Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells. Basic mechanisms of electrical field generation in the tissue. Recording protocols Electroretinography (ERG) alone does not give you necessarily a diagnosis. Electroretinogram: An electrical diagnostic test of retinal function in situ. Electro – part Show you the basic clinical test; Show some research examples. The Eye .
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The electroretinogram ERG is a diagnostic test that measures the electrical activity generated by neural and non-neuronal cells in the retina in response to a light stimulus.
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 Electroretonogram can also be used to monitor disease progression or evaluating for retinal toxicity with various drugs or from a retained intraocular foreign body. James Dewar of Scotland subsequently performed it in humans inhowever widespread clinical application did not occur untilwhen American psychologist Lorin Riggs introduced the basivs electrode.
Many of the observations and analyses that serve as the basis for understanding electrodetinogram ERG components today were conducted by Ragnar Granit for which he won the Nobel Prize for Physiology and Medicine in This wave reflects the hyperpolarization of the photoreceptors due to closure of sodium ion channels in leectroretinogram outer-segment membrane. Absorption of electroretinogrwm triggers the rhodopsin to activate transducin, a G-protein.
This leads to the activation of cyclic guanosine monophosphate phosphodiesterase cGMP PDE eventually leading to a reduction in the level of cGMP within the photoreceptor. This leads to closure of the sodium ion channels resulting in a decrease of inwardly directed sodium ions, or a hyperpolarization of the cell.
The a-wave amplitude is measured from baseline to the trough of the a-wave. The hyperpolarization of the photoreceptor cells results in a decrease in the amount of neurotransmitter released, which subsequently leads to a depolarization of the post-synaptic bipolar cells. The bipolar-cell depolarization increases the level of extracellular potassium, subsequently generating a transretinal current. It is this transretinal current that depolarizes elecctroretinogram radially oriented Muller cells and generates the corneal-positive deflection.
The b-wave amplitude is generally measured from the trough of the a-wave to the peak of the b-wave. This wave is the most common component of the ERG used in clinical and experimental analysis of human retinal function. The c-wave is a reflection of the resulting change in the transepithelial potential due to the hyperpolarization at the apical membrane of the RPE cells and the hyperpolarization of bxsics distal portion of the Muller cells.
The c-wave generally peaks within 2 to 10 seconds following a light stimulus, depending on flash intensity and duration.
Due to elecrtoretinogram c-wave response developing over several seconds, it is susceptible to influences from electrode drift, eye movements, and blinks. Dark adapted Oscillatory potentials: Implicit time or peak time is a measure of the time interval from onset of the stimulus to the peak of the b-wave. For single-flash ERGs, in cases of Xenon flash tube, the duration of flash is less than a milisecond.
Electroretinogram – EyeWiki
Light emitting diodes LEDs produce longer flashes upto 5 milisecond, and peak time in such cases should be measured from the midpoint of the flash to compensate the effect of flash duration on the p eak tim e, according to the current ISCEV guidelines update.
This includes 6 protocols named according to the strength of the stimulus in candela. The rod and cone photoreceptor function responses can be separated using a variety of ERG techniques. Scotopic rod responses are isolated by dark-adaptation for a minimum of 20 minutes per ISCEV standards followed by a short wavelength stimulus as a single flash or 10 Hz flicker. Although the resulting response has rod and cone components, the rod component is dominant and the primary contributor to the increased amplitude and increased implicit time.
Photopic cone responses can be obtained either before or after dark-adaptation. Since rods cannot follow a flicker stimulus greater than 20 Hz, cone photoreceptor function is primarily measured under light-adapted conditions for at least 10 minutes with either single flash stimulus wavelength greater than nm or 30 Hz flicker stimulus.
Photopic responses result in small b-wave amplitudes with a short latency mswhereas scotopic rod conditions produce much larger b-wave amplitudes with a longer latency 60 ms. Oscillatory potentials OP are high-frequency, low-amplitude wavelets on the ascending limb of the b-wave with a frequency of about to Hz. Although it is not known for certain, it is suspected that OPs are generated from the amacrine cells located in the inner retina. The focal ERG fERG; also known as the foveal ERG is used primarily to measure the functional integrity of the fovea and is therefore useful in providing information in diseases limited to the macula.
A variety of techniques have been described in the literature for recording fERGs. Differing field sizes varying from 3 degrees to 18 degrees and light stimulus frequencies have been used in the various methods, however each technique deals with the challenge of limiting amount of light scattered outside the focal test area.
Focal ERG is useful for assessing macular function in conditions such as age-related macular degeneration, however requires good fixation from the subject.
The full-field ERG Ganzfeld; ffERG measures the stimulation of the entire retina with a flashlight source under dark-adapted scotopic and light-adapted photopic types of retinal adaptation. This is useful in detecting disease with widespread generalized retinal dysfunction i. Due to the massed retinal electrical response, small retinal lesions may not be revealed in ffERG recordings. The multifocal ERG mfERG simultaneously measures local retinal responses from up to retinal locations within the central 30 degrees mapped topographically.
This new technology was developed by Erich Sutter in the early s and involves powerful computers and high —intensity display monitors. The light stimuli are presented on a video monitor in one of a large number of arrays consisting of hexagonal elements.
The hexagonal elements in the array are distributed so that the focal retinal responses have an approximately equal signal-to-noise ratio. The central hexagons are smaller than those in the periphery. The elements are stimulated in a pseudo-random sequence of light and dark, called a maximum length sequence or m-sequence.
The resulting waveforms are similar to those of the ffERG: Degree of retinal toxicity related to certain drugs such as hydroxychloroquine or ethambutol is better detected using mfERG compared to ffERG. Early visual field defects due electroretinotram glaucoma may also be detected sooner using mfERG compared to automated perimetry.
The PERG is used to detect subtle optic neuropathies. In demyelinating optic neuropathy, the PERG is relatively normal, while it may be abnormal in ischemic optic neuropathies. P50 evaluates the macular function.
Marked rod dysfunction and elevated threshold of rods and cones on dark adaptation. Normal scotopic a-wave with selectively reduced b-wave; implicit time of b-wave is approximately the same under scotopic and photopic conditions.
Reduced scotopic amplitudes which improve to normal values after longer variable perior of dark adaptation.
Extent of reduced a- and b-wave amplitudes depends on extent of fundus pigmentary changes; longer duration of dark-adaptation may be necessary for scotopic amplitudes to reach normal values. Reduced a- and b-wave amplitudes in both scotopic and photopic conditions; increased rod and cone b-wave implicit times. Normal or mild to moderately reduced photopic and scotopic responses variable depending on extent of fundus involvement.
Electroretinnogram reduction in b-wave amplitude more prominent in scotopic than photopic responses; delayed photopic and scotopic b-wave implicit times. Initially loss of oscillatory potentials in flash ERG with subsequent reduction of b-wave amplitude. Normal ERG in the absence of peripheral retinal neovascularization, reduced amplitudes of ERG components when peripheral retinal neovascularization is present.
Initially decreased oscillatory potentials, later stages involve reduced a- and b-wave amplitudes. Normal ERG responses unless presence of advanced retinopathy; cone function initially more affected than rod function. Transiently decreased ekectroretinogram and b-wave amplitudes under both photopic and scotopic conditions with recovery after 24 hours. Initially a transient supernormal response then negative pattern followed by non-detectable response in severe cases rod function more affected than cones; reduction of b-wave amplitude more than a-wave.