It has been generally believed that the activity of retinal ganglion cells (RGCs) contributes little to shaping the corneal electroretinogram (ERG) elicited by ganzfeld stimuli (full-field ERG). However, a response has been newly identified to originate from RGCs that receive signals from cones [
The PhNR is strongly attenuated in primate’s eyes with experimentally induced glaucoma and also in eyes intravitreally injected with tetrodotoxin [
When the full-field PhNR amplitude was used as a diagnostic tool, the sensitivity and specificity to discriminate glaucomatous from normal eyes were 77% and 90%, respectively [
The focal ERG system originally developed by Miyake et al. [
From these results, it appeared that the focal PhNR is better than the full-field PhNR to discriminate glaucomatous from normal eyes. However, a direct comparative study comparing the diagnostic values of full-field and focal PhNRs obtained from the same eyes has not been reported although studies using different patient populations for the full-field and focal PhNRs have been done [
Thus, the purpose of this study was to compare the ability of the full-field and focal PhNRs to detect glaucomatous eyes at different stages. Importantly, the full-field and focal PhNRs were recorded from the same eyes.
One hundred and three eyes of 103 patients with OAG were studied. Their ages ranged from 37 to 83 years with a mean ± standard deviation of 68.2 ± 9.1 years. The diagnosis of OAG was based on the presence of a glaucomatous optic disc associated with visual field defects measured by SAP. The presence of glaucomatous optic disc was determined by the guideline of Japanese Society of Glaucoma developed in 2005 (
Forty-two eyes of 42 age-matched normal volunteers, ranging in age from 53 to 78 years with a mean of 67.6 ± 7.3 years, were studied. We selected normal eyes from patients with macular hole in the fellow eye which was treated by vitrectomy. They underwent comprehensive ophthalmological examinations including measuring visual acuity by a Snellen chart and observing the ocular fundus by an indirect ophthalmoscope as well as a biomicroscopic slit lamp. In addition, we performed optical coherence tomography and SAP to rule out macular and optic nerve diseases.
This research was conducted in accordance with the Institutional Guidelines of Iwate Medical University, and the procedures conformed to the tenets of the Declaration of Helsinki. An informed consent was obtained from all subjects after a full explanation of the nature of the experiments.
The pupils were maximally dilated to approximately 8 mm in diameter following topical application of a mixture of 0.5% tropicamide and 0.5% phenylephrine HCL. The recordings of the full-field and focal ERGs were made on the same eye on the same day. The stimulus conditions for the recordings of the full-field cone ERGs and focal ERGs have been reported in detail [
The full-field cone ERGs were elicited by red stimuli of 1 600 cd/m2 (
Focal ERGs were recorded from the macular area and from the supero-temporal and infero-temporal areas of the macula. Responses from these areas are designated as the center, superior/temporal, and inferior/temporal responses, respectively (Figure
Ocular fundus photograph showing retinal areas which were stimulated by focal spots with a diameter of 15 degrees.
The responses were digitally band-pass filtered from 0.5 to 1000 Hz for the full-field ERG and from 5 to 500 Hz for the focal ERG. It is often difficult to determine the negative trough of the PhNR especially in cases with reduced PhNR amplitudes. Therefore, we measured the PhNR amplitude at the fixed time points. We determined the time of the maximum amplitude of the PhNR in normal subjects according to the method of Rangaswamy et al. [
Representative full-field cone and focal electroretinograms recorded from a normal subject and a glaucoma patient with advanced visual field defects.
The Humphrey Visual Field Analyzer (Model 750, Humphrey Instruments, San Leandro, CA, USA) was used for SAP. The SITA Standard strategy was applied to program 24-2. From the mean deviation (MD) of the 24-2 program, we classified patients with glaucomatous visual fields into three groups: early (MD
When the fixation loss rate is higher than 20%, the field examination was determined to be unreliable and excluded from the analysis. In addition, when the false-positive or false-negative error rates exceeded 33%, the visual field was not used for the analysis. The interval between the visual field testing and ERG recording was less than 1 month.
We used receiver operating characteristic (ROC) curves to determine the optimal cut-off values that yielded the highest likelihood ratio. The area under the curve (AUC) was used to compare the ROC curves. The comparison between AUCs was made according to the method reported by DeLong et al. [
The full-field and focal ERGs recorded from a normal control and a patient that had advanced glaucoma with a mean deviation
We have plotted the PhNR amplitudes and PhNR/b-wave amplitude ratios against stages of glaucoma in Figures
The PhNR amplitudes of the full-field (a) and focal ERGs (b) center, (c) superior/temporal, and (d) inferior/temporal) are plotted for the normal controls (
The PhNR/b-wave amplitude ratios of the full-field (a) and focal ERGs (b) center, (c) superior/temporal, and (d) inferior/temporal) are plotted for the normal controls (
The PhNR amplitude and PhNR/b-wave amplitude ratio of the full-field ERGs gradually decreased as the stage of glaucoma advanced. On the other hand, the greatest loss of the PhNR amplitude and PhNR/b-wave amplitude ratio of the focal ERG was seen at the early stage of glaucoma. For example, the mean of the focal PhNR amplitude recorded from the center was reduced from 1.24
The full-field PhNR amplitude fell outside the normal range in 29, 48, and 56% of patients of the early, intermediate, and advanced groups. The focal PhNR amplitudes of the central retinal area fell outside the normal range in 62, 61, and 76% of patients of the early, intermediate and advanced groups. The corresponding percentages for the superior/temporal and inferior/temporal focal PhNR amplitudes were 49 and 46% for the early, 59 and 57% for the intermediate, and 85 and 79% for the advanced groups, respectively. Thus, the focal PhNR amplitude showed abnormal values in more patients at any stages than the full-field PhNR amplitude. Similar results were obtained for the PhNR/b-wave amplitude ratio.
The cut-off values were varied by 1.0
Area under the curve of the PhNR amplitude and PhNR/b-wave amplitude ratio.
PhNR amplitude | PhNR/b-wave amplitude ratio | |||
AUC | 95% CI | AUC | 95% CI | |
Full-field ERG | 0.748 | 0.638–0.839 | 0.666 | 0.551–0.768 |
Focal ERG | ||||
Center | 0.866 | 0.759–0.925 | 0.863 | 0.767–0.930 |
Sup/temp | 0.863 | 0.767–0.930 | 0.886 | 0.795–0.947 |
Inf/temp | 0.874 | 0.780–0.938 | 0.924 | 0.841–0.971 |
Full-field ERG | 0.865 | 0.758–0.937 | 0.789 | 0.670–0.880 |
Focal ERG | ||||
Center | 0.906 | 0.808–0.964 | 0.938 | 0.849–0.982 |
Sup/temp | 0.929 | 0.838–0.978 | 0.946 | 0.860–0.987 |
Inf/temp | 0.959 | 0.878–0.992 | 0.942 | 0.854–0.984 |
Full-field ERG | 0.954 | 0.875–0.989 | 0.910 | 0.817–0.965 |
Focal ERG | ||||
Center | 0.951 | 0.871–0.988 | 0.930 | 0.842–0.977 |
Sup/temp | 0.968 | 0.895–0.995 | 0.953 | 0.874–0.989 |
Inf/temp | 0.972 | 0.902–0.996 | 0.972 | 0.901–0.996 |
PhNR: photopic negative response; AUC: area under the curve; CI: confidence interval; sup/temp: superior/temporal; inf/temp: inferior/temporal.
Receiver operating characteristic (ROC) curves for the PhNR amplitude (a) and PhNR/b-wave amplitude ratio (b) of the full-field and focal electroretinograms. Patients with early glaucoma (
Receiver operating characteristic (ROC) curves for the PhNR amplitude (a) and PhNR/b-wave amplitude ratio (b) of the full-field and focal electroretinograms. Patients with intermediate glaucoma (
Receiver operating characteristic (ROC) curves for the PhNR amplitude (a) and PhNR/b-wave amplitude ratio (b) of the full-field and focal electroretinograms. Patients with
In early glaucoma, the focal PhNR amplitude curves were always superior to the full-field PhNR amplitude curves. As a result, the AUC of the focal PhNR amplitude of the inferior/temporal area was significantly larger than that of the full-field PhNR amplitude (Figure
For eyes with intermediate glaucoma, most parts of the ROC curves of the focal ERG amplitudes overlapped the curve of the PhNR amplitude of the full-field ERGs. Thus, there was no significant difference in the AUCs between the focal and full-field PhNR amplitudes (Figure
In eyes with advanced glaucoma, the ROC curves for the PhNR amplitude and PhNR/b-wave amplitude ratio of the focal and full-field ERGs were overlapped (Figure
The sensitivity and specificity were obtained with the optimal cut-off values for the PhNR amplitude (Table
Sensitivity and specificity of the PhNR amplitude to discriminate glaucomatous eyes.
Sensitivity (95%CI) | Specificity (95%CI) | Cut-off value ( | |
---|---|---|---|
Full-field ERG | 38.1 (23.6–54.4) | 92.3 (79.1–98.3) | 22 |
Focal ERG | |||
Center | 69.1 (52.9–82.4) | 95.2 (83.8–99.3) | 0.7 |
Sup/temp | 63.4 (46.9–77.9) | 97.6 (87.1–99.6) | 0.5 |
Inf/temp | 56.1 (46.9–77.9) | 95.2 (83.8–99.3) | 0.7 |
Combined | 88.1 (74.4–96.0) | 90.5 (87.7–99.6) | |
Full-field ERG | 59.3 (38.8–77.6) | 92.3 (79.1–98.3) | 22 |
Focal ERG | |||
Center | 64.3 (44.1–81.3) | 95.2 (83.8–99.3) | 0.7 |
Sup/temp | 75.0 (55.1–89.3) | 97.6 (87.1–99.6) | 0.5 |
Inf/temp | 67.9 (47.7–84.1) | 95.2 (83.8–99.3) | 0.7 |
Combined | 92.9 (87.7–99.6) | 90.5 (87.7–99.6) | |
Full-field ERG | 66.7 (48.2–82.0) | 92.3 (79.1–98.3) | 22 |
Focal ERG | |||
Center | 88.2 (72.5–96.6) | 95.2 (83.8–99.3) | 0.7 |
Sup/temp | 90.9 (75.6–98.0) | 97.6 (87.1–99.6) | 0.5 |
Inf/temp | 90.9 (75.6–98.0) | 95.2 (83.8–99.3) | 0.7 |
Combined | 97.1 (87.7–99.6) | 90.5 (87.7–99.6) |
PhNR: photopic negative response; CI: confidence interval; sup/temp: superior/temporal; inf/temp: inferior/temporal.
Sensitivity and specificity of the PhNR/b-wave amplitude ratio to discriminate glaucomatous eyes.
Sensitivity (95%CI) | Specificity (95%CI) | Cut-off value | |
---|---|---|---|
Full-field ERG | 23.8 (12.1–39.5) | 97.4 (86.5–99.6) | 0.19 |
Focal ERG | |||
Center | 61.9 (45.6–76.4) | 97.6 (87.4–99.6) | 0.22 |
Sup/temp | 75.6 (59.7–87.6) | 97.6 (87.1–99.6) | 0.23 |
Inf/temp | 73.1 (57.1–85.3) | 95.2 (83.8–99.3) | 0.29 |
Combined | 97.6 (87.7–99.6) | 92.9 (87.7–99.6) | |
Full-field ERG | 40.7 (22.4–61.2) | 97.4 (86.5–99.6) | 0.20 |
Focal ERG | |||
Center | 67.9 (47.7–84.1) | 97.6 (87.4–99.6) | 0.22 |
Sup/temp | 85.7 (67.3–95.9) | 97.6 (87.1–99.6) | 0.23 |
Inf/temp | 78.6 (59.0–91.7) | 95.2 (83.8–99.3) | 0.29 |
Combined | 96.4 (87.7–99.6) | 92.9 (87.7–99.6) | |
Full-field ERG | 69.7 (51.3–84.4) | 97.4 (86.5–99.6) | 0.20 |
Focal ERG | |||
Center | 70.6 (52.5–84.9) | 97.6 (87.4–99.6) | 0.22 |
Sup/temp | 90.9 (75.6–98.0) | 95.6 (87.1–99.6) | 0.23 |
Inf/temp | 90.9 (75.6–98.0) | 95.2 (83.8–99.3) | 0.29 |
Combined | 97.1 (87.7–99.6) | 92.9 (87.7–99.6) |
PhNR: photopic negative response; CI: confidence interval; sup/temp: superior/temporal; inf/temp: inferior/temporal.
In patients with mild defects of the visual field, the sensitivities of the focal PhNR amplitudes were significantly higher than those of the full-field PhNR amplitudes (
In intermediate and advanced glaucoma, the sensitivities of the focal PhNRs were generally higher than those of the full-field PhNRs. A significant difference was found between the focal and full-field PhNRs in the PhNR/b-wave amplitude ratio obtained from the superior/temporal and inferior/temporal areas in intermediate glaucoma (
In advanced glaucoma, there was no significant difference in the sensitivity between the full-field and focal PhNRs.
We compared diagnostic abilities between the full-field and focal PhNRs in detecting glaucomatous eyes. Our results demonstrated that the AUCs and sensitivities were higher for the focal PhNR than for the full-field PhNR at the early and intermediate stages of glaucoma. This suggests that the focal PhNR is a good indicator to detect the functional loss in early and intermediate glaucoma.
The AUCs of the focal PhNRs were better for identifying eyes with early and intermediate glaucoma than those of the full-field PhNRs. On the other hand, there was no significant difference in the AUCs between the focal and full-field PhNRs in advanced glaucoma. When the combined criterion for the focal PhNR was used, the sensitivity increased to 88.1% and 97.6% for the focal PhNR amplitude and PhNR/b-wave amplitude ratio, respectively, even in early glaucoma, while the sensitivities for the PhNR amplitude and amplitude ratio of the full-field ERG were 38.1% and 23.8%. These findings indicate that the focal PhNR is a better indicator than the full-field PhNR in detecting functional changes in early and intermediate glaucoma.
We selected the optimal cut-off value with the highest likelihood ratio which maximally reduces false positive cases. This then kept the specificity high for both PhNR parameters. The disadvantage of the combined criterion is that it lowers the specificity as reported although a high sensitivity was obtained [
We have reported that a curvilinear relationship existed between the retinal sensitivity (in decibels) measured by perimetry and the focal PhNR amplitude [
It is essential that the ocular fundus is visible to be able to record the focal PhNRs reliably because the stimulus areas stimulated must be monitored during the recordings using an infrared fundus camera. It is impossible to record the focal ERG in patients with dense opacities of the ocular media, such as cataracts and vitreous opacities. Furthermore, opacities of the ocular media can produce stray-light that makes the focal stimulus larger. Therefore, we have excluded patients with clinically significant cataracts that affected vision. On the other hand, the stray-light effect is negligible for the full-field ERGs. In cases with severe opacity of the ocular media, the full-field PhNRs would be more reliable than the focal PhNR.
Intersession variability is represented by the coefficients of variation (CV = standard deviation/mean
The recording and stimulus conditions of the focal ERG were different from those of full-field ERG, which may explain why the focal PhNR was better than the full-field PhNR in diagnosing early or intermediate glaucoma. First, we set the low cut filters at 0.5 Hz and 5 Hz for the full-field and focal ERGs, respectively. The higher cut-off frequency (5 Hz) used to record the focal PhNR was necessary to eliminate the drifts in the baseline. Thus, some of the low frequency components of the PhNR were reduced as shown in monkeys [
Second, the full-field ERGs were elicited by red stimuli on a blue background (R/B) while the focal ERGs were elicited by white stimuli on a white background (W/W). The R/B stimuli have been shown to be a very good combination to elicit large and reliable PhNRs [
Therefore, the differences in the recording and stimulus conditions do not seem to be able to explain the current results in which the focal PhNR was more sensitive than the full-field PhNR in diagnosing early and intermediate glaucoma.
The results of this study indicate that the PhNRs of the full-field and focal ERGs represent functional loss of RGCs in glaucoma at different stages of glaucoma. The focal ERG has the diagnostic ability with high sensitivity and specificity in detecting glaucomatous eyes at the early and intermediate stages, especially when the combined criterion is used. There was no difference in the diagnostic value between the full-field and focal PhNRs in advanced glaucoma. Thus, the focal PhNR can be a good functional parameter to detect early or intermediate glaucoma.
This paper was supported by a Grant-in-Aid for Scientific Research C from Ministry of Education, Science, and Culture in Japan no. 20592056, Grant from Keiryokai Research Foundation no. 102, grant from The Imai Memorial Fund for Research.