Electroretinograms

The electroretinogram (ERG) is an electrical response of the retina to photic stimulation. A flash of light or bright appearance of a pattern elicits a biphasic negative/positive waveform. The a-wave originating in the receptor level of rods and cones is the initial large negative wave. The b-wave originating in the midretina is the following large positive component. Protocols include full-field and multifocal ERGs. Full-field ERGs are recorded if one is most interested in the global health of the retina, such as in retinitis pigmentosa. A limitation of the full-field ERG is that the recording is a massed potential from the whole retina. Multifocal ERGs can map small scotomas in the central 40+ degrees of visual field. Abbreviated methods are used with infants and in the operating room as part of an exam under anesthesia. The effects of toxic medications can be detected and quantified using ERGs. The ERG is useful to assess cases of retinal foreign bodies and trauma to estimate the extent of retinal dysfunction. Foreign bodies affect retinal function depending on the extent of trauma to the retina and the location and composition of the object. Systemic metabolic disorders usually reduce ERG b-wave amplitudes, particularly the scotopic dim flash ERGs.