Journal of Refractive Surgery

Original Article Supplemental Data

Prevalence of Glaucoma in Myopic Corneal Refractive Surgery Candidates in Hong Kong China

Sayantan Biswas, Mphil; Vishal Jhanji, FRCOphth; Christopher K. S. Leung, MD

Abstract

PURPOSE:

To estimate the prevalence of glaucoma in myopic individuals in Hong Kong China seeking corneal refractive surgery and factors associated with the occurrence of glaucoma.

METHODS:

Six hundred twenty-nine eyes of 315 myopic individuals seeking corneal refractive surgery in a tertiary institution in Hong Kong China, were consecutively recruited. Glaucoma was defined by the presence of abnormal retinal nerve fiber layer (RNFL) and neuroretinal rim measurements obtained with spectral-domain optical coherence tomography, as well as narrowed neuroretinal rim and optic disc excavation observed in color optic disc stereophotographs. Factors associated with glaucoma were analyzed with univariable and multivariable mixed-effects logistic regression analysis.

RESULTS:

The mean age and spherical equivalent were 36.3 ± 9.3 years (range: 21.5 to 60.1 years) and −6.59 ± 2.34 diopters (D) (range: −2.25 to −17.13 D), respectively. Twenty-three eyes of 16 myopic individuals (5.1%; 95% confidence interval [CI]: 3.2% to 8.1%) had glaucoma, of which 10 eyes of 7 myopic individuals (2.2%; 95% CI: 1.1% to 4.5%) also had visual field defects by standard automated perimetry (5 had mild and 5 had moderate visual field loss). None of the myopic individuals were aware of the diagnosis at the time of examination. The inferotemporal sector of the optic disc was the most frequent location where RNFL thinning was found and the superonasal visual field was the most common region where visual field defects were located. The only biometric factor associated with glaucoma was intraocular pressure (odds ratio: 1.77; 95% CI: 1.14 to 2.75).

CONCLUSIONS:

Glaucoma was present in 5.1% of myopic individuals seeking corneal refractive surgery in Hong Kong China. Corneal refractive surgeons should be watchful in the diagnostic evaluation of glaucoma among myopic corneal refractive surgery candidates.

[J Refract Surg. 2016;32(5):298–304.]

Abstract

PURPOSE:

To estimate the prevalence of glaucoma in myopic individuals in Hong Kong China seeking corneal refractive surgery and factors associated with the occurrence of glaucoma.

METHODS:

Six hundred twenty-nine eyes of 315 myopic individuals seeking corneal refractive surgery in a tertiary institution in Hong Kong China, were consecutively recruited. Glaucoma was defined by the presence of abnormal retinal nerve fiber layer (RNFL) and neuroretinal rim measurements obtained with spectral-domain optical coherence tomography, as well as narrowed neuroretinal rim and optic disc excavation observed in color optic disc stereophotographs. Factors associated with glaucoma were analyzed with univariable and multivariable mixed-effects logistic regression analysis.

RESULTS:

The mean age and spherical equivalent were 36.3 ± 9.3 years (range: 21.5 to 60.1 years) and −6.59 ± 2.34 diopters (D) (range: −2.25 to −17.13 D), respectively. Twenty-three eyes of 16 myopic individuals (5.1%; 95% confidence interval [CI]: 3.2% to 8.1%) had glaucoma, of which 10 eyes of 7 myopic individuals (2.2%; 95% CI: 1.1% to 4.5%) also had visual field defects by standard automated perimetry (5 had mild and 5 had moderate visual field loss). None of the myopic individuals were aware of the diagnosis at the time of examination. The inferotemporal sector of the optic disc was the most frequent location where RNFL thinning was found and the superonasal visual field was the most common region where visual field defects were located. The only biometric factor associated with glaucoma was intraocular pressure (odds ratio: 1.77; 95% CI: 1.14 to 2.75).

CONCLUSIONS:

Glaucoma was present in 5.1% of myopic individuals seeking corneal refractive surgery in Hong Kong China. Corneal refractive surgeons should be watchful in the diagnostic evaluation of glaucoma among myopic corneal refractive surgery candidates.

[J Refract Surg. 2016;32(5):298–304.]

Myopia is epidemic in Asia. With a prevalence of up to 70% in schoolchildren1,2 and up to 40% in adults,3–5 detecting and treating ocular complications related to myopia poses a major public health challenge. An important blinding complication in association with myopia is primary open-angle glaucoma. In the Blue Mountains Eye Study, the odds ratio (OR) of primary open-angle glaucoma was 2.3 (95% confidence interval [CI]: 1.3 to 4.1) for eyes with low myopia (spherical equivalent [SE] between −1.00 and −3.00 diopters [D]) and 3.3 (95% CI: 1.7 to 6.4) for eyes with moderate to high myopia (SE ≤ −3.00 D) compared with eyes without myopia.6 In the Beijing Eye Study, the frequency of glaucoma for eyes with high myopia (≤−6.00 D) was greater than for eyes with low myopia (<−0.50 to −3.00 D) (OR: 3.5; 95% CI: 1.71 to 7.25), although it was not significantly different from that for eyes with moderate myopia (between −3.00 and −6.00 D).7 In the Singapore Indian Eye Study, each millimeter increase in axial length was associated with a 1.43-fold (95% CI: 1.13 to 1.80) increase in risk of primary open-angle glaucoma.8 By contrast, in the Ocular Hypertension Treatment Study, a prospective study evaluating the risk of primary open-angle glaucoma in patients with ocular hypertension, myopia was not a predictive factor for development of primary open-angle glaucoma.9 Likewise, there was a lack of association between myopia and the prevalence of self-reported glaucoma in the United States National Health and Nutrition Examination Survey.10 The presence of parapapillary atrophy and tilted optic disc configuration in myopia can blur the distinction between glaucomatous and nonglaucomatous optic discs, which may account for the discrepancy in studies investigating the association between myopia and glaucoma.11,12

Myopic individuals seeking corneal refractive surgery is an important target group at risk of glaucoma. They are relatively young and have a relatively high degree of myopia, thereby having a high risk of blindness due to glaucoma if the disease is not detected and managed at its early stage. However, detecting glaucoma in myopic individuals is challenging for corneal refractive surgeons and glaucoma specialists. Missing a diagnosis of glaucoma has undesirable consequences for both patients (visual loss) and physicians (medical allegation).

The objectives of this study were to investigate the prevalence of glaucoma, determined by the presence of retinal nerve fiber layer (RNFL) and neuroretinal rim abnormalities using spectral-domain optical coherence tomography (OCT) and color optic disc stereophotography in myopic corneal refractive surgery candidates and to investigate biometric factors associated with the occurrence of glaucoma in Hong Kong China.

Patients and Methods

Study Group

Three hundred fifteen myopic individuals with spherical equivalent between −2.25 and −17.13 D seeking corneal refractive surgery at the University Eye Center, the Chinese University of Hong Kong from January 2013 to April 2015 were consecutively enrolled. All myopic individuals had autorefraction, subjective refraction, and anterior and posterior segment examination of both eyes under slit-lamp biomicroscopy. The intraocular pressure (IOP), central corneal thickness, and axial length were measured with Goldmann applanation tonometry, ultrasound pachymetry, and partial coherence laser interferometry (IOLMaster; Carl Zeiss Meditec, Jena, Germany), respectively. All eyes examined in the study were phakic and had corrected distance visual acuity of at least 20/40. No individuals had a history of macular disease, refractive or intraocular surgery, or neurological disease. Color optic disc stereophotographs were obtained with a fundus camera (TRC-50DX; Topcon Corporation, Tokyo, Japan). The RNFL and optic nerve head were imaged by spectral-domain OCT (Cirrus HD-OCT; Carl Zeiss Meditec). Gonioscopy and standard white-on-white automated perimetry were performed for myopic individuals with evidence of glaucomatous optic disc changes (described below). The study was conducted in accordance with the tenets of the Declaration of Helsinki and approved by the local research ethics committee with written informed consent obtained.

OCT Imaging of the RNFL and Optic Disc

The Cirrus HD-OCT “Optic Disc Cube” scan was used to image and measure the RNFL and neuroretinal rim in the 6 × 6 mm2 optic disc region (200 × 200 pixels). The RNFL and neuroretinal rim analysis reports generated from the instrument were then examined for RNFL and neuroretinal rim abnormalities. RNFL abnormalities were detected using the RNFL thickness map and the RNFL thickness deviation map (described below). The RNFL thickness in the 6 × 6 mm2 optic disc region was examined because RNFL abnormalities can be missed if analysis is limited to the circumpapillary RNFL measurement.13 Neuroretinal rim abnormalities were detected in the circumpapillary neuroretinal rim thickness profile. The neuroretinal rim thickness was measured as the distance from the termination of Bruch's membrane (disc margin) to the inner limiting membrane that resulted in the best overall estimate of the total amount of neuroretinal rim tissue.14 Only images with signal strength of 6 or greater were included. Saccadic eye movement was detected with the line-scanning ophthalmoscope overlaid with OCT en face images. OCT images with poor signal strength, motion artifact, and missing data were identified by the operator with rescanning performed in the same visit.

Visual Field Testing

Perimetry was performed with standard automated white-on-white perimetry (Humphrey Field Analyzer II-I, SITA standard 24-2 strategy; Carl Zeiss Meditec) in eyes with glaucomatous optic disc changes (described below). A reliable visual field test had fixation losses and false-negative and false-positive errors of 20% or less. A visual field defect was defined as having three or more significant (P < .05) non-edge contiguous points with one or more at the P < .01 level on the same side of the horizontal meridian in the pattern deviation plot in a reliable visual field and confirmed with another examination.

Diagnosis of Glaucoma

Glaucomatous optic disc changes were defined by the presence of RNFL abnormalities in the RNFL thickness map and the RNFL thickness deviation map, with corresponding neuroretinal rim abnormalities in the circumpapillary neuroretinal rim thickness profile and corresponding narrowed neuroretinal rim and optic disc excavation in color optic disc stereophotographs evaluated by a glaucoma specialist (CKSL) (Figure A, available in the online version of this article). The RNFL thickness deviation map (50 × 50 superpixels) encoded a superpixel in yellow if the RNFL thickness of the superpixel was below the lower 95th percentile of the normative data and in red if the RNFL thickness of the superpixel was below the lower 99th percentile of the normative data. The RNFL thickness map (200 × 200 pixels) presented the RNFL thickness distribution in a color coded map with warm colors representing thick and cool colors representing thin RNFL measurements. RNFL abnormalities were defined when at least 20 contiguous superpixels (instrument default) were encoded in red in the RNFL thickness deviation map with corresponding RNFL loss in the superotemporal and/or inferotemporal RNFL bundles in the RNFL thickness map. OCT neuroretinal rim abnormalities were defined when the circumferential extent of the neuroretinal rim thickness was below the lower 95th percentile of the normative data for 20° or more in the circumpapillary neuroretinal rim profile. Glaucoma was diagnosed when there was evidence of glaucomatous optic disc changes with or without visual field defects. Perimetric glaucoma was diagnosed when the glaucomatous optic disc changes were associated with corresponding visual field defects.


Examples illustrating a (A) glaucomatous eye and a (B) normal eye. Glaucoma was diagnosed when narrowed neuroretinal rim and optic disc excavation was detected in color optic disc stereophotographs with corresponding retinal nerve fiber layer (RNFL) abnormalities observed in both the RNFL thickness map and the RNFL thickness deviation map. In panel (A), narrowed neuroretinal rim and optic disc excavation were found at the inferotemporal sector of the optic disc. Corresponding inferotemporal RNFL abnormalities were evident in the RNFL thickness map and the RNFL thickness deviation map. In panel (B), although RNFL abnormalities were found in the RNFL thickness deviation map, there was no corresponding RNFL loss in the RNFL thickness map. The superotemporal and inferotemporal RNFL bundles appeared normal and symmetric in the RNFL thickness map. The convergence of the superotemporal bundles towards the macula rendered the superior quadrant encoded as “abnormal” in the RNFL thickness deviation map. No neuroretinal rim loss or optic disc excavation was detected in the color optic disc photograph.

Figure A.

Examples illustrating a (A) glaucomatous eye and a (B) normal eye. Glaucoma was diagnosed when narrowed neuroretinal rim and optic disc excavation was detected in color optic disc stereophotographs with corresponding retinal nerve fiber layer (RNFL) abnormalities observed in both the RNFL thickness map and the RNFL thickness deviation map. In panel (A), narrowed neuroretinal rim and optic disc excavation were found at the inferotemporal sector of the optic disc. Corresponding inferotemporal RNFL abnormalities were evident in the RNFL thickness map and the RNFL thickness deviation map. In panel (B), although RNFL abnormalities were found in the RNFL thickness deviation map, there was no corresponding RNFL loss in the RNFL thickness map. The superotemporal and inferotemporal RNFL bundles appeared normal and symmetric in the RNFL thickness map. The convergence of the superotemporal bundles towards the macula rendered the superior quadrant encoded as “abnormal” in the RNFL thickness deviation map. No neuroretinal rim loss or optic disc excavation was detected in the color optic disc photograph.

Statistical Analysis

Statistical analyses were performed using Stata software version 14.0 (StataCorp, College Station, TX). The sample size of a prevalence study was calculated with reference to an expected prevalence (P), the desired level of precision (e), and the desired confidence level (Z) as described by Cochran:15



n
=


Z
2


p
(
1
−
p
)



e
2

Findings from meta-analysis indicated that the pool prevalence of primary open-angle glaucoma in Asia was 2.34%16 and that myopic eyes had an approximately two-fold increase in risk of developing glaucoma compared with non-myopic eyes.17 The prevalence of glaucoma in this study was estimated to be 4.68% (ie, p = 2 × 2.34%). Taking Z = 1.96 (ie, 95% to be the desired confidence level) and the level of precision to be p/2 (ie, e = 4.68%/2),18 the sample size (n) required for the study was 313. Both eyes were analyzed in the study. Univariable and multivariable mixed-effects logistic regression analyses were performed to investigate factors associated with glaucoma with adjustment of correlation between fellow eyes. Factors with a P value of less than .05 in the univariable model were analyzed in the multivariable model. A P value of less than .05 was considered statistically significant.

Results

A total of 629 eyes from 315 myopic individuals with spherical equivalent between −2.25 and −17.13 D (mean: −6.59 ± 2.34 D) and axial length between 23.5 and 30.3 mm (mean: 26.1 ± 1.2 mm) were consecutively included (one eye was excluded because of the failure to obtain an OCT image with a signal strength ≥ 6). A scatter plot of spherical equivalent and axial length is shown in Figure B (available in the online version of this article). A total of 53.4% of eyes had high myopia (SE < −6.00 D); 43.9% had moderate myopia (−3.00 D ≤ SE ≤ −6.00 D); and 2.7% had mild myopia (−0.50 D < SE < −3.00 D). The mean age, IOP, and central corneal thickness were 36.3 years (range: 21.5 to 60.1 years), 13.2 mm Hg (range: 8 to 28 mm Hg), and 551.8 µm (range: 444 to 663 µm), respectively. Table 1 shows the demographics and other biometric measurements.


A scatter plot of the distribution of axial length and spherical equivalent of all myopic eyes included in the study (629 eyes from 315 myopic individuals). D = diopters

Figure B.

A scatter plot of the distribution of axial length and spherical equivalent of all myopic eyes included in the study (629 eyes from 315 myopic individuals). D = diopters


Demographics and Biometric Measurements of All Myopic Individuals

Table 1:

Demographics and Biometric Measurements of All Myopic Individuals

Prevalence of Glaucoma in Myopia

Twenty-three eyes of 16 myopic individuals (5.1%; 95% confidence interval [CI]: 3.2% to 8.1%) had glaucomatous optic disc changes in which 10 eyes of 7 myopic individuals (2.2%; 95% CI: 1.1% to 4.5%) also had visual field defects. Five eyes had mild (MD ≥ −6 dB) and 5 had moderate (−6 dB > MD > −12 dB) visual field loss. No individuals were aware of the diagnosis of glaucoma at the time of examination. None had angle closure. Five eyes were found to have RNFL abnormalities without glaucomatous optic disc changes. These eyes were not considered as glaucoma. The frequency distribution plots of the locations of the RNFL abnormalities (superpixels encoded in red in the RNFL thickness deviation map) (n = 23) and visual field defects (test locations with P < .05 in the pattern deviation plot) (n = 10) are shown in Figure C (available in the online version of this article). The inferotemporal meridians at 252° to 279° were the most frequent locations where RNFL defects were detected, which corresponded to the superonasal visual field where visual field defects were most commonly found.


Frequency distribution plots constructed by overlaying the (A) retinal nerve fiber layer (RNFL) thickness deviation maps (ie, superpixels encoded in red) of the 23 eyes with glaucoma and (B) the standard automated perimetry pattern deviation plots (ie, test locations with P < .05) of the 10 eyes with perimetric glaucoma. The inferotemporal meridians at 252° to 279° were the most frequent locations where RNFL abnormalities were found, which corresponded with the superonasal visual field being the most commonly affected region.

Figure C.

Frequency distribution plots constructed by overlaying the (A) retinal nerve fiber layer (RNFL) thickness deviation maps (ie, superpixels encoded in red) of the 23 eyes with glaucoma and (B) the standard automated perimetry pattern deviation plots (ie, test locations with P < .05) of the 10 eyes with perimetric glaucoma. The inferotemporal meridians at 252° to 279° were the most frequent locations where RNFL abnormalities were found, which corresponded with the superonasal visual field being the most commonly affected region.

Factors Associated With Glaucoma in Myopic Patients

The mean age, IOP, central corneal thickness, spherical equivalent, and axial length in eyes with and without glaucoma are shown in Table 2 (glaucomatous eyes, n = 23) and Table 3 (perimetric glaucomatous eyes, n = 10). In the glaucoma group (n = 23), although IOP and spherical equivalent were associated with glaucoma in the univariable logistic regression analyses, only IOP was associated with glaucoma in the multivariable analysis. In the perimetric glaucoma group (n = 10), IOP was the only factor associated with glaucoma in the univariable and multivariable analyses. The odds ratio was 1.77 (95% CI: 1.14 to 2.75) in the glaucoma group and 2.10 (95% CI: 1.20 to 3.67) in the perimetric glaucoma group for each 1 mm Hg increase in IOP. The mean IOP in the glaucoma and perimetric glaucoma groups was 15.6 ± 4.8 and 17.3 ± 6.2 mm Hg, respectively. Notably, only 2 eyes in the glaucoma group had IOP greater than 21 mm Hg.


Factors Associated With Glaucoma (n = 23) Analyzed With Univariable and Multivariable Mixed-Effects Logistic Regression Analysis

Table 2:

Factors Associated With Glaucoma (n = 23) Analyzed With Univariable and Multivariable Mixed-Effects Logistic Regression Analysis


Factors Associated With Perimetric Glaucoma (n = 10) Analyzed With Univariable Mixed-Effects Logistic Regression Analysis

Table 3:

Factors Associated With Perimetric Glaucoma (n = 10) Analyzed With Univariable Mixed-Effects Logistic Regression Analysis

Discussion

With consecutive examination of 315 myopic corneal refractive surgery candidates in a tertiary institution in Hong Kong China, we showed that 16 myopic individuals (5.1%) had primary open-angle glaucoma in at least one eye. Among the 23 eyes of the 16 myopic individuals with glaucoma, 10 eyes of 7 (2.2%) myopic individuals had visual field defects, of which 5 had mild (MD ≥ −6 dB) and 5 had moderate (−6 dB > MD > −12 dB) visual field loss. Notably, none of the myopic individuals were aware of the diagnosis at the time of examination. They were relatively young (mean age: 36.3 ± 9.3 years) and the IOP was below 21 mm Hg in most eyes (21 of 23; 91.3%). Refractive surgeons should be vigilant in detecting glaucoma in myopic individuals seeking corneal refractive surgery.

The pool prevalence of primary open-angle glaucoma has been estimated to be 2.34% (95% CI: 0.96% to 4.55%) in Asia for individuals aged 40 to 80 years.16 For example, in the Liwan Eye Study, the prevalence of primary open-angle glaucoma in Southern Chinese individuals was 2.1% (95% CI: 1.4% to 2.8%) for adults aged 50 years and older.19 In the Beijing Eye Study, the prevalence of primary open-angle glaucoma in Northern Chinese individuals was 2.6% (95% CI: 2.1% to 3.0%) for adults aged 40 years and older.20 In the current study, the prevalence of primary open-angle glaucoma in myopic Hong Kong Chinese individuals with a mean age of 36.3 ± 9.3 years was 5.1% (95% CI: 3.1% to 8.2%). However, we did not observe a significant association between glaucoma and the degree of myopia (measured in axial length and spherical equivalent) in a myopic population (Tables 23). This result corroborates with the finding of the Beijing Eye Study in which the frequency of glaucoma for eyes with high myopia less than −6.00 D was not significantly different from that for eyes with moderate myopia between −3.00 and −6.00 D (P = .075).7 In other words, although myopic individuals may have a higher prevalence of glaucoma compared with the general population, the degree of myopia does not appear to influence the odds of developing glaucoma in Chinese individuals.

In many population studies investigating the prevalence of glaucoma, glaucoma was commonly diagnosed with reference to the cup-to-disc ratio. For example, in the International Society of Geographical and Epidemiological Ophthalmology classification scheme, the diagnosis of glaucoma is established when the cup-to-disc ratio or cup-to-disc ratio asymmetry was 97th percentile or greater for the normal population with the presence of visual field abnormality (category 1 diagnosis) and 99th percentile or greater for the normal population if the test subjects cannot complete visual field testing (category 2 diagnosis).21 Because the optic discs often assume a tilted configuration with extensive parapapillary atrophy in myopic eyes,22 accurate measurement of the cup-to-disc ratio is difficult. Further, physiological variation in the configurations of optic disc in myopic eyes can confound the differentiation between glaucomatous and normal optic discs. In this study, we therefore also included spectral-do-main OCT analysis of the RNFL and neuroretinal rim to corroborate optic disc changes detected in optic disc stereophotographs. The inferotemporal sector being the most common area where RNFL abnormalities were detected (Figure CA) corresponded with the superonasal visual field being the most common region where visual field defects were found (Figure CB). The inferotemporal sector of the optic disc and the superonasal visual field are typically affected in glaucoma. Our finding supports the notion that spectral-domain OCT is useful to detect RNFL damage in myopic eyes. In a previous study, we demonstrated that the superotemporal and inferotemporal RNFL bundles tend to converge temporally with increasing degree of myopia, which may result in false-positive detection of RNFL abnormalities in the RNFL thickness deviation map.23 For this reason, we not only studied RNFL abnormalities in the RNFL thickness deviation map but also examined the corresponding loss of the superotemporal and inferotemporal RNFL bundles in the RNFL thickness map (Figure A). Glaucoma is characterized by both RNFL thinning and neuroretinal rim narrowing. Including RNFL and neuroretinal rim abnormalities for glaucoma diagnosis would strengthen the diagnostic specificity and is relevant in a study investigating the prevalence of glaucoma in myopic eyes.

We adopted stringent criteria to define glaucoma because optic disc and visual field changes developed in myopic eyes may not always be associated with glaucoma. In a retrospective case series, Doshi et al. reported that 16 young Chinese patients with a mean age of 38.9 years previously diagnosed as having glaucoma or glaucoma suspect showed no evidence of progression in the visual field defects or the optic disc for up to 7 years.24 In another retrospective study, Ohno-Matsui et al. reviewed the medical records of 492 eyes of 308 patients with high myopia (defined as refractive error < −8.00 D or axial length ≥ 26.5 mm).25 They found that new visual field defects developed in only 13.2% of eyes over a mean follow-up of 11.6 years and these visual field defects could not be explained by glaucoma. Tilting of the optic disc and parapapillary atrophy developed in association with myopia may increase the strain of the optic nerve head for a given level of IOP, leading to RNFL, optic disc, and visual field changes similar to, but not associated with, glaucoma. In this study, 5 eyes were found to have RNFL abnormalities without glaucomatous optic disc changes. These eyes were not considered as glaucoma.

The only biometric variable significantly associated with glaucoma was IOP. Although IOP has been consistently shown to be a major risk factor in clinical trials for glaucoma treatment,9,26–28 it is worth noting that that the mean IOP for eyes with glaucomatous optic disc changes in this study was 15.6 ± 4.8 mm Hg (range: 10 to 28 mm Hg) and that only 2 of the 23 eyes (8.7%) had IOP of 21 mm Hg or greater. In other words, IOP measured with the Goldmann applanation tonometry remained within the normal range in most patients with myopic glaucoma. Normal tension glaucoma has been reported to account for 85% of primary open-angle glaucoma in South China,19 90% in North China,29 82% in South India,30 85% in Malay,31 77% in South Korea,32 and 78% in Japan.33 With a high prevalence of normal tension glaucoma in Asia,34 our findings may not be generalized to other ethnic groups. Yet, it is notable that the mean central corneal thickness of the 23 eyes diagnosed as having glaucoma was 536.1 ± 37.3 µm. The IOP could be underestimated with Goldmann applanation tonometry.

Our study had limitations. Although our study is not population-based, we specifically targeted a group at risk of glaucoma and performed an in-depth analysis of the RNFL and optic disc. The investigation of glaucoma prevalence in myopic individuals seeking corneal refractive surgery is of direct relevance and interest to refractive surgeons and glaucoma specialists. To our knowledge, our data are the first to report the prevalence of glaucoma in myopic corneal refractive surgery candidates, although the estimated prevalence may not be generalized to other refractive surgery clinics or other geographic locations. Differentiating glaucomatous from non-glaucomatous optic discs in eyes with myopia is complex and challenging. Although defining glaucoma with reference to both spectral-domain OCT RNFL and neuroretinal rim analysis and optic disc stereophotographic assessment offered a high specificity for detection of glaucoma, optic disc and RNFL changes might not be concomitantly detected and the prevalence of glaucoma could be underestimated. Notably, the application of the RNFL thickness deviation map and the circumpapillary neuroretinal rim thickness normative profile to detect RNFL and neuroretinal abnormalities could be limited by the fact that the Cirrus HD-OCT normative data were derived from normal individuals without high myopia. We therefore also examined the RNFL thickness maps and color optic disc stereophotographs to corroborate any detected RNFL and neuroretinal rim abnormalities. The development and application of a high myopic normative database would be relevant to reliably determine RNFL and neuroretinal rim abnormalities in eyes with high myopia. Perimetry was only performed in eyes with glaucomatous optic disc changes because visual field defects developed in the absence of glaucomatous optic disc changes would be unlikely to be related to glaucoma.

A relatively high prevalence of glaucoma (5.1%) was observed in myopic corneal refractive surgery candidates in Hong Kong China. None of the myopic individuals were aware of the diagnosis at the time of examination and the IOP in most myopic individuals was normal. Refractive surgeons and glaucoma specialists should collaborate and perform careful examination of the optic disc and the RNFL for myopic individuals seeking corneal refractive surgery.

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Demographics and Biometric Measurements of All Myopic Individuals

ParameterMean ± SD (Range)
Patients/eyes315/629
Gender (male/female)92/223
Age (y)36.3 ± 9.3 (21.5 to 60.1)
Spherical equivalent (diopters)−6.59 ± 2.34 (−17.13 to −2.25)
Axial length (mm)26.1 ± 1.2 (23.5 to 30.3)
Intraocular pressure (mm Hg)13.2 ± 2.4 (8 to 28)
Central corneal thickness (µm)551.8 ± 31.7 (444 to 663)
Average RNFL thickness (µm)90.4 ± 8.5 (64.0 to 119.0)

Factors Associated With Glaucoma (n = 23) Analyzed With Univariable and Multivariable Mixed-Effects Logistic Regression Analysis

ParameterNo Glaucoma, Mean ± SDGlaucoma, Mean ± SDUnivariable Analysis, Odds Ratio (95% CI)PMultivariable Analysis, Odds Ratio (95% CI)P
Patients/eyes299/60616/23
Age (years)36.3 ± 9.2 (21.5 to 60.1)37.4 ± 10.3 (22.4 to 54.4)1.02 (0.94 to 1.11).594
IOP (mm Hg)13.1 ± 2.2 (8 to 21)15.6 ± 4.8 (10 to 28)1.77 (1.15 to 2.71).0091.77 (1.14 to 2.75).011
CCT (µm)552.4 ± 31.4 (444 to 663)536.1 ± 37.3 (444 to 599)0.97 (0.94 to 1.01).126
Spherical equivalent (D)−6.54 ± 2.33 (−17.13 to −2.25)−7.88 ± 2.25 (−12.0 to −4.50)0.69 (0.49 to 0.95).0240.63 (0.39 to 1.03).064
Axial length (mm)26.1 ± 1.1 (23.5 to 30.3)27.0 ± 1.2 (25.1 to 28.9)3.03 (0.84 to 10.85).089

Factors Associated With Perimetric Glaucoma (n = 10) Analyzed With Univariable Mixed-Effects Logistic Regression Analysis

ParameterWithout Perimetric Glaucoma, Mean ± SDWith Perimetric Glaucoma, Mean ± SDOdds Ratio (95% CI)P
Patients/eyes308/6197/10
Age (years)36.2 ± 9.2 (21.5 to 60.1)41.5 ± 9.0 (33.5 to 54.4)1.10 (0.93 to 1.31).254
IOP (mm Hg)13.1 ± 2.3 (8 to 21)17.3 ± 6.2 (10 to 28)2.10 (1.20 to 3.67).010
CCT (µm)552.1 ± 31.5 (444 to 663)531.4 ± 39.8 (444 to 578)0.96 (0.90 to 1.02).163
Spherical equivalent (D)−6.58 ± 2.34 (−17.13 to −2.25)−7.50 ± 2.45 (−12 to −4.5)0.71 (0.37 to 1.35).294
Axial length (mm)26.1 ± 1.2 (23.5 to 30.3)26.4 ± 0.80 (25.1 to 27.7)1.71 (0.52 to 5.64).379
Authors

From the Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, People's Republic of China.

Dr. Leung is a speaker for and has received material support in the form of equipment and a research grant from Carl Zeiss Meditec. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (CKSL); data collection (SB, VJ); analysis and interpretation of data (SB, CKSL); writing the manuscript (SB, CKSL); critical revision of the manuscript (VJ, CKSL); statistical expertise (SB, CKSL); administrative, technical, or material support (VJ, CKSL); supervision (CKSL)

Correspondence: Christopher K. S. Leung, MD, Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, People's Republic of China. E-mail: tlims00@hotmail.com

Received: August 31, 2015
Accepted: January 27, 2016

10.3928/1081597X-20160229-01

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