–84 146 54.1 (45.9–62.3) 147 73.5 (66.3–80.7)
85–89 50 64.0 (50.2–77.8) 70 80.0 (70.4–89.6)
90+ 18 72.2 (49.3–95.1) 15 73.3 (48.0–98.7)
n = number of persons
* 95% Confidence Limits
Cataract FMioguunrtea i1n ps rEeyvea lSetnucdey in cross-sections I and II of the Blue
Cataract prevalence in cross-sections I and II of the Blue
Mountains Eye Study.
0
10
20
30
40
50
cortical PSC nuclear any
cataract
Cataract type
%
Cross-section I
Cross-section II
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prevalence of any cataract (including cataract surgery) was
relatively stable over the 6-year period.
Although different population-based studies used different
grading systems to assess cataract [15], the overall
prevalence of the three cataract types were similar across
different study populations [12,16-23]. Most studies have
suggested that nuclear cataract is the most prevalent type
of cataract, followed by cortical cataract [16-20]. Ours and
other studies reported that cortical cataract was the most
prevalent type [12,21-23].
Our age-specific prevalence data show a reduction of
15.9% in cortical cataract prevalence for the 80–84 year
age group, concordant with an increase in cataract surgery
prevalence by 9% in those aged 80+ years observed in the
same study population [10]. Although cortical cataract is
thought to be the least likely cataract type leading to a cataract
surgery, this may not be the case in all older persons.
A relatively stable cortical cataract and PSC prevalence
over the 6-year period is expected. We cannot offer a
definitive explanation for the increase in nuclear cataract
prevalence. A possible explanation could be that a moderate
level of nuclear cataract causes less visual disturbance
than the other two types of cataract, thus for the oldest age
groups, persons with nuclear cataract could have been less
likely to have surgery unless it is very dense or co-existing
with cortical cataract or PSC. Previous studies have shown
that functional vision and reading performance were high
in patients undergoing cataract surgery who had nuclear
cataract only compared to those with mixed type of cataract
(nuclear and cortical) or PSC [24,25]. In addition, the
overall prevalence of any cataract (including cataract surgery)
was similar in the two cross-sections, which appears
to support our speculation that in the oldest age group,
nuclear cataract may have been less likely to be operated
than the other two types of cataract. This could have
resulted in an increased nuclear cataract prevalence (due
to less being operated), compensated by the decreased
prevalence of cortical cataract and PSC (due to these being
more likely to be operated), leading to stable overall prevalence
of any cataract.
Possible selection bias arising from selective survival
among persons without cataract could have led to underestimation
of cataract prevalence in both surveys. We
assume that such an underestimation occurred equally in
both surveys, and thus should not have influenced our
assessment of temporal changes.
Measurement error could also have partially contributed
to the observed difference in nuclear cataract prevalence.
Assessment of nuclear cataract from photographs is a
potentially subjective process that can be influenced by
variations in photography (light exposure, focus and the
slit-lamp angle when the photograph was taken) and
grading. Although we used the same Topcon slit-lamp
camera and the same two graders who graded photos
from both surveys, we are still not able to exclude the possibility
of a partial influence from photographic variation
on this result.
A similar gender difference (women having a higher rate
than men) in cortical cataract prevalence was observed in
both surveys. Our findings are in keeping with observations
from the Beaver Dam Eye Study [18], the Barbados
Eye Study [22] and the Lens Opacities Case-Control
Group [26]. It has been suggested that the difference
could be related to hormonal factors [18,22]. A previous
study on biochemical factors and cataract showed that a
lower level of iron was associated with an increased risk of
cortical cataract [27]. No interaction between sex and biochemical
factors were detected and no gender difference
was assessed in this study [27]. The gender difference seen
in cortical cataract could be related to relatively low iron
levels and low hemoglobin concentration usually seen in
women [28]. Diabetes is a known risk factor for cortical
Table 3: Gender distribution of cataract types in cross-sections I and II.
Cataract type Gender Cross-section I Cross-section II
n % (95% CL)* n % (95% CL)*
Cortical Male 1496 21.1 (19.0–23.1) 1328 20.4 (18.2–22.6)
Female 1939 25.9 (23.9–27.8) 1785 26.2 (24.2–28.3)
PSC Male 1500 6.5 (5.2–7.7) 1314 6.4 (5.1–7.7)
Female 1944 6.2 (5.1–7.2) 1753 5.7 (4.6–6.7)
Nuclear Male 1106 17.6 (15.4–19.9) 1225 22.5 (20.1–24.8)
Female 1395 19.5 (17.4–21.6) 1635 25.0 (22.9–27.1)
n = number of persons
* 95% Confidence Limits
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cataract but in this particular population diabetes is more
prevalent in men than women in all age groups [29]. Differential
exposures to cataract risk factors or different dietary
or lifestyle patterns between men and women may
also be related to these observations and warrant further
study.
Conclusion
In summary, in two population-based surveys 6 years
apart, we have documented a relatively stable prevalence
of cortical cataract and PSC over the period. The observed
overall increased nuclear cataract prevalence by 5% over a
6-year period needs confirmation by future studies, and
reasons for such an increase deserve further study.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
AGT graded the photographs, performed literature search
and wrote the first draft of the manuscript. JJW graded the
photographs, critically reviewed and modified the manuscript.
ER performed the statistical analysis and critically
reviewed the manuscript. PM designed and directed the
study, adjudicated cataract cases and critically reviewed
and modified the manuscript. All authors read and
approved the final manuscript.
Acknowledgements
This study was supported by the Australian National Health & Medical
Research Council, Canberra, Australia (Grant Nos 974159, 991407). The
abstract was presented at the Association for Research in Vision and Ophthalmology
(ARVO) meeting in Fort Lauderdale, Florida, USA, May 2005.
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BMC Ophthalmology 2006, 6:17 http://www.biomedcentral.com/1471-2415/6/17
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