Objective functional ancillary testing of the retina and/or retinal pigment epithelium (RPE) that records electrical responses based on various stimuli and assists in the diagnosis and management of various retinal diseases, especially inherited retinal dystrophies, autoimmune disorders, inflammations, and medication toxicities.
Key Features
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Electroretinography (ERG) records the electrical response evoked from the entire retina by a brief flash of light and consists of an a -wave, which represents the photoreceptor response, and a b -wave, which represents the combined response of the Müller and bipolar cells.
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The multifocal ERG (mfERG) is a record of multiple local ERG responses elicited from the central 40° of the retina.
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Electrooculography (EOG) records the standing electrical potential generated by the RPE.
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The International Society for Clinical Electrophysiology of Vision (ISCEV) provides worldwide standardized clinical protocols for electrophysiological testing, including guidelines for ERG, mfERG, and EOG testing.
Introduction
Electrophysiology encompasses several objective examination techniques that measure the function of the retina by measuring action potentials caused by particular patterns of light stimulation within the retina. Clinical electroretinography (ERG) is useful to determine the existence of retinal degenerative conditions caused by hereditary, toxic, metabolic, retinal vascular, or inflammatory etiologies, being particularly valuable in determining the abnormal nature of what clinically appears to be a normal retina. Full-field ERG represents a mass-evoked response of the outer retinal layers, which reflects total retinal function. The multifocal (mf)–ERG represents a cone-generated response of localized retinal function in the central macula, which is useful in establishing the presence of macular dysfunction. Electro-oculography (EOG) represents the standing electrical potential of the entire eye, which reflects the pigment epithelium. These electrophysiological results should be considered in conjunction with those of complete medical and ophthalmic evaluations, including a careful history and laboratory testing, when indicated.
Full-Field ERG
The full-field, or Ganzfeld, ERG measures a mass response generated by cells from the entire retina. Photoreceptors generate the initial negative component, or a -wave, whereas Müller cells and bipolar cells are responsible for the later, positive, b -wave. Both a -wave and b -wave are best illustrated in the maximal combined rod-cone response. The ganglion cell layer does not contribute to the ERG. The full-field ERG is useful for establishing generalized loss of rod or cone function, or both. Patients who have focal macular disorders do not have abnormalities of full-field ERG amplitude, nor do patients who have diseases of the inner retina, optic nerve, or cortical conditions.
Starting in 1989, the International Society for Clinical Electrophysiology of Vision (ISCEV) has put forth standardized basic ERG protocols to allow comparison across laboratories, and these protocols are periodically updated. The most recent ISCEV standard for full-field clinical ERG includes six protocols, which are named according to the stimulus (flash strength in candela-seconds per meter squared [cd/s/m 2 ]) and the adaptation state ( Fig. 6.9.1 ). In preparation, the pupils should be maximally dilated, and the pupil size should be noted before and after the responses are recorded. Prior to recording scotopic electroretinograms, the patient should have 20 minutes of dark adaptation, and prior to recording scotopic electroretinograms, the patient should have 10 minutes of light adaptation. Following dark adaptation, weaker flashes of light stimuli should be presented prior to stronger flashes. Likewise, fluorescein angiography (FA) and fundus photography should be avoided directly prior to testing. Contact electrodes (connected to the positive input) are utilized and include corneal contact lenses, conjunctival conductive fibers, and lower eyelid skin electrodes. The cornea is protected with nonirritating conductive solution, and topical anesthesia is employed, as necessary. Reference electrodes (connected to the negative input) are incorporated either as a contact lens–speculum assembly or as skin electrodes placed near each orbital rim. A separate electrode is used as the common electrode and is attached to the earlobe, mastoid, or forehead. Full-field (Ganzfeld) stimulation is used for uniform retinal illumination with a fixation spot. The maximum flash duration is 5 milliseconds (ms). The flash strength for ERG testing is described in units of cd/s/m 2 , with a weak flash stimulus strength of 0.010 photopic cd/s/m 2 , a standard flash stimulus of 3 cd/s/ m 2 , and a strong flash stimulus of 10 photopic cd s m −2 . Light adaptation and background luminance are set at 30 cd/s/m 2 . Stimulus and response names are described by the state of light adaptation and the flash strength in photopic cd/s/m 2 .
Normal values for full-field ERG vary by laboratory, so normal intralaboratory value ranges should be given with every ERG report, and reference values should be adjusted for age. Ocular pigmentation, high refractive errors, and time of recording should be noted.
An electrophysiological response may be affected in both amplitude and/or timing ( Fig. 6.9.2 ). The a -wave amplitude is measured from the prestimulus baseline to the a -wave trough, and the b -wave amplitude is measured from the a -wave trough to the b -wave peak. Timing includes the peak time (also called implicit time, t ) and is measured from the time of the flash to the peak of the wave of interest (see Fig. 6.9.2 , arrows ). The amplitude of a flicker ERG is measured from the trough to the peak of a typical wave, and the implicit time is measured from the midpoint of the stimulus to the following peak. With regard to measuring oscillatory potentials, most clinical applications are noting the presence and waveform of the peaks in comparison with reference data.
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