Glaucoma is an ocular disease which causes progressive optic nerve fiber damage and leads to visual field loss1; therefore, early glaucoma detection is very important for early treatment.1–4 It has been demonstrated that structural damage to the optic nerve head (ONH) and peripapillary retinal nerve fiber layer (RNFL) might occur before any detectable functional visual loss.2–4 Therefore, detection of ONH and RNFL damage is crucial for early diagnosis of glaucoma.5–7 With the advancement of optical imaging technology, Stratus OCT (Carl Zeiss Meditec, Inc., Dublin, CA) and GDx VCC are very popular instruments in helping clinicians to understand the RNFL thickness in glaucoma management. However, different instrument designs have many limitations as far as we know; and the diagnostic capability in different stage of glaucoma disease and in different ethnic group varies among the studies.8–11 Furthermore, there are still few studies which aim to compare the RNFL thickness differences between OCT and GDx in early glaucoma (EG), ocular hypertensive (OH) or glaucoma suspect (GS) eyes to date.12–15 Therefore, in the current study, we aimed to compare the relationship between Stratus OCT and GDx VCC in RNFL thickness measurement in EG, OH and GS eyes in a Chinese population. In addition, the diagnostic ability of the two instruments in each group will be evaluated.

One eye each of 170 subjects (50 eyes with EG, 32 eyes with OH and 38 eyes with GS and 50 healthy eyes) was included in the prospective cross-sectional study. All recruited cases were examined in the glaucoma service, department of Ophthalmology, China Medical University Hospital since January 2007 to December 2008. Informed consent was obtained from all participants, and the study was approved by the Institutional Review Board of the China Medical University Hospital. This research follows the tenets of the Declaration of Helsinki.

Subjects with a best-corrected visual acuity of less than 20/40, a spherical equivalent outside ±5.0 diopters, and a cylinder correction >3.0 diopters were excluded. All subjects underwent a complete ophthalmic examination, including slit lamp biomicroscopy, measurement of intraocular pressure, stereoscopic fundus examination and standard full threshold automated perimetry (30-2 mode, Humphrey Field Analyzer, Humphrey Instrument, Dublin, CA).

Inclusion criteria for healthy eye included no history of eye disease, no family history of glaucoma, intraocular pressure lower than 21mmHg when measured by Goldmann applanation tonometry, open angle on gonioscopy, and normal optic disk appearance based on clinical stereoscopic examination. A normal result on the glaucoma hemifield test and corrected pattern standard deviation (HFA, program 30-2) within normal limits were required. Subjects with healthy eyes were volunteers from the staff or their family members at the China Medical University Hospital.

Patients in the glaucoma group met the following criteria: at least one of their optic disks was glaucomatous in appearance with notching or thinning of the neuroretinal rim16; a corresponding nerve fiber bundle visual field defect, as described by Keltner et al.,17 with standard automated perimetry; gonioscopy revealed open angles. We defined EG eyes as having a mean deviation (MD) that was not as severe as −6dB on Humphrey Field Analyzer according to the criteria of Hodapp–Parrish–Anderson.18 GS eyes were defined as the presence of an abnormal disk consistent with glaucoma as determined by the same experienced glaucoma specialist along with a normal visual field. OH eyes were defined as subjects with intraocular pressure higher than 22mmHg, an open angle along with normal visual field.

Previous studies reported good diagnostic accuracy in glaucoma detection with different imaging machines.8,9,11,20–22 Some studies evaluated and compared the diagnostic power between Stratus OCT and GDx VCC in different stages of glaucoma.12–15 In one recent study by Zarei et al., they reported that many GDx VCC parameters were significantly correlated with those of the OCT in patients with juvenile glaucoma23 (r: 0.80 for average thickness; 0.86 for superior thickness; 0.75 for inferior thickness). In their study, the glaucomatous eyes had VF loss of −6.04±2.76dB (MD) and 5.59±3.17dB (PSD).23 In another study by Leung et al.,24 they reported that significant correlations were observed between Stratus OCT and GDx VCC RNFL measurements (average RNFL thickness, r: 0.852). In their glaucoma and glaucoma suspect group, the visual field severity is around −7.80±7.86dB. Compared to other studies, our result shows that GS group and OH group were not well correlated in RNFL thickness measured by the Stratus OCT and GDx VCC, and the diagnostic capability was quite poor in differentiating healthy from OH or GS eyes. Possible reasons might come from different scanning technique from the design of two machines themselves,25 different study population and with very mild visual field severity in our OH and GS groups.

The role of OCT and GDx in early stage glaucoma diagnosis has been well reported.1,5,13,26 Here we again report that the best parameter with largest AROC was nerve fiber indicator (AROC, 0.798) for GDx VCC and average thickness (AROC, 0.787) for OCT in differentiating healthy from EG group. Similar reports also agree that Stratus OCT and GDx VCC has moderate sensitivity with high specificity for the early glaucoma detection.18,19,26,27 However, the diagnostic capability of two techniques is quite poor in OH and GS group from our result (OH vs healthy, AROC, 0.510–0.645; GS vs healthy, AROC, 0.510–0.689). We think that the result also implies the truth that the role of the two imaging machines in OH or GS group at the current stage is still uncertain because the concept of structural damage might precede functional damage in glaucoma is still a controversy.28 But the RNFL thickness measurement by these two machines could be used as a tool for follow-up of OH or GS group in real practice is well accepted.29

There are still some limitations in this cross-sectional study. First, entry bias comes from the selection of study cohort. For example, one recent study reported that RNFL analysis by OCT is more suitable for the glaucoma assessment in the tilted disk compared with GDx-VCC.30 Furthermore, due to the strict criteria of imaging data, we might delete any case with poor imaging signal, which might influence the outcome. Second, the visual field criteria that we used in this study are the ones proposed by Hodapp–Parrish–Anderson18 and Caprioli et al.,19 To avoid the bias coming from visual field interpretation in very early stage of glaucoma, all the study participants were strictly evaluated by the same Dr. (Chen HY). Although we have tried to reduce the bias in disease grouping, this is an inevitable problem in this kind of imaging studies. At last, comparisons across studies are difficult, because of differences in population demographics, the definition and severity of glaucoma. However, the results can be used as the basis for further improving the diagnostic accuracy of glaucoma in the Chinese population in the near future.

In summary, the diagnostic ability of Stratus OCT and GDx VCC was moderate in early glaucoma detection; RNFL thickness measurement was highly correlated between the two machines in early glaucomatous eyes. The role of imaging machine in evaluating OH or GS eye needs to be addressed in the future. When managing the case with OH or GS eye at the current stage, we should be more cautious in interpreting the different imaging data from the two machines.

This research follows the tenets of the Declaration of Helsinki. Informed consent was obtained from all participants, and the study was approved by the Institutional Review Board of the CMUH.

The authors have no conflict of interests to declare.