The electrical response is a result of a retinal potential generated by light-induced changes in the flux of transretinal ions, primarily sodium and potassium. Most often, ERGs are obtained using electrodes embedded in a corneal contact lens, which measure a summation of retinal electrical activity at the corneal surface. The ERG can provide important diagnostic information on a variety of retinal disorders including, but not limited to congenital stationary night blindness, Leber congenital amaurosis, and cancer-associated retinopathy. Moreover, an ERG can also be used to monitor disease progression or evaluating for retinal toxicity with various drugs or from a retained intraocular foreign body.
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Creel Donnell J. Creel 1. Introduction Electrophysiological testing of patients with retinal disease began in clinical departments in the late nineteen forties. Under the influence of the Swedish pioneers, Holmgren and Granit , the electroretinogram was being dissected into component parts and early intraretinal electrode studies were beginning to tell which cells or cell layers gave rise to the various components.
A detailed discussion of the electroretinogram, or ERG as it is commonly abbreviated, is found in the accompanying chapter by Ido Perlman. The changes in the standing potential across the eyeball were recorded by skin electrodes during simple eye movements and after exposure to periods of light and dark.
Over the years ERG recording techniques have become progressively more sophisticated in the clinical setting. With the advent of perimetry, optical coherence tomography OCT and pattern ERG techniques, more precise mapping of dysfunctional areas of the retina is now possible. The mfERG provides a detailed assessment of the health of the central retina. Where the previous chapter The electroretinogram: ERG, Ido Perlman presents the basic science behind the waveforms and components of the massed ERG response, in this chapter the intention is to show the clinical use of the various electrophysiological tests.
The electroretinogram ERG. The global or full-field electroretinogram ERG is a mass electrical response of the retina to photic stimulation.
The ERG is a test used worldwide to assess the status of the retina in eye diseases in human patients and in laboratory animals used as models of retinal disease. The basic method of recording the electrical response known as the global or full-field ERG is by stimulating the eye with a bright light source such as a flash produced by LEDs or a strobe lamp.
The flash of light elicits a biphasic waveform recordable at the cornea similar to that illustrated below Fig 1. The two components that are most often measured are the a- and b-waves. The a-wave is the first large negative component, followed by the b-wave which is corneal positive and usually larger in amplitude. The ERP latency is less than 1 microsecond.
Due to photovoltaic effects ERPs are best recorded using no metal such as with cotton wick contact shown in Fig 4. The ERP is difficult to record and is not commonly used clinically. Early receptor potential occurring in first two milliseconds prior to a-wave. Figure 1b depicts a concocted ERG of all components if one would stimulate the retina with a long light pulse producing an off response.
Hypothetical ERG showing all components if stimulus included a long pulse of light. Two principal measures of the ERG waveform are taken: 1 The amplitude a from the baseline to the negative trough of the a-wave, and the amplitude of the b-wave measured from the trough of the a-wave to the following peak of the b-wave; and 2 the time t from flash onset to the trough of the a-wave and the time t from flash onset to the peak of the b-wave Fig.
In contrast, the b-wave reflects the health of the inner layers of the retina, including the ON bipolar cells and the Muller cells Miller and Dowling, Two other waveforms that are sometimes recorded in the clinic are the c-wave originating in the pigment epithelium Marmor and Hock, and the d-wave indicating activity of the OFF bipolar cells see Figure 3.
Later we shall discuss some wavelets that occur on the rising phase of the b-wave known as oscillitatory potentials OPs. OPs are thought to reflect activity in amacrine cells Fig. A laboratory should test normal subjects to ensure their testing environment produces similar results. A normal ERG in a newborn infant can be small amplitude the first couple of months. The ERG attains peak amplitude in adolescence and slowly declines in amplitude throughout life Weleber, After age years the amplitude of the ERG declines even more.
Implicit times slow gradually from adolescence through old age as well. Below are two figures illustrating how the b-wave attenuates in amplitude with age and slows in its implicit time Fig.
There is considerable variation among individuals but the linear regression line in each figure indicates the trend of aging affects on the ERG. ERG recording electrodes. The ERG can be recorded several ways. The pupil is usually dilated. There are a number of corneal ERG electrodes that are in common use. Some are speculum structures Fig. Some versions use carbon, wire or gold foil to record electrical activity. There are also cotton wick electrodes Fig.
Most electrodes are monopolar, i. Some are bipolar with the reference electrodes built into a metal surface on a speculum. We use Burian speculum electrodes when possible.
Sizes are available down to a size that fits in the eye of most full-term babies. When the eye is too small for speculum recording electrodes we use the ERG Jet type most of the time. The ERG can also be recorded using skin electrodes placed just above and below the eye, or below the eye and next to the lateral canthus.
Since skin electrodes are not in direct contact with the eye there is significant attenuation in amplitude of the ERG, so a number of individual responses to flash stimulation are usually averaged by computer.
Pictured in Figure 6 is a comparison of bright white flash ERGs recorded from the same person using three types of recording devices and an averaged ERG from skin electrodes.
Follow sterilization recommended by the manufacturer. Do not leave electrodes in this solution for more than a few minutes. Light stimulation for ERGs. There are also several methods of stimulating the eye. Some laboratories use a strobe lamp that is mobile and can be easily placed in front of a person whether sitting or reclining Fig.
The mobility of a strobe lamp or an array of LEDs is a necessity in some situations such as at the hospital bedside or in the operating room. The Ganzfeld stimulation globe For patients over 5 years of age most laboratories use a Ganzfeld globe with a chin rest and fixation points Fig. The Ganzfeld allows the best control of background illumination and stimulus flash intensity. Either strobe lamp or Ganzfeld methods of flash presentation can be used to record the ERG following a single flash or to average responses to several flashes with the aid of a computer.
Clinical decisions can be made from ERGs generated by either methodology. Testing infants for ERGs Infants up to about 2 years of age can usually be tested without sedation by the parent holding them bundled in a blanket.
It is difficult to convince a child less than 5 years of age to allow a contact lens or speculum recording electrode in their eye. Alternatively, the child is sedated or anesthetized.
Few laboratories have Ganzfeld stimulators that can be tilted and placed over the face of a sedated patient and it is difficult to use such equipment in the operating room. Thus flash stimuli with sedated patients are usually delivered with a strobe lamp Fig.
Mesopic single flashes, oscillatory potentials and 30 Hz flicker can be used to evaluate retinal function. It is difficult to completely darken the O. Anesthesia affects the ERG varying with type and depth of anesthesia. Light levels of anesthesia have little affect and most anesthetics do not affect a-waves or implicit times. Coordinate with anesthesiologists to attain a light level of anesthesia. Separating rod and cone ERGs Most disorders of the retina are detected by an attenuation of amplitude.
Implicit times, of both a- and b-waves are also affected in some conditions. Implicit times and amplitudes vary depending upon whether the eye is dark adapted or not, and brightness and color of the light stimulus. These parameters allow separation of rod and cone activity in any duplex retina.
Rods and cones differ in number, peak color sensitivity, threshold and recovery. There are about million rods in each retina and about million cones see Facts and Figures chapter. Because of sheer numbers, the ERG following a white flash is dominated by the mass response of the rods.
By manipulating adaptation level and background illumination, flash intensity, color of the flash and rate of stimulation, rod and cone activity can be significantly isolated. Using color stimuli Peak wavelength sensitivity for rods is around nm and the peak sensitivity of cones as a group is about nm Tennis ball yellow Fig. By using color filters such as the Kodak Blue and Red Wratten series, or color flashes generated by LEDs Figure 9a , you can isolate rod and cone ERGs using dim flash stimuli into photopic cone and scotopic rod signals as illustrated in Figure 9b.
Dim red flashes stimulate both rod and cone function producing a small photopic component bx and larger rod b-wave. Rods are about three log units more sensitive than cones. However cones recover faster than rods. Even under ideal conditions rods cannot follow a flickering light up to 20 per second whereas cones can easily follow a 30 Hz flicker, which is the rate routinely used to test if a retina has good cone physiology Fig.
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
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. However, by using scotopically matched backgrounds, one can see that mechanisms intrinsic to the cone system itself are also involved Such an experiment is illustrated in Fig. Therefore, the ERP is believed to reflect dipole changes in the visual pigment molecules because of conformational changes that are elicited by photon absorption.
The Electroretinogram and Electro-oculogram: Clinical Applications by Donnell J. Creel
Creel Donnell J. Creel 1. Introduction Electrophysiological testing of patients with retinal disease began in clinical departments in the late nineteen forties. Under the influence of the Swedish pioneers, Holmgren and Granit , the electroretinogram was being dissected into component parts and early intraretinal electrode studies were beginning to tell which cells or cell layers gave rise to the various components. A detailed discussion of the electroretinogram, or ERG as it is commonly abbreviated, is found in the accompanying chapter by Ido Perlman. The changes in the standing potential across the eyeball were recorded by skin electrodes during simple eye movements and after exposure to periods of light and dark. Over the years ERG recording techniques have become progressively more sophisticated in the clinical setting.
Nijora They replaced the vitreous humor of cats with heavy oil to abolish current flow from the retina to distant sites and thereby ensured large potential recordings of local ERG from the retinal surface. Amplitude and Implicit Time Measurements The most common ERG response from a human, which is elicited with a full-field Ganzfeld flash of light, contains the a-wave and the b-wave as shown in Fig. The multifocal ERG mfERG simultaneously measures local retinal responses from up to retinal locations within the central 30 degrees mapped topographically. Such analysis can also be used to compare ERG data between laboratories as shown in Fig. For the fraction of the unit of energy erg, see micro-erg. Figure 10 A typical early receptor potential ERP of the human eye that was elicited by a very bright light flash arrow. It is normally masked throughout most of its period by the large, positive b-wave.