Technical aspects of multifocal ERG recording

A simplification of intricate multifocal electroretinogram understanding: A mini-review

Variation of multifocal electroretinogram (mfERG) data presentation in existing scientific publications is a challenge for eye care practitioners to apply the scientific information for evidence-based practice in patient management. This review offers an overview of the mfERG data presentation types. Eight types of data presentation in the form of a table, scatter plot, line graph, bar graph/box plot, single waveform/a group of waveforms, trace array topography, three-dimensional topography, and two-dimensional topography are identified. The table format is used to provide the exact values. Line graphs, scatter, and box plots offer information about the relationship of mfERG values. Waveforms are helpful for comparison between groups or conditions. Topographies outline the retinal, especially the specific localized retinal abnormalities. An infographic of fundamental mfERG electrical response with definitions and clinical indications is provided to bridge the gap between researchers and clinicians to facilitate efficient clinical application.

Fundus motion during mfERG testing

PURPOSE

The purpose of this study was to assess eye movements during a multifocal ERG (mfERG) recording. This study evaluated the relationship between bivariate contour ellipse areas (BCEAs), mfERG amplitudes (Amps) and mfERG implicit times (ITs) with repeat testing and experienced subjects.

METHODS

Thirty subjects were selected (15 experienced to ocular procedures and 15 novices). All were confirmed to have healthy retinas and at least 20/25 vision. MfERGs with a stimulus near 100% contrast and 4-min m-sequence were recorded on two different days using our common clinical technique, which did not constrain the head. VERIS with fundus monitoring system was used for recording with a Burian-Allen electrode. An external camera captured the fundus during each mfERG recording. The optic nerve head position was tracked in each video using a custom algorithm in order to determine BCEAs. Each subject performed one mfERG on two different days. MfERGs were analyzed for Amps and ITs for the fovea and whole eye.

RESULTS

There was no correlation between the mfERG metrics and BCEAs with repeat testing. There were also no differences between the experienced and novice subjects for mfERG Amps, ITs or BCEAs. Eye movements between visits were highly correlated (multiple r = 0.67). BCEAs were larger during mfERGs (1.04 ± 0.8 deg²) than those observed in previous literature using brief viewing tasks (< 0.3 deg²). The proportion of time spent fixating within 1.0 and 2.0 degrees of the central hexagon was 68 and 93%, respectively.

CONCLUSIONS

This study is the first to evaluate the stability of the retina while recording a mfERG in healthy subjects and indicates that the center of fixation during a mfERG stays within the central hexagon. Eye stability during an initial recording is the best indicator of stability on the second recording. The amount of movement during these recordings did not seem to affect the mfERG Amps or ITs. These data suggest clinical confidence with mfERGs when recording novice patients.

Variability of the multifocal electroretinogram based on the type and position of the electrode

PURPOSE

To evaluate and compare the wave amplitude of multifocal electroretinogram (mfERG) measurements and discomfort from Dawson-Trick-Litzkow (DTL) electrode located on the cornea (cDTL) and on the conjunctival fornix (fDTL) and ERG-jet contact lens electrode (CL).

METHODS

Thirty-six patients and 18 healthy volunteers were evaluated with three different methods (cDTL, fDTL and CL). Wave amplitude, number of artifacts, number of electrode dislocations and level of discomfort obtained with each electrode were compared. These variables were also compared between patients and healthy volunteers (control group). Comparisons between wave amplitudes were made using intraclass correlation coefficient (ICC).

RESULTS

Responses by the three tested electrode were better correlated (ICC) in patient (0.616-0.756) than in control groups (0.342-0.679). CL provided the highest wave amplitude in both groups (p < 0.005), but it was associated with higher discomfort (p < 0.001) and the highest rate of dislocations (72 and 100 % in control and patients, respectively). Looking at the differences obtained by each electrode between both groups, CL seems to be able to differentiate patient from control in the ring 1 of the mfERG. By contrast, fDTL gave the lowest wave amplitude in both groups, but it had the advantage to, apparently, discriminate between patients and control group in rings 1 and 2. cDTL produced more artifacts than the other electrodes in both groups (p < 0.001).

CONCLUSIONS

Although wave amplitude measurements with different electrodes were correlated, they cannot be directly compared, so it is mandatory to create an appropriate normative database with each electrode. Despite providing the lowest amplitudes, fDTL seems to offer the best features to perform mfERG regarding discomfort, number of artifacts and diagnostic capability.

Fixation stability and implication for multifocal electroretinography in patients with neovascular age-related macular degeneration after anti-VEGF treatment

PURPOSE

To quantify fixation stability in patients with neovascular age-related macular degeneration (nAMD) at baseline, 3 and 6 months after anti-vascular endothelial growth factor (anti-VEGF) treatment and furthermore asses the implications of an unsteady fixation for multifocal electroretinography (mfERG) measurements.

METHODS

Fifty eyes of 50 nAMD patients receiving intravitreal anti-VEGF treatment with either bevacizumab or ranibizumab and eight eyes of eight control subjects were included. Fixation stability measurements were performed with the Eye-Link eyetracking system and the retinal area in degrees² (deg²) containing the 68 % most frequently used fixation points (RAF68) was calculated. MfERG P1 amplitude and implicit time were analyzed in six concentric rings and as a summed response. Patients were examined at baseline, 3 and 6 months. Four different mfERG recordings were performed for the control subjects to mimic an involuntary unstable fixation: normal central fixation, 2.4°, 4.8°, and 7.1° fixation instability.

RESULTS

For control subjects, a fixation instability of 2.4° (corresponding to the central hexagon) did not reduce mfERG ring amplitudes significantly, whereas 4.8° and 7.1° fixation instability reduced the amplitudes significantly in rings 1 and 2 (p < 0.001) as well as in the peripheral rings in the 7.1° instability condition (p < 0.001). Fixation stability improved non-significantly for patients at 3 and 6 months. The size of the retinal area of fixation was at baseline, 3 and 6 months negatively correlated to visual acuity (VA) (r_baseline = -0.65, r_{3 months} = -0.60, and r_{6 months} = -0.66 respectively, p < 0.001) and mfERG amplitudes of the three innermost rings (r_baseline = -0.29, p = 0.042, r_3 months = -0.43, p = 0.003 and r_6 months = -0.31, p = 0.042). The VA cutoff for a fixation area less than 5 deg² (approximately the central hexagon) was 65, 77, and 68 ETDRS letters (corresponding a maximal Snellen equivalent of 0.31) at baseline, 3 and 6 months, respectively.

CONCLUSIONS

MfERG amplitudes in recordings of nAMD patients are at substantial risk of being reduced due to poor fixation as a large number of patients may use a fixation area of more than 5 deg². Fixation monitoring during recording as well as interpretation of results should be performed with care, especially in patients with poor visual acuity.

The effect of filtering on the two-global-flash mfERG: identifying the optimal range of frequency for detecting glaucomatous retinal dysfunction

PURPOSE

To study the effects of filtering bandwidth on the two-global-flash multifocal electroretinogram (mfERG) responses in primary open-angle glaucoma (POAG) compared with control subjects.

METHODS

A two-global-flash mfERG (VERIS 6.06™, FMS III) was recorded in 20 healthy subjects and 22 POAG patients with a band-pass filter (BPF) of 1-300 Hz (103 Hexagons, M-sequence stimulus: Lmax 100 cd/m(2), Lmin < 1 cd/m(2), global flash: 200 cd/m(2)). The root-mean-square average of the central 10° was calculated. Three response epochs were analysed: the response to the focal flash, at 15-45 ms (DC), and the following two components induced by the effects of the preceding focal flash on the response to the global flashes at 45-75 ms (IC1) and at 75-105 ms (IC2). The following BPF settings were analysed: 1-300 Hz, 3-300 Hz, 10-300 Hz, 100-300 Hz, 200-300 Hz, 1-10 Hz, 1-100 Hz and 1-200 Hz.

RESULTS

Filtering at 1-300 Hz showed significantly lower responses in POAG than in control subjects (p < 0.001) for all epochs analysed. At 1-100 Hz, this also held true even though the difference between the groups became smaller. At 1-10 Hz, responses were extremely small and did not differ between POAG and control (p > 0.5). This would suggest a filter setting of 10-300 Hz for mfERG recordings in POAG. However, when a filter setting of 10-300 Hz was compared to 1-300 Hz, with a filter setting of 10-300 Hz, the DC in POAG differed more (p < 0.0001) from normal than with 1-300 Hz (p = 0.0002). For IC1 and IC2, the stronger difference between POAG and control was found with 1-300 Hz (p < 0.0001) rather than with 10-300 Hz (p < 0.0001 and p = 0.0005, respectively). For the 'oscillatory potentials' at 100-300 Hz, POAG and control differed significantly in IC1 and IC2 (p < 0.05), but not in DC (p = 0.8). However, filtering at 200-300 Hz did not show a difference between POAG and control (p > 0.5). Thus, we applied a filter setting of 1-200 Hz, which seemed to be most sensitive in detecting glaucomatous retinal dysfunction (p < 0.0001).

CONCLUSIONS

A filter setting of 1-200 Hz appears most sensitive to detect glaucomatous damage if using a two-global-flash mfERG: using a band-pass filter a with lower low-frequency cut-off, containing the 10 Hz component, may be especially important in the small induced components that show glaucomatous damage most sensitively. High frequencies of 100-300 Hz also contain information that differentiates glaucoma from normal and thus should be included in the analysis.

Multifocal electroretinography

Multifocal electroretinography (mfERG) provides functional and objective evidence of retinal dysfunction. We have found mfERG to be especially useful in the management of occult outer retinopathy and Stargardt's disease.

Comparing the Multifocal Electroretinogram Topography before and after Cataract Surgery

PURPOSE

To investigate whether multifocal electroretinogram (mfERG) topography would be affected by nuclear cataract.

METHODS

Multifocal electroretinograms were recorded from 10 elderly subjects (10 eyes) with nuclear cataract of grade five (LOCS III) before and after cataract surgery (phacoemulsification). Their visual acuity before the cataract surgery was between 6/12 and 6/18. The postoperative period was from 2 to 3 months. None of the subjects had any significant eye disease apart from cataract. The mfERG responses were grouped into six concentric rings for analysis. Both the amplitudes and the latencies of N1 and P1 of first-order responses were used for analysis.

RESULTS

N1 amplitude only from ring 1 increased significantly after cataract surgery. P1 amplitude from ring 1 and ring 2 also increased significantly after cataract surgery. The latencies of neither N1 nor P1 from all rings changed significantly.

CONCLUSIONS

Nuclear cataract could affect the topography of mfERG, so clinicians should be aware of the effects of cataract when interpreting mfERG responses in cataract patients.

A comparison of the suitability of cathode ray tube (CRT) and liquid crystal display (LCD) monitors as visual stimulators in mfERG diagnostics

The aim of this study was to determine up to which extent the specific characteristics of cathode ray tube (CRT) and liquid crystal display (LCD) monitors influence the retinal biosignal when used as stimulators in ocular electrophysiology. In a conventional CRT monitor, each pixel lights up only for a duration of a few milliseconds during each frame. In contrast, liquid crystal displays are quasi-static, i.e. each pixel has a constant luminance during the whole length of the frame, but lights up only with a certain delay after the trigger. These different display characteristics may affect the mfERG signal. The temporal and spatial luminance distributions of a CRT and an LCD monitor were measured in white flashes. The total amount of emitted light was calculated by integration of the intensity versus time curves. By means of an mfERG recording system (RETIsystem, Roland Consult, Brandenburg, Germany) first-order kernel (FOK) mfERG signals were computed and then analysed using customized MATLAB (TheMathWorks, Natick, MA, USA) software. With the two stimulator monitors, differences in the mfERG signal were observed. The latencies of mfERG responses recorded with the LCD monitor were significantly increased by 7.1 ms for N1 and 9.5 ms for P1 compared to the CRT. Due to a higher luminance, the N1 amplitude was significantly higher by approx. 2 dB in measurements with the LCD monitor while no significant difference could be detected with regard to the more contrast sensitive P1 amplitude. When using LCD monitors as stimulators the increase in latencies and differences in the luminance versus time profile must be taken into account. Prior to clinical application, the establishment of guidelines for the use of LCD monitors is recommended.

Intra-sessional and inter-sessional variability of multifocal electroretinogram

The aim of this study was to investigate the intra-sessional and inter-sessional variability of the multifocal electroretinogram (mfERG). Right eyes of 20 healthy adult subjects with uncorrected visual acuity of 1.0 were studied. mfERG recordings were obtained in two sessions by using jet corneal electrodes. The responses were averaged over five retinal regions, the central hexagon (CH: 0.0-2.3 degrees ) and four concentric rings [ring 1 (R1: 2.3-7.4 degrees ), ring 2 (R2: 7.4-12.0 degrees ), ring 3 (R3: 12.0-19.4 degrees ), and ring 4 (R4: 19.4-30.0 degrees )]. In the first session, recordings were repeated while the electrodes stayed in place (the first and the second recordings). Second sessions (third recordings) were performed 1 week later. Individual coefficient of variation (CV; standard deviation/mean) for P1 and N1 amplitudes and implicit times were calculated for the first to second and first to third recordings to assess intra-sessional and inter-sessional variability, respectively, and were evaluated by Wilcoxon-paired samples test. Regarding implicit time and amplitude of mfERG recordings, there was no significant difference between the first to second and first to third recordings (P > 0.05, Bonferroni-corrected Wilcoxon test). CV values significantly decreased with eccentricity in both intra-sessional and inter-sessional comparisons (P < 0.05, Wilcoxon test). However, partial ring averaging showed no significant difference (P > 0.05) between CV values for single hexagons in each ring. Intra-sessional CV values were significantly lower than corresponding inter-sessional values (P < 0.05, Wilcoxon test). Our results support the importance of electrode replacement on variability. The increased number of hexagons that are averaged is the main factor that explains the lower variability in the more peripheral rings.

Flash electroretinography: normative values with surface skin electrodes and no pupil dilation using a standard stimulation protocol

OBJECTIVES

To demonstrate the clinical feasibility of using surface electrodes for recording flash electroretinography (ERG), using the stimulation standards of the International Society for the Clinical Electrophysiology of Vision (ISCEV), without pupillary dilation.

METHODS

Eighteen physiologically normal volunteers were tested. Surface recording electrodes were placed over the middle third of the lower eyelid of each eye, and the reference electrode placed on the temple. The recording filters and method of stimulation for the standard flash cone, 30 Hz flicker, rod, standard combined and oscillatory responses were those recommended by the ISCEV. Four patients with a history of visual dysfunction were also examined.

RESULTS

The amplitudes of the responses were noted to have a high signal to noise ratio, allowing us to average only a few responses. The waveform configurations were almost identical to those published using other recording methods. Patients with retinitis pigmentosa and early onset retinal dystrophy had abnormal responses, whereas patients with Stargardt's Syndrome and spasm of accommodation had responses within normal limits.

CONCLUSIONS

This study has established the clinical feasibility of performing flash ERG using surface skin electrodes, with no pupillary dilation and using the stimulation standards of the ISCEV.

Simultaneous structural and functional imaging of the macula using combined optical coherence tomography ophthalmoscope and multifocal electroretinogram

Multimodal imaging (MMI) is a new imaging technique that combines an electrophysiological test (multifocal electroretinogram) with an optical coherence tomography ophthalmoscope. MMI allows the two technologies to operate simultaneously yet independently to provide accurate correlation of macular structure and function at multiple sites. Preliminary work with MMI in patients with macular holes showed that functional deficits existed beyond the geographical area of structural alterations and that macular hole size did not always correlate with the severity of functional loss. Functional information may therefore be important when assessing prognosis in these patients. We envisage that MMI may prove a useful new tool in the assessment of many macular pathologies.

The clinical applications of multifocal electroretinography: a systematic review

Multifocal electroretinography (mfERG) is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG therefore allows topographic mapping of retinal function in the central 40-50 degrees of the retina. The strength of mfERG lies in its ability to provide objective assessment of the central retinal function at different retinal areas within a short duration of time. Since the introduction of mfERG in 1992, mfERG has been applied in a large variety of clinical settings. This article reviews the clinical applications of mfERG based on the currently available evidence. mfERG has been found to be useful in the assessment of localized retinal dysfunction caused by various acquired or hereditary retinal disorders. The use of mfERG also enabled clinicians to objectively monitor the treatment outcomes as the changes in visual functions might not be reflected by subjective methods of assessment. By changing the stimulus, recording, and analysis parameters, investigation of specific retinal electrophysiological components can be performed topographically. Further developments and consolidations of these parameters will likely broaden the use of mfERG in the clinical setting.

The rod-mediated multifocal electroretinogram in aging and in early age-related maculopathy

PURPOSE

To measure function with the rod-mediated multifocal electroretinogram (mfERG) in younger and older subjects with normal vision and with early age-related maculopathy (ARM).

METHODS

Thirty subjects were studied: 10 healthy subjects with a mean age of 31 years (young group), 10 healthy subjects with a mean age of 71 years (old group), and 10 early ARM subjects with a mean age of 71 years (early ARM group). The influence of cataract was approximated by retesting five subjects of the young group through an 0.3 neutral density filter (ND filter group). We analyzed first-order N1P1-amplitude and P1-implicit time (P1-IT) mfERG responses and correlated them with funduscopic changes as defined by the Age-Related Eye Disease Study (AREDS) group.

RESULTS

Averaged concentric ring P1-ITs were significantly delayed in the old (p = 0.02) and early ARM (p < 0.001) compared with the young group and in the early ARM group compared with the old and ND group (p < or = 0.01). There were no significant differences in N1P1-amplitudes between groups, but there was a significant location effect for all groups with highest mean amplitudes for the most peripheral ring of hexagons (p < 0.01). Significantly delayed overall P1-ITs (p < 0.05) were correlated with progressive funduscopic changes.

CONCLUSIONS

Aging and early ARM affects the rod-mediated mfERG, and there is good correlation with funduscopic changes. Although a lens effect cannot be excluded, a neuronal transmission alteration at the postreceptoral level is suggested.

Effects of unsteady fixation on multifocal electroretinogram (mfERG)

PURPOSE

To investigate the effect of unsteady fixation on the multifocal electroretinogram (mfERG) measurement in normal subjects.

METHODS

The mfERGs of 20 subjects with normal vision (mean age=23.5 years) were recorded with different levels of voluntary eye movements made to mimic unsteady fixation. Subjects were required to move their fixation regularly every 2 s between the center and the ends of a fixation cross, so that 51.2% of the time fixation was at the center and 12.2% of the time it was at each end of the fixation cross. Four different conditions were performed: central fixation (without voluntary eye movements) and with 2 degrees, 4 degrees and 6 degrees magnitude of unsteady fixation. First-order kernel mfERG findings are presented.

RESULTS

Analysis of the ring responses indicated that the central mfERG amplitude was most affected by unsteady fixation. There was significantly reduced amplitude for 4 degrees unsteady fixation and as expected, this reduction became larger with 6 degrees unstable fixation. However, there was no significant effect on the center hexagon amplitude for 2 degrees unsteady fixation. The amplitudes of the ring-2 responses were only affected in the 6 degrees unsteady fixation condition. No significant change in implicit time was found for any level of unsteady fixation.

CONCLUSION

These results suggest that mfERG amplitude is not substantially affected if fixation is maintained within the central stimulus hexagon. We conclude that, for patients with poor fixation, the accuracy of mfERG results may be difficult to interpret and the use of a fixation-monitoring system is desirable for ideal measurement. The depth of depression at the blind spot area may be another useful parameter to interpret the accuracy of mfERG results in patients with poor fixation.

Effects of reference electrode location on monopolar-derived multifocal electroretinograms in cynomolgus monkeys

The purpose of this study is to determine the effect of reference electrode location on the multifocal electroretinographic waveform. Multifocal electroretinograms (mfERGs) were recorded from 20 ocularly normal cynomolgus monkeys. The corneal electrode was an ERG-jet referenced to an ipsilaterally (outer canthus) situated subdermal needle electrode and to the contralateral corneal electrode. Testing was monocular and recordings from both montages were obtained simultaneously. The stimulus array consisted of 103 equal-sized hexagonal elements, which subtended +/-44 degrees about the central visual axis. Mean luminance of the display was 100 cd/m2. First-order (K1) and second-order (first slice) kernels (K2.1) of the mfERG were grouped in (a) 4 rings, representing the central 56 degrees of visual field and (b) in 15-element quadrants. The mfERG waveform measures included amplitude, implicit time, and root mean square (RMS) of the oscillatory potentials (OP) and response waveform. K1 and K2.1 ring and quadrant amplitudes were larger with the contralateral than with the ipsilateral reference, but more notably signal-to-noise ratios (S:N) of the response waveform were always larger with the ipsilateral reference. Implicit times were longer for the contralateral than ipsilateral reference montage. K1 and K2.1 implicit times in males were longer than in females. Quadrant groupings revealed generally larger K1 and K2.1 amplitudes in nasal than in temporal retina.

Aging and mfERG topography

AIM

To study the effect of aging retina on the multifocal electroretinogram (mfERG).

METHODS

A total of 18 young subjects (age 18-24 years) and 36 elderly subjects (aged 60-85 years) with intraocular lenses (IOLs) were recruited for this study. No subjects had significant eye diseases or media opacities. mfERG was measured in standard conditions using the VERIS system (version 4.1). There were three groups of 18 subjects: (1) 18-25 years, (2) 60-70 years, and (3) 75-85 years. mfERG responses were grouped into central, paracentral, and peripheral regions for analysis. The N1 amplitude, P1 amplitude, N1 latency, and P1 latency of the first-order responses were analysed.

RESULTS

Age had no effect on P1 latency, N1 amplitude, and P1 amplitude; however, N1 latencies from central to peripheral regions were significantly longer for group 3 than for group 1.

CONCLUSIONS

This study suggests that measured age-related decreases in mfERG responses are due to optical factors (decrease in retinal light levels, scatter) before the age of 70 years, but neural factors significantly affect mfERG topography after the age of 70 years.

Cone- and rod-mediated multifocal electroretinogram in early age-related maculopathy

PURPOSE

To investigate the cone- and rod-mediated multifocal electroretinograms (mfERG) in early age-related maculopathy (early ARM).

METHODS AND SUBJECTS

We investigated the cone- and rod-mediated mfERG in 17 eyes of 17 subjects with early ARM and 16 eyes of 16 age-matched control subjects with normal fundi. All subjects had a visual acuity of 6/12 or better. We divided the ARM subjects into two groups based on drusen size and retinal pigment epithelium abnormalities-a less advanced (ARM1) and a more advanced (ARM2) group. The mfERG data were compared to templates derived from the control group. We analysed the mfERG results for the central and peripheral fields (CP method) and the superior and inferior fields (SI method).

RESULTS

While the mean cone results showed no statistically significant difference between the groups, the rods showed significantly delayed responses in the ARM1 group for the CP and the SI methods, but not in the ARM2 group, although there was a trend of longer latencies compared to the control group.

CONCLUSION

Our results show a functional impairment of the rods in early ARM subjects. As there is histopathological evidence showing earlier rod than cone impairment in early ARM, following the rod function with the mfERG might be helpful in diagnosis or for monitoring the progression of early ARM.

The effects of pupil size on the multifocal electroretinogram

This is an investigation of the effect of changing the pupil diameter on the P1 amplitude and latency of the multifocal electroretinogram (mfERG). MfERGs were recorded using a custom built wide field electrophysiological system. An array of 61 empirically scaled hexagons was used to stimulate the visual field. The duration of overall recording period was 8 min, segmented into 16 intervals each lasting 30 s. A combination of mydriatics and miotics were used to pharmacologically alter the pupil size and diameters between 1 and 10 mm were measured. There was a reduction in mfERG P1 amplitude in some cases greater than 50% (mfERG P1 amplitude 53 nV at 8 mm to 25 nV at 1 mm), with a change in pupil diameter of 7 mm. The mfERG P1 latency increased in some cases by as much as 8 ms in the central 40 degrees (mfERG P1 latency 39 ms at 8 mm to 47 ms at 1 mm). These results suggest that pupil size has significant effects on mfERG P1 amplitude and latency.

Towards optimal filtering of "standard" multifocal electroretinogram (mfERG) recordings: findings in normal and diabetic subjects

AIMS

To study the effects of two commonly used pre-amplifier filtering bandwidths on normal multifocal electroretinogram (mfERG) responses and their comparative abilities to detect retinal disease.

METHODS

103 standard mfERGs were recorded simultaneously in two channels with different pre-amplifier settings (10-100 Hz and 10-300 Hz) from one eye of each of 20 normal subjects, 17 diabetics with non-proliferative diabetic retinopathy (NPDR), and 12 diabetics without retinopathy. Signal to noise ratios (SNR) of the normal subjects' first order mfERGs were compared between channels. All subjects' amplitudes and implicit times were derived using a "template stretching" method. For comparison, implicit time was also measured using a "template sliding" method. mfERG amplitudes and implicit times were compared between the channels and among subject groups.

RESULTS

Normal mean amplitudes and implicit times were similar for the two channels. However, normal 10-100 Hz recordings had significantly higher SNR and lower intersubject variability than 10-300 Hz recordings. In NPDR, the 10-100 Hz channel identified significantly more implicit time and amplitude abnormalities. In the diabetics without retinopathy, 10-100 Hz filtering identified significantly more implicit time abnormalities than 10-300 Hz filtering. For both filter settings, diabetic implicit times were more often abnormal than amplitudes. The 10-100 Hz channel was superior for both implicit time measurements.

CONCLUSION

Standard mfERGs recorded from normal eyes and filtered 10-100 Hz contain less noise, higher SNR, and less intersubject variability than those filtered at 10-300 Hz. This underlies the finding that the 10-100 Hz filter setting identifies more retinal dysfunction than the 10-300 Hz setting.

MfERG waveform characteristics in the RS1h mouse model featuring a 'negative' ERG

Several retinal disorders lead to a relatively greater attenuation of the b-wave compared to the a-wave of the electroretinogram (ERG), a constellation called 'negative' ERG. To determine the waveform characteristics of multifocal ERGs (mfERGs) and their dependence on recording parameters in such a case, we studied the Rs1h(-/Y) mouse, the model for x-linked juvenile retinoschisis. mfERGs were recorded with a VERIS 4 system connected to a piggyback stimulator prototype that added the stimulus to the optical pathway of a HRA scanning-laser ophthalmoscope (SLO) by means of a wavelength-sensitive mirror. Real-time fundus visualization was achieved with the infrared laser of the SLO (835 nm). High-pass filter settings and the time interval used by the 'artefact removal' feature were varied to study their influence on the waveform. The mfERG in the Rs1h(-/Y) mouse had a 'negative' shape. However, the high-pass filter setting had to be lowered from the usual 10 Hz down to about 2 Hz in order to obtain that result, otherwise the negative shape was lost and mainly a positive peak remained. Similarly, a short time interval used by the 'artefact removal' feature also removed the negative shape. The Rs1h(-/Y) mouse was found to be a valuable model of diseases with a 'negative' waveform shape also in mfERG. Our results underline the importance of a lower high-pass filter cutoff frequency when recording mfERGs in such disorders. In addition, if the 'artefact removal' feature is used, it should be verified that it doesn't distort the waveform shape.

Topographic mapping of retinal function with the SLO-mfERG under simultaneous control of fixation in Best's disease

PURPOSE

To introduce the scanning laser ophthalmoscope-evoked mfERG (SLO-mfERG) as a new method to measure focal retinal function.

METHODS

Sixty-two healthy individuals and 12 patients with Best's disease were examined. mfERGs were recorded using a scanning laser ophthalmoscope as a stimulator and trigger device (He-Neon 632.8 nm) as well as a fundus-monitoring system (Infrared 730 nm).

RESULTS

Amplitudes in the central concentric area were found to be significantly lower in patients with Best's disease than in healthy controls, while no significant differences were found for the more peripheral areas.

CONCLUSION

SLO-mfERG is a reliable new technique for topographic mapping of retinal function under simultaneous control of fixation.

Preoperative and postoperative assessment by multifocal electroretinography in the management of optic disc pits with serous macular detachment

PURPOSE

To evaluate the macular function by means of multifocal electroretinogram (MF ERG) in eyes with congenital optic disc pit and serous macular detachment. The evaluation was performed before and after the successful surgical intervention.

DESIGN

Nonrandomized, comparative (self-controlled), interventional trial.

PARTICIPANTS

Ten patients (10 eyes) with optic disc pit with serous macular detachment were treated with the macular buckling procedure and followed up from March 1999 through May 2001.

METHODS

In all patients included in the study, MF ERG was recorded before and after treatment. For recording, the VERIS III system (Visual Evoked Response Imaging System; Tomey, Nagoya, Japan) was used.

MAIN OUTCOME MEASURES

The retinal response densities were studied before and after treatment in areas 1 and 2 and were compared with visual acuity.

RESULTS

The MF ERG before treatment showed decreased retinal response densities in all 10 cases. Twelve months after the surgical intervention, the electrical response densities in areas 1 (foveal area) and 2 (parafoveal area) improved. Further improvement was noted in 3 of the 10 patients who completed a follow-up of 18 months after treatment. In 8 of the 10 patients, postoperative visual acuity also increased. In the remaining two patients, visual acuity did not change after treatment, despite improvement of the retinal response densities.

CONCLUSIONS

In eyes with congenital optic disc pit with serous macular detachment, the decrease in retinal electrophysiologic response before treatment was not limited to the fovea, but also involved the perifoveal area. In all 10 eyes 12 months after treatment, the electrical activities in areas 1 and 2 improved. This improvement was not always followed by an increase in visual acuity. In 2 of the 10 patients, visual acuity remained unchanged. The values of retinal response densities before treatment cannot be used alone as a prognostic factor for the postoperative functional retinal results. It should also be stressed that the results of this study cannot be compared with the results of studies where spontaneous resolution of the macular detachment occurred.

Spectral characteristics of light sources for S-cone stimulation

PURPOSE

Electrophysiological investigations of the short-wavelength sensitive pathway of the human eye require the use of a suitable light source as a S-cone stimulator. Different light sources with their spectral distribution properties were investigated and compared with the ideal S-cone stimulator.

METHODS

First, the theoretical background of the calculation of relative cone energy absorption from the spectral distribution function of the light source is summarized. From the results of the calculation, the photometric properties of the ideal S-cone stimulator will be derived. The calculation procedure was applied to virtual light sources (computer generated spectral distribution functions with different medium wavelengths and spectrum widths) and to real light sources (blue and green light emitting diodes, blue phosphor of CRT-monitor, multimedia projector, LCD monitor and notebook display). The calculated relative cone absorbencies are compared to the conditions of an ideal S-cone stimulator.

RESULTS

Monochromatic light sources with wavelengths of less than 456 nm are close to the conditions of an ideal S-cone stimulator. Spectrum widths up to 21 nm do not affect the S-cone activation significantly (S-cone activation change < 0.2%). Blue light emitting diodes with peak wavelength at 448 nm and spectrum bandwidth of 25 nm are very useful for S-cone stimulation (S-cone activation approximately 95%). A suitable display for S-cone stimulation is the Trinitron computer monitor (S-cone activation approximately 87%). The multimedia projector has a S-cone activation up to 91%, but their spectral distribution properties depends on the selected intensity. LCD monitor and notebook displays have a lower S-cone activation (< or = 74%).

CONCLUSION

Carefully selecting the blue light source for S-cone stimulation can reduce the unwanted L-and M-cone activation down to 4% for M-cones and 1.5% for L-cones.

Scanning laser ophthalmoscope-evoked multifocal-ERG (SLO-m-ERG) by using short m-sequences

PURPOSE

To demonstrate the possibility of topographic mapping of retinal function under simultaneous control of fixation in humans, by scanning laser ophthalmoscope evoked multifocal electroretinography (SLO-m-ERG).

METHODS

A confocal scanning laser ophthalmoscope was used as a stimulator and trigger unit to take m-ERGs. Short m-sequences based on a modified algorithm were used, with the advantage that each measurement cycle can be evaluated separately. We examined 78 normal subjects; in 62 a distortion factor of 1:1 was applied, and a factor of 1:4 in 16.

RESULTS

The recorded amplitudes decreased with eccentricity, approximately following the decrease of retinal cone density. Amplitudes were higher in the central hexagonal element in the group with 1:4 distortion than in the group with the 1:1 distortion setting.

CONCLUSIONS

SLO-m-ERG is a reliable technique for topographic mapping of retinal function under simultaneous control of fixation.

Aging-related changes in the multifocal electroretinogram

The multifocal electroretinogram (MERG) was recorded in the central 36 degrees-diameter field in 26 young (19-30 yr) and 20 old (60-74 yr) adults in normal retinal health according to a fundus grading scale. The mean amplitude densities of the first-order and second-order responses in this retinal region were lower for old adults compared with young. The aging-related reduction of the first-order amplitude density was greatest at the fovea and decreased as a function of eccentricity. Similarly, the amplitudes of the first negative and positive peaks of the first-order MERG waveform were reduced with age, and the pattern of reduction followed a similar eccentricity dependency. The aging-related changes in the MERG waveform may be due to slowed temporal adaptation in the aged retina.

The impact of fixation on the multifocal electroretinogram

There are a number of variables which can influence the quality of multifocal ERG waveforms. In common with visual field measurements, fixation quality may be an important parameter on the integrity of the acquired data. A low cost, fixation-monitoring device was used to assess fixation quality on a group of normal volunteers. Data was successfully acquired while five subjects viewed a fixation target for a period of time equal to that of a single multifocal recording segment. The target was presented on a stationary grey background and as the central fixation mark on a 61-element multifocal flicker stimulus. The results show no significant difference with or without the flickering pattern. The percentage of samples falling within 1.2 degrees of the point of fixation was 51%. This suggests that fixation quality is adequate for scaled stimuli where the central element subtends 2.4 degrees. High resolution stimuli of less than 2.4 degrees may be more susceptible to fixation fluctuations during the recording process.

Assessing retinal function with the multifocal technique

With the multifocal technique, as developed by Erich Sutter and colleagues, scores of focal electroretinogram (ERG) responses can be obtained in a matter of minutes. Although this technique is relatively new, it has already provided insights into the mechanisms of retinal disease. However, because it is new, there also remain questions about how it works and what it measures. This chapter considers some of these insights and some of these questions. The first part (Section 2) describes how the multifocal ERG (mERG) is recorded and considers its relationship to the full-field ERG. The mERG responses are shown to be from relatively local regions of the retina and are comprised of the same components as the full-field ERG. The diagnostic advantage of the mERG as compared to the full-field ERG is also illustrated. In Section 3, the effects of damage to different cell layers of the retina are shown to affect the mERG differently, and these changes are summarized within a conceptual framework. It is argued, for example, that when diseases of the receptor outer segment, like retinitis pigmentosa, result in small, depressed mERG responses, then the damage is, as expected, at the outer segment. However, when these diseases result in mERG responses that are reasonably large but very delayed, then the damage is beyond the outer segment, probably in the outer plexiform layer. The implicit time of the mERG, not amplitude, is the more sensitive measure of damage in degenerative diseases of the receptors. On the other hand, diseases, like glaucoma, which act on the ganglion axon, do not result in easily identified changes to the mERG unless inner retinal damage is involved as well. Inner retinal damage changes the waveform of the mERG and decreases the naso-temporal variation normally observed. Finally, diseases, like diabetes, that act on more than one layer of the retina can have a range of effects. In Section 4, recent work with the monkey mERG is reviewed, with emphasis on the relevance to human diseases. For example, blocking the sodium-based action potentials produced by ganglion and amacrine cells eliminates the naso-temporal variation in the monkey mERG and these altered mERG responses resemble those from some patients with diabetes or glaucoma. Finally, in Section 5 the second-order kernel is described. The presence of a second-order kernel has important implications for understanding the shape of the mERG response (first-order kernel). Full-field simulations of the mERG paradigm illustrate that the first-order kernel is comprised of responses with different waveforms. Further, it is argued that the nonlinear, adaptive mechanisms that produce the second-order kernel are involved in shaping the time course of the response. Patients with large, but abnormally delayed mERG responses (first-order kernel), do not have a detectable second-order kernel. It is speculated that a markedly diminished second-order kernel is diagnostic of outer plexiform layer damage, not inner plexiform layer damage as is commonly assumed.