- Email: [email protected]
a Wave Ratio as a Function of Intensity in Central Retinal Vein Occlusion
Electroretinogram b-Wave Implicit Time and bja Wave Ratio as a Function of Intensity in Central Retinal Vein Occlusion MICHAEL E. BRETON, PhD, ALBERT W. SCHUELLER, BS, DAN P. MONTZKA, MD
Abstract: The ability of electroretinogram (ERG) b-wave implicit time and b/a wave ratio to predict iris neovascular response was analyzed as a function of stimulus intensity over a 3.6 log unit intensity range in 39 patients with central retinalvein occlusion (CRVO). Predictive power for CRVO patients was evaluated using ROC area at intensities of 1.23, 1.83,2.43, and 3.03 effective log quanta/ rod, where reliable data for both parameters were obtainable from most patients. The relative predictive power of b-wave implicit time and b/a wave ratio were shown to vary with stimulus intensity. The predictive power of b-wave implicit time, as measured by ROC area, declined to below significance at high intensity (above 1.83 log quanta/rod), while b/a wave ratio performed best at middle intensities (1.83 and 2.43 log quanta/rod) and not as well at high and low intensities. Further analysis of statistical behavior of both ERG parameters was obtained from the t statistic. Insight into the mechanism influencing predictive power of b-wave implicit time was derived from measurements on normal adults and CRVO patients with response data taken at high intensities. These results suggest that an optimal stimulus intensity range can be found for these ERG parameters in the evaluation of CRVO. Ophthalmology 1991; 98:1845-1853
Central retinal vein occlusion (CRVO) may lead to a dangerous neovascularization of the iris (NVI) in approximately 20% of patientsY The risk of this compli-
Originally received March 18. 1991. Revision accepted August 7, 1991. From the Scheie Eye Institute. Department of Ophthalmology, University of Pennsylvania, Philadelphia. Presented in part at the Noninvasive Assessment of the Visual System Topical Meeting, Santa Fe, February 1991. Supported in part by NIH grant EY05649 (Dr. Breton). Dr. Breton owns more than 1% of LKC Technologies Inc, the manufacturer of the ERG equipment used in this study. Reprint requests to Michael E. Breton, PhD, Scheie Eye Institute, 51 N 39th St, Philadelphia, PA 19104.
cation in patients who are ischemic can be greatly reduced with laser therapy." It is therefore clinically important to identify those patients at high risk so that they may be followed closely or treated before serious pathologic changes occur. Although fluorescein angiography is used routinely to estimate the degree of retinal ischemia present in CRVO and, therefore, the risk of NVI, it has been shown by some to have problems with reliability and repeatability." The electroretinogram (ERG), especially the b-wave feature, is an indicator of visual function dependent on the ischemic status of the inner retina. For this reason, the ERG has been studied as a possible prognostic indicator in CRVO. Although others have noted changes in the ERGs of vein occlusion patients.l" the work of Sabates et al,? which demonstrated a parametric relation between the b/a wave ratio and prognosis in patients with questionable perfusion status, led to renewed interest in 1845
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the ERG as an indicator of ischemic status and risk of NVI in CRVO. The effectiveness of the ERG as a predictor of the eventual development of NVI in CR VO has been supported by several recent studies that evaluated the predictive power of Naka-Rushton fit parameters (Rmax and Log K) as well as b-wave implicit times and b/a wave ratio. Breton et al 8 and Johnson et al? reported significant predictive power for the intensity-response curve fitting parameters Log K and Rmax, as well as for the b/a wave amplitude ratio and 30 Hz implicit time. Kaye and Harding'? reported that the b-wave implicit time parameter outperformed several other parameters as a predictor of NVI and that the separation between rubeotic and nonrubeotic patients did not change over a 1 log unit range of intensities. Johnson et al? also reported that bwave implicit time is an effective predictor ofNVI. These two reports differ from that of Breton et al. 8 They did not find b-wave implicit time (at their highest intensity) to have predictive power comparable to the other ERG parameters and deleted it from their analysis. This report examines the hypothesis that the change in dynamic behavior of b-wave implicit time as a function of intensity can affect its predictive power in CRVO and can account for the result obtained by Breton et al," showing no predictive power for b-wave implicit time at their highest intensity. Others have investigated the behavior of b-wave implicit time in normal subjects as a function of intensity and report that implicit time decreases steadily as stimulus intensity increases.!':'? Additionally, Breton and Montzka.P studying healthy humans, and Fox and Farber," studying rats, reported that the b-wave implicit time function levels out or saturates at a high intensity. Breton and Montzka'? reported that the intensity at which the implicit time function levels out is close to the intensity that is calculated to produce rod photocurrent amplitude saturation. The behavior of implicit time in healthy subjects and its relation to changes in both receptor and inner retinal function are important in explaining the change in predictive power ofb-wave implicit time as a function of stimulus intensity. This report also compares the performance of an amplitude-based parameter, the b/a ratio, to that of b-wave implicit time as a function of intensity. Our results demonstrate a difference in the behavior of the predictive powers of these two parameters as a function of stimulus intensity. Information gained from analysis of the predictive power of these two parameters also suggests that an optimal stimulus intensity range can be found for each parameter.
METHODS SUBJECfS
A total of 39 patients with CRVO were included in the analysis, 17 of which were retained from a previous study." All patients were recruited from the Retina Service of the Scheie Eye Institute and were diagnosed by a member of 1846
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the Retina Service as having a CRVO, based on a complete ocular examination. All patients, without signs of background diabetic retinopathy, diagnosed as having CRVO during the 31f2 year study period (n = 60) were referred for prospective study. Patients were excluded from the analysis because of: (1) incomplete follow-up (n = 13), (2) treatment with laser photocoagulation before developing rubeosis because they were judged to be clinically at high risk (n = 7), and (3) development of neovascular changes of the disc (n = 1). Systemic hypertensive status was not monitored. All patients were followed for at least 6 months. Neovascularization of the iris later developed in 12 of the 39 patients included in the study. The average age of the patient cohort was 67.3 ± 14 years. Four additional CR VO patients (not included in the 39 patients described above and hereafter referred to as the high-intensity CRVO patients) who were judged clinically to be at low to moderate risk of developing NVI also were selected for study. These 4 patients had higher intensity data that could be used to characterize the highintensity behavior ofb-wave implicit time in typical fellow and CRVO eyes. These patients were aged 55, 67, 72, and 83 years. Another set of comparison data was taken from 4 younger normal control subjects, ranging in age from 18 to 27 years, in smaller intensity steps and up to higher intensities. These data along with those of the older fellow eyes are used to show the dynamic behavior of b-wave implicit time as a function of intensity. These control subjects also had an ophthalmologic examination to rule out ocular disease and abnormal visual function. Informed consent was obtained from all subjects in accordance with the requirements of the Institutional Review Board. ERG EQUIPMENT
All CRVO ERG data were recorded using an LKC Systems UTAS E-lOOO instrument (Gaithersburg, MD), which consists of recording amplifiers, an interface for computer and stimulators, a Ganzfeld for full visual field stimulus presentation, and a xenon flash unit. Data from the 4 younger normal subjects and from the fellow eyes of the 4 high-intensity CRVO patients were taken on an EPIC-lOOO (LKC) system that is similar in design and capability to the UTAS E-lOOO. Both systems use expansion boards on the bus for analog-to-digital conversion of the responses and control of stimulators. The recording amplifiers are AC coupled and use electronic bandpass filters. Filter settings for the recordings were set at 0.3 Hz for the low frequency cutoff and 500 Hz for the high frequency cutoff. Stimulus attenuation was accomplished with Kodak Wratten neutral density filters. The stimulus intensity for the Grass PS-22 flash with no filters in place (0 decibel [dB] condition) was 0.5 log cd - sec/rrr' measured as the luminance of the reflective surface of the Ganzfeld. An additional Vivitar photoflash unit was used to present high-intensity stimuli up to 2.9 log cd - sec/ rrr'. The pulse width of the PS-22 is approximately 10 usee. The pulse width of the Vivitar flash is approximately 1 msec.
BRETON et al •
B·WAVE IMPLICIT TIME IN CRVO
canthus, with the ground electrode on the forehead. Intensity-response data were taken using neutral density filters from 36 dB attenuation (-0.57 log quanta absorbed per rod , - 3. 1 log cd - sec/m 2) to 0 dB (3.03 log quanta/ rod , 0.5 log cd - sec/rrr') with a white flash stimulus in steps of 6 dB with background off. Electroretinogram responses from the 4 control subjects and the 4 high-intensity CRVO patients were obtained using a similar protocol. Data for these subjects and patients were taken at more closely spaced intensity steps than for the cohort of 39 CRVO patients. Intensities greater than 3.03 log quanta/rod or 0.5 log cd - sec/m2 (the highest intensity used with the CRVO patients) were presented using the high-intensity Vivitar xenon flash unit at the end of each session. An interstimulus interval was incorporated between higher intensity flash stimuli to allow for complete recovery of the ERG waveform. This procedure allowed sampling ofb-wave implicit time up to intensities of 5.4 log quanta/rod or 2.9 log cd - sec/rrr'.
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Time (rns) Fig I. An intensity response waveform series for a typic al CRVO patient (age 67) with b-wave peaks mark ed for the fellow eye (filled circles) and for the CRVO eye (open circles) superim posed at each intensity. B-wave peaks are displa ced to longer time s for the CR VO eye compared with the fellow eye and move to shorter implicit times for both eyes as intensit y increases. There is a n abrupt decline in rate of peak movement at about 2 log quanta/rod.
ERG PROCEDURE
Electroretinograms were performed bilaterally on the cohort of 39 CRVO patients within 2 weeks oftheir initial visit (before any neovascular complications or treatment) and at irregular intervals thereafter, depending on clinical status. Single and repetitive flashed stimuli were presented in a highly reflective white Ganzfeld using an automated sequence (UTAS recordings) or predetermined manual sequence (EPIC recordings). Corneal electrodes were unipolar, disposable ERG-jet type. Testing was preceded by 30 minutes of dark adaptation. Pupils were fully dilated with 1% tropicamide and 10% Neo-Synephrine. Reference electrodes were placed near the corresponding temporal
The ERGs used in the anal ysis of predicti ve power were taken exclusi vely from the first examination after onset ofCRVO symptoms. The b-wave implicit time was measured as the time from stimulus onset to the peak of the b-wave feature (first major positive polarity response after the initial negative polarity a-wave feature). Figure 1 shows a full intensity series for both eyes of one of the highintensity CRVO patients. Waveforms for both eyes are superimposed based on the prestimulus baseline (note: onl y 7 of the 20 msec of prestimulus baseline recorded are shown in Fig 1). Stimulus intensity in log quanta/rod is marked on the ord inate for each waveform pair. Bwave peaks are marked with a filled circle for the fellow eye and an open circle for CRVO eye. In general , the bwave peaks for the CRVO eye are displaced to longer times and peak impli cit times for both eyes move to the left as intensity increases. This figure also shows the tendency ofb-wave implicit time to saturate as intensity increases abo ve about 2 log quanta/rod. The data shown in the figure extend to intensities much higher than those analyzed for the patient cohort. At low and middle intensities, where the patient cohort data are analyzed, cursor placement is unambiguous. However, at the highest intensities (4.23 log quanta/ rod and greater), oscillatory potentials introduce problems in terms of unambiguously locating the b-wave peak. It is possible that the true b-wave peak falls in between the oscillatory peaks. Because this problem with cursor placement does not occur until almost 2 log units above the intensity of implicit time saturation, it does not affect the statistics ofour anal yzed data and therefore does not affect our conclusions. We show the data at the highest intensities for this typical patient merely to be complete and to leave no doubt that implicit time does saturate for both affected and unaffected eyes. 1847
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Fig 2. B-wave implicit time declines with increasin g stimulus intensity for the three CRYO categories: CR YO eyes with neovascular response (n = 12), CRYO eyes with no neovascula r response (n = 27), and the unaffected fellow eyes for all subjects (n = 39) (error bars ± standard error ). At th e higher inten sities, where predictive power as measured by ROC area is lost, the error bars overlap for the NYI and non-NYI response groups.
Table 1. t Statistic Values with Levels of Significance* Log Quanta/Rod
1.23 b-Wave implicit time Significance b/a Wave ratio Significance
2.86
P < 0.01 1.16 NS
1.83
2.43
2.3 0.92 P < 0.025 NS 2.3 3.29 P < 0.025 P < 0.005
3.03 0.38 NS
2.60 P < 0.01
NS = not significant. * Degrees offreedom = 33 at 1.23 log quanta/rod ; otherwise, degrees of freedom = 37.
The b/a ratio was defined as the overall amplitude of b-wave divided by the distance from the baseline to the negative a-wave peak. Although CRVO patient data were recorded in 6-dB steps of attenuation from 36 dB to 0 dB for a maximum number of 7 data points in the intensity response, we restricted our anal ysis to the top 4 points (18 dB = 1.23 log quanta/rod = -1 .32 log cd - sec/nr'; 12 dB = 1.83 log quanta/rod = -0.72 log cd - sec/rrr' ; 6 dB = 2.43 log quanta/rod = -0.12 log cd - sec/rrr' ; and 0 dB = 3.03 log quanta/rod = 0.50 log cd - sec/rrr'), B-wave implicit time and b/a wave ratio measurements made in th is patient cohort below this level are unreliable . Higher intensity data were taken on the EPIC system and are used only in the analysis of b-wave implicit time. B-WAVE IMPLICIT TIME FOR CRVO PATIENTS
Figure 2 shows the b-wave implicit times for the cohort of39 CRVO patients anal yzed prospectively. The CRVO eyes that developed NVI are shown as squares (n = 12), 1848
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Fig 3. The b/a wave ratio as a function of inten sity for the three CR YO categories as in Figure 2 (error bars ± I stand ard error). As stimulus intensity increases, separation of group means declines. Inset, the enlarged view of the higher intensity data points shows that standard errors of the mean also dimin ish so that relative separation of NYI affected to no naffected CRYO eyes is main tain ed. However, the fellow eyes are indistingu ishable from the nonaffected CRYO eyes.
the CR VO eyes that did not develop NVI are shown as diamonds (n = 27), and fellow eyes are shown as triangles (n = 39). For all categories, implicit times decreased at a relatively high rate and then leveled off as a function of increasing stimulus intensity. The three eye conditions have the effect of separating the response functions vertically such that the more severely affected eyes are associated with longer implicit times. However, a most interesting result is the convergence of the implicit times for the two CRVO eye conditions as intensity increases. At the highest intensity, eyes that eventually developed NVI show implicit times that are indistinguishable from those that did not. This result is further supported by the t statistic values shown in Table 1. The two lowest intensities show a clear separation of these two CRVO categories on the basis of b-wave implicit time. The fellow eyes maintain clear separation from the CRVO eyes at all intensities and appear nearly parallel to the CRVO eyes without NVI. BI A
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Figure 3 shows b/a wave ratios for the same eye conditions as in Figure 2. As with b-wave implicit time , all three eye conditions show vertical separation at the lowest intensity, but tend to converge at higher intensities. However, the mean separation at the lowest intensity is not significant, while that at the intermediate and highest intensities is significant (Table 1). Th e inset shows an enlarged view of the higher intensity data. In this case, in contrast to the results for b-wave implicit time , the error bars for b/a ratio do not overlap for the two CR VO eye conditions (NVI versus non-NVI). Fellow eye values do not differ significantly from those of the CRVO eyes without NVI at the thre e higher intensities. Consistent with
BRETON et al •
B-WAVE IMPLICIT TIME IN CRVO
1.23 Log Quanta/Rod b-Wave Implicit Time 12
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Fig 4. Histogram comparison of b-wave implicit times for NVI affected and non-NY! affected subjects at the four intensities: A, 1.23 log quanta/ rod or -1.32 log cd - scc/rrr'; B, 1.83 log quanta/rod or -0.72 log cd-rsec/trr'; C, 2.43 log quanta/rod or -0. I 2 log cd-rsec/rrr': D, 3.03 log quanta/ rod or 0.50 log cd-sec/m" Histogram bars are centered to the left and right of the upper limits of their respective bins (bin width = 5 msecs) (i.e., bars at 80 msec include those patients who had b-wave implicit times in the 76 to 80 msec range). Overlap of the distributions increases as a function of increasing intensity.
the presumed site of inner retinal compromise, which would tend to reduce b-wave amplitude relative to a-wave amplitude, the eyes with NVI fall below the other two categories at all intensities. RECEIVER OPERATING CHARACTERISTIC AREA
The receiver operating characteristic (ROC) is a plot of true-positive versus false-positive results in discriminating NVI affected from non-NVI affected patients. The area under the ROC curve correlates with the ability of the
parameter plotted to discriminate between 2 categories based on that parameter, with an area of 0.5 equal to chance discrimination. 15- 1? This statistical technique quantifies the degree of separation between two distributions and presents a formal method for determining significance of a discrimination level from chance or in comparison to other conditions. It is especially useful when substantial overlap is present and when examination of histograms may not be informative, as is true for many of the experimental conditions represented in this study (Fig 4). The histograms presented in Figure 4 show the 1849
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chance (P < 0.02). Receiver operating characteristic values at the highest and lowest intensities were not significantly above chance discrimination.
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Table 2. ROC Areas with Levels of Significance above Chance Log Quanta/Rod
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0.76
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number ofCRVO eyesin the NVI and non-NVI categories falling at different b-wave implicit time values and as a function of intensity. Figures 4A through 4D show data for intensities of 1.23 up to 3.03 log quanta/rod. Figure 5 shows how ROC area changes as a function of stimulus intensity for the two parameters, b-wave implicit time and b/a wave ratio, while Table 2 shows the ROC area values and statistical significance compared with chance. For b-wave implicit time, ROC area increases as stimulus intensity decreases. For the highest intensity, the ROC area, and therefore the ability ofb-wave implicit time to discriminate NVI affected from non-NVI affected patients, is 0.56 and is not significantlygreater than chance alone. At 0.6 log units below the highest intensity, the ROC area increases to 0.64, but is still below significance. At the two lower intensities, ROC area increases to 0.72 and 0.76, both of which are significantly above chance (P < 0.01), and appears to level off. Receiver operating characteristic area for the b/a wave ratio shows a pattern different from that ofb-wave implicit time. Peak values of 0.72 and 0.73 occur at the middle intensities of 2.43 log quanta/rod and 1.83 log quanta/ rod, respectively. These values are significantly above 1850
The t statistic values for b-wave implicit time and b/a ratio, shown in Table 1, are a measure of the separation of mean values for NVI affected compared with non-NVI affected eyes as a function of stimulus intensity. For bwave implicit time, the t statistic values, like the ROC areas, increase as intensity decreases. The t values for intensities of 3.03 and 2.43 log quanta/rod are not significant, whereas the values for intensities of 1.83 and 1.23 log quanta/rod are significant (P < 0.025). For the b/a ratio, the t values also tend to correlate with the ROC areas in that the lowest t value corresponds to the lowest stimulus intensity, and the greatest t value occurs at an intermediate intensity. B-WAVE IMPLICIT TIME AT HIGHER INTENSITIES
The data for the 4 young control subjects and 4 highintensit y CRVO patients (both CRVO and fellow eye) are plotted individually in Figures 6A and 68. The intensity range for these data is much greater (6 log units) than that shown for CRVO patients in Figure 2 (1.8 log units). The normal data (Fig 6A) indicate two regions with nearly constant rates of decline in b-wave implicit time with intensity. Below an intensity of ahout 2.0 log quanta/rod, the rate of decline is approximately - 25 msec/log quanta/ rod. At intensities higher than 2.0 log quanta/rod, the rate of decline in b-wave implicit time undergoes an abrupt change to a nearly constant rate of - 2.0 msec/log quanta/ rod. The reduction in the rate of change ofb-wave implicit time beyond 2 log quanta/rod is about a factor of 10, based on independent linear regressions in each region. The data from the 4 high-intensity CRVO patients (Fig 6B) show an overall pattern similar to the control subjects and show the probable high-intensity behavior of the CRVO patient cohort used in the analysis for whom ERG data above 3.03 log quanta/rod were not available. This inference is supported by the trend of the averaged data for CRVO NVI affected and non-NVI affected eyes to level out at 2.43 and 3.03 log quanta/rod in Figure 2. The fellow eyes in Figure 6B (filled circles) fall below the CRVO eyes (open circles) at all intensities, however both eyes exhibit a sharp reduction in their rates of decline with intensity at about 2.0 log quanta/rod. We note that both the normal subjects and the high-intensity CR va patients show this sharp reduction at nearly the same intensity despite the age difference.
DISCUSSION We have used two statistical methods to evaluate the prognostic ability of b-wave implicit time and b/a wave ratio to discriminate NVI affected from non-NVI affected CRVO patients. These parameters showed predictive
BRETON et al
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a-WAVE IMPUCIT TIME IN CRVO
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power that varied significantly with intensity. More specifically, b-wave implicit time loses its ability to predict neovascular outcome for our patients and recording conditions above a stimulus intensity corresponding to absorption of approximately 2 log quanta/rod. In addition, the predictive power ofb-wave implicit time and b/a wave ratio behave differently as a function of intensity. The best predictive performance for b/a wave ratio occurred
at middle intensities and declined at both high and low intensities. B-WAVE IMPLICIT TIME
In an analysis of the b-wave implicit time function in younger normal subjects (Fig 6), Breton and Montzka' r conclude that decline in implicit time with increasing in1851
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tensity is driven by increasing amplitude of the rod signal to the inner retina. This analysis is consistent with a serial two stage retinal response model, such as that proposed by Johnson and Hood,21 with the first stage corresponding to the photoreceptor response and the second stage to inner retinal response. The b-wave implicit time function levels out at approximately 2 log quanta absorbed per rod, a value very close to the quantal absorption calculated to produce photocurrent amplitude saturation in mammalian rods by Penn and Hagins'? and by Baylor et al. 20 Thus, the leveling out of the b-wave implicit time function may be an indicator of the intensity at which rod photocurrent amplitude saturation occurs.':' Our CRVO patient data show different patterns of increased b-wave implicit time (Fig 2), for the CRVO eyes that developed NVI and for those that did not. The nonNVI affected eyes show a large increase in b-wave implicit time at each intensity compared with the nonaffected fellow eyes, producing a nearly parallel vertical shift of the implicit time intensity response function. This is consistent with decreased sensitivity of the inner retina, corresponding to a loss at the second stage, with little or no change in photoreceptor sensitivity. In comparison, the implicit times for the NVI eyes are longer at lower intensities, but converge with the values for the non-NVI affected eyes at the higher intensities. This could be consistent with either a nonparallel vertical shift or a horizontal shift to the right relative to the nonNVI affected and fellow eyes. Since the implicit time function of the NVI eyes has not clearly leveled off at the highest intensities used in this study, it is not certain from the data which explanation best applies. A horizontal shift to the right relative to the non-NVI affected eyes would imply some loss of sensitivity at the photoreceptor stage (without a change in maximum response amplitude) in addition to inner retinal loss. A photoreceptor shift to the right, when combined with a vertical shift resulting from loss of inner retinal sensitivity, could account for the relative positioning of the b-wave implicit time function of the NVI affected compared with non-NVI affected eyes for our data. However, if the nonparallel vertical shift is occurring, then a nonlinear gain change at the inner retinal stage would be required to explain the results. The analysis of CRVO patients reported by Breton et al" that found no predictive power for b-wave implicit time was based on data taken at a stimulus intensity beyond the point of b-wave implicit time convergence for their patient cohort. The difference between Johnson et al? and Breton et al may be partly explained by the use of higher intensities in the study by Breton et al. The fact that the study by Johnson et al reports analyzing b-wave implicit time data at a stimulus intensity approximately 0.2 log units below Breton et al raises the possibility that Johnson et al used stimulus intensities that were effectively lower than the intensity level ofb-wave implicit time saturation. Since the point of convergence in the non-NVI and NVI implicit time functions is abrupt, small deviations in intensity above and below the point of convergence could have significant effects on the predictive power of b-wave implicit time. However, other factors such as patient selection procedures also must be considered. 1852
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Kaye and Harding'? appear to have used stimulus intensities that were at or above levels which cause the bwave implicit time functions for NVI affected and nonNYI affected eyes to converge for our patient cohort. Thus, an explanation based on intensity would not help in explaining the difference between their study and Breton et al." Kaye and Harding's'? implicit time results are different from Breton et al in that a large separation is present between the NVI affected and non-NYI affected eyes even at higher intensities. This pattern of response is similar to what is seen in the present study for non-NVI affected compared with fellow eyes. Also different from Breton et al, the non-NVI affected CRVO eyes have an implicit time function almost indistinguishable at high intensity from that of the fellow eyes (Fig 2). These results are consistent with very limited retinal damage being present in the majority of the Kaye and Harding CRVO eyes, the exception being those eyes that developed NVI complications. This result may in part be due to the Kaye and Harding patient selection criteria. These criteria appear to be heavily weighted toward including clinically nonischemic eyes. Because rubeosis iridis developed in seven of these eyes, it is apparent that the clinical selection method was less than perfect in selecting patients at risk for NVI, an important fact motivating the study. The patient selection criteria could have contributed to altering the statistical distribution of implicit times in the direction observed within the CRVO group because: (1) the majority ofpatients were nonischemic and showed implicit times in the CRVO eye close to values in the fellow eye, (2) a minority of patients were effectively ischemic (although clinically they appeared not to be), showed longer implicit times and developed NVI. The study by Breton et al" excluded no patients because they were judged to be clinically at high risk for NYI. In the expanded data set for the current study, seven patients of a consecutive series of 60 referrals were excluded because they were judged clinically to be ischemic and at high risk. However, others, who might also have been considered ischemic on clinical grounds, were not excluded because an important goal of the study was to follow as many patients as possible without treatment. This raises the possibility that the original Breton et al patients as well as the present expanded cohort are more heterogeneous with respect to ischemic status than the Kaye and Harding cohort and that this factor contributes to the differences in the underlying implicit time distributions and to the differences noted at higher intensities. BfA
WAVE RATIO
The behavior of the b/a wave ratio must appeal to a different type of explanation to account for loss of predictive power at lower intensities. At the lowest intensity, the b/a wave ratio did not show significant ability to discriminate NVI affected from non-NVI affected patients as measured by t statistic or ROC area (Fig 3). This decline in predictive power may reflect greater reduction in amplitude and increase in variability of the a-wave feature relative to the b-wave feature. However, at higher inten-
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s-WAVE IMPLICITTIME IN CRVO
sities, amplitude saturation of the photoreceptors and the changes this event brings about in the dynamic interaction of the underlying a-wave and b-wave components, may play a role in reducing the predictive value of the b/a wave ratio. SUMMARY
These results suggest that optimum levels of stimulus intensity may be defined for ERG parameters by choosing intensities that maximize ROC area and the statistical separation of the disease state from normal as measured by the t statistic. Optimal stimulus values may be different for the various ERG parameters. The finding that the predictive value of the b/a wave ratio declines at lower intensities and that the predictive value of b-wave implicit time declines at higher intensities may also provide a rationale for selecting an optimal intensity for evaluating both parameters. The optimal range for exam intensities, as indicated by the intensities used in this study, appears to be near 12 dB of attenuation for b/a wave ratio and 18 dB of attenuation for b-wave latency, on our scale. This corresponds to illuminance values of -0.7 log cd - sec/ m 2 (1.83 log quanta/rod) and -1.3 log cd - sec/m? (1.23 log quanta/rod), respectively. Our results suggest a range of best intensity for both bwave implicit time and b/a wave ratio that falls below the point at which b-wave implicit time levels off for control subjects and somewhat above the intensity at which the a-wave feature first appears (to allow reliable measurement of the b/a wave ratio). This range of intensity yields high predictive performance for both b-wave implicit time and b/a wave ratio for our patient cohort.
ACKNOWLEDGMENTS The authors thank Charles E. Metz, Pu-Lan Wang, long-Her Shen, and Helen B. Kronman for their ROC software "Labroc l ," which made calculation of ROC areas and statistical significances possible and easy.
REFERENCES 1. Hayreh SS, Rojas R, Podhajsky P, et al. Ocular neovascularization with retinal vascular occlusion-III. Incidence of ocular neovascularization with retinal vein obstruction. Ophthalmology 1983; 90:488506. 2. Magargal LE, Donoso LA, Sanborn GE. Retinal ischemia and risk of neovascularization following central retinal vein obstruction. Ophthalmology 1982; 89:1241-5.
3. Magargal LE, Brown GC, Augsburger JJ, Donoso LA. Efficacy of panretinal photocoagulation in preventing neovascular glaucoma following ischemic central retinal vein obstruction. Ophthalmology 1982; 89: 780-4. 4. Welch JC, Augsburger JJ. Assessment of angiographic retinal capillary nonperfusion in central retinal vein occlusion. Am J Ophthalmol1987; 103:761-6. 5. Henkes HE. Electroretinography in circulatory disturbances of the retina. I. Electroretinogram in cases of occlusion of central retinal vein or of one of its branches. Arch Ophthalmol1953; 49:190-201. 6. Sakaue H, Katsumi 0, Hirose T. Electroretinographic findings in fellow eyes of patients with central retinal vein occlusion. Arch Ophthalmol 1989; 107:1459-62. 7. Sabates R, Hirose T, McMeel JW. Electroretinography in the prognosis and classification of central retinal vein occlusion. Arch Ophthalmol 1983; 101:232-5. 8. Breton ME, Quinn GE, Keene SS, et al. Electroretinogram parameters at presentation as predictors of rubeosis in central retinalvein occlusion patients. Ophthalmology 1989; 96: 1343-52. 9. Johnson MA, Marcus S, Elman MJ, McPhee TJ. Neovascularization in central retinal vein occlusions: electroretinographic findings. Arch Ophthalmol1988; 106:348-52. 10. Kaye SB, Harding SP. Early electroretinography in unilateral central retinal vein occlusion as a predictor of rubeosis iridis. Arch Ophthalmol 1988; 106:353-6. 11. Massof RW, Wu L, Finkelstein 0, et al. Properties of electroretinographic intensity-response functions in retinitis pigmentosa. Doc Ophthalmol 1984; 57:279-96. 12. Johnson EP. The character of the b-wave in the human electroretinogram. Arch Ophthalmol 1958; 60:565-91. 13. Breton ME, Montzka D. Isolation of receptor photocurrent response in the gross electroretinogram. In: Noninvasive Assessment Visual System: Topical meeting, Feb 5-8, 1990, Incline, Nevada. Washington, DC: Optical Society of America, 1990; 58-61 (1990 technical digest series; v, 3). 14. Fox DA, Farber DB. Rods are selectively altered by lead: I. Electrophysiology and biochemistry. Exp Eye Res, 1988; 46:597-611. 15. Massof RW, Emmel TC. Criterion-free parameter-free distribution-independent index of diagnostic test performance. Appl Optics 1987; 26: 1395-408. 16. Metz CE, Wang pol, Kronman HB. A new approach for testing the significance of differences between ROC curves measured from correlated data. In: Deconinck F, ed. Information Processing in Medical Imaging. Boston: Martinus Nijhoff, 1984; 432-45. 17. Swets JA, Pickett RM. Evaluation of Diagnostic Systems: Methods from Signal Detection Theory. New York: Academic Press, 1982. 18. Granit R. The components of the retinal action potential in mammals and their relation to the discharge in the optic nerve. J Physiol (Lond) 1933; 77:207-39. 19. Penn RD, Hagins WA. Kinetics of the photocurrent of retinal rods. Biophys J 1972; 12:1073-94. 20. Baylor DA, Nunn BJ, Schnapf JL. The photocurrent, noise and spectral sensitivity of rods of the monkey Macaca fascicularis. J Physiol (Lond) 1984; 357:575-607. 21. Johnson MA, Hood DC. A theoretical interpretation of ERG abnormalities in central retinal vein occlusion. In: Noninvasive Assessment of the Visual System: Topical Meeting, Feb. 16-18, 1988, Incline Village, Nevada. Washington, DC: Optical Society of America. 1988; 3: 84-87 (1988 technical digest series; v. 3).
1853
Abstract: The ability of electroretinogram (ERG) b-wave implicit time and b/a wave ratio to predict iris neovascular response was analyzed as a function of stimulus intensity over a 3.6 log unit intensity range in 39 patients with central retinalvein occlusion (CRVO). Predictive power for CRVO patients was evaluated using ROC area at intensities of 1.23, 1.83,2.43, and 3.03 effective log quanta/ rod, where reliable data for both parameters were obtainable from most patients. The relative predictive power of b-wave implicit time and b/a wave ratio were shown to vary with stimulus intensity. The predictive power of b-wave implicit time, as measured by ROC area, declined to below significance at high intensity (above 1.83 log quanta/rod), while b/a wave ratio performed best at middle intensities (1.83 and 2.43 log quanta/rod) and not as well at high and low intensities. Further analysis of statistical behavior of both ERG parameters was obtained from the t statistic. Insight into the mechanism influencing predictive power of b-wave implicit time was derived from measurements on normal adults and CRVO patients with response data taken at high intensities. These results suggest that an optimal stimulus intensity range can be found for these ERG parameters in the evaluation of CRVO. Ophthalmology 1991; 98:1845-1853
Central retinal vein occlusion (CRVO) may lead to a dangerous neovascularization of the iris (NVI) in approximately 20% of patientsY The risk of this compli-
Originally received March 18. 1991. Revision accepted August 7, 1991. From the Scheie Eye Institute. Department of Ophthalmology, University of Pennsylvania, Philadelphia. Presented in part at the Noninvasive Assessment of the Visual System Topical Meeting, Santa Fe, February 1991. Supported in part by NIH grant EY05649 (Dr. Breton). Dr. Breton owns more than 1% of LKC Technologies Inc, the manufacturer of the ERG equipment used in this study. Reprint requests to Michael E. Breton, PhD, Scheie Eye Institute, 51 N 39th St, Philadelphia, PA 19104.
cation in patients who are ischemic can be greatly reduced with laser therapy." It is therefore clinically important to identify those patients at high risk so that they may be followed closely or treated before serious pathologic changes occur. Although fluorescein angiography is used routinely to estimate the degree of retinal ischemia present in CRVO and, therefore, the risk of NVI, it has been shown by some to have problems with reliability and repeatability." The electroretinogram (ERG), especially the b-wave feature, is an indicator of visual function dependent on the ischemic status of the inner retina. For this reason, the ERG has been studied as a possible prognostic indicator in CRVO. Although others have noted changes in the ERGs of vein occlusion patients.l" the work of Sabates et al,? which demonstrated a parametric relation between the b/a wave ratio and prognosis in patients with questionable perfusion status, led to renewed interest in 1845
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the ERG as an indicator of ischemic status and risk of NVI in CRVO. The effectiveness of the ERG as a predictor of the eventual development of NVI in CR VO has been supported by several recent studies that evaluated the predictive power of Naka-Rushton fit parameters (Rmax and Log K) as well as b-wave implicit times and b/a wave ratio. Breton et al 8 and Johnson et al? reported significant predictive power for the intensity-response curve fitting parameters Log K and Rmax, as well as for the b/a wave amplitude ratio and 30 Hz implicit time. Kaye and Harding'? reported that the b-wave implicit time parameter outperformed several other parameters as a predictor of NVI and that the separation between rubeotic and nonrubeotic patients did not change over a 1 log unit range of intensities. Johnson et al? also reported that bwave implicit time is an effective predictor ofNVI. These two reports differ from that of Breton et al. 8 They did not find b-wave implicit time (at their highest intensity) to have predictive power comparable to the other ERG parameters and deleted it from their analysis. This report examines the hypothesis that the change in dynamic behavior of b-wave implicit time as a function of intensity can affect its predictive power in CRVO and can account for the result obtained by Breton et al," showing no predictive power for b-wave implicit time at their highest intensity. Others have investigated the behavior of b-wave implicit time in normal subjects as a function of intensity and report that implicit time decreases steadily as stimulus intensity increases.!':'? Additionally, Breton and Montzka.P studying healthy humans, and Fox and Farber," studying rats, reported that the b-wave implicit time function levels out or saturates at a high intensity. Breton and Montzka'? reported that the intensity at which the implicit time function levels out is close to the intensity that is calculated to produce rod photocurrent amplitude saturation. The behavior of implicit time in healthy subjects and its relation to changes in both receptor and inner retinal function are important in explaining the change in predictive power ofb-wave implicit time as a function of stimulus intensity. This report also compares the performance of an amplitude-based parameter, the b/a ratio, to that of b-wave implicit time as a function of intensity. Our results demonstrate a difference in the behavior of the predictive powers of these two parameters as a function of stimulus intensity. Information gained from analysis of the predictive power of these two parameters also suggests that an optimal stimulus intensity range can be found for each parameter.
METHODS SUBJECfS
A total of 39 patients with CRVO were included in the analysis, 17 of which were retained from a previous study." All patients were recruited from the Retina Service of the Scheie Eye Institute and were diagnosed by a member of 1846
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the Retina Service as having a CRVO, based on a complete ocular examination. All patients, without signs of background diabetic retinopathy, diagnosed as having CRVO during the 31f2 year study period (n = 60) were referred for prospective study. Patients were excluded from the analysis because of: (1) incomplete follow-up (n = 13), (2) treatment with laser photocoagulation before developing rubeosis because they were judged to be clinically at high risk (n = 7), and (3) development of neovascular changes of the disc (n = 1). Systemic hypertensive status was not monitored. All patients were followed for at least 6 months. Neovascularization of the iris later developed in 12 of the 39 patients included in the study. The average age of the patient cohort was 67.3 ± 14 years. Four additional CR VO patients (not included in the 39 patients described above and hereafter referred to as the high-intensity CRVO patients) who were judged clinically to be at low to moderate risk of developing NVI also were selected for study. These 4 patients had higher intensity data that could be used to characterize the highintensity behavior ofb-wave implicit time in typical fellow and CRVO eyes. These patients were aged 55, 67, 72, and 83 years. Another set of comparison data was taken from 4 younger normal control subjects, ranging in age from 18 to 27 years, in smaller intensity steps and up to higher intensities. These data along with those of the older fellow eyes are used to show the dynamic behavior of b-wave implicit time as a function of intensity. These control subjects also had an ophthalmologic examination to rule out ocular disease and abnormal visual function. Informed consent was obtained from all subjects in accordance with the requirements of the Institutional Review Board. ERG EQUIPMENT
All CRVO ERG data were recorded using an LKC Systems UTAS E-lOOO instrument (Gaithersburg, MD), which consists of recording amplifiers, an interface for computer and stimulators, a Ganzfeld for full visual field stimulus presentation, and a xenon flash unit. Data from the 4 younger normal subjects and from the fellow eyes of the 4 high-intensity CRVO patients were taken on an EPIC-lOOO (LKC) system that is similar in design and capability to the UTAS E-lOOO. Both systems use expansion boards on the bus for analog-to-digital conversion of the responses and control of stimulators. The recording amplifiers are AC coupled and use electronic bandpass filters. Filter settings for the recordings were set at 0.3 Hz for the low frequency cutoff and 500 Hz for the high frequency cutoff. Stimulus attenuation was accomplished with Kodak Wratten neutral density filters. The stimulus intensity for the Grass PS-22 flash with no filters in place (0 decibel [dB] condition) was 0.5 log cd - sec/rrr' measured as the luminance of the reflective surface of the Ganzfeld. An additional Vivitar photoflash unit was used to present high-intensity stimuli up to 2.9 log cd - sec/ rrr'. The pulse width of the PS-22 is approximately 10 usee. The pulse width of the Vivitar flash is approximately 1 msec.
BRETON et al •
B·WAVE IMPLICIT TIME IN CRVO
canthus, with the ground electrode on the forehead. Intensity-response data were taken using neutral density filters from 36 dB attenuation (-0.57 log quanta absorbed per rod , - 3. 1 log cd - sec/m 2) to 0 dB (3.03 log quanta/ rod , 0.5 log cd - sec/rrr') with a white flash stimulus in steps of 6 dB with background off. Electroretinogram responses from the 4 control subjects and the 4 high-intensity CRVO patients were obtained using a similar protocol. Data for these subjects and patients were taken at more closely spaced intensity steps than for the cohort of 39 CRVO patients. Intensities greater than 3.03 log quanta/rod or 0.5 log cd - sec/m2 (the highest intensity used with the CRVO patients) were presented using the high-intensity Vivitar xenon flash unit at the end of each session. An interstimulus interval was incorporated between higher intensity flash stimuli to allow for complete recovery of the ERG waveform. This procedure allowed sampling ofb-wave implicit time up to intensities of 5.4 log quanta/rod or 2.9 log cd - sec/rrr'.
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Time (rns) Fig I. An intensity response waveform series for a typic al CRVO patient (age 67) with b-wave peaks mark ed for the fellow eye (filled circles) and for the CRVO eye (open circles) superim posed at each intensity. B-wave peaks are displa ced to longer time s for the CR VO eye compared with the fellow eye and move to shorter implicit times for both eyes as intensit y increases. There is a n abrupt decline in rate of peak movement at about 2 log quanta/rod.
ERG PROCEDURE
Electroretinograms were performed bilaterally on the cohort of 39 CRVO patients within 2 weeks oftheir initial visit (before any neovascular complications or treatment) and at irregular intervals thereafter, depending on clinical status. Single and repetitive flashed stimuli were presented in a highly reflective white Ganzfeld using an automated sequence (UTAS recordings) or predetermined manual sequence (EPIC recordings). Corneal electrodes were unipolar, disposable ERG-jet type. Testing was preceded by 30 minutes of dark adaptation. Pupils were fully dilated with 1% tropicamide and 10% Neo-Synephrine. Reference electrodes were placed near the corresponding temporal
The ERGs used in the anal ysis of predicti ve power were taken exclusi vely from the first examination after onset ofCRVO symptoms. The b-wave implicit time was measured as the time from stimulus onset to the peak of the b-wave feature (first major positive polarity response after the initial negative polarity a-wave feature). Figure 1 shows a full intensity series for both eyes of one of the highintensity CRVO patients. Waveforms for both eyes are superimposed based on the prestimulus baseline (note: onl y 7 of the 20 msec of prestimulus baseline recorded are shown in Fig 1). Stimulus intensity in log quanta/rod is marked on the ord inate for each waveform pair. Bwave peaks are marked with a filled circle for the fellow eye and an open circle for CRVO eye. In general , the bwave peaks for the CRVO eye are displaced to longer times and peak impli cit times for both eyes move to the left as intensity increases. This figure also shows the tendency ofb-wave implicit time to saturate as intensity increases abo ve about 2 log quanta/rod. The data shown in the figure extend to intensities much higher than those analyzed for the patient cohort. At low and middle intensities, where the patient cohort data are analyzed, cursor placement is unambiguous. However, at the highest intensities (4.23 log quanta/ rod and greater), oscillatory potentials introduce problems in terms of unambiguously locating the b-wave peak. It is possible that the true b-wave peak falls in between the oscillatory peaks. Because this problem with cursor placement does not occur until almost 2 log units above the intensity of implicit time saturation, it does not affect the statistics ofour anal yzed data and therefore does not affect our conclusions. We show the data at the highest intensities for this typical patient merely to be complete and to leave no doubt that implicit time does saturate for both affected and unaffected eyes. 1847
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Fig 2. B-wave implicit time declines with increasin g stimulus intensity for the three CRYO categories: CR YO eyes with neovascular response (n = 12), CRYO eyes with no neovascula r response (n = 27), and the unaffected fellow eyes for all subjects (n = 39) (error bars ± standard error ). At th e higher inten sities, where predictive power as measured by ROC area is lost, the error bars overlap for the NYI and non-NYI response groups.
Table 1. t Statistic Values with Levels of Significance* Log Quanta/Rod
1.23 b-Wave implicit time Significance b/a Wave ratio Significance
2.86
P < 0.01 1.16 NS
1.83
2.43
2.3 0.92 P < 0.025 NS 2.3 3.29 P < 0.025 P < 0.005
3.03 0.38 NS
2.60 P < 0.01
NS = not significant. * Degrees offreedom = 33 at 1.23 log quanta/rod ; otherwise, degrees of freedom = 37.
The b/a ratio was defined as the overall amplitude of b-wave divided by the distance from the baseline to the negative a-wave peak. Although CRVO patient data were recorded in 6-dB steps of attenuation from 36 dB to 0 dB for a maximum number of 7 data points in the intensity response, we restricted our anal ysis to the top 4 points (18 dB = 1.23 log quanta/rod = -1 .32 log cd - sec/nr'; 12 dB = 1.83 log quanta/rod = -0.72 log cd - sec/rrr' ; 6 dB = 2.43 log quanta/rod = -0.12 log cd - sec/rrr' ; and 0 dB = 3.03 log quanta/rod = 0.50 log cd - sec/rrr'), B-wave implicit time and b/a wave ratio measurements made in th is patient cohort below this level are unreliable . Higher intensity data were taken on the EPIC system and are used only in the analysis of b-wave implicit time. B-WAVE IMPLICIT TIME FOR CRVO PATIENTS
Figure 2 shows the b-wave implicit times for the cohort of39 CRVO patients anal yzed prospectively. The CRVO eyes that developed NVI are shown as squares (n = 12), 1848
1.83
2.43
3.0 3
Lo g Quont o/Rod
Lo g Quant a/ Ro d
Fig 3. The b/a wave ratio as a function of inten sity for the three CR YO categories as in Figure 2 (error bars ± I stand ard error). As stimulus intensity increases, separation of group means declines. Inset, the enlarged view of the higher intensity data points shows that standard errors of the mean also dimin ish so that relative separation of NYI affected to no naffected CRYO eyes is main tain ed. However, the fellow eyes are indistingu ishable from the nonaffected CRYO eyes.
the CR VO eyes that did not develop NVI are shown as diamonds (n = 27), and fellow eyes are shown as triangles (n = 39). For all categories, implicit times decreased at a relatively high rate and then leveled off as a function of increasing stimulus intensity. The three eye conditions have the effect of separating the response functions vertically such that the more severely affected eyes are associated with longer implicit times. However, a most interesting result is the convergence of the implicit times for the two CRVO eye conditions as intensity increases. At the highest intensity, eyes that eventually developed NVI show implicit times that are indistinguishable from those that did not. This result is further supported by the t statistic values shown in Table 1. The two lowest intensities show a clear separation of these two CRVO categories on the basis of b-wave implicit time. The fellow eyes maintain clear separation from the CRVO eyes at all intensities and appear nearly parallel to the CRVO eyes without NVI. BI A
RATIO
Figure 3 shows b/a wave ratios for the same eye conditions as in Figure 2. As with b-wave implicit time , all three eye conditions show vertical separation at the lowest intensity, but tend to converge at higher intensities. However, the mean separation at the lowest intensity is not significant, while that at the intermediate and highest intensities is significant (Table 1). Th e inset shows an enlarged view of the higher intensity data. In this case, in contrast to the results for b-wave implicit time , the error bars for b/a ratio do not overlap for the two CR VO eye conditions (NVI versus non-NVI). Fellow eye values do not differ significantly from those of the CRVO eyes without NVI at the thre e higher intensities. Consistent with
BRETON et al •
B-WAVE IMPLICIT TIME IN CRVO
1.23 Log Quanta/Rod b-Wave Implicit Time 12
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Fig 4. Histogram comparison of b-wave implicit times for NVI affected and non-NY! affected subjects at the four intensities: A, 1.23 log quanta/ rod or -1.32 log cd - scc/rrr'; B, 1.83 log quanta/rod or -0.72 log cd-rsec/trr'; C, 2.43 log quanta/rod or -0. I 2 log cd-rsec/rrr': D, 3.03 log quanta/ rod or 0.50 log cd-sec/m" Histogram bars are centered to the left and right of the upper limits of their respective bins (bin width = 5 msecs) (i.e., bars at 80 msec include those patients who had b-wave implicit times in the 76 to 80 msec range). Overlap of the distributions increases as a function of increasing intensity.
the presumed site of inner retinal compromise, which would tend to reduce b-wave amplitude relative to a-wave amplitude, the eyes with NVI fall below the other two categories at all intensities. RECEIVER OPERATING CHARACTERISTIC AREA
The receiver operating characteristic (ROC) is a plot of true-positive versus false-positive results in discriminating NVI affected from non-NVI affected patients. The area under the ROC curve correlates with the ability of the
parameter plotted to discriminate between 2 categories based on that parameter, with an area of 0.5 equal to chance discrimination. 15- 1? This statistical technique quantifies the degree of separation between two distributions and presents a formal method for determining significance of a discrimination level from chance or in comparison to other conditions. It is especially useful when substantial overlap is present and when examination of histograms may not be informative, as is true for many of the experimental conditions represented in this study (Fig 4). The histograms presented in Figure 4 show the 1849
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chance (P < 0.02). Receiver operating characteristic values at the highest and lowest intensities were not significantly above chance discrimination.
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Table 2. ROC Areas with Levels of Significance above Chance Log Quanta/Rod
1.23 b-Wave implicit time Significance* b/a Wave ratio Significance*
0.76
P < 0.01
0.56 NS
1.83
2.43
3.03
0.73
0.64
0.53
0.73 P < 0.01
0.72 P < 0.02
0.64
P < 0.01
NS
NS NS
ROC = receiver operating characteristic; NS = not significant. * Calculated with Labroc1 software (see Acknowledgment section).
number ofCRVO eyesin the NVI and non-NVI categories falling at different b-wave implicit time values and as a function of intensity. Figures 4A through 4D show data for intensities of 1.23 up to 3.03 log quanta/rod. Figure 5 shows how ROC area changes as a function of stimulus intensity for the two parameters, b-wave implicit time and b/a wave ratio, while Table 2 shows the ROC area values and statistical significance compared with chance. For b-wave implicit time, ROC area increases as stimulus intensity decreases. For the highest intensity, the ROC area, and therefore the ability ofb-wave implicit time to discriminate NVI affected from non-NVI affected patients, is 0.56 and is not significantlygreater than chance alone. At 0.6 log units below the highest intensity, the ROC area increases to 0.64, but is still below significance. At the two lower intensities, ROC area increases to 0.72 and 0.76, both of which are significantly above chance (P < 0.01), and appears to level off. Receiver operating characteristic area for the b/a wave ratio shows a pattern different from that ofb-wave implicit time. Peak values of 0.72 and 0.73 occur at the middle intensities of 2.43 log quanta/rod and 1.83 log quanta/ rod, respectively. These values are significantly above 1850
The t statistic values for b-wave implicit time and b/a ratio, shown in Table 1, are a measure of the separation of mean values for NVI affected compared with non-NVI affected eyes as a function of stimulus intensity. For bwave implicit time, the t statistic values, like the ROC areas, increase as intensity decreases. The t values for intensities of 3.03 and 2.43 log quanta/rod are not significant, whereas the values for intensities of 1.83 and 1.23 log quanta/rod are significant (P < 0.025). For the b/a ratio, the t values also tend to correlate with the ROC areas in that the lowest t value corresponds to the lowest stimulus intensity, and the greatest t value occurs at an intermediate intensity. B-WAVE IMPLICIT TIME AT HIGHER INTENSITIES
The data for the 4 young control subjects and 4 highintensit y CRVO patients (both CRVO and fellow eye) are plotted individually in Figures 6A and 68. The intensity range for these data is much greater (6 log units) than that shown for CRVO patients in Figure 2 (1.8 log units). The normal data (Fig 6A) indicate two regions with nearly constant rates of decline in b-wave implicit time with intensity. Below an intensity of ahout 2.0 log quanta/rod, the rate of decline is approximately - 25 msec/log quanta/ rod. At intensities higher than 2.0 log quanta/rod, the rate of decline in b-wave implicit time undergoes an abrupt change to a nearly constant rate of - 2.0 msec/log quanta/ rod. The reduction in the rate of change ofb-wave implicit time beyond 2 log quanta/rod is about a factor of 10, based on independent linear regressions in each region. The data from the 4 high-intensity CRVO patients (Fig 6B) show an overall pattern similar to the control subjects and show the probable high-intensity behavior of the CRVO patient cohort used in the analysis for whom ERG data above 3.03 log quanta/rod were not available. This inference is supported by the trend of the averaged data for CRVO NVI affected and non-NVI affected eyes to level out at 2.43 and 3.03 log quanta/rod in Figure 2. The fellow eyes in Figure 6B (filled circles) fall below the CRVO eyes (open circles) at all intensities, however both eyes exhibit a sharp reduction in their rates of decline with intensity at about 2.0 log quanta/rod. We note that both the normal subjects and the high-intensity CR va patients show this sharp reduction at nearly the same intensity despite the age difference.
DISCUSSION We have used two statistical methods to evaluate the prognostic ability of b-wave implicit time and b/a wave ratio to discriminate NVI affected from non-NVI affected CRVO patients. These parameters showed predictive
BRETON et al
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a-WAVE IMPUCIT TIME IN CRVO
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power that varied significantly with intensity. More specifically, b-wave implicit time loses its ability to predict neovascular outcome for our patients and recording conditions above a stimulus intensity corresponding to absorption of approximately 2 log quanta/rod. In addition, the predictive power ofb-wave implicit time and b/a wave ratio behave differently as a function of intensity. The best predictive performance for b/a wave ratio occurred
at middle intensities and declined at both high and low intensities. B-WAVE IMPLICIT TIME
In an analysis of the b-wave implicit time function in younger normal subjects (Fig 6), Breton and Montzka' r conclude that decline in implicit time with increasing in1851
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tensity is driven by increasing amplitude of the rod signal to the inner retina. This analysis is consistent with a serial two stage retinal response model, such as that proposed by Johnson and Hood,21 with the first stage corresponding to the photoreceptor response and the second stage to inner retinal response. The b-wave implicit time function levels out at approximately 2 log quanta absorbed per rod, a value very close to the quantal absorption calculated to produce photocurrent amplitude saturation in mammalian rods by Penn and Hagins'? and by Baylor et al. 20 Thus, the leveling out of the b-wave implicit time function may be an indicator of the intensity at which rod photocurrent amplitude saturation occurs.':' Our CRVO patient data show different patterns of increased b-wave implicit time (Fig 2), for the CRVO eyes that developed NVI and for those that did not. The nonNVI affected eyes show a large increase in b-wave implicit time at each intensity compared with the nonaffected fellow eyes, producing a nearly parallel vertical shift of the implicit time intensity response function. This is consistent with decreased sensitivity of the inner retina, corresponding to a loss at the second stage, with little or no change in photoreceptor sensitivity. In comparison, the implicit times for the NVI eyes are longer at lower intensities, but converge with the values for the non-NVI affected eyes at the higher intensities. This could be consistent with either a nonparallel vertical shift or a horizontal shift to the right relative to the nonNVI affected and fellow eyes. Since the implicit time function of the NVI eyes has not clearly leveled off at the highest intensities used in this study, it is not certain from the data which explanation best applies. A horizontal shift to the right relative to the non-NVI affected eyes would imply some loss of sensitivity at the photoreceptor stage (without a change in maximum response amplitude) in addition to inner retinal loss. A photoreceptor shift to the right, when combined with a vertical shift resulting from loss of inner retinal sensitivity, could account for the relative positioning of the b-wave implicit time function of the NVI affected compared with non-NVI affected eyes for our data. However, if the nonparallel vertical shift is occurring, then a nonlinear gain change at the inner retinal stage would be required to explain the results. The analysis of CRVO patients reported by Breton et al" that found no predictive power for b-wave implicit time was based on data taken at a stimulus intensity beyond the point of b-wave implicit time convergence for their patient cohort. The difference between Johnson et al? and Breton et al may be partly explained by the use of higher intensities in the study by Breton et al. The fact that the study by Johnson et al reports analyzing b-wave implicit time data at a stimulus intensity approximately 0.2 log units below Breton et al raises the possibility that Johnson et al used stimulus intensities that were effectively lower than the intensity level ofb-wave implicit time saturation. Since the point of convergence in the non-NVI and NVI implicit time functions is abrupt, small deviations in intensity above and below the point of convergence could have significant effects on the predictive power of b-wave implicit time. However, other factors such as patient selection procedures also must be considered. 1852
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Kaye and Harding'? appear to have used stimulus intensities that were at or above levels which cause the bwave implicit time functions for NVI affected and nonNYI affected eyes to converge for our patient cohort. Thus, an explanation based on intensity would not help in explaining the difference between their study and Breton et al." Kaye and Harding's'? implicit time results are different from Breton et al in that a large separation is present between the NVI affected and non-NYI affected eyes even at higher intensities. This pattern of response is similar to what is seen in the present study for non-NVI affected compared with fellow eyes. Also different from Breton et al, the non-NVI affected CRVO eyes have an implicit time function almost indistinguishable at high intensity from that of the fellow eyes (Fig 2). These results are consistent with very limited retinal damage being present in the majority of the Kaye and Harding CRVO eyes, the exception being those eyes that developed NVI complications. This result may in part be due to the Kaye and Harding patient selection criteria. These criteria appear to be heavily weighted toward including clinically nonischemic eyes. Because rubeosis iridis developed in seven of these eyes, it is apparent that the clinical selection method was less than perfect in selecting patients at risk for NVI, an important fact motivating the study. The patient selection criteria could have contributed to altering the statistical distribution of implicit times in the direction observed within the CRVO group because: (1) the majority ofpatients were nonischemic and showed implicit times in the CRVO eye close to values in the fellow eye, (2) a minority of patients were effectively ischemic (although clinically they appeared not to be), showed longer implicit times and developed NVI. The study by Breton et al" excluded no patients because they were judged to be clinically at high risk for NYI. In the expanded data set for the current study, seven patients of a consecutive series of 60 referrals were excluded because they were judged clinically to be ischemic and at high risk. However, others, who might also have been considered ischemic on clinical grounds, were not excluded because an important goal of the study was to follow as many patients as possible without treatment. This raises the possibility that the original Breton et al patients as well as the present expanded cohort are more heterogeneous with respect to ischemic status than the Kaye and Harding cohort and that this factor contributes to the differences in the underlying implicit time distributions and to the differences noted at higher intensities. BfA
WAVE RATIO
The behavior of the b/a wave ratio must appeal to a different type of explanation to account for loss of predictive power at lower intensities. At the lowest intensity, the b/a wave ratio did not show significant ability to discriminate NVI affected from non-NVI affected patients as measured by t statistic or ROC area (Fig 3). This decline in predictive power may reflect greater reduction in amplitude and increase in variability of the a-wave feature relative to the b-wave feature. However, at higher inten-
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sities, amplitude saturation of the photoreceptors and the changes this event brings about in the dynamic interaction of the underlying a-wave and b-wave components, may play a role in reducing the predictive value of the b/a wave ratio. SUMMARY
These results suggest that optimum levels of stimulus intensity may be defined for ERG parameters by choosing intensities that maximize ROC area and the statistical separation of the disease state from normal as measured by the t statistic. Optimal stimulus values may be different for the various ERG parameters. The finding that the predictive value of the b/a wave ratio declines at lower intensities and that the predictive value of b-wave implicit time declines at higher intensities may also provide a rationale for selecting an optimal intensity for evaluating both parameters. The optimal range for exam intensities, as indicated by the intensities used in this study, appears to be near 12 dB of attenuation for b/a wave ratio and 18 dB of attenuation for b-wave latency, on our scale. This corresponds to illuminance values of -0.7 log cd - sec/ m 2 (1.83 log quanta/rod) and -1.3 log cd - sec/m? (1.23 log quanta/rod), respectively. Our results suggest a range of best intensity for both bwave implicit time and b/a wave ratio that falls below the point at which b-wave implicit time levels off for control subjects and somewhat above the intensity at which the a-wave feature first appears (to allow reliable measurement of the b/a wave ratio). This range of intensity yields high predictive performance for both b-wave implicit time and b/a wave ratio for our patient cohort.
ACKNOWLEDGMENTS The authors thank Charles E. Metz, Pu-Lan Wang, long-Her Shen, and Helen B. Kronman for their ROC software "Labroc l ," which made calculation of ROC areas and statistical significances possible and easy.
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3. Magargal LE, Brown GC, Augsburger JJ, Donoso LA. Efficacy of panretinal photocoagulation in preventing neovascular glaucoma following ischemic central retinal vein obstruction. Ophthalmology 1982; 89: 780-4. 4. Welch JC, Augsburger JJ. Assessment of angiographic retinal capillary nonperfusion in central retinal vein occlusion. Am J Ophthalmol1987; 103:761-6. 5. Henkes HE. Electroretinography in circulatory disturbances of the retina. I. Electroretinogram in cases of occlusion of central retinal vein or of one of its branches. Arch Ophthalmol1953; 49:190-201. 6. Sakaue H, Katsumi 0, Hirose T. Electroretinographic findings in fellow eyes of patients with central retinal vein occlusion. Arch Ophthalmol 1989; 107:1459-62. 7. Sabates R, Hirose T, McMeel JW. Electroretinography in the prognosis and classification of central retinal vein occlusion. Arch Ophthalmol 1983; 101:232-5. 8. Breton ME, Quinn GE, Keene SS, et al. Electroretinogram parameters at presentation as predictors of rubeosis in central retinalvein occlusion patients. Ophthalmology 1989; 96: 1343-52. 9. Johnson MA, Marcus S, Elman MJ, McPhee TJ. Neovascularization in central retinal vein occlusions: electroretinographic findings. Arch Ophthalmol1988; 106:348-52. 10. Kaye SB, Harding SP. Early electroretinography in unilateral central retinal vein occlusion as a predictor of rubeosis iridis. Arch Ophthalmol 1988; 106:353-6. 11. Massof RW, Wu L, Finkelstein 0, et al. Properties of electroretinographic intensity-response functions in retinitis pigmentosa. Doc Ophthalmol 1984; 57:279-96. 12. Johnson EP. The character of the b-wave in the human electroretinogram. Arch Ophthalmol 1958; 60:565-91. 13. Breton ME, Montzka D. Isolation of receptor photocurrent response in the gross electroretinogram. In: Noninvasive Assessment Visual System: Topical meeting, Feb 5-8, 1990, Incline, Nevada. Washington, DC: Optical Society of America, 1990; 58-61 (1990 technical digest series; v, 3). 14. Fox DA, Farber DB. Rods are selectively altered by lead: I. Electrophysiology and biochemistry. Exp Eye Res, 1988; 46:597-611. 15. Massof RW, Emmel TC. Criterion-free parameter-free distribution-independent index of diagnostic test performance. Appl Optics 1987; 26: 1395-408. 16. Metz CE, Wang pol, Kronman HB. A new approach for testing the significance of differences between ROC curves measured from correlated data. In: Deconinck F, ed. Information Processing in Medical Imaging. Boston: Martinus Nijhoff, 1984; 432-45. 17. Swets JA, Pickett RM. Evaluation of Diagnostic Systems: Methods from Signal Detection Theory. New York: Academic Press, 1982. 18. Granit R. The components of the retinal action potential in mammals and their relation to the discharge in the optic nerve. J Physiol (Lond) 1933; 77:207-39. 19. Penn RD, Hagins WA. Kinetics of the photocurrent of retinal rods. Biophys J 1972; 12:1073-94. 20. Baylor DA, Nunn BJ, Schnapf JL. The photocurrent, noise and spectral sensitivity of rods of the monkey Macaca fascicularis. J Physiol (Lond) 1984; 357:575-607. 21. Johnson MA, Hood DC. A theoretical interpretation of ERG abnormalities in central retinal vein occlusion. In: Noninvasive Assessment of the Visual System: Topical Meeting, Feb. 16-18, 1988, Incline Village, Nevada. Washington, DC: Optical Society of America. 1988; 3: 84-87 (1988 technical digest series; v. 3).
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