Mijind In general, when retinal function deteriorates, the light-induced electrical activity in the retina reduces. Degree of retinal toxicity related to certain drugs such as hydroxychloroquine or ethambutol is better detected using mfERG compared to ffERG. Original article contributed by: Measurements with potassium-sensitive microelectrode in the photoreceptor layer shows a light-induced decrease in the slectroretinogram concentration of potassium ions, due to light-induced electrical activity in the photoreceptors. Figure 8 The effects of barium ions on the ERG responses of the rabbit.
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The electrical basis of ERG recordings. ERG responses are recorded with an active extracellular electrode positioned either on the cornea, in the vitreous or at different levels inside the retina. Extracellular recording of electrical activity of living tissue is rendered possible when electrical currents spread along an extracellular matrix with electrical resistance. Similarly, extracellular currents from all retinal cell types will sum up only if they are directed radially.
In contrast, lateral currents will cancel each other since the retinal lateral arrangement is completely symmetrical. Therefore, when a homogenous light stimulation is directed at the whole retina, only radial extracellular currents are formed. We assume that a light stimulus elicits extracellular electrical currents that flow from sources to sinks.
These currents will flow through different pathways including local and remote ones. We can divide these currents into two principal pathways, the local one A and the remote one B as shown schematically in figure 3a. The current in pathway A, flows through a local route remaining entirely within the retina, while the current flowing through pathway B leaves the retina through the vitreous and anterior ocular tissue and returns to the retina through the sclera, the choroid and the pigment epithelium layer.
The light-induced current flowing through pathway B can be recorded in a noninvasive manner, with extra-ocular electrodes, as illustrated in figure 3a. An electrical scheme of the resistances through which currents IA and IB figure 3a flow when the retina is stimulated with light. The current source I, represents the electrical current that is generated in the retina in response to a light stimulus.
Figure 3b shows an equivalent electrical circuit of the eye Rodieck, A light stimulus elicits an extracellular current source I that divide into two pathways; one flowing through the retina local pathway, IA in Fig. Each tissue e. Therefore, the voltage difference between points A and B can be calculated for the local or remote pathways. When using two electrodes to record light-induced electrical activity of the retina, the largest light-induced potential change will be monitored if the measurement is done between points A and B, which are on the two sides of the cells producing the electrical response.
However, when the electroretinogram is recorded from humans or from laboratory animals during chronic experiment, the electrodes cannot be inserted into the retina. The alternative is to record from extraocular sites by placing both active and reference electrodes outside the eye.
In general, when retinal function deteriorates, the light-induced electrical activity in the retina reduces. The currents IA and IB will be smaller and the ERG will be smaller too thus, indicating retinal pathology However, we have to remember that the magnitude of the different resistances and more so, the relationships between them can also affect the ERG that is measured with extra-ocular electrodes. The division of the current originating from the light-induced retinal activity into the local and remote pathways depends upon the relative resistances of the two pathways.
The pigment epithelium layer R-membrane offers the highest resistance to electrical current along the ocular tissues Brindley, ; Brindely and Hamasaki, ; Byzov, ; Ogden and Ito, as denoted by a large resistor R6 in figure 3b.
Therefore, any change in the magnitude of this resistor will affect the distribution of currents between the retinal pathway IA and the remote pathway IB.
This change will be reflected in the ERG responses that are measured with extra-ocular electrodes. Such changes in the distribution of resistances may account for species differences in the magnitude of the ERG responses and for intra-subject differences within a given species. The importance of the resistances of the ocular tissues has been recognized by Arden and Brown They replaced the vitreous humor of cats with heavy oil in order to abolish current flow from the retina to distant sites and thereby ensured large potential recordings of local ERG from the retinal surface.
In the clinical environment, it is well documented that the ERG can be reduced significantly in patients with giant retinal tears who have undergone vitrectomy surgery and injection of silicon oil into the vitreous. Since silicon oil does not conduct electric currents, the resistance of the vitreous increases by several folds causing the current IB to be so reduced that the ERG becomes very small in amplitude Doslak et al.
The origin of the major ERG waves. However, the cellular origin of the different components needs to be understood. Basically, two types of approaches, physiological and pharmacological, have been used to dissect out these cellular origins.
The physiological experiments are based on the assumption that the generators of specific ERG components are located in specific retinal layers and therefore, when these are passed by the intra-retinal microelectrode, the polarity of the specific ERG waves will reverse. These current source-density analyses have indeed revealed the anatomical location within the retina of the different ERG components. The pharmacological approaches to ERG analyses are based on retinal physiology and biophysics.
In these experiments, specific agonists and antagonists of cellular mechanisms are applied and their effects on the ERG then analyzed. In the following section, the sources of the a-, b- and c-waves will be discussed, not according to the order of their timing in the ERG response but according to the retinal level at which they are generated, starting from the most distal layer, the pigment epithelium.
The first indication of this was reported by Noell He showed that systemic injection of sodium azide elicited an electrical potential from the retina similar to the ERG c-wave. The azide-induced potential was not influenced by iodoacetic acid, which is known to destroy photoreceptors, or by cutting the optic nerve, which causes degeneration of the ganglion cells.
This interpretation of the c-wave origin was proven directly when intracellular recordings were made from pigment epithelial cells. The potential changes that were recorded from these cells in response to light stimuli were identical in shape and temporal properties to the ERG c-wave Steinberg et al. Furthermore, when the retina was separated from the pigment epithelium, the ERG response contained normal a- and b-wave, but the c-wave disappeared.
Figure 4 shows ERG recording from the skate eyecup that consists of a-wave, b-wave and c-wave upper trace. When the retina is separated from the sclera and pigment epithelium, the ERG response contains only the a- and b-waves. Furthermore, aspartate, by blocking transmission from photoreceptors to bipolar cells, completely removes the b-wave Fig. ERGs recording from the skate. The recording was done from the whole eyecup upper trace , following the separation of the retina from the pigment epithelium middle trace and after exposing the retina to aspartic acid lower trace Pepperberg et al.
The pigment epithelium cells are functionally asymmetrical cells with their basal membrane toward the choroid less permeable to potassium ions than the apical membrane retinal side. This asymmetry causes a constant potential difference between the retina and the choroid with the retinal side positive relative to the choroidal side. The standing potential is very sensitive to the extracellular concentration of potassium ions. Any change in concentration of potassium ions at one side will be expressed in a change in the whole trans-epithelial potential.
Measurements with potassium-sensitive microelectrode in the photoreceptor layer shows a light-induced decrease in the extracellular concentration of potassium ions, due to light-induced electrical activity in the photoreceptors.
The reduction in the extracellular concentration of potassium ions near the apical membrane of the pigment epithelial cells is expressed as an increase in the trans-epithelial potential with the retinal side becoming more positive relative to the choroidal side. To simplify the comparison, the KRG responses are inverted, thus a positive deflection in this figure means a reduction in extracellular concentration of potassium ions.
The intensity series Fig. The intensity-response curves show excellent correlation between peak amplitude of the c-wave and peak reduction in potassium concentration Fig. Three different intensities of the light stimulus were applied. B Comparing time-to-peak upper panel and peak amplitude lower panel of the ERG c-wave and the KRG Oakley and Green, Although the c-wave originates from the pigment epithelium, it depends upon the integrity of the photoreceptors, because light absorption in the photoreceptors triggers the chain of events leading to the decrease in extracellular concentration of potassium ions.
Therefore, the ERG c-wave can be used to assess the functional integrity of the photoreceptors, the pigment epithelial cells and the interactions between them. The most important information on the origin of these waves was obtained from ERG recordings with intra-retinal microelectrodes Tomita, ; Brown and Wiesel, a, b; Brown and Murakami, a; Brown, These studies suggested the photoreceptor layer as the origin of the fast P-III wave.
Differential recording in the rat retina using two microelectrodes revealed that the a-wave resulted from extracellular radial current. A pharmacological approach to the study of the origin of the ERG a-wave became possible when the identity of the neurotransmitter released from the photoreceptors became known. Since L-glutamate is the neurotransmitter of the photoreceptors, exposing the retina to agonists or antagonists of L-glutamate can effectively block synaptic transmission from the photoreceptors and isolate the contribution of the photoreceptors to the ERG.
This can be achieved by exposing the isolated skate retina to L-aspartate, an excitatory acidic amino acid Fig. Figure 6 shows ERG responses from dark-adapted rabbits that were recorded 3 hours after intravitreal injection of L-glutamate or 2-amino-phosphonobutyric acid APB into one eye and saline into the fellow control eye A and B respectively.
The experimental drug was injected into the right eye lower trace and saline into the left eye upper trace as a control. However, by separating the retina from the pigment epithelium, P-I can be eliminated and by exposing the retina to drugs, such as aspartic acid, that block synaptic transmission from the photoreceptors to the neurons in the inner nuclear layer, the P-III component can be isolated and studied Witkovsky et al.
The data in figures 4 and 6 clearly indicate that the b-wave originates in retinal cells that are post-synaptic to the photoreceptors. Blocking synaptic transmission from the photoreceptors to second order retinal neurons by saturating the post-synaptic receptors with L-aspartate Fig. In fact, any procedure that blocks synaptic transmission from the photoreceptors, like superfusion with cobalt ions or with high magnesium low calcium solutions, will eliminate the ERG b-wave Furakawa and Hanawa, ; Sillman et al.
The b-wave is also eliminated when the blood flow through the central retinal artery is blocked either intentionally in laboratory animals Noell, ; Brown and Watanabe, , or in human patients Nilsson, Since the neural retina is supplied by the retinal vasculature and the photoreceptors by the choroidal plexus, this experimental manipulation, or pathological cases, effectively eliminates light-induced electrical activity in the neural retina from the photoreceptors.
Applying sink-source analysis to electrophysiological recordings of the intra-retinal ERG responses at different retinal depths further reveals the location of the b-wave P-II generators. Faber was the first to calculate the extracellular currents that underlie the ERG b-wave of the rabbit eye. He reported that a sink for the b-wave was in the distal part of the retina, most probably in the outer plexiform layer, while the source was distributed proximally and distally to the sink.
The most effective are potassium ions Miller, Studies were done in mudpuppy Karwoski and Proenza, ; Karwoski et al. These and other studies reported a light-induced increase in extracellular potassium in the outer and inner plexiform layers.
This increase was thought most likely due to leakage from depolarizing retinal neurons. It was assumed that the origin of potassium increases in the outer plexiform layer was bipolar cells, most specifically ON-center bipolar cells that were depolarized by light Dick and Miller, In the inner plexiform layer, the increase in extracellular potassium resulted from light-induced activity of amacrine and ganglion cells Karwoski and Proenza, ; Dick and Miller, Depth recordings of extracellular concentration of potassium and of local field potentials are shown in figure 7A Karwoski et al.
Current source-density analysis of data like these Fig. The pathways of the extracellular currents that have been suggested to underlie the generation of the ERG b-wave. The two sinks OPL and IPL reflect the increase in extracellular potassium ions due to light-induced electrical activity. The vitreous serves as a large current source due to the high potassium conductance of the endfeet of the Muller cells Newman, More evidence concerning the source of the ERG b-wave was gained with specific agonists and antagonists to glutamate receptors.
Exposing the vertebrate retina to 2-aminophosphonobutyric acid APB , a specific agonist of glutamate metabotropic receptors, eliminates the ERG b-wave Gurevich and Slaughter, as shown in figure 6B. Since APB-sensitive metabotropic glutamate receptors are found only in ON-center bipolar cells Slaughter and Miller, , this finding constitutes a clear indication of the involvement of these bipolar cells in the generation of the b-wave.
In primates, Sieving and coauthors propose that the b-wave of the photopic ERG response are mainly contributed by the ON-center bipolar cells but are opposed by OFF-center bipolar cells.
Experiments with injection of barium ions into the vitreous of rabbits did not eliminate the ERG b-wave. Under certain conditions barium ions even caused augmentation of the b-wave as shown in figure 8 Lei and Perlman, This was not the case as shown by the ERG response in figure 8. The effects of barium chloride solution injected into the vitreous of one eye, while saline was injected into the vitreous of the other eye, were tested.
The ERGs of the experimental eye that was being injected with barium chloride are augmented compared to those of the control eye. The effects of barium ions on the ERG responses of the rabbit.
The Electroretinogram and Electro-oculogram: Clinical Applications by Donnell J. Creel
What is electroretinography? An electroretinography ERG test, also known as an electroretinogram, measures the electrical response of the light-sensitive cells in your eyes. These cells are known as rods and cones. They form part of the back of the eye known as the retina. There are around million rods in the human eye and six to seven million cones.
ELECTRORETINOGRAM BASICS PDF
Rod phototransduction in retinitis pigmentosa: It is clear from Fig. Scotopic threshold response STR of the human electroretinogram. The elcetroretinogram of the b-wave is measured from the trough of the a-wave to the peak more Signal transmission along retinal rods and the origin of the electroretinographic a-wave. Furthermore, the temporal properties of P-II are of value to the clinician. The ERG originates from extracellular currents that are generated in response to a light stimulus. The visual system in vertebrates can be roughly divided into two subsystems, the rod system night vision and the cone system bsaics vision. National Academy Press; The waves are called a- b- and c-waves.