Skin ERGs: their effectiveness in paediatric visual assessment, confounding factors, and comparison with ERGs recorded using various types of corneal electrode

Comparison of scotopic and photopic ERGs recorded using a variety of ERG electrodes show that contact lens electrodes produce the largest ERGs, and these are about 30-50% larger compared with those recorded with foil or thread electrodes, which contact a smaller part of the cornea. Flash ERGs recorded from infra-orbital skin electrodes are about one eighth the size of those recorded with contact lens electrodes, and around one quarter the size of those recorded with foil or fibre corneal electrodes. Reliable ERGs can be obtained in young children from infra-orbital electrodes sited centrally, within 1 cm of the rim of the eyelid, when signal averaging is used. Stimulation under fully darkened laboratory conditions with red and dim blue flashes permit assessment of cone and rod function, respectively. Factors such the recording electrode position, electrode derivation, upward rotation of the eye, eyelid closure and markedly constricted pupils can degrade the skin ERG. There are strong diagnostic advantages in recording the VEP concurrently with the skin ERG, particularly in young children. Examples of recordings in Leber's amaurosis, delayed visual maturation, albinism, optic nerve hypoplasia, achromatopsia add X-linked congenital stationary nightblindness are shown to demonstrate how ERG/VEP recordings can help in distinguishing between these conditions.

Electrodes and the recording of the human electroretinogram (ERG)

The development of electrodes for recording the human electroretinogram is reviewed. The salient features of the different types in use are described and evaluated. The relation performance of different types is used as a basis for examining the dipole model of the origin of electroretinographic signals and suggestions are made for imposing it.

Gold foil electrodes: a two-center study of electrode reliability

Using the routine techniques employed at our two centers for recording pattern electroretinograms and a variety of methods, we investigated the reliability of the gold foil electrode over repeated uses. We found no significant difference between the pattern electroretinograms recorded with new electrodes and those that had had such extensive use in patients that they were believed unsuitable for further clinical use. We therefore continue to recommend the gold foil electrode for repeated use.

The gold foil electrode in pattern electroretinography

A recent study found that the gold foil electrode produces large pattern electroretinogram amplitudes, but the test-retest reliability was low. In a three-center study, we observed that 90% of 29 patients who were tested with gold foil electrodes used three times appeared to have markedly lower amplitudes than when tested with new electrodes during the same session. Across study centers, the mean of the new electrode recordings was 3.78 microV (standard deviation, 1.13 microV), versus 2.93 microV (1.29 microV) for used electrodes. This 0.85-microV reduction (22%) was statistically significant (F = 7.10 p = 0.01). Electrodes used three times demonstrated an average change in the coefficient of variation of 14% (standard deviation/mean = coefficient of variation; new, 1.13/3.78 = 30%; used, 1.29/2.93 = 44%). Two of the study sites (Houston/Indianapolis) conducted test-retest pattern electroretinograms on a total of 18 patients and found the mean evoked potential to be 3.55 microV with new electrodes and 2.82 microV with used electrodes. The coefficient of variation for the test-retest data was 30% and 47% for new and used electrodes, respectively. Light microscopy showed small cracks on the surface of the electrode, with the number and configuration of the cracks varying in each electrode. The presence of cracks is further complicated by their proximity to the tear film. These sources of variation can result in significantly different impedances. We propose that constant flexion, as a result of patient blinking, causes cracks in the thin gold surface of the electrode. Used electrodes will produce lower pattern electroretinogram amplitudes and poor test-retest reliability.(ABSTRACT TRUNCATED AT 250 WORDS)

[Electroretinography study of unanesthetized young children]

BACKGROUND

The electroretinogram (ERG) ist an important examination method for the evaluation of retinal dysfunction in children. However it is often difficult and sometimes almost impossible to examine especially younger children with usual methods. Because of its risks, general anesthesia is rarely used.

METHODS

ERG examinations were performed without general anesthesia or sedation with DTL (Dawson-Trick-Litzkow) electrodes. These are conjunctival electrodes made of thin microfibers. Several additional modifications of the standard examination technique were utilized, which enabled us to examine children of almost any age. 10 volunteers and 34 children between 6 month and 9 years of age were examined.

RESULTS

62 eyes of 34 children were evaluated. The diagnostic problem could be solved in three quarters of the patients with the ERG.

CONCLUSION

With the DTL electrode and some methodical changes in the standard ERG technique it is possible to perform ERG's in little children without anesthesia or sedation. This method can be applied in children over 2 years of age. Below this age an sedation of any kind is necessary.

A new system for recording electroretinographical potentials in animals

Electroretinography can provide meaningful evidence of oculotoxic changes. We describe a new recording system specially constructed for animal experiments. It meets the standards for electroretinographical investigations and recording systems set up by international committees. The recording apparatus has a resolution of 0.49 microV (amplitude) and 1 ms with a sensitivity range of +/- 1 mV full scale and a band-pass of 0.03 to 1,000 Hz. The signals are digitized and processed by a computer. The calibration symbol is faithfully represented as a rectangular impulse with an amplitude of 100 microV and a period of 100 ms. Normal values were calculated for 25-week-old albino Wistar rats. The normal range is defined as the mean +/- 2 SD for A and B wave parameters. These normal values can serve as reference values for future studies in oculotoxicity.

[Digital data collection and analysis in clinical electrophysiology. A PC program for electrophysiology]

BACKGROUND

The number of data acquired in electrophysiology and the need for immediate analysis and interpretation justified the use of computerized signal acquisition and -analysis. Since such techniques are well known and established in computerized perimetry, the user of EOG, ERG, and VEP will profit from such an approach.

METHOD

A computer program for signal acquisition and -analysis of clinical electrophysiology is described. The software is written in BASIC for IBM compatible PC/XT/AT computers. The program supports all electrophysiological examinations (EOG, ERG, VEP). It contains a subroutine for automated EOG recording and -analysis, allows measurements of amplitude and latency of standardized Ganzfeld-ERG and pattern-ERG. Options for recording and analysis of further stimuli are prepared. The possibility of determination of area under the curve is implemented. In the VEP subroutine evaluation of amplitude, latency, area under the curve, and Fast-Fourier-Transformation--necessary for steady-state-VEP--is installed.

CONCLUSION

The program demonstrates that with the now available low cost Personal Computer technology it is possible to build up individual software systems for clinical electrophysiology supporting and optimizing all recording, analysing, and administrative procedures.

Recording the oscillatory potentials of the electroretinogram with the DTL electrode

Suprathreshold photopic oscillatory potentials recorded with a DTL electrode were compared to those obtained with a Lovac corneal electrode. The overall oscillatory potential response (sum of oscillatory potentials) recorded with the DTL electrode was half of that obtained with the Lovac electrode. However, there was no evidence of a selective attenuation (or amplification) of any given oscillatory potential with the DTL electrode. Similarly, the oscillatory potential relative amplitude ratios and the peak times of the oscillatory potentials were identical for both electrodes. Our findings clearly indicate that the DTL electrode is adequate to record the high-frequency oscillatory potentials. Given the low cost and ease of use, as well as the disposable nature of the DTL electrode, we believe that electroretinographic specialists should seriously consider a wider utilization.

Corneal electrode for recording electroretinograms in rats

The corneal contact lens electrode, because of its convenience, has replaced the cotton-wick electrode for recording electroretinograms from patients and animals such as dogs, rabbits, and cats. The cotton-wick electrode, however, remains popular for rat electroretinogram measurements because small contact lens corneal electrodes that fit rat eyes are difficult to fabricate. We prepared corneal electrodes from disposable needles for use in recording electroretinograms from rats. The electrodes were readily prepared, were inexpensive, and were used successfully in six rats.

[In incipient glaucoma the pattern electroretinogram displays diffuse, retinal damage]

The pattern electroretinogram (PERG), a noninvasive tool to measure the activity of retinal ganglion cells, shows changes in early glaucoma. We wanted to investigate whether or not the PERG is preferentially affected when the Bjerrum area is stimulated. We recorded the PERG from 17 eyes in 17 patients with initial stages of visual field defects due to open-angle glaucoma and from 10 eyes in 10 normal subjects. Counter-phasing checkerboard patterns were used as visual stimuli with a check size of 0.8 degrees at 16 reversals/s and 98% contrast. Three visual field extents were stimulated: (1) full screen (31 degrees x 27 degrees), (2) central stimulation with a diameter of 14 degrees, (3) peripheral stimulation = full screen with central mask of 14 degrees in diameter. The latter stimulus covers the regions where traditionally glaucomatous defects are expected early. In glaucoma patients, there was a significant decrease in the PERG amplitude compared to normals. This reduction was very similar for all three stimuli (42%, 52% and 47%, respectively). Thus, the PERG amplitude is reduced to a similar degree both in regions of the visual field where early damage is found (Bjerrum area) and in the central area. This result suggests that in early glaucoma there is focal as well as diffuse retinal damage.

[Developments in ophthalmologic electrophysiology]

During the last years electrophysiology in ophthalmology has been facilitated to a big extent by computerized methods. Thus it has become a suitable method for an increasing number of relevant diagnostic problems. To make comparisons between test results of different diagnostic places easier an international standard concerning the methodology of the electroretinogram (ERG) has been elaborated by the "International Society of Clinical Electrophysiology in Vision" (ISCEV). Aim of the present article is to give a short description of the ERG-standard and to review recent developments in clinical electrophysiology in ophthalmology (ca. since 1989).

A new focal pattern stimulator under direct observation of ocular fundus

A new focal pattern stimulator was developed. Since this system enables direct observation of ocular fundus during examination, it may be useful for the objective assessment of visual function in patients with various kinds of eye diseases.

The electroretinogram in infants and young children

Electroretinography (ERG), especially when combined with visual evoked potential recording, can provide valuable information to the pediatric clinician assessing a young child with nystagmus who appears not to see. ERG changes are described in clinical conditions affecting rod and/or cone function, vitreoretinal disease, maculopathies, and disorders involving the retina and central nervous system. The role of ERG in monitoring treatment, and in excluding retinal disease prior to eye surgery, is also covered.

Electrode comparison in pattern electroretinography

In recent years, there has been great interest in recording the pattern electroretinogram (PERG) in glaucomatous and diabetic populations. The Dawson, Trick, and Litzkow thread electrode (DTLTE) and the gold foil electrode (GFE), commonly used for recording PERGs, were compared for variations in amplitude of response, test-retest variability, and patient comfort. Two study centers collected data on a total of 32 normal subjects. The subjects from the London center showed a slight (but not significant) preference for the DTLTE, and the Houston subjects also found the DTLTE to be significantly more comfortable (chi-square = 39, P less than 0.001). In both study groups, the GFE was found to produce a statistically larger amplitude of response than that obtained with the DTLTE. Significant differences were found regardless of the slow (transient, 3.1 Hz; F = 6.24; P = 0.0192) or fast (steady state, 8.3 Hz; F = 18.38; P = 0.0001) stimulus-presentation rate. Larger differences between the two electrodes occurred under steady-state conditions. Although there is no consensus as to the optimum recording conditions to obtain the subtle PERG, it appears the the GFE records larger responses than the DTLTE. However, test-retest data confirmed that the GFE records twice the amplitude of the DTLTE, and it also produced twice the variability (average percent difference over time for GFE, 15%; for DTLTE, 8%).

New noncorneal HK-loop electrode for clinical electroretinography

A new noncorneal electrode for clinical electroretinography was developed. It consists of a thin wire forming a loop modeled to fit into the lower conjunctival sac. Electrical contact is made with the scleral conjunctiva through an exposed portion of otherwise insulated wire. The recorded pattern electroretinograms are in the same amplitude range as if recorded by the gold foil electrode, while the flash electroretinograms with the new electrode are of about two-thirds the amplitude of corneal electrodes. The new electrode is more durable and hence less expensive than gold foil electrodes and can likewise be used without topical anesthetic. Cleaning is easy and effective. The electrode rarely causes discomfort and produces stable responses for at least 2 hours. The electrode aims to match stability of skin electrodes with sensitivity of fragile foil and fiber electrodes.

Mapping of VEPs and ERG responses to punctual stimulations in the visual field

Visual field projection maps were obtained using retinal and cortical EP amplitude values. Single meridian investigations were also performed. Stimulation was done with punctual photic stimuli delivered by the LED perimeter built for the purpose. Schedule with 28 stimulation loci was adopted. The B wave of the average ERG and P100 component of the cortical VEP were measured. The maps were obtained by interpolation of amplitude values on a grid of 21 x 21 characters. By comparing the retinal and cortical visual field maps, conclusions regarding the normality of visual structures (peripheral or central) can be drawn. Our procedure adds to the previous data on visual field investigation (cynetic or static) important elements concerning vision electrophysiology.

M-wave of the toad electroretinogram

1. In the retina, two distinct, light-evoked releases of K+ have been described. One takes place in the outer plexiform layer (OPL) and is termed the "distal K+ increase." The other takes place in the inner plexiform layer (IPL) and is termed the "proximal K+ increase." Although the distal K+ increase generates the electroretinogram (ERG) b-wave, the contribution of the much larger proximal K+ increase to the ERG is less well understood. In this paper we detail our investigation of the proximal K+ increase and its contribution to the ERG. We describe an ERG component, the M-wave, which had not heretofore been observed in the diffuse-flash, vitreal ERG. 2. We studied the proximal K+ increase and the ERG M-wave in the isolated retina preparation of the toad, Bufo marinus. We used K(+)-sensitive microelectrodes, as well as conventional intra- and extracellular microelectrodes, to record K+ changes, the local (or intraretinal) ERG, the vitreal ERG, and Müller cell responses. 3. As in earlier studies of the amphibian and cat M-wave, we readily observed an M-wave in the intraretinal, or local, ERG (LERG). The M-wave we studied had characteristics similar to those of M-waves that were previously described. Specifically, we found that the M-wave was generated by a Müller cell response to the proximal K+ increase and that both the proximal K+ increase and the LERG M-wave were spatially tuned. 4. We used the aspartate receptor agonist, N-methyl-DL-aspartate (NMA), to reveal that an M-wave is present in the vitreal ERG. Researchers who previously investigated the M-wave were unable to identify an M-wave in the vitreal ERG. We found that the toad ERG M-wave was a small, positive potential that was partially obscured by the much larger b-wave and slow PIII components. 5. We observed that picrotoxin (PTX) had an excitatory effect on inner retina, as evidenced by an enhanced proximal K+ increase and an enhanced M-wave. This result indicates that it is likely that GABAergic inhibition in inner retina plays an important role in retinal processing in the toad. 6. At threshold, we found that the ERG consisted mainly of an M-wave, indicating that the amphibian threshold ERG is driven by proximal retina. This result is analogous to previous observations of the threshold ERG in cat. However, in cat, the M-wave and threshold response have been described as distinct ERG components.(ABSTRACT TRUNCATED AT 400 WORDS)

[Effect of UV-B radiation on the parameters of rabbit electroretinogram]

Rabbit's eyes were exposed to UV-B-light (lambda = 280-320 nm) at biologically effective doses of 20 to 1300 J/M2. The amplitudes of a- and b-waves of the ERG were found to decrease during a period of 7-20 days to reach approximately 50% of the initial level at a maximum dose. By day 30 the wave amplitudes returned to the control. The latencies of both ERG waves were invariable throughout the entire period of observation. The postirradiation changes in the ERG wave amplitudes were most pronounced during the attenuating photokeratitis.

[Recording system for the scotopic threshold response]

The scotopic threshold response (STR) is a newly investigated component of the electroretinogram. Since the STR reflects post-photoreceptoral processing in the retina at intensities near absolute threshold, the STR would be a useful index to evaluate the rod pathway in the proximal retina. A conventional ERG setup, however, involves difficulties in STR recording. Therefore we constructed a new recording system for the purpose. The characteristics of the apparatus are as follows: 1) A diffusing sphere provides full-field stimuli ranging from a very dim light near absolute threshold to a very bright light. 2) A remote-control system for changing filters is used to avoid breakdown of the dark-adapted status. 3) A software artifact rejection routine is used to exclude responses influenced by eye movements and blinks. Using the apparatus we have examined the human STR and generally confirmed the previous findings.

Local ERG for clinical examination of eye diseases

We used a suction electrode with a frosted front window to record a full-field (general) ERG, and a suction electrode with a short-focus lens and a red-green light diode to record a local ERG. As the suction electrode moved with the eye, the stimulus struck the macula in all positions of gaze. Our data showed that only mildly intense stimuli with an angular size of 10 to 15 degrees induced a retinal response, with a macular contribution of 70%. Patients with various dystrophic changes of the retina, such as Stargardt's dystrophy, vitelliform dystrophy of the Best type, X-linked juvenile retinoschisis, and age-related macular dystrophy of different stages, were examined. In patients with retinitis pigmentosa, the cones were involved in the pathologic process to varying degrees; the behavior of the local ERGs helped determine the final visual acuity. We also observed a reduction in the local ERG in patients with Stargardt's dystrophy that disappeared as the disease progressed. In age-related macular dystrophy, a disturbance of the electrogenesis mainly occurred in the central retina in advanced stages of the disease.

Microcomputer-based recording system for clinical electrophysiology

We developed a personal computer-based system for clinical electrophysiologic measurements. The computer interfaced with a commercially available A/D converter, a low-noise isolation preamplifier, filter circuits, pattern and Ganzfeld stimulators, and a hardcopy unit. Separate programs were developed for electroretinography (ERG), pattern ERG and simultaneous visual evoked potential (VEP), flash and pattern-shift VEP, and electro-oculographic measurements. The complete control of the applied hardware (eg, stimulus control, automatic gain, and filter selection) is a common feature of the computer programs. These programs provide oscilloscopic functions, overload protection, artifact elimination, averaging, automatic peak latency and amplitude determination, baseline correction, smoothing, and digital filtering. The results can be presented on matrix, laser printers, or digital plotters. The hardware components and the features of the driver software are demonstrated on normal and pathologic signals.

[The UTAS-E1000 System in electrophysiological studies of the visual system. I. Characteristics of the system and its possibilities and equipment]

The paper presents the technical and medical characteristics of the computerized system UTAS-E1000 (made by LKC, USA) destined for the electrodiagnostics of the visual system. Discussed are the properties of the construction which decide on the possibility of automatization of electrodiagnostic tests and also personal experience on the development of the system in order to increase its scope of application.

[The UTAS-E1000 System in electrophysiological studies of the visual system. II. Programming and clinical use]

The authors discuss a set of electrodiagnostic tests of the UTAS-E1000 system and their personal work on new tests. They present the scope of applications of the equipment in examinations of some pathological conditions of the visual system and--on the basis of their personal experience--the possibilities of introduction of new electrodiagnostic tests.