Utility values for glaucoma patients in Korea
Utility values for glaucoma patients in Korea
Seulggie Choi 1 2
Jin A. Choi 0 1
Jin Woo Kwon 0 1
Sang Min Park 1 2
Donghyun Jee 0 1
0 St. Vincent's Hospital, College of Medicine, Division of Vitreous and Retina, Department of Ophthalmology, Catholic University of Korea , Suwon , Republic of Korea, 3 Seoul National University Hospital, Department of Family Medicine , Seoul , Republic of Korea
1 Editor: Gianni Virgili, Universita degli Studi di Firenze , ITALY
2 Seoul National University Graduate School, Department of Biomedical Sciences , Seoul , Republic of Korea
The present study assessed utility values of Korean glaucoma patients. The quality of life
determined by EQ-5D-3L in Korean glaucoma patients was higher compared to those in
other countries. Patient demographics as well as measures of disease severity were
important factors in determining the quality of life within glaucoma patients.
Funding: This study was funded by the National
Evidence-based Healthcare Collaborating Agency of
Korea (Grant title: Cost-effectiveness analysis of
glaucoma screening and management, Grant
number: HC16C2299, URL:http://www.neca.re.kr/
eng/). The funding organization had no role in
study design, data collection, and analysis,
decision to publish, or preparation of the
Glaucoma is the second leading cause of blindness in the world, affecting more than 70 million
]. Particularly, the number of glaucoma patients is expected to reach 37 million by
2020 in Asia alone, constituting 47% of the world's population of glaucoma patients [
]. Due to
the increased life expectancy, the prevalence of glaucoma has increased in Korea, with recent
studies reporting prevalence rates of 3.5% [
] and 4.5% [
] for POAG and PACG, respectively.
Furthermore, the prevalence of glaucoma in Korea is expected to continue to increase, with a
recent study reporting a 54% increase in glaucoma prevalence from 2008 to 2013 [
One of the primary goals of managing glaucoma is maintaining the quality of life of the
patients at a socially acceptable cost. Determining the quality of life for patients with a disease
of rising prevalence has important clinical and public health implications [
]. Utility value,
which determines a patient's perception of the quality of life from a scale of 0 to 1, quantifies
the strength of one's preference for a health state. Furthermore, utility values can be used to
compare the quality of life among different groups and are thus useful in determining the
impact a disease has on the health status of patients.
While previous studies have determined the utility values of glaucoma patients in a number
of different countries [
], there are limited studies investigating the quality of life of glaucoma
patients in Korea. Therefore, we used a nationwide representative survey to determine utility
values of glaucoma patients as a part of the economic evaluation of glaucoma management in
We used the Korean National Health and Nutrition Examination Survey (KNHANES)
database conducted from 2008 to 2012 for this study. KNHANES is a nationwide population-based
cross-sectional survey consisting of health records from health interviews and examinations
]. From 2008 to 2012, the Korean Ophthalmologic Society participated in KNHANES,
thereby including ophthalmologic interviews and examinations conducted by trained
ophthalmologists within the survey [
For the evaluation of glaucoma, slit-lamp examinations and intraocular pressure (IOP)
measurement using a Goldmann applantation tonometer were conducted. A digital
non-mydriatic fundus camera (TRC-NW6S, Topcon) and a digital camera (Nikon D-80, Nikon Inc.,
Tokyo, Japan) were used to capture digital fundus images of all participants under
physiological mydriasis. If the participants had an IOP of greater than or equal to 22 mmHg or a
glaucomatus optic disc, visual field testing using the frequency doubling technology (FDT)
Humphrey Matrix (Carl Zeiss Meditec Inc., Dublin, CA, USA) was conducted. A
glaucomatous optic disc was defined as having: (a) a vertical or horizontal cup-to-disc ratio of 0.5 or
greater, (b) a retinal nerve fiber layer defect, (c) an optic disc hemorrhage, or (d) violated the
ISNT (inferior, superior, nasal, and temporal) rule. Finally, visual acuity was measured at 4
meters with an international standard vision chart based on the LogMAR Scale (Jin's Vision
Chart, Seoul, Korea).
Glaucoma was defined according to the International Society for Geographical and
Epidemiological Ophthalmology diagnostic criteria [
]. Specifically, glaucoma was defined as the
presence of optic nerve damage (vertical or horizontal cup-to-disc ratio of 0.6 or greater, disc
hemorrhage, or retinal nerve fiber layer defect) and an abnormal FDT testing result. Among
glaucoma patients, primary open angle glaucoma (POAG) patients were defined as those with
a peripheral anterior chamber depth of greater than 1/4th of the corneal thickness, while
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primary angle-closure glaucoma (PACG) patients were defined as those with a peripheral
anterior chamber depth of 1/4th of the corneal thickness or less. FDT score was calculated by
adding the number of abnormal locations from FDT testing, after which the scores were divided
into quartiles. Finally, vision loss was defined when visual acuity was less than 0.1 .
Among the 27,088 participants aged 19 years or more with available data on slit-lamp
examinations and fundus images, 26,230 participants who did not meet the criteria for having
glaucoma were excluded, resulting in 858 glaucoma patients. Among them, 12 participants
without values on education status and 13 participants without values on household income
were excluded, ultimately resulting in a study population of 833 glaucoma patients.
Measurement of utility value
Developed by the EuroQol Group, EQ-5D-3L is a generic preference-based measure consisting
of five questions that reflect the current health status of the patient [
]. The questions are
composed of five dimensions: mobility, self-care, usual activities, pain/discomfort, and
anxiety/depression. Each question has three levels that indicate (a) no problem, (b) some problems,
or (c) severe problems. EQ-5D-3L have been used widely as a tool of assessing the health status.
The Korean EQ-5D-3L questionnaire was developed following the EuroQol group procedure
by the Korean Centers for Disease Control and Prevention . The Korean EQ-5D-3L utility
score ranges from -0.171 (worst health status) to 1.000 (best health status)
Demographics and measures of glaucoma severity
Among patient demographics, age (years, less than 50, 50±59, 60±69, and 70 or more), sex
(men and women), education (elementary school or lower, middle school, high school,
technical college, and college or higher), employment status (yes and no), household income (1st,
2nd, 3rd, and 4th quartiles), and marital status (yes and no) were determined by a questionnaire.
Glaucoma subtype (POAG and PACG), better eye cup-to-disc ratio (<0.6 and 0.6), better
and worse eye FDT score (1st, 2nd, 3rd, and 4th quartiles), and vision loss (no vision loss,
unilateral vision loss, and bilateral vision loss) were determined.
The proportions of the patient demographics and measures of glaucoma severity were
calculated. For each patient demographic and measure of glaucoma severity, the adjusted means and
95% confidence intervals (CI) of EQ-5D-3L utility values were calculated. Adjusted mean values
were calculated by linear regression analysis. In Model 1, covariates age and sex were adjusted.
In Model 2, education, employment status, household income, martial status, glaucoma
subtype, better eye cup-to-disc ratio, and vision loss were additionally adjusted. Statistical
significance was assumed at a p value of less than 0.05 in a two-sided manner. All statistical analyses
conducted in this study were done with STATA 13.0 (StataCorp LP, College Station, TX, USA).
All participants of KNHANES from 2008 to 2012 provided informed consent before the
survey. No approval from the Institutional Review Board was needed as KNHANES is publicly
available from the Korea Centers for Disease Control and Prevention.
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Acronyms: EQ-5D-3L, three level version of EuroQol-5D; SD, standard deviation; POAG, primary open angle
glaucoma; PACG, primary angle-closure glaucoma; FDT, frequency doubling technology
61.3 years. There were similar numbers of men (416 patients, 49.1%) and women (417 patients,
50.1%). For education, the greatest proportion of the highest level of education was elementary
school or lower (35.2%), while 17.3% of the patients graduated from college or higher. Most
patients were POAG patients (809 patients, 97.1%), while only 24 patients (2.9) were PACG
patients. Finally, the proportions of patients with no vision loss, unilateral vision loss, and
bilateral vision loss were 95.3%, 4.3%, and 0.4%, respectively.
The adjusted mean utility values according to sociodemographic characteristics are shown
in Table 2. When the utility values were determined according to age, the adjusted mean
EQ5D-3L score for the youngest group (less than 50 years) was 0.96 (95% CI 0.94±0.99), while the
mean EQ-5D-3L score for the oldest group (70 years or more) was 0.86 (95% CI 0.80±0.92).
There was a statistically significant trend towards decreased utility values with increasing age
(p for trend 0.005). The adjusted mean utility value for patients within the lowest quartile of
household income was 0.87 (95% CI 0.83±0.90) while patients within the highest quartile had
an adjusted utility value of 0.96 (95% CI 0.93±0.99). Increasing quartiles of household income
was associated with improved utility values (p for trend <0.001). Finally, patients who were
not married had significantly lower adjusted mean utility values (0.87, p value 0.014) compared
to married patients (0.93).
Table 3 demonstrates the adjusted mean utility values according to glaucoma subtype,
better eye cup-to-disc ratio, FDT score, and vision loss. Compared to POAG patients, PACG
patients had significantly higher adjusted mean utility values (p value 0.012). Furthermore,
while the utility value for patients within the first quartile of FDT score in the worse eye was
0.94 (95% CI 0.91±0.96), the utility value for those within the fourth quartile was 0.88 (95% CI
0.84±0.92), with a p value of 0.024. Finally, patients with no vision loss had an adjusted mean
utility value of 0.92 (95% CI 0.91±0.94), while patients with bilateral vision loss had a mean
utility value of 0.83 (95% CI 0.76±0.90). Patients with bilateral vision loss had significantly
lower utility values compared to patients without vision loss (p value 0.013).
We have shown that the mean EQ-5D-3L score of 833 glaucoma patients in Korea was 0.8968
(SD 0.1597). Glaucoma patients who are older, have low income status, and are not married
had lower utility values. Finally, glaucoma patients with PACG, more advanced degrees of
glaucoma severity, and with vision loss had lower utility values.
Several previous studies have used EQ-5D to determine utility values for glaucoma patients
[15±18]. Aspinall and colleagues, who investigated the quality of life of glaucoma patients,
determined that the mean EQ-5D score for 72 glaucoma patients in the United Kingdom was
0.76 (SD 0.19) [
]. Another study comparing the sensitivity of EQ-5D, Short Form-6D, and
Time Trade Off utility values among POAG patients showed that the mean EQ-5D score for
131 POAG patients in the United Kingdom was 0.8 (SD 0.2) [
]. Similarly, two separate
studies evaluating utility values among glaucoma patients showed mean EQ-5D scores for
glaucoma patients in Sweden and Europe were 0.80 (SD 0.23) and 0.65 (SD 0.28), respectively [
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Interestingly, the utility values for glaucoma patients in the United Kingdom, Sweden, and
Europe were all lower compared to that among Korean glaucoma patients determined in our
study. One possible contributing factor is the active promotion of awareness for glaucoma by a
number of ophthalmologic societies in Korea. This could increase the awareness of glaucoma
among the general population and thus make it more likely for early diagnosis. Earlier
diagnosis of glaucoma may lead to easier management of the disease and thus yield less symptoms
and complications. Furthermore, increased awareness may prompt glaucoma patients to
adhere to medications and management regimens, thereby yielding more favorable outcomes.
However, the exact reasons for the higher utility values among Korean glaucoma patients
compared to those in other countries cannot be determined from our results and merit further
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A number of previous studies have determined utility values for glaucoma patients
according to patient demographics such as age, income, sex, and education [19±21]. In two separate
studies, there was no significant difference in utility values according to age among glaucoma
patients (p values 0.69 and 0.46, respectively) [
]. In contrast, we found that older
glaucoma patients had lower utility scores compared to younger patients (p for trend 0.005). While
the reasons for the differing results of utility values according to age are unknown, it is
reasonable to assume that older glaucoma patients may have been diagnosed with glaucoma for
longer durations, likely resulting in more advanced stages of the disease. Greater degrees of
glaucoma progression or severity among older patients may result in more symptoms and
complications, contributing to the lower utility values compared to young patients. The
reasons as to why utility values did not differ according to age among glaucoma patients in
previous studies are unclear and merit further evaluation.
In contrast to the results from our study, a previous study investigating the utility values
among glaucoma patients revealed that there was no significant difference in utility values
according to income [
]. Although the reasons for the discrepancy between studies in the
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association of income and utility value among glaucoma patients are unclear, it is reasonable
to assume that patients with higher income may have more regular access to high quality care,
which could result in better management of glaucoma and improved quality of life. Among
previous studies that examined utility values for glaucoma patients according to sex, one
found no difference in utility values [
], another showed that men had lower utility values
], while another revealed that women had lower utility values [
]. In our study, we found
that there was no significant difference in utility values for men and women.
We could not find any significant difference in utility values according to education, which
is in contrast to a previous study also showing lower utility values for those with lower levels of
education among 213 Chinese glaucoma patients [
]. Although patients with higher
education may be expected to have higher utility values as such patients may be more self-aware of
the early signs of glaucoma and thus may be more likely to be diagnosed early, our results
suggest that other factors highly correlated with education, such as household income, may act
more strongly on utility values among glaucoma patients. Aside from age and income, we have
added to previous studies by showing that utility values according to martial status are also
Similar to the results from our study, a previous study has shown that PACG patients had
better utility values compared to POAG patients [
]. This may be due to the fact that POAG
and PACG are different in how the symptoms are presented. While POAG patients tend to
have no symptoms until the disease has progressed, PACG patients tend to have immediate
symptoms such as eye pain and blurred vision. Due to the acute nature in which PACG
symptoms are presented, PACG patients may seek early medical care, which could result in
improvement of symptoms [
]. In contrast, POAG patients suffer from the relatively
delayed presentation of symptoms and thus may be associated with lower utility values
compared to PACG patients.
Previous studies have shown that utility values differ according to glaucoma severity in
terms of visual acuity, mean deviation index, and pattern standard deviation [
Similarly, we have shown that utility values according to FDT score and vision loss are different
among glaucoma patients. As FDT score may be considered as a measure of the degree of
visual field seeing capacity, glaucoma patients with advanced cases of glaucoma may have
lower utility values. Similarly, as visual acuity is directly related to the ability to conduct
everyday activities, patients with bilateral vision loss could result in decreased quality of life.
There are several limitations to consider when interpreting the results of this study. First,
the cross-sectional nature of this data makes it difficult to rule out the possibility of reverse
causality, in which decreased utility values may be the cause of differences in patient
demographics, rather than the other way around. Second, EQ-5D is known to have a ceiling effect
problem in which as much as 65% of the general population report perfect EQ-5D scores [
Therefore, studies using other tools of measuring utility are needed to confirm the findings of
this study. Third, visual field defect was determined by the FDT Humphrey Matrix, rather
than the Zeiss-Humphrey field analyzer as suggested in guidelines [
]. Use of the FDT
Humphrey Matrix may have led to an overestimation of glaucoma patients, leading to a subsequent
greater utility value for glaucoma patients compared to that in previous studies. Furthermore,
mean deviation could not be determined, which is an important indicator of glaucoma
progression and future studies using the Zeiss-Humphrey field analyzer are needed to further
validate the findings of this study. Finally, other serious comorbidities that may affect the utility
values of glaucoma patients were not considered. However, a previous study has shown that
systemic comorbidities does not have an impact on the quality of life of patients with
ophthalmologic diseases [
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Despite these limitations, our study was based on a nationally representative data with a
study population of 833 glaucoma patients, which is larger than most previous studies.
Furthermore, we took into account a greater number of demographics, such as employment
status, household income, and marital status, which few previous studies have investigated upon.
Finally, despite its ceiling effect problem, EQ-5D is nevertheless a widely used tool for
measuring utility values and the results of this study could later be used as a primary source of QALY
estimation for cost-effective analyses.
The utility value for glaucoma patients in Korea was higher compared to glaucoma patients in
other countries. Glaucoma patients who are older, have low income status, and are not
married had lower utility values. Furthermore, glaucoma patients who were diagnosed with
PACG, with a greater degree of disease severity, and with bilateral vision loss had lower quality
Conceptualization: Sang Min Park, Donghyun Jee.
Data curation: Seulggie Choi, Sang Min Park, Donghyun Jee.
Funding acquisition: Donghyun Jee.
Investigation: Seulggie Choi, Sang Min Park, Donghyun Jee.
Methodology: Jin A. Choi, Jin Woo Kwon.
Resources: Donghyun Jee.
Supervision: Sang Min Park, Donghyun Jee.
Formal analysis: Seulggie Choi, Jin A. Choi, Jin Woo Kwon, Sang Min Park, Donghyun Jee.
Validation: Jin A. Choi, Jin Woo Kwon, Sang Min Park, Donghyun Jee.
Writing ± original draft: Seulggie Choi, Sang Min Park, Donghyun Jee.
Writing ± review & editing: Seulggie Choi, Jin A. Choi, Jin Woo Kwon, Sang Min Park,
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1. Quigley HA , Broman AT . The number of people with glaucoma worldwide in 2010 and 2020 . Br J Ophthalmol. 2006 ; 90 ( 3 ): 262 ± 7 . Epub 2006/02/21. https://doi.org/10.1136/bjo. 2005 .081224 PMID: 16488940; PubMed Central PMCID : PMCPMC1856963 .
2. Kim CS , Seong GJ , Lee NH , Song KC , Namil Study Group KGS. Prevalence of primary open-angle glaucoma in central South Korea the Namil study . Ophthalmology . 2011 ; 118 ( 6 ): 1024 ± 30 . Epub 2011/ 01/29. https://doi.org/10.1016/j.ophtha. 2010 . 10 .016 PMID: 21269703 .
3. Kim YY , Lee JH , Ahn MD , Kim CY , Namil Study Group KGS. Angle closure in the Namil study in central South Korea . Arch Ophthalmol . 2012 ; 130 ( 9 ): 1177 ± 83 . Epub 2012/09/12. https://doi.org/10.1001/ archophthalmol. 2012 .1470 PMID: 22965594 .
4. Seo SJ , Lee YH , Lee SY , Bae HW , Hong S , Seong GJ , et al. Estimated Prevalence of Glaucoma in South Korea Using the National Claims Database . J Ophthalmol . 2016 ; 2016 : 1690256 . Epub 2016/06/ 02. https://doi.org/10.1155/ 2016 /1690256 PMID: 27247797; PubMed Central PMCID : PMCPMC4876228 .
5. Zanlonghi X , Arnould B , Bechetoille A , Baudouin C , Bron A , Denis P , et al. [ Glaucoma and quality of life] . J Fr Ophtalmol . 2003 ; 26 Spec No 2 :S39 ± 44 . Epub 2003/12/04. PMID: 14646830 .
6. Tosh J , Brazier J , Evans P , Longworth L . A review of generic preference-based measures of healthrelated quality of life in visual disorders . Value Health . 2012 ; 15 ( 1 ): 118 ± 27 . Epub 2012/01/24. https:// doi.org/10.1016/j.jval. 2011 . 08 .002 PMID: 22264979; PubMed Central PMCID : PMCPMC3268858 .
7. Kweon S , Kim Y , Jang MJ , Kim Y , Kim K , Choi S , et al. Data resource profile: the Korea National Health and Nutrition Examination Survey (KNHANES) . Int J Epidemiol . 2014 ; 43 ( 1 ): 69 ± 77 . Epub 2014/03/04. https://doi.org/10.1093/ije/dyt228 PMID: 24585853; PubMed Central PMCID : PMCPMC3937975 .
8. Yoon KC , Mun GH , Kim SD , Kim SH , Kim CY , Park KH , et al. Prevalence of eye diseases in South Korea: data from the Korea National Health and Nutrition Examination Survey 2008 ± 2009 . Korean J Ophthalmol. 2011 ; 25 ( 6 ): 421 ± 33 . Epub 2011/12/02. https://doi.org/10.3341/kjo. 2011 . 25 .6.421 PMID: 22131780; PubMed Central PMCID : PMCPMC3223710 .
9. Foster PJ , Buhrmann R , Quigley HA , Johnson GJ . The definition and classification of glaucoma in prevalence surveys . Br J Ophthalmol . 2002 ; 86 ( 2 ): 238 ± 42 . Epub 2002/01/30. PMID: 11815354; PubMed Central PMCID : PMCPMC1771026 .
10. Chon B , Qiu M , Lin SC . Myopia and glaucoma in the South Korean population . Invest Ophthalmol Vis Sci . 2013 ; 54 ( 10 ): 6570 ± 7 . Epub 2013/09/12. https://doi.org/10.1167/iovs.13-12173 PMID: 24022009 .
11. Rim TH , Nam JS , Choi M , Lee SC , Lee CS . Prevalence and risk factors of visual impairment and blindness in Korea: the Fourth Korea National Health and Nutrition Examination Survey in 2008±2010 . Acta Ophthalmol . 2014 ; 92 ( 4 ):e317± 25 . Epub 2014/01/31. https://doi.org/10.1111/aos.12355 PMID: 24475752 .
12. Rabin R , de Charro F. EQ -5D: a measure of health status from the EuroQol Group . Ann Med . 2001 ; 33 ( 5 ): 337 ± 43 . Epub 2001/08/09. PMID: 11491192 .
13. Brooks R. EuroQol : the current state of play . Health Policy . 1996 ; 37 ( 1 ): 53 ± 72 . Epub 1996/06/06. PMID: 10158943 .
14. Lee YK , Nam HS , Chuang LH , Kim KY , Yang HK , Kwon IS , et al. South Korean time trade-off values for EQ-5D health states: modeling with observed values for 101 health states . Value Health . 2009 ; 12 ( 8 ): 1187 ± 93 . Epub 2009/08/08. https://doi.org/10.1111/j.1524- 4733 . 2009 . 00579 . x PMID : 19659703 .
15. Aspinall PA , Johnson ZK , Azuara-Blanco A , Montarzino A , Brice R , Vickers A . Evaluation of quality of life and priorities of patients with glaucoma . Invest Ophthalmol Vis Sci . 2008 ; 49 ( 5 ): 1907 ± 15 . Epub 2008/04/26. https://doi.org/10.1167/iovs.07-0559 PMID: 18436824 .
16. Bozzani FM , Alavi Y , Jofre-Bonet M , Kuper H . A comparison of the sensitivity of EQ-5D, SF-6D and TTO utility values to changes in vision and perceived visual function in patients with primary open-angle glaucoma . BMC Ophthalmol . 2012 ; 12 : 43 . Epub 2012/08/23. https://doi.org/10.1186/ 1471 -2415-12-43 PMID: 22909264; PubMed Central PMCID : PMCPMC3552875 .
17. Thygesen J , Aagren M , Arnavielle S , Bron A , Frohlich SJ , Baggesen K , et al. Late-stage, primary openangle glaucoma in Europe: social and health care maintenance costs and quality of life of patients from 4 countries . Curr Med Res Opin . 2008 ; 24 ( 6 ): 1763 ± 70 . Epub 2008/06/19. https://doi.org/10.1185/ 03007990802111068 PMID: 18559164 .
18. Kobelt G , Jonsson B , Bergstrom A , Chen E , Linden C , Alm A. Cost-effectiveness analysis in glaucoma: what drives utility? Results from a pilot study in Sweden . Acta Ophthalmol Scand . 2006 ; 84 ( 3 ): 363 ± 71 . Epub 2006/05/18. https://doi.org/10.1111/j.1600- 0420 . 2005 . 00621 . x PMID : 16704699 .
19. Paletta Guedes RA , Paletta Guedes VM , Freitas SM , Chaoubah A . Utility values for glaucoma in Brazil and their correlation with visual function . Clin Ophthalmol . 2014 ; 8 : 529 ± 35 . Epub 2014/03/22. https:// doi.org/10.2147/OPTH.S60105 PMID: 24648717; PubMed Central PMCID : PMCPMC3958501 .
20. Saw SM , Gazzard G , Au Eong KG , Oen F , Seah S. Utility values in Singapore Chinese adults with primary open-angle and primary angle-closure glaucoma . J Glaucoma . 2005 ; 14 ( 6 ): 455 ± 62 . Epub 2005/ 11/09. PMID: 16276277 .
21. Gupta V , Srinivasan G , Mei SS , Gazzard G , Sihota R , Kapoor KS . Utility values among glaucoma patients: an impact on the quality of life . Br J Ophthalmol . 2005 ; 89 ( 10 ): 1241 ± 4 . Epub 2005/09/20. https://doi.org/10.1136/bjo. 2005 .068858 PMID: 16170108; PubMed Central PMCID : PMCPMC1772859 .
22. Cheng HC , Guo CY , Chen YJ , Chen MJ , Ko YC , Huang N , et al. Patient-Reported Vision-Related Quality-of-Life Differences between Primary Angle-Closure Glaucoma and Primary Open-Angle Glaucoma . PLoS One . 2016 ; 11 ( 9 ): e0163123 . Epub 2016 /10/01. https://doi.org/10.1371/journal.pone.0163123 PMID: 27690232; PubMed Central PMCID : PMCPMC5045164 .
23. Chen PP , Lin SC , Junk AK , Radhakrishnan S , Singh K , Chen TC . The Effect of Phacoemulsification on Intraocular Pressure in Glaucoma Patients: A Report by the American Academy of Ophthalmology . Ophthalmology. 2015 ; 122 ( 7 ): 1294 ± 307 . Epub 2015/05/07. https://doi.org/10.1016/j.ophtha. 2015 . 03 . 021 PMID: 25943711 .
24. Alsagoff Z , Aung T , Ang LP , Chew PT . Long-term clinical course of primary angle-closure glaucoma in an Asian population . Ophthalmology . 2000 ; 107 ( 12 ): 2300 ± 4 . Epub 2000/11/30. PMID: 11097612 .
25. McDowell I . Measuring health: a guide to rating scales and questionnaires . 3rd ed. Oxford; New York: Oxford University Press; 2006 . xvi, 748 p. p.
26. Brown MM , Brown GC , Sharma S , Hollands H , Landy J . Quality of life and systemic comorbidities in patients with ophthalmic disease . Br J Ophthalmol . 2002 ; 86 ( 1 ):8± 11 . Epub 2002/01/22. PMID: 11801494; PubMed Central PMCID : PMCPMC1770989 .