Methods

We retrospectively reviewed medical records in this cross-sectional study. Our study included 184 NTG eyes from 71 white patients who visited the Jules Stein Eye Institute, University of California, Los Angeles, and 113 patients who visited the Seoul National University Hospital, Seoul, Korea, for glaucoma and who did not have other ocular conditions aside from glaucoma. We also analyzed 30 normal control eyes from 30 patients at each institution who were diagnosed as healthy. We included only 1 eye of each patient in our analysis. If both eyes were eligible, we randomly selected 1 eye. For simplicity, we use the terminology Korean and Western in this study. Korean subjects were frequency matched for glaucoma severity with Western patients based on visual field mean deviation (MD). This study with retrospective review of patient data was approved by the Institutional Review Boards of the Seoul National University Hospital Clinical Research Institute and the University of California at Los Angeles, was performed in accordance with the tenets of the Declaration of Helsinki, and is in compliance with the Health Insurance Portability and Accountability rules and regulations governing such work.

Patient demographic characteristics, including gender, age, and medical histories, were collected. Each patient underwent a complete ophthalmic examination, including funduscopy. Gonioscopic angle evaluation was performed to exclude angle-closure patients. Inclusion criteria included patients with glaucoma with glaucomatous optic nerve head damage, retinal nerve fiber layer abnormalities that corresponded to the visual field defects, or both. Exclusion criteria included a history of intraocular disease, ocular injuries, intraocular or extraocular surgery, laser treatment, optic nerve disease other than glaucoma, or abnormal optic nerve shape (e.g., coloboma, optic nerve hypoplasia, morning glory disc anomaly, etc.). Eyes with IOPs of more than 22 mm Hg and best-corrected visual acuity of 20/40 or worse also were excluded.

We measured RNFL thickness with the same imaging devices at both sites, the Stratus optical coherence tomography (OCT) device (version 4.0; Carl Zeiss Meditec, Dublin, California, USA) and the Heidelberg Retina Tomography (HRT III) device (Heidelberg Engineering, Heidelberg, Germany). Fundus photographs of the retina and stereo disc photographs were obtained with a Zeiss fundus camera. Visual field tests were performed using the 24-2 program of the Humphrey visual field analyzer (Zeiss Inc, San Leandro, California, USA) with the standard Swedish interactive threshold algorithm. Central corneal thickness (CCT) was measured with an ultrasonic pachymeter (Cinescan; Quantel Medical, Bozeman, Montana, USA), and the average of 3 measurements was recorded. Measurements of axial length were performed with the IOLMaster (Carl Zeiss Meditec, Jena, Germany).

We analyzed the data with chi-square and independent t tests using a commercially available statistical software package (SPSS version 17.0; SPSS, Inc, Chicago, Illinois, USA). All P values were 2-sided and were considered statistically significant when less than .05.

Results

Demographic characteristics for the NTG patients are shown in Table 1 . There was a significant difference in age, with Western NTG patients being older ( P < .001). The gender, refractive error, baseline IOP, and MD value and pattern standard deviation value of the visual field did not show clinically significant difference. Of the baseline characteristics ( Table 2 ), Korean NTG patients have significantly less family history ( P < .001). There were no clinically significant differences in the prevalence of systemic hypertension, hypotension, diabetes, migraine, or cold hands. Several parameters differed significantly between groups. Of the ocular parameters, the cornea was thicker in Western NTG patients than in Korean NTG patients ( P < .001). Disc hemorrhage, peripapillary atrophy, and axial length did not show significant differences ( Table 3 ). The HRT parameters for NTG patients are shown in Table 4 . Cup-shape measure parameters showed that Korean NTG patients have significantly steeper cups ( P = .001), whereas the disc area and rim area were not different between the 2 groups. Of the OCT parameters, all tested OCT thickness parameters were significantly different ( Table 5 ; P < .05). In multivariate analysis after adjusting for age, sex, disc area, MD, pattern standard deviation, and spherical equivalent ( Table 6 ), cup-shape measure, average RNFL thickness assessed by OCT, and central corneal thickness showed significant differences between the groups ( P < .05). Figures 1 and 2 show the scatterplots of cup-shape measure obtained by HRT and the average RNFL thickness measured by OCT versus MD of the visual field, respectively. In comparison with the control groups, normal Korean subjects have significantly thicker RNFL than normal Western subjects ( Table 7 ; P < .05).

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