Intraretinal analysis of the a-wave of the electroretinogram (ERG) in dark-adapted intact cat retina

It has often been assumed that the recovery of the a-wave from its trough is caused by the intrusion of the b-wave. This study examined the recovery following the a-wave trough using intraretinal recordings in dark-adapted intact cat retina. Adult cats were anesthetized and paralyzed. The vitreal ERG was recorded between the vitreous humor and a reference electrode near the eye. Intraretinal recordings were made by referencing a microelectrode to the vitreal electrode. Bright flashes of diffuse white light were used to elicit a- and b-waves. Intravitreal injections of 2-amino-4-phosphonobutyrate (APB), cis 2,3-piperidine dicarboxylic acid (PDA), and kynurenic acid (KYN) were used to block the responses of bipolar and horizontal cells. Intravitreal injections of UL-FS 49 or DK-AH 269 were used to block Ih, a hyperpolarization-activated potassium current. Since the microelectrode was referenced to the vitreal electrode, recordings from the inner retina showed only the oscillatory potentials and b-waves. In the inner retina, the potential was flat until the b-wave became measurable, approximately 17 ms from the onset of the flash. The a-wave started to appear as the microelectrode reached the photoreceptors and its amplitude increased with depth until the microelectrode reached the choroid. The a-wave peaked at approximately 8 ms in response to flashes that saturated its amplitude and then began to recover well before any inner retinal responses were apparent. After injections of APB, PDA, and KYN, vitreal and intraretinal recordings showed only the a-wave, which consisted of an increase to peak at approximately 10 ms followed by a recovery to a plateau which was reached at approximately 25 ms. Blockers of Ih reduced the recovery, but did not eliminate it. The a-wave peaks and partially recovers before the b-wave intrudes. Both phases survive blockers of second-order neurons which implies that the photoreceptors generate both the rising and recovery phase of the a-wave. The recovery phase may be due to a current generated by the inner segment of photoreceptors.

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.

[Changes in the oscillatory potentials in the electroretinogram during the evolution of diabetic retinopathy]

Oscillatory potentials where intensely studied in diabetic patients with various degrees of retinopathy. Classical reports pointed out the early reduction of their amplitude, as well as of their latency, while a and b waves remained still normal. A prognostic value was attached to these modifications, the reduction of oscillatory potentials being correlated with a ten time higher probability of developing a high risk retinopathy. 27 diabetic patients where investigated electroretinographically by us. No significant variation of the amplitude of oscillatory potentials was observed during the progression of diabetic retinopathy.

[Fourier analysis of macular local electroretinogram in retinal detachment]

OBJECTIVE

To understand the sites and physiological characteristics of macular dysfunction in retinal detachment (RD) by evaluating fundamental harmonic (1f) and second harmonic (2f) components of flicker local electroretinogram (LERG).

METHODS

LERG were recorded in 128 eyes of 120 patients to a flicker sinusoidally to 31Hz with a hand-held stimulator with direct visualization of the fundus through dilated pupils.

RESULTS

Compared to controls, patients showed losses of both fundamental and second harmonic amplitudes. The abnormality rates of fundamental amplitude in groups with RD affected time < 1 weeks, 1 approximately weeks, 2 approximately weeks, 4 approximately weeks and > 8 weeks were 15%, 25%, 47%, 76% and 91% respectively; and the abnormality of rates fundamental-second harmonic amplitude ratios (1f/2f) were 5%, 14%, 28%, 64% and 87% respectively. On average, 2f decreased more quikly than 1f and 1f/2f tended to increase with the course of disease. The results show both 1f and 2f descend in the RD eyes with or no macular detachment (P < 0.05). 2f just descend obviously in the 48 macular detachment eyes (P < 0.01), so that 1f/2f ascend in these eyes(P < 0.05). There was a significant relation between amplitudes of LERG in the detached eyes before surgery and best postoperational correct vision in the attached eyes.

CONCLUSION

In the eyes with retinal detachment, both receptoral and postreceptoral sites contribute to macular dysfunction. The level of macular dysfunction in the eyes with or no macular detachment is different. It would be of clinical value to obtain a fundamental-second harmonic ratio in order to assess the disturbance in the different site of macular. In our opinion: macular flicker local electroretinogram is an important indicator of prognosis to the functional recovery of the attached eye after corrective surgery.

A contact lens electrode with built-in high intensity white light-emitting diodes. A contact lens electrode with built-in white LEDs

We determined the clinical usefulness of a new contact lens electrode with built-in, white light-emitting diodes (LEDs) for full-field electroretinograms (ERGs). Three, high-brightness white LEDs were incorporated into a contact lens electrode and served as the source for the stimulus and the background. The stimulus intensity, stimulus duration and background illumination were regulated by a small LED control device. Maximum stimulus and background intensities were 3.9 and 3.6 log cd/m2, respectively. We successfully recorded intensity-response series for scotopic and photopic ERGs. We also recorded a duration-series for photopic ERGs, and ERGs that were comparable to the ISCEV standardized ERGs. The compactness and ease of using this system suggest that it will be clinically useful under different conditions.

[The disposing techniques of evoked potentials]

This paper is to bring forward the new disposing techniques of evoked potentials which include four aspect techniques of the averaging, the recording, digital sampling and filters about the averaging, evoked potential amplitude, evoked potential latency, evoked potential recording, and evoked potential generations. The technique of the averaging including signal filtering and a periodic averaging, can enhance EP dedection. The commercial EP machines also plot changes in latency between serial EP studies in order to detect trends in peak latency. The modern digital EP recording device consists of sensory stimator, recording amplifiers with analog filters, an analog-to-digital converter, a digital signal averager, and a display and storage system. A sample-and-hold function is one of the recent developments which used EP collectors that provide simultaneous recording with multiple channels employing different time and voltage scales and sampling rates. The EP data may be further processed following A-D conversion by digital filters.

Multi-electrode stimulation and recording in the isolated retina

As part of an exploration of the feasibility of an epi-retinal prosthesis, we developed an experimental method to electrically stimulate and record from retinal neurons using a micro-fabricated multi-electrode array. An isolated retina is placed on an array of 10 microm diameter disk electrodes with the ganglion cell side of the retina facing the electrode surfaces. The retina is bathed in oxygenated Ames' medium and warmed in order to sustain it in vitro for the duration of an experiment, typically 4-9 h. To reduce stimulus artifacts, the electrodes are grouped into two clusters - one used for stimulation and the other for recording--spaced several hundred microns apart, and electrodes are insulated with both silicon nitride and a 10 microm thick layer of polyimide. Stimuli are delivered to the array using an optically isolated current source stimulator, and the resulting responses recorded with an eight channel nerve response amplifier. Stimulation and recording are performed under computer control. A variety of physiologic measurements is described in order to illustrate the strengths and drawbacks of this method.

Comparison between Dawson, Trick, and Litzkow electrode and contact lens electrodes used in clinical electroretinography

PURPOSE

To determine the reliability of the Dawson, Trick, and Litzkow (DTL) electrode in electroretinographic recordings (ERG) in subjects of all ages, we evaluated ERG responses obtained with the contact lens (CL) and DTL electrodes and estimated a DTL/CL ratio for each sample.

METHODS

Seventy-nine volunteers aged 2-84 years (79 normal eyes) were recruited for the study. Electroretinographic recordings elicited by monocular single-flash stimuli delivered by an automatic ERG recording system were recorded using both CL and the DTL electrodes (CL-ERG and DTL-ERG, respectively).

RESULTS

The relative amplitude of the DTL-ERG to the CL-ERG was 79.6-99.8% (mean = 93.4%) for the a-wave and 84.4-106.3% (mean = 92.3%) for the b-wave. The relative latency of the DTL-ERG to the CL-ERG was 86. 0-107.6% (mean = 98.2%) for the a-wave and 96.1-113.0% (mean = 97. 9%) for the b-wave. The a- and b-wave amplitudes differed significantly between DTL and CL electrodes only in the 40- to 49-year-old age groups (P <.05). Regression analysis indicated moderate to strong relationships between the electrodes for amplitude (a-wave, r = 0.690; b-wave, r = 0.824) and latency (a-wave, r = 0.667; b-wave, r = 0.725).

CONCLUSION

The DTL electrode has as high stability as the conventional CL electrodes for ERG recordings in most age groups.

Standard for pattern electroretinography. International Society for Clinical Electrophysiology of Vision

The pattern electroretinogram (PERG) is a retinal response evoked by viewing an alternating checkerboard or grating. It receives clinical and research attention because it can provide information about inner retinal cells and the macula. However, clinicians may have trouble choosing between different techniques for recording the PERG that have been described in the literature. The International Society for Clinical Electrophysiology of Vision has prepared a standard for a basic PERG recording procedure to aid new users in obtaining reliable responses and to encourage more uniformity among existing users.

Comparison of guinea pig electroretinograms measured with bipolar corneal and unipolar intravitreal electrodes

This study considers the precision and accuracy of bipolar corneal electrodes compared with unipolar intravitreal methods in collecting electroretinographic (ERG) recordings from a small animal. Flash ERGs were obtained from 9 adult guinea pigs on three occasions. Corneal bipolar (Burian-Allen) electrodes were used to collect data on the first two occasions whereas unipolar intravitreal electrodes were used on the last. We identified the a-wave, b-wave, oscillatory potentials, PIII and PII responses. Intensity-response functions were fit using a Naka-Rushton relationship with a bootstrap estimating the 95% confidence limits. Discrepancy analysis was applied to determine the coefficient of agreement. We found significantly larger amplitudes with unipolar intravitreal electrodes (ANOVA; a-wave, p<0.002; b-wave, p<0.001; Oscillatory potentials (OPs), p<0.005) especially at high intensities. Implicit times showed little differences between electrodes for the a-wave, significantly faster (p<0.03) b-waves at some intensities, and significantly slower (p<0.005) OP implicit times across all intensities. The PIlI amplitude (log microV), sensitivity and timing were not significantly different (p>O.05) if expressed in logarithmic units but PII amplitude (log microV) was significantly smaller with corneal electrodes. We suggest that a conversion factor (x1.35) should be applied to data collected with bipolar corneal electrodes to estimate the amplitudes of the modelled parameters accurately. The corneal electrode gave a precision of +/-39 microV which yields a statistical power of 0.90 for a sample size of 7 subjects. We conclude that bipolar corneal electrodes provide smaller electroretinogram amplitudes due to their location and reduced span of the retinal generators.

[The electroretinogram (ERG) of the mouse: normal values, optimal stimulation and recording]

PURPOSE

The electroretinogram (ERG) is an appropriate method to evaluate the retinal function in a variety of animal models. In this study we present suitable conditions of stimulation and recording in the dark-adapted mouse.

METHODS

Mice (n = 15) were dark-adapted during 14 hours and anesthetized with a single intraperitoneal injection of xylazine/ketamine. Pupils were dilated and a d.c.-silk-silver electrode or a AgCl-contact-lens electrode was placed on the cornea. The electroretinogram (ERG) was obtained by Ganzfeld stimulation over a range of 6 log units of intensity (8 x 10(-2) - 8 x 104 cd/m2). Intensity, duration and the interval of the light stimuli were varied separately.

RESULTS

Reproducible values of the intensity-response functions are obtained for the a-, b- and c-waves of the ERG under well controlled adaptation- and stimulus-conditions. C-wave amplitudes are best evaluated using d.c.-recording and a stimulus duration of 4 seconds. The position of the d.c.-silk-silver electrode on the cornea can affect the ERG-amplitudes. Using a contact-lens electrode, the recorded b-wave amplitudes are on average 20% below those recorded with a centrally positioned d.c.-silk-silver electrode. Stimulus-intervals of at least 60 seconds are recommended at high intensities.

CONCLUSIONS

An unequivocal assessment of retinal function requires reproducible ERG-values over a wide range of intensities. To obtain these, well controlled and standardized experimental conditions are required.

[From the symptom to electroretinography diagnosis]

Retinal function can be documented noninvasively and objectively by electroretinography, complementing clinical examinations. Symptoms of nightblindness and of dayblindness with photoaversion, nystagmus, poor vision in infants or unclear visual field defects are meaningful indications for ERG testing. We use standardized (ISCEV) full-field single flash ERGs to evaluate the function of the rod- and of the cone-system. In infants, general anesthesia is useful to combine an abbreviated ERG protocol with ophthalmoscopy and fundus photography. ERG testing facilitates to distinguish between functional deficits in the rod- and cone-system, between congenital-stationary retinal dysfunction and progressive retinal heredo-degenerations. Frequently a functional deficit of the retina without ophthalmoscopic changes can be assessed. These entities include achromatopsia, congenital stationary night blindness, early stages of retinitis pigmentosa (RP) or progressive cone dystrophy, as well as toxic retinal changes. Congenital amaurosis Leber (LCA), infantile RP, Usher's syndrome and retinal involvement in other neuropediatric or metabolic syndromes can be diagnosed or excluded by ERG recording early-on. Synoptic evaluation of the full-field ERG, pattern-ERG and VEP completes neuro-ophthalmological screening.

A simple and stable d.c.electrode for ocular electrophysiology

We describe the fabrication of a simple silver-silk electrode which permits remarkably stable d.c.recording of the electroretinogram (ERG) and the optic nerve response (ONR). A saline soaked wick of surgical silk, guided into a polyethylene tube connects the tissue to a coil of Ag/AgCl wire placed in a small glass vial, which is filled with 0.9% NaCl. The vial that holds the tube and the wire is closed with a rubber cap allowing easy refilling with NaCl. Examples of the usefulness of the new silver-silk electrode are shown. We applied it in experimental work in the isolated arterially perfused cat eye for d.c.recordings of the ERG and the optic nerve response (ONR), and also in vivo, in anesthetized mice to record c-waves.

[Simplified electroretinography protocol and diagnosis of retinal dystrophies in children]

PURPOSE

To report results of a simplified electroretinogram in children.

PATIENTS

124 children under 6 years of age with nystagmus, blindness, neurological disease, cone rod dystrophy in the family, or abnormal fundus appearance were examined.

METHOD

The electroretinogram was recorded by corneal electrodes in an awake state without sedation. A light-emitting orange diode stimulator was used. Stimulation was performed subsequently after 3 minutes of light adaptation and after 8 minutes of darkness. If the electroretinogram was abnormal, a second recording was done a few weeks later using the same method. In some cases, the children were re-examined, and a ganzfeld stimulation ERG was recorded.

RESULTS AND DISCUSSION

This method allowed a reliable diagnosis of photoreceptor dystrophy: Leber's congenital amaurosis with or without rare metabolic diseases, X link pigmentary retinopathy or retinal dystrophy with general disease in children. It helped to differentiate isolated retinal pigment changes with normal electroretinogram from functional retinal impairment and rod-cone dystrophy from other retinal dystrophies.

[Multifocal electroretinogram (MF-ERG) in diagnosis of macular changes. Example: senile macular degeneration]

PURPOSE

Small areas of retinal pathology may pose diagnostic difficulties. The noninvasive multifocal electroretinogram (MF-ERG) provides a topographical mapping of retinal function. Its role in the diagnosis of macular diseases is examined in age-related macular degeneration (AMD). AMD is a main cause of central visual loss in the elderly population, affecting the second eye in 75%.

METHODS

MF-ERG recordings of three patients with AMD were compared to the findings of fundus photography and fluorescein angiography. During the MF-ERG recordings the central 50 degrees of the retina was stimulated. The visual stimulus consisted of 241 hexagons that alternated, independently and pseudorandomly, between black and white according to a special predetermined binary sequence. Local retinal response components were extracted using the Fast m-Transform Algorithm.

RESULTS

Three of six eyes had undergone cataract surgery with implantation of a posterior chamber lens (PCL). In accordance with an increase in light transmission through PCLs, these eyes showed an increase in the MF-ERG responses. MF-ERG allowed accurate topographic mapping of focal areas of retinal dysfunction in all patients tested. There was good correspondence to anatomical changes detected by fluorescein angiography.

CONCLUSION

The high resolution of the MF-ERG enables detection of small areas of retinal pathology. It thus presents a clinically useful, noninvasive method in the early diagnosis and follow-up of macular disease.

Reproducibility of ERG responses obtained with the DTL electrode

Previous investigators have suggested that the DTL fibre electrode might not be suitable for the recording of replicable electroretinograms. We present experimental evidence that when used adequately, this electrode does permit the recording of highly reproducible retinal potentials.

Comparisons of contact lens, foil, fiber and skin electrodes for patterns electroretinograms

Pattern electroretinograms are small physiologic signals that require good patient cooperation and long recording times, particularly when conditions are not optimal. Six electrodes were compared to evaluate their efficacy. Pattern electroretinograms were recorded in eight healthy volunteers to high-contrast, pattern-reversal checks (40' width) with Burian-Allen, DTL fiber, C-glide, gold foil, HK loop and skin electrodes. Raw data for 320 reversals were analyzed off-line to evaluate signal amplitude, quality, P50 and N95 peak times, artifact rate and electrical noise. Insertion time, impedance and subjective comfort were also assessed. The Burian-Allen contact lens electrode gave the largest signal and lowest impedance but was the least comfortable and had the highest artifact rate (p < 0.01). A skin electrode on the lower eyelid produced the smallest pattern electroretinogram with the poorest quality (p < 0.05). The four other electrodes were foil or fiber electrodes in contact with the tear film, conjunctiva and/or the inferior cornea. The signal from these showed only minor differences. When electrodes are compared for pattern electroretinograms recording, the foil and fiber electrodes do not differ substantially but contact lens and skin electrodes show substantial disadvantages.

The c-wave of the electroretinogram recorded under clinical conditions from rabbits

To determine whether large and repeatable c-waves can be recorded from rabbits with equipment already in use in clinical electroretinographic laboratories, the Burian-Allen electrode, connected bipolarly or monopolarly, was used to record electroretinograms from pigmented rabbits. The Jet electrode was also used. The c-waves elicited by long-duration (4-second) stimuli were compared to those elicited by stroboscopic stimuli. In addition, the c-waves recorded with direct-coupled amplification were compared to those recorded with condenser-coupled amplification (one-half-amplitude bandpass=0.1 Hz). The b-wave amplitude was not altered by the amplifier coupling or by the two stimulus durations. The largest c-waves were elicited by 4-second-duration stimuli and recorded with direct-coupled amplification. Although the c-wave amplitude was reduced by stroboscopic stimuli and by condenser coupling, large and repeatable c-waves were elicited by stroboscopic stimuli and recorded with condenser-coupled amplification. A comparison of stimulus duration and amplifier coupling showed that the stimulus duration was more important in recording large-amplitude c-waves. Similar results were obtained with the Jet electrode. We conclude that repeatable and large c-waves can be elicited by a stroboscopic stimuli and can be recorded with condenser-coupled amplification with good low-frequency response from rabbits.

[Interdisciplinary diagnosis and therapy of traumatic optic nerve damage]

Traumatic optic nerve lesions (TONL) still pose a large clinical problem concerning early detection and treatment. Neuro-ophthalmology provides reliable tests to detect afferent lesions but these methods are limited to just 30% of the severely injured patients. Especially in the patient with multiple injuries, optic nerve injuries are hardly predictable. In the latter group we established well-known neurophysiological methods for early detection of afferent disorders of the visual pathway, i.e. flash-VEP ERG. Apart from these diagnostic problems of TONL, controversy still surrounds the appropriate treatment of TONL--whether conservative or surgical or even combined treatment should be advocated. Our aim was to establish a reliable diagnostic schedule, based on the combination of neuro-ophthalmological, spiral-CT and clinical findings, and a treatment plan, so that in any patient there is a distinct guideline as to whether there is a need for early treatment of the peripheral visual pathway or not. In 52 patients who were assessed by the above-mentioned schedule, we could detect any of the 20 afferent disorders of the peripheral visual pathway. Although it is difficult to prove therapeutic effects on the injured optic nerve, immediate combined conservative treatment plus optic nerve decompression helped in three patients, who reported unilaterally no light-perception at admission, to regain at least partial recovery of afferent function of the visual pathway. Most of the trauma units still handle the problem of optic nerve trauma with a 'wait and see' policy. This is not regarded as an up-to-date option, since there are alternatives, and these will be outlined.

[The effect of light intensity and neostigmine on the electroretinogram of Drosophila melanogaster]

In this article is presented the influence upon the electroretinogram, of some external factors acting at different levels of the visual system in the fruit fly, Drosophila melanogaster. The experiments revealed the effects generated by the light intensity modification as well as by the injection of a neuropharmaceutic--the neostigmine. The increase of the light stimuli intensity induced the same change of the rate between the amplitudes of the main electroretinographic components as the neostigmine does. It was used the "flickering" excitation regime, utilizing a data acquisition system adapted to the principal experimental device.

[Objective assessment of visual field defects using multifocal electroretinography]

BACKGROUND

Multifocal electroretinography allows simultaneous recording of 61 focal electroretinographic signals from the retina of the posterior pole. The function of the outer retinal layers can be mapped for a visual field of 30.5 degrees radius. We herein describe the topography of such potentials in patients with hemianopic and concentric visual field defects.

SUBJECTS AND METHODS

Six patients with visual field defects caused by chorioretinal and central visual pathways diseases were examined using multifocal ERG.

RESULTS

In 30 normal volunteers in the entire 30 degrees visual field clear signals were obtained. In the patients with visual field defects caused by retinal diseases in areas with reduced light sensitivity diminished electroretinographical activity was found. In contrast, in patients with bitemporal hemianopsia due to a chiasmal lesion no correlation between visual field and magnitude of focal ERGs was seen.

CONCLUSIONS

In retinal disorders defects in multifocal ERG presented the similar pattern as scotomata in perimetry. The patient with visual field defects due to disturbances in the chiasma exhibited a normal ERG-topography. In patients with visual field defects multifocal ERG supported differentiation of the location of the lesion.

Pattern ERGs from isoluminant gratings; poor selectivity compared with VEPS

The PERGs elicited by low contrast achromatic (luminance-modulated) and chromatic (isoluminant) gratings were studied as a function of the presentation mode (rapid onset, offset, reversal), departure from isoluminance, contrast and spatial frequency; predominantly phasic-type responses were found in all cases. We propose that the PERGs for low and moderate contrast, achromatic and chromatic grating stimuli, are generated by non-linear, transient retinal cells of the magno-system. Parvo retinal cells seem to contribute to the response only at higher chromatic contrasts. The transient nature of the PERG obtained from isoluminant gratings makes it doubtful that these signals represent the activity of colour-related processes.

The repeatability and variability of the multifocal electroretinogram for four different electrodes

Multifocal electroretinograms were recorded from twelve subjects on three separate days using the Visual Evoked Response Imaging System (VERIS). The subjects were 18-40 years-of-age. Each recording session was separated by an interval of between one and 16 weeks. Four electrode types were compared: the JET contact lens electrode, the gold foil electrode, the DTL thread electrode and the carbon fibre (c-glide) electrode. The b-wave amplitude density (nV/sq.deg) recorded with each electrode was measured. The mean amplitude density recorded at each session for each subject and each electrode was calculated and compared. There were no significant differences in amplitude density for the ERGs recorded with each type of electrode over the three days. The coefficient of variation for each electrode and each subject was then calculated using the results for the three days. The coefficients of variation for each electrode showed significant differences (One-way ANOVA: F = 4.51, d.f. = 3.44, P < 0.008). The c-glide electrode results had the highest variability and post hoc tests showed that its coefficient of variation was significantly different from those of the JET and gold foil electrodes but not from that of the DTL thread electrode.