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Bjo08600238 PDF
Bjo08600238 PDF
PERSPECTIVE
Br J Ophthalmol 2002;86:238–242
This review describes a scheme for diagnosis of search is that the term glaucoma is reserved for
glaucoma in population based prevalence surveys. people with established, visually significant, end
organ damage. In the public health context, glau-
Cases are diagnosed on the grounds of both structural coma can be seen as an optic neuropathy
and functional evidence of glaucomatous optic associated with characteristic structural damage
neuropathy. The scheme also makes provision for to the optic nerve and associated visual dysfunc-
tion that may be caused by various pathological
diagnosing glaucoma in eyes with severe visual loss processes.
where formal field testing is impractical, and for blind
Structural damage—optic neuropathy
eyes in which the optic disc cannot be seen because of The feature that differentiates glaucoma from
media opacities. other causes of visual morbidity is a characteristic
.......................................................................... pattern of damage to the optic nerve head. This is
most easily recognised at the superior and inferior
poles of the optic disc. The vertical cup:disc ratio
n appropriate case definition is the keystone
A
(VCDR) has proved to be a simple, relatively
of epidemiological research whether meas- robust index of glaucomatous loss of the neu-
uring prevalence, studying risk factors, or roretinal rim. As with intraocular pressure, VCDR
conducting clinical trials. This reconsideration of is a continuous variable within the population.
the definition and classification of glaucoma was One approach would be to determine the range
prompted by our experiences of cross sectional of CDR in people with normal visual function
prevalence research in Africa and Asia, and by the (normal visual field) in a population. This group
difficulty we experienced in identifying and classi- of individuals will therefore be “hypernormal,” as
fying cases and in making valid comparisons with those with visual dysfunction due to causes other
previously published data. The proposed definition than glaucoma would be excluded. The choice of
of glaucomatous optic neuropathy has evolved where to place the division between “normal”
from one initially developed for the Kongwa Eye and “abnormal” is, for the time being, arbitrary
Study in Tanzania.1 At the same time, work in and partially flawed by the fact that there is over-
Mongolia and Singapore,2 3 where there was a high lap between the range of CDRs in those with and
prevalence of primary angle closure glaucoma without glaucomatous visual loss. Faced with this
(PACG), had prompted a re-examination of the dilemma, we propose that the statistical conven-
definition of this condition. We were concerned tion that a probability of <5% representing a sig-
that in previous reports subjects with “latent angle nificant deviation from normal be invoked.
closure glaucoma” had been classified as cases of Therefore, the CDR above which 2.5% of the nor-
established glaucoma, despite having normal mal population lie defines the “upper limit of
visual function. This may result in misinterpreta- normal” (the other 2.5% falling below the normal
tion of the estimates of visual morbidity attribut- distribution). By using the 97.5th percentile, one
able to glaucoma, especially as PACG is believed to avoids making the assumption that CDR is
be at least as prevalent as primary open angle normally distributed (it has been found to be
glaucoma (POAG).4 Gaussian in some studies, but not in others). We
At the biennial congress of the International also suggest using the 97.5th percentile value for
Society for Geographical and Epidemiological CDR asymmetry as a second criterion for abnor-
Ophthalmology held in Leeuwenhorst, the Neth- mality. Examples of what these criteria might be
erlands, in June 1998, a group interested in glau- for some populations are shown in Table 1.
coma epidemiology met to discuss the prototype
system. This has since been discussed further, and Functional damage
See end of article for
authors’ affiliations various experts in the fields of glaucoma research While most published definitions of glaucoma
....................... and clinical practice consulted. (The appendix include the presence of “characteristic visual field
lists participants and co-opted advisers.) The defects,” many authors fail to provide quantita-
Correspondence to:
Paul J Foster, Department views presented here are, however, those of the tive, clearcut descriptions of what this means. The
of Epidemiology and authors. Our aim has been to present a practical broadly accepted principles are summarised in
International Eye Health,
framework which can be tested and discussed Table 2.
Institute of Ophthalmology,
Bath Street, London further.
EC1V 9EL, UK; .................................................
p.foster@ucl.ac.uk PROPOSED DEFINING FEATURES OF
Abbreviations: PACG, primary angle closure glaucoma;
Accepted for publication
GLAUCOMA POAG, primary open angle glaucoma; VCDR, vertical
3 August 2001 The fundamental concept of the proposed classi- cup:disc ratio; IOP, intraocular pressure; PAC, primary
....................... fication for cross sectional epidemiological re- angle closure; PAS, peripheral anterior synechiae
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The definition and classification of glaucoma in prevalence surveys 239
Table 1 Vertical cup:disc ratio (VCDR) distribution in people with normal visual
fields in one African and three Asian countries
CDR CDR asymmetry
*Field testing was carried out in subjects with either CDR >0.35 or IOP > 18 mm Hg. Data presented
include those who did not meet these criteria, and therefore did not undergoing field testing.
†Field testing carried out on all subjects.
100
Table 2 Characteristics of glaucomatous field
defects
Cumulative percentage
80
(1) Asymmetrical across the horizontal midline (in early/moderate
cases)
(2) Located in the mid-periphery (in early/moderate cases) 60
(3) Clustered in neighbouring test points
(4) Reproducible on at least two occasions
40
(5) Not explained by any other disease
(6) Considered a valid representation of the subjects functional status
(based on performance indices such as false positive rate) 20
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Cup:disc ratio
These principles fail to account for the possibility of diffuse
damage to the visual field in glaucoma. While some diffuse Figure 1 The cumulative percentage of vertical CDR distribution
loss clearly must occur, its magnitude and importance in spe- among subjects able to complete visual field testing in a population
cific glaucoma diagnosis are difficult to determine.5 Following survey3 in whom a reproducible visual field defect (glaucoma
hemifield test “outside normal limits” and a four point cluster (p<5%)
consultation with a group of researchers interested in the psy-
on the pattern deviation plot) was identified. The data shown here
chophysics of glaucoma, we have adopted the following as the are based on 61 of 67 eyes. Fives eyes were excluded because lens
“gold standard” of glaucomatous visual field loss. The opacity was sufficient to account for the field defect. One eye was
glaucoma hemifield test graded “outside normal limits” and a excluded because diabetic retinopathy was present. Several eyes
cluster of three contiguous points at the 5% level on the with severe visual field loss were not able to produce a reliable
pattern deviation plot, using the threshold test strategy with visual field test result.
the 24-2 test pattern of the Zeiss-Humphrey field analyser 2.
This is not intended to indicate that this device is the only Levels of evidence
acceptable tool for field analysis. However, we consider it the It is therefore envisaged that cases of glaucoma would be clas-
standard against which others should be validated. sified according to three levels of evidence. The highest level of
Glaucoma certainty requires optic disc abnormalities (VCDR >97.5th
The relation between VCDR and proved visual field abnormali- percentile in the normal population) and visual field defect
ties is complex. Some eyes have reproducible visual field compatible with glaucoma. In the second, if a visual field test
defects although they have a CDR that lies within the range could not be performed satisfactorily, a severely damaged optic
defined as normal by the criteria we have selected. Figure 1 disc (VCDR > 99.5th percentile of the normal population)
shows the relation between CDR and the cumulative probabil- would be sufficient to make the diagnosis. Lastly, if the optic
ity of a reproducible field defect CDR among Chinese disc could not be examined because of media opacity (and,
Singaporeans (unpublished data, Paul Foster, Steve Seah, Sin- hence, no field test was also possible), an IOP exceeding the
gapore National Eye Centre, 2001). 99.5th percentile of the normal population, or evidence of pre-
We further propose that an individual with field loss who vious glaucoma filtering surgery, may be taken as sufficient for
meets the stated criteria (and optic disc meeting criteria for a diagnosis of glaucoma (see Table 3 for summary).
abnormality) in one eye has glaucoma. This takes account of
the fact that damage is often present in one eye before the CLASSIFICATION ACCORDING TO MECHANISM OF
other. However, we appreciate that this monocular based defi- DAMAGE
nition may not be representative of a subject’s functional POAG and the role of IOP
capacity.6 Although the level of intraocular pressure (IOP) is one of the
We have not sought to specify that the visual field defect most consistent risk factors for the presence of glaucoma, the
should be “consistent” with the pattern of structural damage concept that statistically raised IOP is a defining characteristic
to the optic nerve—for example, requiring that inferior field for glaucoma has been almost universally discarded. This is
loss must be matched with superior optic disc rim loss. This based on several population based studies that document the
may lower the specificity of the definition, although we believe typical disc and field damage of glaucoma in people with a
the interobserver variation in making this judgment is poten- statistically normal IOP and, conversely, people with statisti-
tially so great as to introduce greater weakness to the scheme. cally elevated IOP and no evidence of optic neuropathy. We
We therefore suggest that structural features exceeding the propose to follow this current convention except for category
specified limits, combined with a field defect that meets the 3 diagnosis, as detailed above.
above criteria, will constitute the definition of glaucoma dam- POAG is therefore optic nerve damage meeting any of the
age. three categories of evidence above, in an eye which does not
www.bjophthalmol.com
240 Foster, Buhrmann, Quigley, et al
Table 3 The diagnosis of glaucoma in cross Table 4 Classification of primary angle closure
sectional prevalence surveys (The diagnosis is made (PAC)
according to three levels of evidence) (1) Primary angle closure suspect
Category 1 diagnosis (structural and functional evidence) An eye in which appositional contact between the peripheral iris and
Eyes with a CDR or CDR asymmetry >97.5th percentile for the posterior trabecular meshwork is considered possible (see footnote).
normal population, or a neuroretinal rim width reduced to <0.1 CDR (2) Primary angle closure (PAC)
(between 11 to 1 o’clock or 5 to 7 o’clock) that also showed a An eye with an occludable drainage angle and features indicating
definite visual field defect consistent with glaucoma. that trabecular obstruction by the peripheral iris has occurred, such as
Category 2 diagnosis (advanced structural damage with unproved peripheral anterior synechiae, elevated intraocular pressure, iris
field loss) whorling (distortion of the radially orientated iris fibres), “glaucomfleken”
If the subject could not satisfactorily complete visual field testing but lens opacities, or excessive pigment deposition on the trabecular
had a CDR or CDR asymmetry > 99.5th percentile for the normal surface. The optic disc does not have glaucomatous damage.
population, glaucoma was diagnosed solely on the structural (3) Primary angle closure glaucoma (PACG)
evidence. PAC together with evidence of glaucoma, as defined above.
In diagnosing category 1 or 2 glaucoma, there should be no
alternative explanation for CDR findings (dysplastic disc or marked In epidemiological research this has most often been defined as an
anisometropia) or the visual field defect (retinal vascular disease, angle in which >270° of the posterior trabecular meshwork (the part
macular degeneration, or cerebrovascular disease). which is often pigmented) cannot be seen. This definition is arbitrary
Category 3 diagnosis (Optic disc not seen. Field test impossible) and its evaluation in longitudinal study is an important priority.
If it is not possible to examine the optic disc, glaucoma is diagnosed if: Producing a more evidence based definition of this parameter is a
(A) The visual acuity <3/60 and the IOP >99.5th percentile, or major research priority.
(B) The visual acuity <3/60 and the eye shows evidence of glaucoma
filtering surgery, or medical records were available confirming
glaucomatous visual morbidity.
but they deserve to be considered separately for the purposes of
the definitions we have intended to construct. This classification
scheme is summarised in Table 4.
have evidence of angle closure on gonioscopy, and where there Glaucoma with secondary ocular pathology
is no identifiable secondary cause. Not all prevalence studies of glaucoma have separated primary
and secondary glaucoma in consistent fashion, if they have
Primary angle closure and narrow drainage angles done so at all. None the less, the estimated proportion of glau-
The current classification of PACG is largely based on clinical coma damage that is clearly secondary to other ocular or sys-
observations in European derived people, among whom the temic disease, or to trauma, may represent as much as 20% of
condition is scarce. While the acute, symptomatic phase is all glaucoma. While we argue above for elimination of IOP as
dramatic, it occurs in only a minority of those with PACG a defining feature of primary OAG or ACG, secondary
diagnosed in population based surveys in African and Asian glaucoma is properly considered to represent those eyes in
settings.2 3 7 8 Rather, a chronic, asymptomatic form of PACG which a second form of ocular pathology has caused IOP above
predominates. Thus, a full re-evaluation of the definition of the normal range, leading to optic nerve damage. We propose
this disease is appropriate, with emphasis placed on visual loss that the diagnosis of secondary glaucoma only be based on the
rather than symptomatic disease. presence of optic neuropathy, in so far as it is possible to
We propose that it would be useful to distinguish between determine this, in the presence of a second ocular pathological
the mechanism by which IOP becomes elevated and the process. These processes may include one of the following:
resultant damage that is caused by PACG. To do this, people (1) neovascularisation
meeting gonioscopic criteria for narrow angles and with (2) uveitic
evidence of significant obstruction of the functional trabecular
(3) trauma
meshwork by the peripheral iris would be classified as having
primary angle closure (PAC). Those in whom PAC had led to (4) lens related.
significant glaucomatous damage to the optic nerve would be There are arguments for and against including people with
defined as having PACG. This is not intended to indicate that glaucoma and pigment dispersion syndrome or pseudoexfolia-
those with PAC do not require treatment. It is intended to dif- tion syndrome as cases of secondary glaucoma. They have
ferentiate between those with and without damaged visual been omitted from the list above, on the premise that they
function attributable to glaucomatous optic neuropathy. represent a variant of POAG, although this view remains to be
People with PAC and other causes of visual loss, such as iris fully vindicated. It must be recognised that many eyes with
damage, non-glaucomatous optic atrophy, lens opacity, and secondary glaucoma have opaque media, precluding optic disc
corneal endothelial failure should be separately identified. and visual field examinations. Hence, many of the secondary
This approach to classification differs from the scheme found glaucoma examples will be diagnosed with the category 3
in most textbooks in which people with a narrow drainage information detailed above, when optic neuropathy is inferred
angle and either raised IOP or peripheral anterior synechiae from reduced visual acuity and a relative afferent pupil defect,
(PAS) are said to have primary angle closure “glaucoma.” Thus, in the presence of raised IOP. Furthermore, a substantial
in this new concept, PAC includes both asymptomatic people number of these people are affected unilaterally compared to
with occludable angles who have not had an acute attack, and bilateral involvement in primary glaucoma.
those with PAC who have had an attack that was treated On the other hand, there will be eyes with processes such as
promptly but suffered no detectable nerve damage. As many as pseudoexfoliation or uveitis with IOP above the normal range,
60–75% of people suffering an acute, symptomatic episode of but in which the disc is visible and seen to be normal. For con-
angle closure recover without optic disc or visual field sistency, people with eyes with these features will be
damage,9 10 at least in the short term. If one intends the term categorised as secondary ocular hypertensives, or secondary
glaucoma to signify a disease characterised by an irreversible glaucoma suspects.
defect in visual function, then many people suffering sympto-
matic episodes of high IOP or those with narrow drainage Glaucoma suspects
angles who are as yet asymptomatic do not meet this criterion Our categorisation aims to separate an examined population
for nerve injury. They share anatomical and physiological char- into those who did not have glaucoma, those who had one of
acteristics with those whose angle closure has led to field loss, the defined forms of glaucoma, and those who had some
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The definition and classification of glaucoma in prevalence surveys 241
Greenland 30
+ (with PT or IOP) + A − − + +
Alaska31 + + (with VF) A + + (with IOP) − −
China32 + + + − − − −
Tibet33 + + + − − − +
Japan34 − + (with gonio) + (with IOP) − − − −
South Africa7 + + A + (with VF loss) + (with disc) − −
Mongolia2 + + A + (with VF loss) + (with disc) − −
Taiwan8 − + A − − + +
A = absolute requirement, + = additional feature sufficient for diagnosis combined with absolute requirement, − = not required/not done
PT = provocative testing, VF = visual fields, IOP = intraocular pressure, gonio = findings on gonioscopic examination.
www.bjophthalmol.com
242 Foster, Buhrmann, Quigley, et al
Reliability of test results is an important consideration. The 5 Asman P, Heijl A. Diffuse visual field loss and glaucoma. Acta
Ophthalmol Scand 1994;72:303–8.
standards for check trials provided by the Zeiss-Humphrey
6 Viswanathan AC, McNaught AI, Poinoosawmy D, et al. Severity and
instrument may not be appropriate. Fixation loss scores may stability of glaucoma. Patient perception compared with objective
bear little relation to fixation accuracy,28 and are especially measurement. Arch Ophthalmol 1999;117:450–4.
sensitive to mis-plotting of the blind spot. Furthermore, the 7 Salmon JF, Mermoud A, Ivey A, et al. The prevalence of primary
angle-closure glaucoma and open angle glaucoma in Mamre, Western
precision of false negative and false positive indices is very Cape, South Africa. Arch Ophthalmol 1993;111:1263–9.
poor, given the number of test points. The 95% CIs around a 8 Congdon N, Quigley HA, Hung PT, et al. Screening techniques for
33% false negative rate may be from 13% to 53%.29 The angle-closure glaucoma in rural Taiwan. Acta Ophthalmol
software for calculation of false negative and positive 1996;74:113–19.
9 Douglas GR, Drance SM, Schulzer M. The visual field and nerve head
responses in the newer HFA 2 machine has been modified, in angle-closure glaucoma. A comparison of the effects of acute and
although only limited independent evaluation of this is chronic angle closure. Arch Ophthalmol 1975;93:409–11.
currently available. 10 Dhillon B, Chew PT, Lim ASM. Field loss in primary angle-closure
The failure to include field testing in some surveys in devel- glaucoma. Asia-Pacific J Ophthalmol 1990;2:85–7.
11 Jonas JB, Gusek GC, Guggenmoos-Holzmann I, et al. Size of the optic
oping countries is likely to lead to an underestimation of glau- nerve scleral canal and comparison with intravital determination of optic
coma prevalence. While reliance on the optic nerve appearance disc dimensions. Graefes Arch Clin Exp Ophthalmol 1988;226:213–15.
is not ideal, it would identify the more advanced cases and 12 Jonas JB, Gusek GC, Naumann GO. Optic disc, cup and neuroretinal
rim size, configuration and correlations in normal eyes [published errata
provide at least a minimum estimate of glaucoma prevalence. appear in Invest Ophthalmol Vis Sci 1991;32:1893 and
It would include those at highest risk for total blindness in 1992;32:474–5]. Invest Ophthalmol Vis Sci 1988;29:1151–8.
their lifetimes. 13 Jonas JB, Muller-Bergh JA, Schlotzer-Schrehardt UM, et al.
This scheme is not definitive, but is intended as an Histomorphometry of the human optic nerve. Invest Ophthalmol Vis Sci
1990;31:736–44.
operational approach to identifying, in cross sectional surveys, 14 Jonas JB, Gusek GC, Guggenmoos-Holzmann I, et al. Correlations of
those suffering visual disability from glaucoma and to the neuroretinal rim area with ocular and general parameters in normal
standardise our enumeration and evaluation of the cause of eyes. Ophthalmic Res 1988;20:298–303.
their disease. The usefulness of the proposed system for 15 Jonas JB, Schmidt AM, Muller-Bergh JA, S et al. Human optic nerve fiber
count and optic disc size. Invest Ophthalmol Vis Sci 1992;33:2012–18.
comparison between studies must now be validated by 16 Montgomery DM. Clinical disc biometry in early glaucoma.
subsequent research. Ophthalmology 1993;100:52–6.
17 Spencer AF, Vernon SA. Optic disc measurement with the Zeiss four
mirror contact lens. Br J Ophthalmol 1994;78:775–80.
ACKNOWLEDGMENTS 18 Garway-Heath DF, Ruben S, Viswanathan AC, et al. Vertical cup/disc
We thank all those mentioned in the appendix for participating in this ratio in relation to optic disc size: its value in the assessment of the
work, and wish to emphasise that the views expressed in this manu- glaucoma suspect. Br J Ophthalmol 1998;82:1118–24.
script are solely those of the authors, and not those of the ISGEO or an 19 Varma R, Tielsch JM, Quigley HA, et al. Race-, age-, gender-, and
other organisation or individual. refractive error-related differences in the normal optic disc. Arch
Ophthalmol 1994;112:1068–76.
20 Beck RW, Messner DK, Musch DC, et al. Is there a racial difference in
APPENDIX physiologic cup size? Ophthalmology 1985;92:873–6.
ISGEO Glaucoma classification working group, 27 and 28 June 1998, 21 Chi T, Ritch R, Stickler D, et al. Racial differences in optic nerve head
Leeuwenhorst, Netherlands: co-chair: Gordon J Johnson, Harry A Qui- parameters. Arch Ophthalmol 1989;107:836–9.
gley; rapporteurs: Ralf Buhrmann, Paul J Foster; group members: 22 Wolfs RC, Borger PH, Ramrattan RS, et al. Changing views on
Poul-Helge Alsbirk, Michelle Coffey, Lalit Dandona, Paulus TVM de open-angle glaucoma: definitions and prevalences—The Rotterdam
Jong, Fridbert Jonasson, Paul Mitchell, Ian Murdoch, R Pararajasega- Study. Invest Ophthalmol Vis Sci 2000;41:3309–21.
ram, RS Ramrattan, Poul Roux, Ravi Thomas, Bjorn Thylefors, Roger 23 Erie JC, Hodge DO, Gray DT. The incidence of primary angle-closure
glaucoma in Olmstead County, Minnesota. Arch Ophthalmol
Wolfs; co-opted advisers: Anders Heijl, David Henson, Roger A Hitchings,
1997;115:177–81.
Chris A Johnson, Gottfried Naumann, John F Salmon; co-authors of 24 Clemmesen V, Alsbirk PH. Primary angle-closure glaucoma (ACG) in
studies from which data are presented: Bangladesh: M Rahman, N Rahman, Greenland. Acta Ophthalmol 1971;49:47–58.
AU Zahman, A Zia; Mongolia: J Bassanhu, J Devereux, D Uranchimeg, 25 Alsbirk PH. Anatomical risk factors in primary angle-closure glaucoma.
PS Lee, D Machin; Singapore: SKL Seah, F Oen, TP Ng, D Machin, A ten year follow up survey based on limbal and axial anterior chamber
J Devereux, J Hall, J Hee, SJ Chew, PT Khaw; Tanzania: Y Barron, SK depths in a high risk population. Int Ophthalmol 1992;16:265–72.
West, MS Oliva, BBO Mmbaga. 26 Johnson CA, Cioffi GA, Drance SM, et al. A multicenter comparison
study of the Humphrey field analyzer I and the Humphrey field analyzer
II. Ophthalmology 1997;104:1910–17.
..................... 27 Bengtsson B, Heijl A, Olsson J. Evaluation of a new threshold visual
Authors’ affiliations field strategy, SITA, in normal subjects. Swedish Interactive Thresholding
P J Foster, G J Johnson, Department of Epidemiology and International Algorithm. Acta Ophthalmol Scand 1998;76:165–9.
Eye Health, Institute of Ophthalmology, Bath Street, London EC1V 9EL, 28 Henson D, Evans J, Chauhan BC, et al. Influence of fixation accuracy on
UK threshold variability in patients with open angle glaucoma. Invest
Ophthalmol Vis Sci 1996;37:444–50.
R Buhrmann, University of Ottawa Eye Institute, Ottawa, Ontario,
29 Vingrys AJ, Demirel S. False-response monitoring during automated
Canada
perimetry. Optom Vis Sci 1998;75:513–17.
H A Quigley, Dana Center for Preventive Ophthalmology, Johns Hopkins
30 Alsbirk PH. Anterior chamber depth and primary angle-closure
University, Baltimore, Maryland, USA glaucoma. I. An epidemiologic study in Greenland Eskimos. Acta
Ophthalmol 1975;53:89–104.
REFERENCES 31 Arkell SM, Lightman DA, Sommer A, et al. The prevalence of glaucoma
1 Buhrmann RR, Quigley HA, Barron Y, et al. Prevalence of glaucoma in among eskimos of Northwest Alaska. Arch Ophthalmol
a rural East African population. Invest Ophthalmol Vis Sci 1987;105:482–5.
2000;41:40–8. 32 Hu Z, Zhao ZL, Dong FT. [An epidemiological investigation of glaucoma
2 Foster PJ, Baasanhu J, Alsbirk PH, et al. Glaucoma in Mongolia—a in Beijing and Shun-yi county.] [Chinese] Chung-Hua Yen Ko Tsa Chih
population-based survey in Hövsgöl Province, Northern Mongolia. Arch [Chinese Journal of Ophthalmology] 1989;25:115–18.
Ophthalmol 1996;114:1235–41. 33 Zhao JL. [An epidemiological survey of primary angle-closure glaucoma
3 Foster PJ, Oen FT, Machin DS, et al. The prevalence of glaucoma in (PACG) in Tibet.] [Chinese] Chung-Hua Yen Ko Tsa Chih [Chinese
Chinese residents of Singapore. A cross-sectional population survey in Journal of Ophthalmology] 1990;26:47–50.
Tanjong Pagar district. Arch Ophthalmol 2000;118:1105–11. 34 Shiose Y, Kitazawa Y, Tsukuhara S, et al. Epidemiology of glaucoma in
4 Quigley HA. Number of people with glaucoma worldwide. Br J Japan—a nationwide glaucoma survey. Jpn J Ophthalmol
Ophthalmol 1996;80:389–93. 1991;35:133–55.
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