View Article PDF

The epidemiology of glaucoma has been defined by several large population-based studies, measuring the prevalence in different contexts (Table 1).[[1–10]] As the incidence of glaucoma increases with age, an increase in the number of affected people is predicted from an ageing global population.[[11]] Another source of increasing glaucoma prevalence is the wide dissemination of imaging technology, most notably optical coherence tomography (OCT), which is both specific and sensitive in detecting early glaucoma and is widely available as an opportunistic screening method in developed countries.[[12]]

View Table 1.

In New Zealand, no glaucoma prevalence data has been collected, and it has been assumed that the New Zealand prevalence is comparable to surveys from Australia.[[7,8]] Establishing the prevalence in New Zealand is important to help estimate the burden of this disease.

In the present study, we measured the prevalence of glaucoma in the well-characterised population-based birth cohort of 45-year-old participants of the Dunedin Multidisciplinary Health & Development Study (Dunedin Study).[[13]] Using OCT allowed us to consider how this new, more sensitive technology affected the prevalence estimate, in comparison to older studies which only used optic disc photography and visual field tests.

Methods

Study design and approvals

This was an observational cross-sectional study. Participants gave written informed consent, and all study protocols were approved by the NZ Health and Disability Ethics Committee.

Study population

Participants are members of the Dunedin Study, a longitudinal investigation of health and behaviour in a population-representative birth cohort of 1,037 individuals (91% of eligible births; 52% male) born between 1 April 1972 and 31 March 1973 in Dunedin, New Zealand. The longitudinal study was established at age 3-years based on residence in the province.[[13]] Assessments were conducted at birth and at ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, 38, and most recently at age 45, when 94% of the 997 participants still alive took part. Each study member was brought to the research unit for a day of interviews and examinations. Ninety-three percent of eligible age 45 participants also completed MRI scanning. The cohort represents the full range of socio-economic status in New Zealand’s South Island, and as adults match the NZ National Health and Nutrition Survey on adult health indicators, eg BMI, smoking, GP visits.[[14]] Study participants are primarily of New Zealand European ethnicity (approximately 93%). Written informed consent was obtained from participants, and the study was approved by the New Zealand Health and Disability Ethics Committee.

Data collection

At age 45-years, the following was assessed: first degree relative with glaucoma; best corrected visual acuity; visual field (VF) on Matrix perimeter (Carl Zeiss Meditec, Dublin, CA, USA); non-cycloplegic autorefraction; central corneal thickness (CCT) and intraocular pressure (IOP) using the Tonoref III (Nidek, Japan); axial length using IOL Master (Carl Zeiss Meditec, Dublin, CA, USA); spectral domain OCT (Cirrus HD-OCT, model 5000; Carl Zeiss Meditec, Dublin, California, USA) retinal nerve fibre layer (RNFL) by optic disc cube 200x200, macular ganglion cell layer by macular cube 512x128, vertical cup-disc-ratio (CDR), and disc area; un-dilated digital fundus photographs of each eye were taken after five minutes of dark adaptation, using an NMR-45 fundus camera (Canon, Japan).

Assessment of glaucoma

The diagnosis of glaucoma can be challenging, particularly in the early stages, and disagreement between methods of diagnosis is common.[[15]] Two masked independent ophthalmologists (GW, JG) viewed the fundus photographs, and subjective comments and diagnostic impressions were recorded, as well as disc damage likelihood scale (DDLS), and vertical CDR (inter-rater agreement was measured).[[16–19]] The DDLS was calculated for medium sized optic discs, as size could not be measured from the photographs. Discs with DDLS >5, CDR >0.5 or comments suspecting glaucoma or asymmetry of CDR ≥0.2 were noted to require further review. These suspect discs were reviewed with IOP, CCT, OCT and VF data to generate a consensus glaucoma status:

“Normal” if no suspicion of glaucoma (other non-glaucoma pathology may be present).

“Ocular hypertension” if IOP >21 mmHg and no optic disc abnormality.

“Glaucoma suspect” if optic disc photograph was suspicious for glaucoma with no more than borderline VF or OCT abnormalities (that is, no corresponding abnormalities, or abnormalities not explained by other disease or pathology).

“Glaucoma” if optic disc photograph was suspicious for glaucoma and there were corresponding abnormalities of the OCT or VF.

Each participant was assigned the glaucoma status of their worse eye.

Data analysis

All data was collated and analysed using Excel (Microsoft, Albuquerque, NM, US). To assess the intra-rater agreement for CDR and DDLS, Bland–Altmann plots were constructed, and the mean bias and limits of agreement (mean difference ± 1.96 standard deviation of differences) were calculated.[[20, 21]] Standard errors of the prevalence estimates were calculated for a binomial distribution to generate 95% confidence intervals (CI).

Results

Of the 938 participants, 891 (95%) were assigned a glaucoma status. The 47 who were not assigned had technical difficulties with eye data collection.

Glaucoma status

The prevalence of each glaucoma status and the 95% confidence intervals are shown in Table 2.

View Table 2.

Among the more suspicious eyes of the 65 participants with glaucoma suspect status: 29 had suspicious discs, of which 22 had borderline abnormalities in OCT; four had borderline abnormalities in VF; one had non-corresponding abnormalities in both OCT and VF; and two had suspicious discs and other risk factors only. The remaining 36 glaucoma suspects had abnormalities in OCT alone, but low suspicion optic disc photographs in both eyes.

Among those with glaucoma status, six had abnormalities in OCT corresponding to the glaucomatous optic disc appearance, and one had abnormalities in both OCT and VF (mild) in their more affected eyes.

There were an additional 73 participants (8.2%, CI 6.3%–10%) with abnormalities on the OCT scan who were not deemed to be glaucoma suspects in either eye (non-pathological abnormalities or artefacts) and hence classified as normal.

Inter-rater agreement of optic disc photographs

Inter-rater reliability was a little greater for CDR (mean difference 0.01, limits of agreement -0.13 to +0.15), as compared with DDLS (mean difference -0.55, limits of agreement -1.9 to +0.8, see Supplemental Figure 1). This indicated that GW rated DDLS scores lower than JG on average.

Clinical parameters

Both eyes were pooled to calculate average disc metrics (mean ± standard deviation and CI). The mean DDLS was 2.5 ± 0.88 (2.4-2.6) and mean CDR was 0.32 ± 0.14 (0.31–0.33). The IOP, CCT, and RNFL had a normal symmetrical distribution in keeping with previous cohorts. Figure 1 depicts the distribution of glaucoma statuses across the complete range of IOPs.

Figure 1: Histogram of intraocular pressures (the higher of the two eyes), with glaucoma status labelled. The higher intraocular pressure in participants with glaucoma are indicated as asterisks.

Discussion

In this observational, cross-sectional study of predominantly white (Pākehā) 45-year-old New Zealanders, we found the prevalence of glaucoma to be 0.79% (CI 0.2–1.4), based on fundus photographs, OCT, and VF results. The prevalence of ocular hypertension was 1.68% (CI 0.8–2.5), and glaucoma suspect status was 7.30% (CI 5.6–9.0). The prevalence aligns with other population-based studies with Caucasian/white participants of the 40–50-year age group (Table 1).

An additional 73 participants (8.2%, CI 6.3%–10%) had at least one abnormal eye on OCT imaging that was deemed to be non-pathological or artefactual. From a total of 139 participants with abnormal OCT, seven were assigned glaucoma status, and 59 were glaucoma suspects including just 23 who would be suspected of glaucoma by disc photography, IOP and visual fields. Clearly, OCT technology is highly sensitive, but this comes with a risk of detecting false positives (artefacts).

In this younger 45-year-old cohort with a low prevalence, all of the participants with glaucoma had normal IOP in both eyes, as did all but one eye of one of the 65 glaucoma suspect (21.3 mmHg). This is a greater proportion with normal IOP than would be expected in a Caucasian cohort, and does not fit easily with the idea that ocular hypertension is presumed to be the pathogenic precursor to glaucoma in many cases.[[7–10]] Possible explanations include that naïve optic disc imaging with OCT detects a broader group of glaucoma cases than previous studies, or that this younger cohort may have a greater prevalence of myopia and thus more similarity to East Asian cohorts (with a very high proportion of normal tension glaucoma). Additionally, no disc haemorrhages were seen in any of the disc photos.

Limitations of this study include the lack of specialist assessment in clinic, gonioscopy or slit lamp examination in the diagnosis, or adhering to protocols for glaucoma diagnosis from other population-based studies. There was potential for non-contact tonometry and non-stereo disc imaging to reduce the diagnostic accuracy, but data collection was standardised and robust, and diagnostic classifications were made by consensus with the best available information. Due to low numbers, the findings should not be generalised to Māori, Pasifika and Asian ethnic groups, who have different prevalence of glaucoma types.[[22]]

The prevalence of glaucoma in 45-year-old New Zealanders appears to lie between 0.2% and 1.4%, consistent with other population-based surveys. Future examinations in the same cohort will detect incident cases over time. This is one of the first population-based studies to include OCT in the diagnosis of glaucoma, highlighting the sensitivity of these devices but also the potential for misinterpretation and over-investigation.[[23]]

Summary

Abstract

Aim

We aimed to estimate the prevalence of glaucoma in New Zealand using a population-based birth cohort of 45-year-olds.

Method

Study members of the Dunedin Multidisciplinary Health & Development Study participated (n=938 out of 1037 births (91%)). The data collected included visual acuity, visual field (VF), refraction, central corneal thickness, intraocular pressure (IOP), axial length, spectral domain optical coherence tomography (OCT), and non-mydriatic fundus photographs. Two ophthalmologists reviewed data independently to generate a consensus glaucoma status: “Normal” if no suspicion of glaucoma; “Ocular hypertension” if IOP >21 mmHg; “Glaucoma suspect” if optic disc photograph was suspicious for glaucoma with no more than borderline or non-corresponding VF or OCT abnormalities; and “Glaucoma” if optic disc photograph was suspicious for glaucoma and there were corresponding abnormalities of the OCT or VF.

Results

Of 891 participants with sufficient data to assign a glaucoma status, 804 were “Normal” (90.2% [CI 88.3–92.2]), 15 were “Ocular hypertension” (1.68% [95% confidence interval (CI) 0.84–2.5]), 65 were “Glaucoma suspect” (7.30% [95% CI 5.6–9.0]), and 7 were classified as “Glaucoma” (0.79% [95% CI 0.21–1.4]). An additional 73 participants (8.2%, [95% CI 6.3%–10%]) had abnormalities on the OCT scan but were not deemed to be glaucoma suspects.

Conclusion

The prevalence of glaucoma in New Zealand is between 0.2% and 1.4%, consistent with other population-based studies in the same age group. The study highlights the sensitivity of OCT and the potential for misinterpretation and over-investigation.

Author Information

Dr Aqeeda Singh, MB ChB: Dunedin School of Medicine, University of Otago, Dunedin, New Zealand. Dr Jesse Gale, MB ChB FRANZCO: Department of Surgery & Anaesthesia, University of Otago, Wellington, New Zealand. Dr Kirsten Cheyne, PhD: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand. Antony Ambler, MSc: Social, Genetic, and Developmental Psychiatry Centre, Institute of Psychiatry, Psychology, and Neuroscience, King’s College London, London, United Kingdom. Prof Richie Poulton, PhD: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand Dr Graham Wilson, MB ChB FRANZCO: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand.

Acknowledgements

We thank the Dunedin Study members, their families and friends for their long-term involvement, and Study Founder, Dr Phil A Silva. The Dunedin Multidisciplinary Health and Development Research Unit is based at University of Otago within the Ngāi Tahu tribal area whom we acknowledge as first peoples, tangata whenua (people of this land). This research received support from the NZ Health Research Council Programme Grant (16–604). The Dunedin Multidisciplinary Health and Development Research Unit is additionally supported by the New Zealand Health Research Council and the New Zealand Ministry of Business, Innovation, and Employment. AS received direct support from the Gordon Sanderson Scholarship from Glaucoma New Zealand. The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Correspondence

Dr Jesse Gale: Department of Surgery & Anaesthesia, University of Otago, Wellington. Private Bag 7343 Wellington South 6242, New Zealand. Phone: +644385554, fax: +6443895725.

Correspondence Email

jesse.gale@otago.ac.nz

Competing Interests

Nil.

1) Bengtsson B. The prevalence of glaucoma. Br J Ophthalmol. 1981;65(1):46-9.

2) Tielsch JM, Sommer A, Katz J, Royall RM, Quigley HA, Javitt J. Racial variations in the prevalence of primary open-angle glaucoma. The Baltimore Eye Survey. JAMA. 1991;266(3):369-74.

3) Klein BE, Klein R, Sponsel WE, Franke T, Cantor LB, Martone J, et al. Prevalence of glaucoma. The Beaver Dam Eye Study. Ophthalmology. 1992;99(10):1499-504.

4) Coffey M, Reidy A, Wormald R, Xian WX, Wright L, Courtney P. Prevalence of glaucoma in the west of Ireland. Br J Ophthalmol. 1993;77(1):17-21.

5) Dielemans I, Vingerling JR, Wolfs RC, Hofman A, Grobbee DE, de Jong PT. The prevalence of primary open-angle glaucoma in a population-based study in The Netherlands. The Rotterdam Study. Ophthalmology. 1994;101(11):1851-5.

6) Giuffre G, Giammanco R, Dardanoni G, Ponte F. Prevalence of glaucoma and distribution of intraocular pressure in a population. The Casteldaccia Eye Study. Acta Ophthalmol Scand. 1995;73(3):222-5.

7) Mitchell P, Smith W, Attebo K, Healey PR. Prevalence of open-angle glaucoma in Australia. The Blue Mountains Eye Study. Ophthalmology. 1996;103(10):1661-9.

8) Wensor MD, McCarty CA, Stanislavsky YL, Livingston PM, Taylor HR. The prevalence of glaucoma in the Melbourne Visual Impairment Project. Ophthalmology. 1998;105(4):733-9.

9) Keel S, Xie J, Foreman J, Lee PY, Alwan M, Fahy ET, et al. Prevalence of glaucoma in the Australian National Eye Health Survey. Br J Ophthalmol. 2019;103(2):191-5.

10) Karvonen E, Stoor K, Luodonpaa M, Hagg P, Kuoppala J, Lintonen T, et al. Prevalence of glaucoma in the Northern Finland Birth Cohort Eye Study. Acta Ophthalmol. 2019;97(2):200-7.

11) Tham YC, Li X, Wong TY, Quigley HA, Aung T, Cheng CY. Global prevalence of glaucoma and projections of glaucoma burden through 2040: a systematic review and meta-analysis. Ophthalmology. 2014;121(11):2081-90.

12) Parikh RS, Parikh S, Sekhar GC, Kumar RS, Prabakaran S, Babu JG, et al. Diagnostic capability of optical coherence tomography (Stratus OCT 3) in early glaucoma. Ophthalmology. 2007;114(12):2238-43.

13) Poulton R, Moffitt TE, Silva PA. The Dunedin Multidisciplinary Health and Development Study: overview of the first 40 years, with an eye to the future. Soc Psychiatry Psychiatr Epidemiol. 2015;50(5):679-93.

14) Poulton R, Hancox R, Milne B, Baxter J, Scott K, Wilson N. The Dunedin Multidisciplinary Health and Development Study: are its findings consistent with the overall New Zealand population? N Z Med J. 2006;119(1235):U2002.

15) Ahmad SS. Glaucoma suspects: A practical approach. Taiwan J Ophthalmol. 2018;8(2):74-81.

16) Danesh-Meyer HV, Gaskin BJ, Jayusundera T, Donaldson M, Gamble GD. Comparison of disc damage likelihood scale, cup to disc ratio, and Heidelberg retina tomograph in the diagnosis of glaucoma. Br J Ophthalmol. 2006;90(4):437-41.

17) Henderer JD. Disc damage likelihood scale. Br J Ophthalmol. 2006;90(4):395-6.

18) Henderer JD, Liu C, Kesen M, Altangerel U, Bayer A, Steinmann WC, et al. Reliability of the disk damage likelihood scale. Am J Ophthalmol. 2003;135(1):44-8.

19) Bayer A, Harasymowycz P, Henderer JD, Steinmann WG, Spaeth GL. Validity of a new disk grading scale for estimating glaucomatous damage: correlation with visual field damage. Am J Ophthalmol. 2002;133(6):758-63.

20) Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8(2):135-60.

21) Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1(8476):307-10.

22) Mathan JJ, Patel DV, McGhee CNJ, Patel HY. Analysis of Glaucoma Subtypes and Corresponding Demographics in a New Zealand Population. Biomed Hub. 2016;1(3):1-8.

23) Michelessi M, Li T, Miele A, Azuara-Blanco A, Qureshi R, Virgili G. Accuracy of optical coherence tomography for diagnosing glaucoma: an overview of systematic reviews. Br J Ophthalmol. 2021;105(4):490-5.

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The epidemiology of glaucoma has been defined by several large population-based studies, measuring the prevalence in different contexts (Table 1).[[1–10]] As the incidence of glaucoma increases with age, an increase in the number of affected people is predicted from an ageing global population.[[11]] Another source of increasing glaucoma prevalence is the wide dissemination of imaging technology, most notably optical coherence tomography (OCT), which is both specific and sensitive in detecting early glaucoma and is widely available as an opportunistic screening method in developed countries.[[12]]

View Table 1.

In New Zealand, no glaucoma prevalence data has been collected, and it has been assumed that the New Zealand prevalence is comparable to surveys from Australia.[[7,8]] Establishing the prevalence in New Zealand is important to help estimate the burden of this disease.

In the present study, we measured the prevalence of glaucoma in the well-characterised population-based birth cohort of 45-year-old participants of the Dunedin Multidisciplinary Health & Development Study (Dunedin Study).[[13]] Using OCT allowed us to consider how this new, more sensitive technology affected the prevalence estimate, in comparison to older studies which only used optic disc photography and visual field tests.

Methods

Study design and approvals

This was an observational cross-sectional study. Participants gave written informed consent, and all study protocols were approved by the NZ Health and Disability Ethics Committee.

Study population

Participants are members of the Dunedin Study, a longitudinal investigation of health and behaviour in a population-representative birth cohort of 1,037 individuals (91% of eligible births; 52% male) born between 1 April 1972 and 31 March 1973 in Dunedin, New Zealand. The longitudinal study was established at age 3-years based on residence in the province.[[13]] Assessments were conducted at birth and at ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, 38, and most recently at age 45, when 94% of the 997 participants still alive took part. Each study member was brought to the research unit for a day of interviews and examinations. Ninety-three percent of eligible age 45 participants also completed MRI scanning. The cohort represents the full range of socio-economic status in New Zealand’s South Island, and as adults match the NZ National Health and Nutrition Survey on adult health indicators, eg BMI, smoking, GP visits.[[14]] Study participants are primarily of New Zealand European ethnicity (approximately 93%). Written informed consent was obtained from participants, and the study was approved by the New Zealand Health and Disability Ethics Committee.

Data collection

At age 45-years, the following was assessed: first degree relative with glaucoma; best corrected visual acuity; visual field (VF) on Matrix perimeter (Carl Zeiss Meditec, Dublin, CA, USA); non-cycloplegic autorefraction; central corneal thickness (CCT) and intraocular pressure (IOP) using the Tonoref III (Nidek, Japan); axial length using IOL Master (Carl Zeiss Meditec, Dublin, CA, USA); spectral domain OCT (Cirrus HD-OCT, model 5000; Carl Zeiss Meditec, Dublin, California, USA) retinal nerve fibre layer (RNFL) by optic disc cube 200x200, macular ganglion cell layer by macular cube 512x128, vertical cup-disc-ratio (CDR), and disc area; un-dilated digital fundus photographs of each eye were taken after five minutes of dark adaptation, using an NMR-45 fundus camera (Canon, Japan).

Assessment of glaucoma

The diagnosis of glaucoma can be challenging, particularly in the early stages, and disagreement between methods of diagnosis is common.[[15]] Two masked independent ophthalmologists (GW, JG) viewed the fundus photographs, and subjective comments and diagnostic impressions were recorded, as well as disc damage likelihood scale (DDLS), and vertical CDR (inter-rater agreement was measured).[[16–19]] The DDLS was calculated for medium sized optic discs, as size could not be measured from the photographs. Discs with DDLS >5, CDR >0.5 or comments suspecting glaucoma or asymmetry of CDR ≥0.2 were noted to require further review. These suspect discs were reviewed with IOP, CCT, OCT and VF data to generate a consensus glaucoma status:

“Normal” if no suspicion of glaucoma (other non-glaucoma pathology may be present).

“Ocular hypertension” if IOP >21 mmHg and no optic disc abnormality.

“Glaucoma suspect” if optic disc photograph was suspicious for glaucoma with no more than borderline VF or OCT abnormalities (that is, no corresponding abnormalities, or abnormalities not explained by other disease or pathology).

“Glaucoma” if optic disc photograph was suspicious for glaucoma and there were corresponding abnormalities of the OCT or VF.

Each participant was assigned the glaucoma status of their worse eye.

Data analysis

All data was collated and analysed using Excel (Microsoft, Albuquerque, NM, US). To assess the intra-rater agreement for CDR and DDLS, Bland–Altmann plots were constructed, and the mean bias and limits of agreement (mean difference ± 1.96 standard deviation of differences) were calculated.[[20, 21]] Standard errors of the prevalence estimates were calculated for a binomial distribution to generate 95% confidence intervals (CI).

Results

Of the 938 participants, 891 (95%) were assigned a glaucoma status. The 47 who were not assigned had technical difficulties with eye data collection.

Glaucoma status

The prevalence of each glaucoma status and the 95% confidence intervals are shown in Table 2.

View Table 2.

Among the more suspicious eyes of the 65 participants with glaucoma suspect status: 29 had suspicious discs, of which 22 had borderline abnormalities in OCT; four had borderline abnormalities in VF; one had non-corresponding abnormalities in both OCT and VF; and two had suspicious discs and other risk factors only. The remaining 36 glaucoma suspects had abnormalities in OCT alone, but low suspicion optic disc photographs in both eyes.

Among those with glaucoma status, six had abnormalities in OCT corresponding to the glaucomatous optic disc appearance, and one had abnormalities in both OCT and VF (mild) in their more affected eyes.

There were an additional 73 participants (8.2%, CI 6.3%–10%) with abnormalities on the OCT scan who were not deemed to be glaucoma suspects in either eye (non-pathological abnormalities or artefacts) and hence classified as normal.

Inter-rater agreement of optic disc photographs

Inter-rater reliability was a little greater for CDR (mean difference 0.01, limits of agreement -0.13 to +0.15), as compared with DDLS (mean difference -0.55, limits of agreement -1.9 to +0.8, see Supplemental Figure 1). This indicated that GW rated DDLS scores lower than JG on average.

Clinical parameters

Both eyes were pooled to calculate average disc metrics (mean ± standard deviation and CI). The mean DDLS was 2.5 ± 0.88 (2.4-2.6) and mean CDR was 0.32 ± 0.14 (0.31–0.33). The IOP, CCT, and RNFL had a normal symmetrical distribution in keeping with previous cohorts. Figure 1 depicts the distribution of glaucoma statuses across the complete range of IOPs.

Figure 1: Histogram of intraocular pressures (the higher of the two eyes), with glaucoma status labelled. The higher intraocular pressure in participants with glaucoma are indicated as asterisks.

Discussion

In this observational, cross-sectional study of predominantly white (Pākehā) 45-year-old New Zealanders, we found the prevalence of glaucoma to be 0.79% (CI 0.2–1.4), based on fundus photographs, OCT, and VF results. The prevalence of ocular hypertension was 1.68% (CI 0.8–2.5), and glaucoma suspect status was 7.30% (CI 5.6–9.0). The prevalence aligns with other population-based studies with Caucasian/white participants of the 40–50-year age group (Table 1).

An additional 73 participants (8.2%, CI 6.3%–10%) had at least one abnormal eye on OCT imaging that was deemed to be non-pathological or artefactual. From a total of 139 participants with abnormal OCT, seven were assigned glaucoma status, and 59 were glaucoma suspects including just 23 who would be suspected of glaucoma by disc photography, IOP and visual fields. Clearly, OCT technology is highly sensitive, but this comes with a risk of detecting false positives (artefacts).

In this younger 45-year-old cohort with a low prevalence, all of the participants with glaucoma had normal IOP in both eyes, as did all but one eye of one of the 65 glaucoma suspect (21.3 mmHg). This is a greater proportion with normal IOP than would be expected in a Caucasian cohort, and does not fit easily with the idea that ocular hypertension is presumed to be the pathogenic precursor to glaucoma in many cases.[[7–10]] Possible explanations include that naïve optic disc imaging with OCT detects a broader group of glaucoma cases than previous studies, or that this younger cohort may have a greater prevalence of myopia and thus more similarity to East Asian cohorts (with a very high proportion of normal tension glaucoma). Additionally, no disc haemorrhages were seen in any of the disc photos.

Limitations of this study include the lack of specialist assessment in clinic, gonioscopy or slit lamp examination in the diagnosis, or adhering to protocols for glaucoma diagnosis from other population-based studies. There was potential for non-contact tonometry and non-stereo disc imaging to reduce the diagnostic accuracy, but data collection was standardised and robust, and diagnostic classifications were made by consensus with the best available information. Due to low numbers, the findings should not be generalised to Māori, Pasifika and Asian ethnic groups, who have different prevalence of glaucoma types.[[22]]

The prevalence of glaucoma in 45-year-old New Zealanders appears to lie between 0.2% and 1.4%, consistent with other population-based surveys. Future examinations in the same cohort will detect incident cases over time. This is one of the first population-based studies to include OCT in the diagnosis of glaucoma, highlighting the sensitivity of these devices but also the potential for misinterpretation and over-investigation.[[23]]

Summary

Abstract

Aim

We aimed to estimate the prevalence of glaucoma in New Zealand using a population-based birth cohort of 45-year-olds.

Method

Study members of the Dunedin Multidisciplinary Health & Development Study participated (n=938 out of 1037 births (91%)). The data collected included visual acuity, visual field (VF), refraction, central corneal thickness, intraocular pressure (IOP), axial length, spectral domain optical coherence tomography (OCT), and non-mydriatic fundus photographs. Two ophthalmologists reviewed data independently to generate a consensus glaucoma status: “Normal” if no suspicion of glaucoma; “Ocular hypertension” if IOP >21 mmHg; “Glaucoma suspect” if optic disc photograph was suspicious for glaucoma with no more than borderline or non-corresponding VF or OCT abnormalities; and “Glaucoma” if optic disc photograph was suspicious for glaucoma and there were corresponding abnormalities of the OCT or VF.

Results

Of 891 participants with sufficient data to assign a glaucoma status, 804 were “Normal” (90.2% [CI 88.3–92.2]), 15 were “Ocular hypertension” (1.68% [95% confidence interval (CI) 0.84–2.5]), 65 were “Glaucoma suspect” (7.30% [95% CI 5.6–9.0]), and 7 were classified as “Glaucoma” (0.79% [95% CI 0.21–1.4]). An additional 73 participants (8.2%, [95% CI 6.3%–10%]) had abnormalities on the OCT scan but were not deemed to be glaucoma suspects.

Conclusion

The prevalence of glaucoma in New Zealand is between 0.2% and 1.4%, consistent with other population-based studies in the same age group. The study highlights the sensitivity of OCT and the potential for misinterpretation and over-investigation.

Author Information

Dr Aqeeda Singh, MB ChB: Dunedin School of Medicine, University of Otago, Dunedin, New Zealand. Dr Jesse Gale, MB ChB FRANZCO: Department of Surgery & Anaesthesia, University of Otago, Wellington, New Zealand. Dr Kirsten Cheyne, PhD: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand. Antony Ambler, MSc: Social, Genetic, and Developmental Psychiatry Centre, Institute of Psychiatry, Psychology, and Neuroscience, King’s College London, London, United Kingdom. Prof Richie Poulton, PhD: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand Dr Graham Wilson, MB ChB FRANZCO: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand.

Acknowledgements

We thank the Dunedin Study members, their families and friends for their long-term involvement, and Study Founder, Dr Phil A Silva. The Dunedin Multidisciplinary Health and Development Research Unit is based at University of Otago within the Ngāi Tahu tribal area whom we acknowledge as first peoples, tangata whenua (people of this land). This research received support from the NZ Health Research Council Programme Grant (16–604). The Dunedin Multidisciplinary Health and Development Research Unit is additionally supported by the New Zealand Health Research Council and the New Zealand Ministry of Business, Innovation, and Employment. AS received direct support from the Gordon Sanderson Scholarship from Glaucoma New Zealand. The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Correspondence

Dr Jesse Gale: Department of Surgery & Anaesthesia, University of Otago, Wellington. Private Bag 7343 Wellington South 6242, New Zealand. Phone: +644385554, fax: +6443895725.

Correspondence Email

jesse.gale@otago.ac.nz

Competing Interests

Nil.

1) Bengtsson B. The prevalence of glaucoma. Br J Ophthalmol. 1981;65(1):46-9.

2) Tielsch JM, Sommer A, Katz J, Royall RM, Quigley HA, Javitt J. Racial variations in the prevalence of primary open-angle glaucoma. The Baltimore Eye Survey. JAMA. 1991;266(3):369-74.

3) Klein BE, Klein R, Sponsel WE, Franke T, Cantor LB, Martone J, et al. Prevalence of glaucoma. The Beaver Dam Eye Study. Ophthalmology. 1992;99(10):1499-504.

4) Coffey M, Reidy A, Wormald R, Xian WX, Wright L, Courtney P. Prevalence of glaucoma in the west of Ireland. Br J Ophthalmol. 1993;77(1):17-21.

5) Dielemans I, Vingerling JR, Wolfs RC, Hofman A, Grobbee DE, de Jong PT. The prevalence of primary open-angle glaucoma in a population-based study in The Netherlands. The Rotterdam Study. Ophthalmology. 1994;101(11):1851-5.

6) Giuffre G, Giammanco R, Dardanoni G, Ponte F. Prevalence of glaucoma and distribution of intraocular pressure in a population. The Casteldaccia Eye Study. Acta Ophthalmol Scand. 1995;73(3):222-5.

7) Mitchell P, Smith W, Attebo K, Healey PR. Prevalence of open-angle glaucoma in Australia. The Blue Mountains Eye Study. Ophthalmology. 1996;103(10):1661-9.

8) Wensor MD, McCarty CA, Stanislavsky YL, Livingston PM, Taylor HR. The prevalence of glaucoma in the Melbourne Visual Impairment Project. Ophthalmology. 1998;105(4):733-9.

9) Keel S, Xie J, Foreman J, Lee PY, Alwan M, Fahy ET, et al. Prevalence of glaucoma in the Australian National Eye Health Survey. Br J Ophthalmol. 2019;103(2):191-5.

10) Karvonen E, Stoor K, Luodonpaa M, Hagg P, Kuoppala J, Lintonen T, et al. Prevalence of glaucoma in the Northern Finland Birth Cohort Eye Study. Acta Ophthalmol. 2019;97(2):200-7.

11) Tham YC, Li X, Wong TY, Quigley HA, Aung T, Cheng CY. Global prevalence of glaucoma and projections of glaucoma burden through 2040: a systematic review and meta-analysis. Ophthalmology. 2014;121(11):2081-90.

12) Parikh RS, Parikh S, Sekhar GC, Kumar RS, Prabakaran S, Babu JG, et al. Diagnostic capability of optical coherence tomography (Stratus OCT 3) in early glaucoma. Ophthalmology. 2007;114(12):2238-43.

13) Poulton R, Moffitt TE, Silva PA. The Dunedin Multidisciplinary Health and Development Study: overview of the first 40 years, with an eye to the future. Soc Psychiatry Psychiatr Epidemiol. 2015;50(5):679-93.

14) Poulton R, Hancox R, Milne B, Baxter J, Scott K, Wilson N. The Dunedin Multidisciplinary Health and Development Study: are its findings consistent with the overall New Zealand population? N Z Med J. 2006;119(1235):U2002.

15) Ahmad SS. Glaucoma suspects: A practical approach. Taiwan J Ophthalmol. 2018;8(2):74-81.

16) Danesh-Meyer HV, Gaskin BJ, Jayusundera T, Donaldson M, Gamble GD. Comparison of disc damage likelihood scale, cup to disc ratio, and Heidelberg retina tomograph in the diagnosis of glaucoma. Br J Ophthalmol. 2006;90(4):437-41.

17) Henderer JD. Disc damage likelihood scale. Br J Ophthalmol. 2006;90(4):395-6.

18) Henderer JD, Liu C, Kesen M, Altangerel U, Bayer A, Steinmann WC, et al. Reliability of the disk damage likelihood scale. Am J Ophthalmol. 2003;135(1):44-8.

19) Bayer A, Harasymowycz P, Henderer JD, Steinmann WG, Spaeth GL. Validity of a new disk grading scale for estimating glaucomatous damage: correlation with visual field damage. Am J Ophthalmol. 2002;133(6):758-63.

20) Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8(2):135-60.

21) Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1(8476):307-10.

22) Mathan JJ, Patel DV, McGhee CNJ, Patel HY. Analysis of Glaucoma Subtypes and Corresponding Demographics in a New Zealand Population. Biomed Hub. 2016;1(3):1-8.

23) Michelessi M, Li T, Miele A, Azuara-Blanco A, Qureshi R, Virgili G. Accuracy of optical coherence tomography for diagnosing glaucoma: an overview of systematic reviews. Br J Ophthalmol. 2021;105(4):490-5.

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The epidemiology of glaucoma has been defined by several large population-based studies, measuring the prevalence in different contexts (Table 1).[[1–10]] As the incidence of glaucoma increases with age, an increase in the number of affected people is predicted from an ageing global population.[[11]] Another source of increasing glaucoma prevalence is the wide dissemination of imaging technology, most notably optical coherence tomography (OCT), which is both specific and sensitive in detecting early glaucoma and is widely available as an opportunistic screening method in developed countries.[[12]]

View Table 1.

In New Zealand, no glaucoma prevalence data has been collected, and it has been assumed that the New Zealand prevalence is comparable to surveys from Australia.[[7,8]] Establishing the prevalence in New Zealand is important to help estimate the burden of this disease.

In the present study, we measured the prevalence of glaucoma in the well-characterised population-based birth cohort of 45-year-old participants of the Dunedin Multidisciplinary Health & Development Study (Dunedin Study).[[13]] Using OCT allowed us to consider how this new, more sensitive technology affected the prevalence estimate, in comparison to older studies which only used optic disc photography and visual field tests.

Methods

Study design and approvals

This was an observational cross-sectional study. Participants gave written informed consent, and all study protocols were approved by the NZ Health and Disability Ethics Committee.

Study population

Participants are members of the Dunedin Study, a longitudinal investigation of health and behaviour in a population-representative birth cohort of 1,037 individuals (91% of eligible births; 52% male) born between 1 April 1972 and 31 March 1973 in Dunedin, New Zealand. The longitudinal study was established at age 3-years based on residence in the province.[[13]] Assessments were conducted at birth and at ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, 38, and most recently at age 45, when 94% of the 997 participants still alive took part. Each study member was brought to the research unit for a day of interviews and examinations. Ninety-three percent of eligible age 45 participants also completed MRI scanning. The cohort represents the full range of socio-economic status in New Zealand’s South Island, and as adults match the NZ National Health and Nutrition Survey on adult health indicators, eg BMI, smoking, GP visits.[[14]] Study participants are primarily of New Zealand European ethnicity (approximately 93%). Written informed consent was obtained from participants, and the study was approved by the New Zealand Health and Disability Ethics Committee.

Data collection

At age 45-years, the following was assessed: first degree relative with glaucoma; best corrected visual acuity; visual field (VF) on Matrix perimeter (Carl Zeiss Meditec, Dublin, CA, USA); non-cycloplegic autorefraction; central corneal thickness (CCT) and intraocular pressure (IOP) using the Tonoref III (Nidek, Japan); axial length using IOL Master (Carl Zeiss Meditec, Dublin, CA, USA); spectral domain OCT (Cirrus HD-OCT, model 5000; Carl Zeiss Meditec, Dublin, California, USA) retinal nerve fibre layer (RNFL) by optic disc cube 200x200, macular ganglion cell layer by macular cube 512x128, vertical cup-disc-ratio (CDR), and disc area; un-dilated digital fundus photographs of each eye were taken after five minutes of dark adaptation, using an NMR-45 fundus camera (Canon, Japan).

Assessment of glaucoma

The diagnosis of glaucoma can be challenging, particularly in the early stages, and disagreement between methods of diagnosis is common.[[15]] Two masked independent ophthalmologists (GW, JG) viewed the fundus photographs, and subjective comments and diagnostic impressions were recorded, as well as disc damage likelihood scale (DDLS), and vertical CDR (inter-rater agreement was measured).[[16–19]] The DDLS was calculated for medium sized optic discs, as size could not be measured from the photographs. Discs with DDLS >5, CDR >0.5 or comments suspecting glaucoma or asymmetry of CDR ≥0.2 were noted to require further review. These suspect discs were reviewed with IOP, CCT, OCT and VF data to generate a consensus glaucoma status:

“Normal” if no suspicion of glaucoma (other non-glaucoma pathology may be present).

“Ocular hypertension” if IOP >21 mmHg and no optic disc abnormality.

“Glaucoma suspect” if optic disc photograph was suspicious for glaucoma with no more than borderline VF or OCT abnormalities (that is, no corresponding abnormalities, or abnormalities not explained by other disease or pathology).

“Glaucoma” if optic disc photograph was suspicious for glaucoma and there were corresponding abnormalities of the OCT or VF.

Each participant was assigned the glaucoma status of their worse eye.

Data analysis

All data was collated and analysed using Excel (Microsoft, Albuquerque, NM, US). To assess the intra-rater agreement for CDR and DDLS, Bland–Altmann plots were constructed, and the mean bias and limits of agreement (mean difference ± 1.96 standard deviation of differences) were calculated.[[20, 21]] Standard errors of the prevalence estimates were calculated for a binomial distribution to generate 95% confidence intervals (CI).

Results

Of the 938 participants, 891 (95%) were assigned a glaucoma status. The 47 who were not assigned had technical difficulties with eye data collection.

Glaucoma status

The prevalence of each glaucoma status and the 95% confidence intervals are shown in Table 2.

View Table 2.

Among the more suspicious eyes of the 65 participants with glaucoma suspect status: 29 had suspicious discs, of which 22 had borderline abnormalities in OCT; four had borderline abnormalities in VF; one had non-corresponding abnormalities in both OCT and VF; and two had suspicious discs and other risk factors only. The remaining 36 glaucoma suspects had abnormalities in OCT alone, but low suspicion optic disc photographs in both eyes.

Among those with glaucoma status, six had abnormalities in OCT corresponding to the glaucomatous optic disc appearance, and one had abnormalities in both OCT and VF (mild) in their more affected eyes.

There were an additional 73 participants (8.2%, CI 6.3%–10%) with abnormalities on the OCT scan who were not deemed to be glaucoma suspects in either eye (non-pathological abnormalities or artefacts) and hence classified as normal.

Inter-rater agreement of optic disc photographs

Inter-rater reliability was a little greater for CDR (mean difference 0.01, limits of agreement -0.13 to +0.15), as compared with DDLS (mean difference -0.55, limits of agreement -1.9 to +0.8, see Supplemental Figure 1). This indicated that GW rated DDLS scores lower than JG on average.

Clinical parameters

Both eyes were pooled to calculate average disc metrics (mean ± standard deviation and CI). The mean DDLS was 2.5 ± 0.88 (2.4-2.6) and mean CDR was 0.32 ± 0.14 (0.31–0.33). The IOP, CCT, and RNFL had a normal symmetrical distribution in keeping with previous cohorts. Figure 1 depicts the distribution of glaucoma statuses across the complete range of IOPs.

Figure 1: Histogram of intraocular pressures (the higher of the two eyes), with glaucoma status labelled. The higher intraocular pressure in participants with glaucoma are indicated as asterisks.

Discussion

In this observational, cross-sectional study of predominantly white (Pākehā) 45-year-old New Zealanders, we found the prevalence of glaucoma to be 0.79% (CI 0.2–1.4), based on fundus photographs, OCT, and VF results. The prevalence of ocular hypertension was 1.68% (CI 0.8–2.5), and glaucoma suspect status was 7.30% (CI 5.6–9.0). The prevalence aligns with other population-based studies with Caucasian/white participants of the 40–50-year age group (Table 1).

An additional 73 participants (8.2%, CI 6.3%–10%) had at least one abnormal eye on OCT imaging that was deemed to be non-pathological or artefactual. From a total of 139 participants with abnormal OCT, seven were assigned glaucoma status, and 59 were glaucoma suspects including just 23 who would be suspected of glaucoma by disc photography, IOP and visual fields. Clearly, OCT technology is highly sensitive, but this comes with a risk of detecting false positives (artefacts).

In this younger 45-year-old cohort with a low prevalence, all of the participants with glaucoma had normal IOP in both eyes, as did all but one eye of one of the 65 glaucoma suspect (21.3 mmHg). This is a greater proportion with normal IOP than would be expected in a Caucasian cohort, and does not fit easily with the idea that ocular hypertension is presumed to be the pathogenic precursor to glaucoma in many cases.[[7–10]] Possible explanations include that naïve optic disc imaging with OCT detects a broader group of glaucoma cases than previous studies, or that this younger cohort may have a greater prevalence of myopia and thus more similarity to East Asian cohorts (with a very high proportion of normal tension glaucoma). Additionally, no disc haemorrhages were seen in any of the disc photos.

Limitations of this study include the lack of specialist assessment in clinic, gonioscopy or slit lamp examination in the diagnosis, or adhering to protocols for glaucoma diagnosis from other population-based studies. There was potential for non-contact tonometry and non-stereo disc imaging to reduce the diagnostic accuracy, but data collection was standardised and robust, and diagnostic classifications were made by consensus with the best available information. Due to low numbers, the findings should not be generalised to Māori, Pasifika and Asian ethnic groups, who have different prevalence of glaucoma types.[[22]]

The prevalence of glaucoma in 45-year-old New Zealanders appears to lie between 0.2% and 1.4%, consistent with other population-based surveys. Future examinations in the same cohort will detect incident cases over time. This is one of the first population-based studies to include OCT in the diagnosis of glaucoma, highlighting the sensitivity of these devices but also the potential for misinterpretation and over-investigation.[[23]]

Summary

Abstract

Aim

We aimed to estimate the prevalence of glaucoma in New Zealand using a population-based birth cohort of 45-year-olds.

Method

Study members of the Dunedin Multidisciplinary Health & Development Study participated (n=938 out of 1037 births (91%)). The data collected included visual acuity, visual field (VF), refraction, central corneal thickness, intraocular pressure (IOP), axial length, spectral domain optical coherence tomography (OCT), and non-mydriatic fundus photographs. Two ophthalmologists reviewed data independently to generate a consensus glaucoma status: “Normal” if no suspicion of glaucoma; “Ocular hypertension” if IOP >21 mmHg; “Glaucoma suspect” if optic disc photograph was suspicious for glaucoma with no more than borderline or non-corresponding VF or OCT abnormalities; and “Glaucoma” if optic disc photograph was suspicious for glaucoma and there were corresponding abnormalities of the OCT or VF.

Results

Of 891 participants with sufficient data to assign a glaucoma status, 804 were “Normal” (90.2% [CI 88.3–92.2]), 15 were “Ocular hypertension” (1.68% [95% confidence interval (CI) 0.84–2.5]), 65 were “Glaucoma suspect” (7.30% [95% CI 5.6–9.0]), and 7 were classified as “Glaucoma” (0.79% [95% CI 0.21–1.4]). An additional 73 participants (8.2%, [95% CI 6.3%–10%]) had abnormalities on the OCT scan but were not deemed to be glaucoma suspects.

Conclusion

The prevalence of glaucoma in New Zealand is between 0.2% and 1.4%, consistent with other population-based studies in the same age group. The study highlights the sensitivity of OCT and the potential for misinterpretation and over-investigation.

Author Information

Dr Aqeeda Singh, MB ChB: Dunedin School of Medicine, University of Otago, Dunedin, New Zealand. Dr Jesse Gale, MB ChB FRANZCO: Department of Surgery & Anaesthesia, University of Otago, Wellington, New Zealand. Dr Kirsten Cheyne, PhD: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand. Antony Ambler, MSc: Social, Genetic, and Developmental Psychiatry Centre, Institute of Psychiatry, Psychology, and Neuroscience, King’s College London, London, United Kingdom. Prof Richie Poulton, PhD: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand Dr Graham Wilson, MB ChB FRANZCO: Dunedin Multidisciplinary Health and Development Research Unit, Department of Psychology, University of Otago, Dunedin, New Zealand.

Acknowledgements

We thank the Dunedin Study members, their families and friends for their long-term involvement, and Study Founder, Dr Phil A Silva. The Dunedin Multidisciplinary Health and Development Research Unit is based at University of Otago within the Ngāi Tahu tribal area whom we acknowledge as first peoples, tangata whenua (people of this land). This research received support from the NZ Health Research Council Programme Grant (16–604). The Dunedin Multidisciplinary Health and Development Research Unit is additionally supported by the New Zealand Health Research Council and the New Zealand Ministry of Business, Innovation, and Employment. AS received direct support from the Gordon Sanderson Scholarship from Glaucoma New Zealand. The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Correspondence

Dr Jesse Gale: Department of Surgery & Anaesthesia, University of Otago, Wellington. Private Bag 7343 Wellington South 6242, New Zealand. Phone: +644385554, fax: +6443895725.

Correspondence Email

jesse.gale@otago.ac.nz

Competing Interests

Nil.

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9) Keel S, Xie J, Foreman J, Lee PY, Alwan M, Fahy ET, et al. Prevalence of glaucoma in the Australian National Eye Health Survey. Br J Ophthalmol. 2019;103(2):191-5.

10) Karvonen E, Stoor K, Luodonpaa M, Hagg P, Kuoppala J, Lintonen T, et al. Prevalence of glaucoma in the Northern Finland Birth Cohort Eye Study. Acta Ophthalmol. 2019;97(2):200-7.

11) Tham YC, Li X, Wong TY, Quigley HA, Aung T, Cheng CY. Global prevalence of glaucoma and projections of glaucoma burden through 2040: a systematic review and meta-analysis. Ophthalmology. 2014;121(11):2081-90.

12) Parikh RS, Parikh S, Sekhar GC, Kumar RS, Prabakaran S, Babu JG, et al. Diagnostic capability of optical coherence tomography (Stratus OCT 3) in early glaucoma. Ophthalmology. 2007;114(12):2238-43.

13) Poulton R, Moffitt TE, Silva PA. The Dunedin Multidisciplinary Health and Development Study: overview of the first 40 years, with an eye to the future. Soc Psychiatry Psychiatr Epidemiol. 2015;50(5):679-93.

14) Poulton R, Hancox R, Milne B, Baxter J, Scott K, Wilson N. The Dunedin Multidisciplinary Health and Development Study: are its findings consistent with the overall New Zealand population? N Z Med J. 2006;119(1235):U2002.

15) Ahmad SS. Glaucoma suspects: A practical approach. Taiwan J Ophthalmol. 2018;8(2):74-81.

16) Danesh-Meyer HV, Gaskin BJ, Jayusundera T, Donaldson M, Gamble GD. Comparison of disc damage likelihood scale, cup to disc ratio, and Heidelberg retina tomograph in the diagnosis of glaucoma. Br J Ophthalmol. 2006;90(4):437-41.

17) Henderer JD. Disc damage likelihood scale. Br J Ophthalmol. 2006;90(4):395-6.

18) Henderer JD, Liu C, Kesen M, Altangerel U, Bayer A, Steinmann WC, et al. Reliability of the disk damage likelihood scale. Am J Ophthalmol. 2003;135(1):44-8.

19) Bayer A, Harasymowycz P, Henderer JD, Steinmann WG, Spaeth GL. Validity of a new disk grading scale for estimating glaucomatous damage: correlation with visual field damage. Am J Ophthalmol. 2002;133(6):758-63.

20) Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8(2):135-60.

21) Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1(8476):307-10.

22) Mathan JJ, Patel DV, McGhee CNJ, Patel HY. Analysis of Glaucoma Subtypes and Corresponding Demographics in a New Zealand Population. Biomed Hub. 2016;1(3):1-8.

23) Michelessi M, Li T, Miele A, Azuara-Blanco A, Qureshi R, Virgili G. Accuracy of optical coherence tomography for diagnosing glaucoma: an overview of systematic reviews. Br J Ophthalmol. 2021;105(4):490-5.

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