Effects of experimental glaucoma in macaques on the multifocal ERG

Multifocal ERGs (MERGs) of 5 adult monkeys (Macaca mulatta) with inner retinal defects caused by laser-induced glaucoma were compared to MERGs from 3 monkeys with inner retinal activity suppressed pharmacologically. MERGs were recorded with DTL fiber electrodes from anesthetized monkeys. Stimuli consisted of 103 equal size hexagons within 17° of the fovea. Stimuli at each location passed through a typical VERIS m-sequence of white (200 cd/m²) and black (12 cd/m²) presentations. In animals with laser-induced glaucoma, visual field sensitivity was assessed by static perimetry using the Humphrey C24-2 full-threshold program modified for animal behavior. Inner retinal (amacrine and ganglion cell) activity was suppressed by intravitreal injection of TTX (4.7–7.6 μM) and NMDA (1.6–5 mM). In normal eyes the first order response (1st order kernel) was larger and more complex, with more distinct oscillations (>60 Hz) in central than in peripheral locations. The 2nd order kernel also was dominated by oscillatory activity. There were naso-temporal variations in both kernels. Pharmacological suppression of inner retinal activity reduced or eliminated the oscillatory behavior, and naso-temporal variations. The 1st order kernel amplitude was increased most and was largest at the fovea. Removed inner retinal responses also were largest at the fovea. The 2nd order kernel was greatly reduced at all locations. In eyes with advanced glaucoma, the effects were similar to those produced by suppressing inner retinal activity, but the later portion of the 1st order kernel waveform was different, lacking a dip after the large positive wave. Visual sensitivity losses and MERG changes both increased over the timecourse of glaucoma, with changes in the MERG being more diffusely distributed across the visual field. We conclude that 1st and 2nd order responses of the primate MERG can be identified that originate from inner retina and are sensitive indicators of glaucomatous neuropathy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detection of retinal dysfunction in vitelliform macular dystrophy using the multifocal ERG (MF-ERG)

Vitelliform macular dystrophy (VMD) is widely known for an abnormal EOG in the presence of a normal ERG. In this study the multifocal electroretinogram (MF-ERG) is described as an additional tool to detect retinal dysfunction in VMD. Three patients aged 30, 37 and 59 years with VMD and a visual acuity of OD: 0.4; OS 0.05 (patient 1), 1.25 OU (patient 2) and OU: 0.6 (patient 3) underwent additional electrophysiological testing with the MF-ERG. A multifocal-ERG of the central 50 degrees of the retina was obtained using the VERIS -system. During recording 103 hexagons flickered according to a binary m-sequence of 2¹⁵. Mean luminance was 100 cd/m², contrast was set at 99%. The MF-ERG recordings were compared to age matched control groups. In all three patients the MF-ERG of the central 6 degrees showed reduced amplitudes for N1P1 (first negative peak to first positive peak) and for P1N2 (P1 to the second negative peak). Implicit times were not affected. Therefore the MF-ERG can detect focal retinal dysfunction in VMD which would not be apparent in the summed retinal response recorded with the ganzfeld ERG. In contrast to other diseases, amplitudes rather than implicit times seem to be affected in the MF-ERG of vitelliform macular dystrophy.     

Technical aspects of multifocal ERG recording

There are a wide range of variables which can influence the quality of the multifocal response. It is possible to place these variables into one of four categories. First, the method of stimulus delivery will determine the field of view, interference levels and the duration of on-state stimulation. Second, data acquisition variables such as electrode type and placement, amplifier specifications and filter bandwidth settings will have a direct impact on waveform shape and on the topographic distribution of signal amplitudes. Third, patient variables such as fixation, pupil dilation and refractive error will also contribute to the multifocal response. Fourth, there are many measurements that can be taken from multifocal recordings. In addition to standard amplitude and implicit time measures (the implicit time measure in the multifocal response is becoming increasingly important particularly in early stages of disease processes), the scalar product measure provides information on waveform shape. The conventional impulse and higher order responses will be different for different modes of stimulation such as Cathode Ray Tube (CRT) and Liquid Crystal Display (LCD) systems and latency shifts will be introduced if not corrected in software. Procedures which could lead to misleading interpretation include artefact rejection, averaging with neighbours and summing of responses. These procedures should be handled with caution. This revised version was published online in July 2006 with corrections to the Cover Date.     

A comparison of two multifocal ERG systems

The First Order Kernel responses (FOK) from a group of 15 normal subjects aged 21–36 yrs were recorded under clinical conditions with the multifocal ERG (MERG) systems VERIS and RETiscan using system-specific software and hardware under various parameter settings. First, the multifocal ERG's of normal subjects were recorded with the standard-parameters of VERIS and RETIscan. Thereafter the VERIS system parameters were set as close as possible to the RETIscan-setup, and the recordings were repeated. The VERIS parameters changed were the luminance- and contrast-settings of the stimulus-monitor and m-sequence-timing. In addition, the effects of different sampling rates were also examined. The consequences of the parameter changes were analyzed by displaying the peak times of the First-Order-Kernel (P1). The parameter-combinations used for recording lead to system-specific results. By varying the luminance/contrast settings and/or the m-sequence-timing the results can be changed. The amplitude of the recorded results can be influenced by the luminance-settings of the stimulus-monitor, and the topography of the P1 peak times is affected by the m-/f-frame-combination of the recording stimulus. With their standard parameters, the two systems give different measures of the P1 peak times. By adjusting the recording parameters, these differences can be reduced significantly. The main parameter here is the m-sequence-timing, although the sampling rate must also be taken into account with regard to the accuracy of the recording results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pattern ERG and glaucomatous visual field defects

In the past five years numerous reports have suggested that ganglion cell function can be tested by means of a specialized form of electroretinography, the socalled pattern electroretinogram (PERG). Because of the important potentials of a ganglion cell test for clinical use this technique has been applied by several investigators to patients with (presumed) ganglion cell dysfunction, especially glaucoma. On grounds of principle we had reason to question whether the reported positive results should be attributed to ganglion cell dysfunction or to other factors such as optical disturbances. We investigated in this study the PERG as a function of visual field loss in glaucoma patients with careful control of optical factors. We did not find changes in PERG as a function of field loss. So either field loss is not related to the mass behaviour of ganglion cells, or ganglion cells are not the prime basis of the PERG. We believe the latter to be true.     

Recognition of small stimulus screen masks using the multifocal ERG

To evaluate the ability of the multifocal ERG (mfERG) to detect small defects in the stimulus array was the objective of this paper. Seven normal subjects had mfERGs recorded with a VERIS system. Stimulus arrays composed of 61, 103 or 241 hexagons were covered in part by small masks of different light transmittance properties. Only masks that covered at least one-half of a single 103 hexagon stimulus cell caused a significant reduction in signal. Different-shaped masks of about 5° diameter were detectable using a 61-hexagon array only when they fully covered a stimulus cell. Detection was better, but marginal for some of the masks, with the 103 hexagon array. The 241 hexagon array showed sharp defects for all masks. Masking the stimulus screen is not equivalent to having a pathologic scotoma, but it demonstrates the greatest possible spatial sensitivity of the mfERG system. Thus, the mfERG appears to be able to detect small retinal lesions if they reduce local retinal function by at least 50% and correspond to at least half the area of one stimulus hexagon. Scotomas 5° or smaller would be best detected using a fine (241 hexagon) stimulus array. With coarser stimulus arrays (e.g. 103 or 61 hexagons), the effect of a small scotoma depends on its location relative to the stimulus cells. These issues should be considered when selecting mfERG recording conditions.     

Effect of optical defocus on multifocal ERG responses

Background: Multifocal electroretinography (mfERG) is a sensitive technique to probe retinal function topographically. Various conditions such as rnacular degeneration decrease the first‐order kernel (K1) response magnitude of mfERG. Previous studies have given inconsistent results on the effect of optical defocus due to poor controls. This study investigated the effect of optical defocus on the K1 response of the central retina using a wellcontrolled method. Methods: Twenty subjects were recruited to undergo mfERG measurement using the VERIS 4.0 system. A four millimetre artificial pupil was fitted before each fullydilated right eye, optically corrected for the viewing distance. The implicit times and response amplitudes of nl (first negative trough) and pl (first positive peak) under three different optical defocus conditions (zero dioptres, +1.00 D and +3.00 D) were measured. Results: The implicit times of nl and pl did not demonstrate any significant variation from the central macula to para‐macula under different optical defocus conditions. The response amplitude of nl did not show any changes from the central macula to para‐macula but the response amplitude of the central macular pl showed a significant reduction by 12 per cent under +1.00 D defocus and +3.00 D defocus. Conclusions: Optical defocus causes mild reduction in mERG at the central macula but there are no significant changes in the periphery. A full optical correction is recommended for mERG measurement to minimise the reduction of the macular response due to optical defocus.     

A comparison of CRT and digital stimulus delivery methods in the multifocal ERG

The purpose of this paper is to compare and evaluate the multifocal ERG response from raster based CRT and Digital Projection (LCD) stimulus delivery systems. A custom built p.c. based multifocal system was used to generate a 61 hexagonal element stimulus array. The stimulus was presented on a high luminance CRT display and on a back projected screen using a Digital polysilicon projection system. A fast response photodiode was used to analyse the stimulus pulse characteristics of both systems. A number of recordings were performed to assess the effect of stimulus delivery on a standard m-sequence response, inserted full-field filler response and on separation of onset and offset components. The pulse width for a CRT system is dependant on the type of phosphor and is typically 2 msec whereas the Digital Projection system produces a 13.3 msec pulse equivalent to the frame rate for the system. Slowing down the m-sequence by a factor of eight results in a pulse width of 106 msec which should enable the recovery of true offset responses. The CRT stimulus consists of a series of eight pulses of 2 msec duration each separated by 11.3 msec. First order responses are larger from the CRT system and second order responses larger from the Digital system. In conclusion, there are fundamental differences in the two delivery systems. The CRT system may have more potential in examining non-linear aspects of the multifocal response. Although both systems may be able to record offset responses, the Digital system will generate true offset responses whereas the CRT system may not allow true separation of these components.     

Effects of pre-adaptation conditions and ambient room lighting on the multifocal ERG

The purpose of the study was to evaluate the effects of pre-adaptation and ambient room luminance on the multifocal ERG (mfERG). We recorded mfERGs on 18 normal subjects (average age 32) using a VERIS system, with either 61 or 103 stimulus hexagons. mfERGs were recorded sequentially under different conditions of pre-adaptation and room lighting. Changing pre-adaptation conditions between darkness for 20 min, or light at 1.43 log cd/m² for 10 min, had essentially no effect on the mfERG, regardless of ambient room lighting. However, mfERG parameters were sensitive to the level of ambient room lighting during the recordings. As room luminance was increased from darkness, there was a gradual attenuation of N1 and P1 amplitudes both centrally and peripherally that approached 25% reduction at 1.6 log cd/m², and a decrease in P1 time-to-peak. These effects were greatest in the blind spot. The mfERG is largely independent of pre-adaptation conditions, but waveform amplitudes and times-to-peak diminish with increasing ambient room luminance. The exaggerated attenuation of signals in the blind spot with room lighting suggests that mfERGs recorded in the dark are contaminated by light scattered to dark-adapted peripheral retina. The most stable mfERG recording condition appears to be a fully lighted room (1.6 log cd/m²).     

Continuous monitoring of the stimulated area in multifocal ERG

Introduction: Multifocal electroretinography (MF-ERG) is widely used in the detection of local retinal dysfunction. However, the position of the stimulus on the retina and the stability of fixation are usually not directly accessible. Thus, devices have been designed for a continuous fundus visualization during recording. Methods: MF-ERGs were recorded with a RetiScan^TM system connected to two different Scanning-laser ophthalmoscopes (SLOs) that use either a red (633 nm) or green (415 nm) laser for stimulation, and a VERIS^TM 4 system connected to a piggyback stimulator prototype that added the stimulus to the optical pathway of the SLO by means of a wavelength-sensitive mirror. Fundus visualization was achieved with the infrared lasers of the SLOs (780 and 835 nm). Results: The most extensive study so far with a green laser stimulus in a cat model of retinal degeneration demonstrated the capability of the device to detect retinal landmarks and the different stages of degeneration. Also, the advantages of exactly reproducible stimulus positioning for averaging within and comparison between disease groups became apparent. The results with the same setup in transgenic mice suggest a pure cone origin of the responses. In humans, recordings show that fixation is sufficiently good in most subjects. It is not clear yet whether red or green laser stimulation (or both) is preferable. The results with the prototype were very similar to the MF-ERGs obtained with a standard CRT screen. Conclusions: All three devices allowed us to record MF-ERGs with continuous fundus monitoring. Although further refinements are necessary, it is obvious that fundus controlled methods will improve the reliability of MF-ERG in future research on glaucoma, transplantation studies, and evaluation of gene therapy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Slow-stimulated Multifocal ERG in High- and Normal-tension Glaucoma

Purpose: To study the ability and sensitivity of the slow stimulation multifocal ERG (mfERG) to detect glaucomatous damage. Methods: Right eyes of 20 patients with normal-tension glaucoma (NTG), 15 patients with high-tension glaucoma (HTG) and 15 healthy volunteers underwent testing with the mfERG (VERIS 4.1^TM). The central 50 degrees of the retina were stimulated by 103 hexagons (m-sequence: 2¹³-1, Lmax: 100 cd/m², Lmin: 1 cd/m², background: 50 cd/m²). Each m-sequence step was followed by 3 black frames (Lmax: < 1 cd/m²). Five response averages of the first order response component (KI) were analyzed: the central 7.5 degrees and the 4 adjoining quadrants. The amplitudes from the first minimum, N1, to the first maximum, P1, and from P1 to the second minimum, N2, were analyzed as well as the latencies of N1, P1, N2 and the latencies of 3 multifocal oscillatory potentials (mfOPs) with their maxima at about 73, 80 and 85 ms. Results: For each parameter the percentage of deviation from the mean of the control group was calculated. These values were then added for each individual to form a deviation index (DI). Seventeen patients (85.0%) with NTG and 3 patients (20.0%) with HTG showed a DI outside the normal range. The major changes were observed in the mfOPs of the NTG patients. MfOPs were then selectively filtered at 100–300 Hz and their scalar product was analyzed over an epoch of 68–105 ms. This confirmed that mfOPs differed significantly from the control in the central 7.5° and, for NTG, in the nasal field. With a logistic regression analysis the mfOPs had a sensitivity to differentiate 85% of the NTG patients and 73% of the HTG patients from normal. Conclusions: Under these conditions, the slow-stimulated mfERG can detect glaucomatous dysfunction in NTG (85.0%). The differences observed between NTG and HTG are in support of a different underlying pathomechanism.     

Effects of APB,PDA, and TTX on ERG responses recorded using both multifocal and conventional methods in monkey. Effects of APB,PDA, and TTX on monkey ERG responses

Results from studies of human subjects suggest that the multifocal ERG technique developed by Erich Sutter and colleagues has considerable potential for assessment of retinal function in both the clinic and laboratory. While the utility of this measure depends to a large extent upon an understanding of the physiological origin for the different response components, relatively little is known in this regard. For the experiments described in this report, we made ERG recordings using both multifocal and conventional methods. Intravitreal injections of APB, PDA, and TTX were used to identify contributions from activity in ON pathway, OFF pathway, and third order retinal neurons, respectively. The results show that photoreceptor activity makes a small direct contribution to 1st and 2nd order multifocal photopic luminance responses. TTX-sensitive activity in third order retinal neurons contributes to both 1st and 2nd order responses with relatively greater contribution to the 2nd order response. Blockade of TTX-sensitive activity in third order cells produces effects on the 2nd order response which are very similar to changes observed in eyes suffering selective loss of retinal ganglion cells resulting from experimental glaucoma. Effects of these intravitreally injected test agents were also determined, in the same recording session, for flash, 30 Hz flicker, and oscillatory potential responses.     

Assessing hydroxychloroquine toxicity by the multifocal ERG

Twenty patients on Plaquenil treatment were evaluated for retinal toxicity using the (EOG) and the mfERG. Group 1 comprises 15 patients (30 eyes) with normal EOG. From these patients 11 (22 eyes) showed normal RRD of mfERG in area 1 and area 2. The rest four patients (8 eyes) the RRD were reduced. Six months after interruption of HC, the mfERG improved in three cases. Group 2 comprises 5 patients (10 eyes) with subnormal EOG. Four (8 eyes) of these showed a decrease of RRD of the mfERG in area 1 and 2. In the rest one (2 eyes) the RRD were normal. Six months after interruption of HC the mfERG and the EOG improved in 2 cases. These results postulate that the mfERG may be used as an alternative method, perhaps more sensitive, for the detection of the HC retinopathy and the follow up of the patients on hydroxychloroquine.     

Multifocal ERG in Subjects with a History of Retinopathy of Prematurity

Purpose: Investigate the function of the central retina in subjects with a history of retinopathy of prematurity (ROP). Methods: Multifocal electroretinogram (mfERG) responses to a scaled array of 103 hexagons were recorded in subjects, aged 11–23 years (N = 11), with a documented history of mild ROP. The amplitude and implicit time of the components (N₁, P₁, N₂) of the first order kernel for six concentric rings were compared to those of control subjects (N = 9). Results: The amplitude of each component varied significantly with eccentricity in both ROP and control subjects and was significantly smaller in the ROP subjects. The discrepancy between ROP and control subjects was greatest for central rings (1–3) and smaller for peripheral rings (4–6). The slopes of the functions summarizing log response density as a function of log eccentricity (degrees visual angle) were significantly shallower in ROP subjects. The implicit time of each component was longer in ROP subjects at all eccentricities. Conclusions: ROP associated alterations in neural retinal development may underlie the subtle macular dysfunction disclosed by the mfERG.     

Assessment of local cone on- and off-pathway function using multifocal ERG technique

There is evidence that recording the photopic ON- and OFF-responses with long-duration stimuli is useful for determining the contribution of the cone ON- and OFF-pathways to the primate photopic electroretinogram (ERG). In this study, the optimal conditions for recording multifocal ON-OFF responses are described, and the technique is applied to normal subjects and two patients with unusual retinal diseases. The results from the normal subjects demonstrated that there were topographical variations of the photopic ERG waveform: when responses were normalized to the ON-response (b-wave) amplitude, the OFF-response (d-wave) amplitude increased with increasing eccentricity. The changes in the waveform in two patients suggested relatively greater defects of the hyperpolarizing or depolarizing bipolar cells. We conclude that the multifocal ERG technique with long-duration stimuli can be a useful tool to assess the function of local cone ON- and OFF-pathways in normal and diseased retinas. This revised version was published online in July 2006 with corrections to the Cover Date.     

正常儿童多焦视网膜电图的特征

目的研究正常儿童多焦视网膜电图的波形特征,为相关临床及科研工作提供依据。方法采用VERISScienceTM4.2多焦电生理系统对30例正常儿童双眼分别进行mfERG的一阶反应检查,对不同离心度及不同视网膜区域的波形特征进行分析。结果mfERG反应振幅密度随离心度增加而减小,上下半侧视网膜反应无明显差异,与颞侧视网膜相比,鼻侧视网膜反应振幅密度降低,潜伏期延长。结论mfERG可用于研究不同离心度和不同视网膜区域的反应,为临床应用提供依据。     

Impact of notch filter use on waveforms of First- and Second-Order-Kernel responses from multifocal ERGs

The filter settings of the amplifier section of ERG recording systems have large impact on the waveforms of the recorded responses. In this study, the effects of a 50 Hz notch filter were assessed for both First-Order-Kernel- (FOK) and Second-Order-Kernel-responses (SOK 1st slice) from multifocal ERGs recorded with a VERIS III system. Amplitude and phase responses of the amplifier section were recorded for typical filter settings with the notch filter active and inactive. Multifocal ERGs (MERGs) from a group of 11 normal subjects were recorded for both amplifier settings, the waveforms resulted from those recordings were compared in the time and frequency domain. To verify the results, the recordings without the notch filter were digitally filtered with a simulated notch filter and compared to the responses recorded with an active analog notch filter.The line filter has the biggest attenuation at 50 Hz with an additional phase jump of 180°. The FOK responses of MERGs are assembled by frequencies below 65 Hz with main spectral components between 19 and 47 Hz. The 1st slice of the SOK consists of frequencies up to 100 Hz with main components between 19 and 84 Hz. Thus, if FOK recordings are to be analyzed, the notch filter of the amplifier can be used in order to cope with noise problems caused by the line frequency of 50 Hz. However, one must be aware that the attenuation of higher frequency components will lead to changes of the waveforms. For SOK analysis, main spectral components are attenuated and/or shifted in phase, which leads to completely different waveforms and severe distortion of the recording results.     

An attempt to detect glaucomatous damage to the inner retina with the multifocal ERG

PURPOSE: To detect glaucomatous damage to the inner retina using the multifocal electroretinogram (mERG). METHODS: The stimulus array consisted of 103 hexagons with a mean luminance of 100 cd/m2 and a contrast of 50%. The mERG was recorded from 13 control subjects, 18 patients with open-angle glaucoma (OAG), 4 glaucoma suspects, and one patient with ischemic optic neuropathy (ION). Individual responses, as well as responses summed within quadrants or across the entire array, were measured in a number of ways. Humphrey visual fields were obtained for all patients, and the mean total deviation (MD) values for the 18 patients with OAG ranged from -2.2 to -18.2 with a mean (SD) of -7.3 (4.5). RESULTS: The mERG measure that best discriminated between the patients and the control subjects was the ratio of the amplitude at 8 msec after the peak response to the amplitude at the peak. Although the value of this ratio fell below the median of the control group for 16 of the 18 OAG patients, only 6 of these patients had ratios that fell below the normal range. Other measures of first- and second-order kernels did not do as well. Both within and across patients, the correlation between local field loss and the mERG ratio measure was poor. Furthermore, although in some patients the mERG waveform is clearly different from normal, in other patients (including the patient with ION) the waveform approximates the normal even in visual field areas with substantial sensitivity loss. CONCLUSIONS: Because glaucomatous damage is known to affect the ganglion cell axon, these data suggest that damage to ganglion cell axons is not a sufficient condition to produce changes in the mERG as measured here and that in patients with clear changes in mERG waveforms, these changes do not appear to be well localized and local waveforms are poorly correlated with local changes in field sensitivity.     

The influence of luminance on the multifocal ERG

Purpose To assess the efficacy of high luminance in increasing the amplitude of the multifocal electroretinogram (mfERG). We examined 5 male and 5 female volunteers in the age of 22–52 years (median 28 years). Three different stimulus luminance levels were applied: the bright areas of the stimulus pattern were set to 150, 300 and 500 cd/m². We recorded the potentials via DTL electrodes using the VERIS Science 4.4 system with 61 hexagons, pupils were dilated. Analysis was based on the 5 ring averages.Results Across all hexagons and subjects, the response density (∼amplitude) rose by 20% when increasing the luminance by a factor of 3.3. The peak times decreased slightly with higher luminance, by less then 1.5 ms.Conclusions Combining the present results with those from two previous studies, the gain (= relative amplitude increase for relative luminance increase) is close to 0.4 over a range of 56–700 cd/m². The stimulus luminance range suggested in the mfERG guidelines seems well chosen.     

The interpretation of multifocal binary kernels

The interpretation of binary kernels derived by means of the mulifocal ERG has frequently been the source of confusion and controversy. The aim of this paper is to provide an intuitive understanding of the concept of kernels and to demonstrate with models and examples, the relationship between the kernels and individual responses. The sample data used to illustrate the various aspects of the technique were derived and analyzed by means of the multifocal VERIS^TM instrument using bipolar Burian-Allen contact lens electrodes. From the kernel data, response waveforms are synthesized that can be readily compared with those obtained with conventional flash, double flash and multi-flash ERG techniques. From the kernels of a single multifocal record, waveforms are synthesized that describe responses at various times after a preceding focal flash. It is shown that the observed adaptive changes in the waveforms depend not only on the flash interval but also on retinal eccentricity. Features on the response waveform exhibit independent adaptive behavior suggesting that they originate from different retinal sources. On one hand, the examples presented here illustrate the rich information on retinal dynamics contained in the kernels. On the other hand, they demonstrate how the interpretation of multifocal data can be facilitated when the kernel series is transformed into representations with more direct physiological appeal. This revised version was published online in July 2006 with corrections to the Cover Date.