Sung, Mi-Sun MD; Kang, Byung-Wan MD; Kim, Hwang-Gyun MD; Heo, Hwan MD; Park, Sang-Woo MD, PhD
Glaucoma is a chronic progressive disease with retinal nerve fiber layer (RNFL) thinning and optic nerve head cupping, which leads to retinal ganglion cell (RGC) death.1,2 Traditionally, glaucoma is diagnosed based on the characteristic optic nerve cupping with corresponding visual field (VF) deficits. The evaluation of RNFL thickness is as important as VF testing for the evaluation of patients with glaucoma. It has been reported that RNFL loss of about 40% to 60% can occur before an appearance of VF defect.3 Hence, detecting the RNFL loss is valuable for the diagnosis and management of glaucoma.4–6
A variety of techniques have been developed to quantitatively measure the RNFL thickness to examine the structural damage caused by glaucoma. Of the techniques developed, optical coherence tomography (OCT) determines the thickness of RNFL, using the coherence interferometry, calculating the echo time delay of backscattered light reflected off each retinal layer.7 Recently developed spectral domain-optical coherence tomography (SD-OCT) provides higher resolution and faster scan speed, compared with time domain-optical coherence tomography (TD-OCT). The 3-dimensional (3D) OCT-2000 (Topcon Inc., Tokyo, Japan), which has recently been developed and marketed, is one of the SD-OCT. The 3D OCT provides a high-resolution objective and quantitative assessment of the macular ganglion cell complex (mGCC), which is the sum of the thicknesses of the retinal nerve fiber, ganglion cell, and inner plexiform layers; the 3 innermost retinal layers damaged preferentially by glaucoma. Since Zeimer et al8 reported a reduced macular thickness in glaucoma, many researchers have shown the repeatability and diagnostic power of the measurement of mGCC or macular thickness, comparable with the measurement of peripapillary retinal nerve fiber layer (pRNFL) thickness, for the diagnosis of glaucoma in patients with different stage of glaucoma, high myopia, and large optic disc.9–16
The purpose of this study is to analyze the repeatability and diagnostic power for the measurement of mGCC and pRNFL thickness, using SD-OCT, and to establish a highly repeatable and more diagnostic parameter in advanced glaucoma.
PATIENTS AND METHODS
Forty eyes of 40 glaucoma patients and 20 eyes of 20 nonglaucoma patients enrolled in the Glaucoma Clinic at Chonnam National University Hospital, from June 2011 to December 2011, were included in this cross-sectional study. Informed consent was obtained from all subjects, in accordance with the Declaration of Helsinki, and the protocol was approved by the institutional review board of Chonnam National University Hospital. The subjects underwent a full ophthalmic examination, including best-corrected visual acuity, manifest refraction, intraocular pressure measurement with Goldmann applanation tonometry, slit-lamp examination, anterior chamber angle examination by gonioscopy, optic nerve head and RNFL examination with a disc photography and red-free RNFL photography, Swedish interactive thresholding algorithm 30-2 perimetry with a Humphrey Field Analyzer (Carl Zeiss Meditec Inc., Dublin, CA), and OCT scanning using the 3D OCT-2000 (Topcon Inc.).
We grouped the eyes into 2 groups: the normal group and glaucoma group. To be included in the study, participants had to have a best-corrected visual acuity of ≥20/32 and spherical refraction within ±6.0 D. The inclusion criteria for eyes in the normal group were healthy subjects with no history of glaucoma, no intraocular surgery, intraocular pressure ≤21 mm Hg, nonglaucomatous optic nerve heads, and normal VF test. Patients with glaucoma were included if they had glaucomatous optic nerve damage with accompanying VF defects. Glaucomatous optic nerve damage was defined as the vertical cup-to-disc ratio of ≥0.7, or >0.2 asymmetry between the vertical cup-to-disc ratio of both eyes, or the presence of focal neural rim notching, or generalized loss of the neural rim. Eyes were excluded if they had history of other eye diseases, like cataract or media opacity, or neurological disease, leading to VF abnormality. If both eyes of the same patient were found to be eligible, 1 eye was randomly selected. A reliable Humphrey VF test was defined as having <20% fixation loss and <33% false-positive and false-negative errors. We defined patients with MD<−10 dB as patients with advanced glaucoma. The included subjects were further divided into 2 subgroups, based on MD: between −20 and −10 dB, and <−20 dB.
After pupil dilation, all measurements were performed by 1 examiner, experienced with taking OCT images, using the 3D OCT. Four scans per eye on 4 days within 1 month were taken to calculate the repeatability of the measurement.
Recent SD-OCT have improved the resolution, scan speed, image segmentation, and reproducibility of the measurement, compared with TD-OCT.17–21 The 3D OCT-2000, a newly developed SD-OCT, has an axial resolution of 5 µm and acquires high-resolution images with 27,000 A-scans per second, resulting in less error caused by eye movement of the patient. Unlike TD-OCT, 3D OCT can provide the mGCC thickness, which is sum of the thickness of the RNFL, ganglion cell layer, and inner plexiform layers inside of a 6×6 mm2 centered on the fovea, and can compare the measured values with an integrated normative database.
The pRNFL were scanned with a 3.4-mm-diameter circle and analyzed with the RNFL scanning protocol. The RNFL thickness analysis program calculates the total and quadrant RNFL thickness. The mGCC were scanned with 1024 A-scans 6 mm in length, which covered an area of 6×6 mm centered on the fovea with 512 A-scans×128 B-scans. The mean mGCC thickness of total, superior, and inferior hemispheres was calculated. Only scans with image quality factor ≥70 and without eye movements or blinking artifacts were used for analysis.
SPSS version 12.0 (SPSS, Chicago, IL) and MedCalc version 9.6 (MedCalc, Mariakerke, Belgium) were used for statistical analyses. One-way analysis of variance or χ2 test was used to compare the different parameters among the normal eyes and different glaucoma subgroups. The repeatability was evaluated by calculating the coefficient of variation (CVw; 100XSw/overall mean) and intraclass correlation coefficient (ICC). ICC is an index of reliability that is used to evaluate the extent to which a specific object can produce similar values when performed at a different time, location, or by a different person assuming the data variables are alike. It is between 0 and 1; the closer the value is to 1, the higher the correlation of units, as well as better the repeatability. Bland and Altman22 reported that the ICC of 0.80 to 1.0 is very reliable, and 0.60 to 0.79 is somewhat reliable. The ability of pRNFL and mGCC thickness to diagnose glaucoma was assessed by calculating the area under the receiver operating characteristic curve (AROC), compared with the 20 normal controls. The Delong test was used to determine the statistical difference between the AROCs.23 Sensitivity and specificity of the best parameters of mGCC and pRNFL were calculated based on an internal normative database at the 5% and 1% level. The values of P≤5% were described as significant.
Forty eyes from 40 glaucoma patients and 20 eyes from 20 nonglaucoma subjects were examined clinically and by a 3D OCT. There was no dropout, and all recruited subjects were included in the final analysis. Eyes with glaucoma were placed into the 2 subgroups (n=20 in each), based on the MD obtained from Humphrey Field Analyzer. As expected, the MD and pattern SD were significantly different between the normal controls and eyes with glaucoma. Differences in pRNFL and mGCC parameters between the normal controls and eyes with glaucoma were statistically significant. Demographic and ocular characteristics of the normal controls and glaucoma subjects are shown in Table 1.
The CVw values and ICC with 95% confidence interval of pRNFL and mGCC measurements, obtained using 3D OCT, are summarized in Table 2. In the normal controls, the repeatability was excellent in most parameters for pRNFL and mGCC thickness. The values of CVw ranged between 1.31% and 2.26% in pRNFL parameters, and 1.41% and 1.50% in mGCC parameters. The values of ICC ranged between 0.924 and 0.985 in RNFL parameters, 0.909 and 0.926 in mGCC parameters. In the group with MD between −20 and −10 dB, the most repeatable pRNFL parameter was total pRNFL thickness with CVw value and ICC value of 2.55% and 0.963%, respectively. For the mGCC parameters, CVw values ranged between 1.30% and 1.72%, and ICC values ranged between 0.959 and 0.968. In the group with MD<−20 dB, the CVw values and ICC values of pRNFL parameters were similar to the group with MD between −20 and −10 dB: ranging between 2.12% and 5.02% and between 0.889 and 0.959, respectively. However, the CVw values and ICC values of mGCC parameters were lower than the group with MD between −20 and −10 dB: ranging between 6.06% and 7.48% and between 0.732 and 0.797, respectively (Table 2).
We compared the AROC of pRNFL parameters with those of the mGCC parameters. The best parameters for discriminating the normal eyes from eyes with glaucoma was superior pRNFL thickness, followed by total mGCC and total pRNFL thickness in the group with between −20 and −10 dB (0.957, 0.916, and 0.911, respectively) (Table 3 and Fig. 1). Using the method of Delong et al23 to compare AROC resulted in P=0.97, 0.24, and 0.57 for pairwise comparisons of superior pRNFL/total mGCC, superior pRNFL/total RNFL, and total mGCC/total RNFL, respectively. In the group with MD <−20 dB, the AROC of total RNFL, total mGCC, and inferior pRNFL were larger than those of the other parameters (0.983, 0.943, and 0.939, respectively) (Table 3 and Fig. 2). Using the method of Delong and colleagues resulted in P=0.28, 0.23, and 0.96 for pairwise comparisons of total RNFL/total mGCC, total RNFL/inferior RNFL, and total mGCC/inferior RNFL, respectively. Table 4 presents the results, regarding the sensitivity and specificity of the best variables in each group, on the basis of internal normative database. All parameters, except total mGCC in the group with MD, between −20 and −10 dB, showed high sensitivities and specificities.
Identifying and consecutive monitoring of the structural and functional damage before the development of irreversible vision loss is the critical part of the management of glaucoma. Especially, the detection of progression in patients with advanced glaucoma is critically important to preserve the remaining visual function. Once progression is confirmed, immediate change of the treatment regimen will be necessary.
VF examination has been an important tool to detect the disease progression. However, in a VF test for patients with advanced glaucoma, more patients have deep scotomas, as the interpretation of VF is more difficult, due to a higher variability of threshold value and failure to reflect the true nature of the disease, such as frequent false-negative error, or miscalculated pattern SD, and collapse of pattern deviation probability plot, especially, in patients with MD<−20 dB.24 Although clinically meaningful testing may be performed in these patients with modifications, such as larger test stimuli, or testing with 10-2 algorithms, etc., fluctuation of VF outcomes in some patients on performing VF examination makes it difficult to detect the disease progression. Therefore, for the evaluation of patients with glaucoma consecutively, the measurement of structural change can be a complementary tool to a VF test in patients with advanced glaucoma.
Papillomacular bundle is relatively resistant to glaucomatous change, and is preserved until the disease reaches advanced stages. Because glaucoma is an irreversible degenerative disease, characterized by the loss of RGCs and axons,10,25 the detection of the change to macular structure, especially mGCC, which is composed of these structures, may be useful in advanced glaucoma. In the current study, mGCC measurements, using 3D OCT, were used to directly examine the central portion of the posterior retina.
In the clinical research, repeatability, representing the degree of an error, is the distribution level of the measured data of a certain object acquired from the same inspector, using the same device several times. Without high repeatability, comparison of previous and present examinations will be meaningless. Therefore, to be used as an objective clinical data to monitor the progression of glaucoma, the repeatability of the measurement of mGCC thickness must be determined.
In the current study for the normal controls, the values of CV and ICC ranged from 1.31% (inferior pRNFL) to 2.26% (total pRNFL), and from 0.985 (inferior pRNFL) to 0.924 (total pRNFL), respectively, in the pRNFL parameters, and from 1.41% (superior mGCC) to 1.50% (inferior mGCC), and from 0.926 (total mGCC) to 0.909 (superior mGCC), respectively, in the mGCC parameters. These results suggest that the reproducibility of the pRNFL and mGCC thickness measurements with the 3D OCT are high and similar to those reported in other SD-OCT studies.26–28 In the group with MD between −20 and −10 dB, the ICC values of all mGCC parameters (0.968, 0.962, and 0.959 in superior, inferior, and total mGCC, respectively) were similar or higher than those of pRNFL parameters (0.963, 0.890, and 0.871 in total, superior, and inferior pRNFL, respectively). Therefore, the mGCC parameters showed a similar or even a slightly better repeatability in this group. However, in the group having MD <−20 dB, the ICC values of all mGCC parameters (0.797, 0.755, and 0.732 in inferior, total, and superior mGCC, respectively) were lower than those of pRNFL parameters (0.959, 0.935, and 0.889 in superior, total, and inferior pRNFL, respectively). From these results, our study demonstrates that the repeatability of the mGCC thickness measurement becomes lower as the severity of glaucoma increases in patients with advanced glaucoma.
To assess the diagnostic power of each parameter, AROC of all parameters were calculated. Of all the parameters in the group of MD between −20 and −10 dB, the AROC of the superior pRNFL (0.957) was the greatest, followed by the total mGCC (0.916) and total pRNFL (0.911), but not in a statistically significant manner. For the group of MD <−20 dB, the AROC of total pRNFL (0.983) was the greatest, followed by total mGCC (0.943) and inferior pRNFL (0.939), and again, there were no significant differences between the parameters.
Usually, diagnostic power is improved in increasing the disease severity for most diagnostic tests.28,29 We found this trend in both the pRNFL and mGCC parameters in patients with advanced glaucoma. However, for the mGCC parameters, the repeatability decreased with increase in the disease severity. We suggest that our finding may be related to 2 anatomic factors. Firstly, extensive structural damages, which are more susceptible to the “floor” effect, happen in patients with far-advanced glaucoma. Secondly, in contrast to the optic nerve head having axonal projections from 100% of the RGCs, only 50% of the RGCs are present in the macular area. Therefore, the measurements of the parameters related with mGCC thickness tend to have less signal-to-noise ratio than those related with pRNFL thickness in patients with far-advanced glaucoma, even with SD-OCT.
Our results differ in some respects from those of the previous observations. Other investigators reported that mGCC and pRNFL thickness had similar repeatability and diagnostic power, irrespective of the disease severity.30–34 The difference from our results may be explained by the differences in the severity of glaucoma in patients included in their studies. For example, the average MD of the VF in our far-advanced glaucoma patients was −26.63 dB, compared with −17.91 dB in severe glaucoma by Kim et al.34 Therefore, it is difficult to directly compare our results with the prior results.
Our study had some limitations. Sample size was small. However, the repeatability of the measurement of pRNFL and mGCC thickness was high in 20 normal controls. As we did not include patients with mild and moderate glaucoma in our study, we could not know the longitudinal trend of repeatability or diagnostic power of mGCC thickness parameters in increasing MD of the VF. Further studies will be needed to evaluate this relationship.
In conclusion, the measurement of pRNFL and mGCC thickness had similarly favorable diagnostic power in patients with advanced glaucoma. However, in the far-advanced group of MD<−20 dB, the repeatability of the mGCC parameters was not as good as that of the pRNFL parameters. With this consideration, we suggest that the evaluation of the mGCC parameters for monitoring of the disease progression in patients with advanced glaucoma is not recommended, because of low repeatability. Our results provide information for clinical validity of the measurement of mGCC in the management of glaucoma.
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