Relationships between the electroretinogram a-wave,b-wave and oscillatory potentials and their application to clinical diagnosis

The electroretinogram is the electrical response of the retina to a light stimulus. The amplitude and temporal pattern of its components, the a-wave, the b-wave and the oscillatory potentials, depend on the functional integrity of the retina, on the intensity of test flash reaching the retina and on the ambient illumination. The latter contributions to the normal variability in the electroretinogram can be circumvented by constructing the relationships between the different electroretinogram waves. The electroretinogram responses were recorded from 18 dark-adapted subjects with normal vision. The slope of the a-wave and the amplitude of the b-waves were measured in the time domain. The oscillatory potentials were isolated by a digital filter and were transformed to the frequency domain for quantitative measurement. The relationship between each pair of variables could be fitted by linear segments. Our findings suggest that this mode of electroretinogram analysis can be useful in localizing the site of action of retinal disorders and that the relationship between the a-wave slope and the power density of the oscillatory potentials is a useful index for identifying disorders of the inner retina.     

Assessing abnormal rod photoreceptor activity with the a-wave of the electroretinogram: Applications and methods

The impact of a disease on phototransduction can be assessed by fitting the leading edge of the rod a-wave to high-energy flashes with a quantitative expression. Two parameters of rod receptor activity are obtained, S (sensitivity) and Rm (maximum response). In this study, the meaning of these parameters and examples of conditions that change them were examined. In addition, a new protocol was developed for obtaining these parameters. A set of three to five white flashes were first presented in the dark and then on an adapting field (30 cd/m²). Subtracting the light-adapted responses from the dark-adapted responses yielded isolated rod a-wave responses. A clinical protocol was developed based on a single white flash energy. It is possible to determine whether a disease is producing a change in S and/or Rm with this single flash energy without the use of any equations.     

Effects of retinal dopamine depletion on the rabbit electroretinogram

This study evaluated the effects of intravitreal injections of 300 g of 6-hydroxydopamine (6-OHDA) on electroretinogram (ERG) amplitudes and implicit response times of adult pigmented rabbits. One eye was injected intravitreally with 300 g 6-OHDA and 600 g ascorbic acid in a 0.3 ml 0.9% NaCl solution; the fellow eye received a similar solution containing only 600 g ascorbic acid. Following this treatment ERG recordings were performed at 1, 4, and 7 days. After the last recordings, animals were sacrificed and retinas were isolated for biochemical analyses. Significant and progressive reductions in retinal concentrations of dopamine (DA) and its main metabolites homovanilic acid (HVA), and dihydroxyphenylacetic acid (DOPAC) were found in treated retinas. Concentrations of norepinephrine (NE), serotonin (5HT), and 5 hydroxyindoleacetic acid (5-HIAA) were not affected, thus demonstrating the specific neurotoxic action of 6-OHDA on retinal dopaminergic neurons. Concurrently, significant increases in ERG a- and b-wave amplitudes as well as in implicit response times were observed. These electrophysiological changes were progressive reaching a maximum 7 days after intravitreal injections. Changes in b-wave amplitudes and response times were more pronounced at low intensities of stimulation. These results clearly show that, in rabbits, selective decreases in retinal DA concentrations result in pronounced ERG changes, which offer additional evidence supporting a role for this transmitter in lateral inhibition in the retina.This work was supported by MRC research Grants No. MT2593 and DG284.     

A new component in the a-wave of the human cone electroretinogram

The human cone electroretinogram (ERG) to a full field flash has been examined on a rod saturating background (17,000 photopic trolands). With strong stimuli, a negative wavelet appears in the falling phase of the a-wave. This response has a latency of 10–12 milliseconds, about 6–8 milliseconds after the start of the a-wave and just before the rising phase of the corneal positive b-wave begins. We suggest that it may represent a hyperpolarizing response of second order retinal neurons.     

Long-term behavior and intra-individual stability of the direct current electroretinogram and of the standing potential in the albino rabbit eye

The direct current electroretinogram (ERG) and the standing potential (SP) were studied in seven albino rabbits under general anesthesia. Identical experiments were performed on 2 consecutive days. After 30 min of dark adaptation, repeated light stimuli of maximal intensity of the system were presented to the eyes. The interstimulus interval was 70 s, and stimulus duration 10 s. Each experiment lasted for almost 3 h. In the first experiment, the b- and c-wave amplitudes measured in response to the second light stimulus were markedly reduced compared to those recorded in response to the first stimulus. Both amplitudes then recovered. The b-wave attained a peak about 20 min after the start of light stimulation. The peak was followed by a trough about 20 min later, and the amplitude then slowly increased. Following the minimum recorded during the second light stimulus, the c-wave amplitude reached a peak about 14 min after the start of stimulation. A trough in the amplitude occurred 20 min later. The amplitude then slowly increased to the end value, which was higher than the initial level. The a-wave behaved similarly to the b-wave, but the changes in most cases did not attain statistical significance. A minimum in the SP occurring at the second light stimulus was followed by a peak about 13 min after the start of light stimulation, and then by a trough about 17 min later. In the second experiment, performed one day after the first, the development of the a-, b-, and c-wave amplitudes and of the SP was similar to that observed during the first experiment, and no statistically significant differences between the two experiments were found. The reactions of the ERG and the SP were thus very stable between identical experiments performed on two consecutive days.     

Concentration-dependent effects of dopamine on the direct current electroretinogram of pigmented rabbits during prolonged intermittent recording

The direct-current (DC) electroretinogram (ERG) was studied in 24 pigmented rabbits. Four experiments were performed, each including six animals. One eye was injected intravitreally with 0.1 ml dopamine (DA) with an estimated concentration in the vitreous body of 0.0025 mM, 0.025 mM, 0.25 mM and 2.5 mM, respectively. The contralateral eye was injected with the same amount of saline. Following the injection the animals were dark adapted for 30 min and then exposed to repeated light stimuli of low intensity for almost 3 hours (series I: 1 stimulus per 3 min, 10 s duration, light intensity 6.8 × 10² lux). After another 30 min period of dark adaptation repeated light stimuli of high intensity were presented to the eyes (series II: 1 stimulus per 70s, 10 s duration, light intensity 6.8 × 10⁴ lux) for 33 min. In the control eyes, a slow increase with time of the a-, b-and c-wave amplitudes was observed during series I. During series II, the a- b-and c-wave amplitudes were markedly reduced between the first and the second light stimulus, but subsequently grew to a peak. The behavior of the ERG in the eyes injected with dopamine was not different from that observed in the control eyes at the lowest concentration of the drug. At higher concentrations the b-and c-wave amplitudes were reduced compared with the control eyes, and did not show the slow increase with time observed in these eyes during series I. Peak responses observed during series II in the control eyes was increasingly suppressed in the eyes treated with dopamine. Results of ERG recordings suggest that dopamine influences retinal adaptation in rabbits in a dose dependent manner.     

Scotopic stimulus/response relations of the B-wave of the electroretinogram

Scotopic b-wave stimulus/response (S/R) functions are abnormal in several human retinal degenerative disorders. However, the mechanisms by which diseases affect the S/R parameters are not yet fully known. Three experiments were done to simulate functional pathologies known to occur in retinal degenerations: 1) attenuated sensitivity of retinal units, 2) loss of rhodopsin, 3) loss of sensitivity with little or no loss of rhodopsin. None of the experimental perturbations of normal function replicated the pattern of S/R abnormalities caused by retinal degenerations. Thus, in the retinal degenerative disorders intrinsic abnormalities of cellular processing must affect the organization of distal retinal function indexed by the b-wave.     

Selective abnormality of the cone B-wave in a patient with retinal degeneration

A 44-year-old woman with midperipheral pigmentary changes that resemble retinitis pigmentosa (RP) is described; unlike typical patients with RP, her photopic b-wave was markedly attenuated as compared with her photopic a-wave. Otherwise, her electroretinogram (ERG) was typical of patients with early stages of photoreceptor degeneration. Scotopic and photopic a-wave amplitudes were reduced about 50%; her scotopic b/a appeared normal.     

Effects of low doses of forskolin on the c-wave of the direct current electroretinogram and on the standing potential of the eye

A marked effect of prostaglandins on the b- and c-waves of the direct current electroretinogram was recently reported by our laboratory. The increased b- and c-wave amplitudes in response to prostaglandins may be mediated by cyclic nucleotides acting on fluid and ion transport across the retinal pigment epithelium (RPE). Forskolin is known to increase cyclic adenosine monophosphate in a number of tissues, among them the RPE. To study possible effects of forskolin on the ERG vitrectomy was performed on rabbit eyes, followed by intraocular irrigation with a forskolin solution (10g/ml PHS). Forskolin produced reversible ERG changes with increase in a- (24%, p < 0.001), b- (25%, p < 0.001) and c-wave (53%, p < 0.001) amplitudes and elevation (about 1.0–1.5mV, p < 0.01) of the standing potential of the eye. The increase in c-wave amplitude was significantly greater than that of the a- (p < 0.05) and b- (p < 0.01) wave amplitudes, which seems to imply a primary or at least major effect on the RPE.     

Two neuronal retinal components of the electroretinogram c-wave

Although the rising phase of the b-wave seems to be generated mainly in the rod bipolar cells and the cone on-bipolar cells, the slow component of the electroretinogram, the c-wave, evidently originates in the Müller cells and the pigment epithelium. The c-wave has three components. One cornea-positive component derives from the pigment epithelium, while a distal cornea-negative component (slow PIII) and a proximal slow component originate in the Müller cells. This third proximal component of the c-wave differs between mammalian species: it is negative in the rat retina, positive in the rabbit and human retina and may be lacking in the cat retina.     

Contribution of post-receporal cells to the cone a-wave of the human electroretinogram in congenital stationary night blindness and autoimmune-like retinopathy

In normal subjects the later part of the cone a-wave to a brief flash increases in amplitude after 50-100 ms darkness due to a contribution from secondary hyperpolarising cells. We recorded these responses along with clinical ON and OFF ERGs in patients with inner retinal dysfunction to see if this part of the a-wave is affected. Patients with autoimmune-like retinopathy and CSNB2 had abnormal ON and OFF responses but the a-wave increased in amplitude in the dark as in normals. Conversely, the OFF-response was normal in CSNB1 but the a-wave did not increase in the dark. Contrary to expectation these results show some hyperpolarising cell function in autoimmune-like disease and CSNB2 and some OFF-pathway abnormality in CSNB1. The a- and d-wave are needed to assess OFF-pathway function.     

Oscillatory potentials of visually inattentive children

The oscillatory potentials (OPs) are probably generated in the proximal retina. The OPs of 20 visually inattentive infants and children were recorded. All 20 had evidence of abnormalities of the visual parts of the brain. The a- and b-waves, indices of distal retinal function, were normal in 10 patients, abnormal in the other 10. Among the patients with abnormal, attenuated a- and b-waves, OP amplitudes were more attenuated than among those with normal a- and b-waves. However, the timing of the OP wavelets was not correlated with distal retinal activity. These results suggest that in humans OP amplitude may be determined by inputs from the distal retina, but OP latency and periodicity are governed by processes within the proximal retina.     

复方血栓通胶囊对视网膜色素变性ERG b波振幅的影响

目的:探讨偶发型视网膜色素变性患者在常规西药治疗的基础上加用复方血栓通胶囊对视网膜电流图b波影响。方法:按准入和排除标准选择偶发型视网膜色素变性患者21例,服用二十二碳六烯酸和β-胡萝卜素6mo后加用复方血栓通胶囊再服用6mo,在研究开始时、6mo时和1a时按国际标准方法进行暗视ERGb波振幅检测,观察用药后b波振幅的变化。结果:总体来讲,服药后患者的b波振幅有所提高,但在病情较重的患者(开始治疗时b波振幅≤100μV)未达到统计学显著差异,而在病情较轻的患者(b波振幅>100μV)却有显著差异,服用复方血栓通胶囊是导致这种显著差异出现的一个原因。认为复方血栓通胶囊有利于轻度视网膜色素变性(retinitis pigmentosa,RP)病情的改善,推测其作用不是通过扩张血管,而主要是通过消除血管周围的沉淀物和渗出物,增加血流灌注,提高光感受器细胞对抗病害的能力。结论:复方血栓通胶囊对轻度偶发型视网膜色素变性有治疗作用。     

The human focal electroretinogram as a function of stimulus area

Focal electroretinograms to on-off luminance modulation (2.7 Hz) of uniform fields with a different area (144-2.25 deg²) centered on the fovea were recorded in two young normal subjects. The electroretinogram responses typically consisted of an a-wave, a b-wave, a PIII component and a d-wave. The concentric reduction of the test field size caused progressive amplitude decrement of all components, while their response density increased. The spatial summation properties of the b-wave differed from those of the a-wave, PIII component and d-wave so that the relative contribution of the b-wave to the electroretinogram waveform depended on the stimulus area. For a critically small field size, the electroretinogram was dominated by the PIII and d-wave components. Only these two electroretinogram components displayed a density profile that could be compared with that of cone distribution.     

Effects of intravitreal perfusion with dopamine in different concentrations on the DC electroretinogram and the standing potential of the albino rabbit eye

The direct current electroretinogram and the standing potential were recorded from both eyes of 23 albino rabbits during intraocular perfusion of one of the eyes, which was vitrectomized, with a physiologic reference solution (PHS). PHS was then replaced by a test solution containing dopamine dissolved in PHS. The fluids were subsequently alternated (PHS-dopamine-PHS). During irrigation with 0.25–0.5 mM dopamine (11 rabbits) the c-wave amplitude was 140% higher (p < 0.001) and during irrigation with 25 mM dopamine (6 rabbits) 85% lower (p < 0.01) than it was during the corresponding initial perfusion with PHS. The simultaneously recorded b-wave amplitude was reduced (0.25–0.5 mM: -22%, p < 0.001; 25 mM: - 69%, p < 0.001) and the SP level increased (0.25–0.5 mM: +2375 V, p < 0.01; 25 mM: +2530 V, p < 0.05) compared with the values obtained during the corresponding preceding irrigation with PHS. Thus the changes in the b- and c-wave amplitudes during perfusion with dopamine were dependent on the concentration of the drug. In the contralateral control eye (23 rabbits) the c-wave amplitude was 21% higher (p < 0.001), the b-wave amplitude 14% higher (p < 0.001) and the standing potential 1007 V higher (p < 0.001) during intravitreal perfusion with dopamine in the other eye than during the preceding irrigation with PHS in that eye, possibly as a result of increasing dark adaptation.     

Contribution of rod and cone pathways to the dark-adapted electroretinogram (ERG) b-wave following retinal degeneration in RCS rats

Although the RCS rat is widely used as a model of progressive photoreceptor loss, it is unclear how the relative rod and cone functions change with age. Rod and cone b-waves were isolated using a double flash ERG paradigm. In contrast to cones, rods never reached normal functional maturity levels, and the ERG b-wave changed from being predominantly rod-driven to being purely cone-driven by age 74 days, at which point, b-waves were progressively replaced by negative STR-like (scotopic threshold response) waves that persisted up to age 180 days. A double flash commonly abolished this wave and unveiled a b-wave.     

Electroretinography and electro-oculography to localize abnormalities in early-stage inflammatory eye disease

Electrophysiological investigations were performed in patients with inflammatory eye disease characterized by the presence of vitreous cells. The eyes were classified into four categories on the basis of fluorescein angiography: 1) no fluorescein leakage from retinal vessels, 2) fluorescein leakage from peripheral retinal vessels, 3) fluorescein leakage from the disc or macular vessels, and 4) fluorescein leakage from retinal vessels associated with pigment epithelial and choroidal changes. The electro-oculogram light rise was abnormally increased in the eyes in category 1, but it progressively declined for those in the other categories. The ratio of the b-wave (postreceptoral component) and a-wave (receptoral component) of the flash electroretinogram was unchanged in all categories, but the electroretinographic amplitudes progressively declined from a somewhat supernormal level in category 1 to subnormal in the other categories. Thus, in inflammatory eye disease, changes in the electrical potentials arising in the pigment epithelium and photoreceptors are the earliest detectable signs. Some biochemical changes in the choroid, pigment epithelium, and the photoreceptors appear to take place before any pathological changes in these structures or in the retinal vessels are detectable by ophthalmoscopy or fluorescein angiography.     

Acid-base balance affects electroretinogram b- and c-wave differentially in the perfused cat eye

The pH concentration strongly affects b- and c-waves of the vitreal direct coupled electroretinogram (DC/ERG) in the arterially perfused cat eye, in which acid-base balance can be changed temporarily excluding extraocular regulation (Niemeyer and Steinberg, 1984; Dawis et al., 1984, 1985; Niemeyer et al., 1984;. To increase pHi we injected NaOH and NH₄Cl or substituted for control perfusate low PCO₂, low HCO ₃ ^- perfusate. This induced an increase in b-wave and a decrease in c-wave amplitude. To decrease pHi we injected HCl and NaHCO ₃ ^- , increased PCO₂, or substituted high PCO₂, high HCO ₃ ^- perfusate. This induced a decrease in b-wave and an increase in c-wave amplitude.Covariation between standing potential and c-wave was found in only three of the seven experimental conditions. The change in standing potential was faster than that in the ERG waves. It is suggested that changes in pH affect primarily the retinal component of the c-wave. The b-wave consistently followed the direction of the expected change in pHi. All effects were reversible.     

The negative response of the flash electroretinogram in glaucoma

The existence of a negative ERG component following the b-wave has been known for a long time. Recently, in unilateral macaque experimental glaucoma, a negative response in flash electroretinograms under scotopic as well as photopic conditions has been shown to be greatly reduced or absent compared to the healthy fellow eye. The aim of this pilot study was to test whether a late negative electroretinogram component is reduced also in human glaucoma patients under different stimulus conditions. Dark-adapted ganzfeld flash electroretinograms were recorded after 30 min of dark using two test conditions, obtained as optimal in pilot studies on controls. Under the scotopic condition I white Xenon-flashes of intensity 0.53 Log photopic Td s were presented on a low white background of 1.38 Log scotopic Td. Under the more photopic condition II orange flashes of intensity –0.37 Log photopic Td s were presented on a blue-adapting background of 2.5 Log scotopic Td. Nine controls and 18 patients with advanced glaucoma were analyzed. The amplitude of the negative response was not significantly reduced in glaucoma patients (condition I: –28.5±23.7 V; condition II: –25.2±19.7 V) compared to controls (condition I: –41.4±36.6 V; condition II: –31.3±26.2 V). The peak latency of the responses under condition I and II did not differ significantly between patients and controls. Thus, the late negative electroretinogram component in ganzfeld flash electroretinograms obtained under scotopic and more photopic conditions does not seem to distinguish as easy between human controls and glaucoma patients as animal experiments suggest.     

Utility in clinical practice of standard vs. high-intensity ERG a-waves

Purpose: Standard ERG a-waves represent contributions from both photoreceptor and inner retinal cells, while the leading edge of the high-intensity a-wave is produced only by photoreceptors. This has raised questions about the value of the a-wave as an indicator of photoreceptor disease, and has led to suggestions for standardizing higher-intensity stimuli. Our objective was to compare the behavior of standard and high-intensity a-waves in clinical practice. Methods: Standard ISCEV (International Society for Clinical Electrophysiology of Vision) a-waves and high-intensity a-wave responses were recorded under scotopic and photopic conditions from normal subjects and from patients with photoreceptor dystrophies and other diseases. Results: The standard scotopic a-wave amplitude followed the high-intensity a-wave closely among patients with different diagnoses, and the results did not change significantly when cone a-waves were subtracted to isolate rod signals. The only exception was one patient with the enhanced S cone syndrome (ESCS) whose dark-adapted responses were cone-driven. Initial peak times clustered in a small range for both standard and high-intensity responses, and were not very sensitive to disease. Conclusion: High-intensity a-waves can show photoreceptor characteristics directly, and may help analyze some rare disorders. However, in our study the amplitude of conventional scotopic a-waves mirrored that of the high-intensity responses quite closely over a wide range of patients. This suggests that for practical purposes even if it is not perfect, the standard ERG is an excellent indicator of photoreceptor disease.