In the clinical setting, only the early electrical responses of the retina (within the initial 200 msec) are measured, because later responses usually are obliterated by eye blinks. Within this 200 msec time frame, two predominant responses occur: the a-wave and the b-wave (Fig. 103.1).

The a-wave is the initial downgoing deflection, and it arises from the photoreceptor cells.² The b-wave is the upgoing deflection that follows the a-wave, and it arises from the Müller cells.³ Although the derivation of the b-wave is from the Müller cells, it reflects such activity from the region of the bipolar cells.⁴

Under certain recording conditions, small wavelets, called oscillations, may be seen riding on the downgoing and upgoing waves (Fig. 103.2). These oscillatory potentials arise from a number of cell types in the midretinal layers.⁴

From the foregoing discussion, it is clear that the ganglion cells play no role in the generation of the ERG. Therefore, diseases affecting only the inner retina or the optic nerve should not alter the ERG. It is also important to realize that the standard clinical ERG is a mass response, reflecting activity from the entire retina. Thus, small localized lesions (e.g., macular degeneration) will not affect the ERG amplitude.

The electrical discharges elicited by the light stimulus are recorded directly from the eye via a contact lens placed on the cornea. The signal then is amplified and visualized on an oscilloscope or directly written out on any x-y plotter. To enhance the signal, the light usually is delivered via a Ganzfeld (full-field) bowl, a hemisphere used to scatter light throughout the entire retina. This method also avoids some of the problems associated with light scatter.

Two major parameters are used to evaluate the ERG response in the clinical setting. The first is the amplitude of the wave, which is measured in microvolts (μV). The amplitude of the a-wave is measured from the baseline to the trough of the a-wave, whereas the b-wave is measured from the trough of the a-wave to the peak of the b-wave (see Fig. 103.1).

The implicit time is the second major parameter. It is defined as the time from the stimulus onset to the peak of the response and is measured in milliseconds. The easiest and most accurate measure of the implicit time is the b-wave under light-adapted or photopic conditions (Fig. 103.3).

Certain stimulus conditions allow the isolation of either the cone or rod responses so that each receptor can be studied independently. Under photopic or light-adapted conditions with a bright background light, the rods are sufficiently dampened so that the only response is from the cones. The cone response is rapid, with a b-wave implicit time usually between 28 and 32 msec. The cone response also can be isolated by using a rapidly flickering light. The cones follow a flickering light of up to 60 to 70 Hz, whereas the rods follow a flickering light only up to 12 to 16 Hz. Therefore, a stimulus flickered at 30 Hz elicits a response only from the cone receptors (Fig. 103.4).

After sufficient dark adaptation (30 min), the rod responses are optimized under these scotopic conditions. A single bright flash gives a response that is a composite of the dark-adapted rods and the dark-adapted cones. This response is much larger and has a longer implicit time than is the pure cone response. How, then, does one look at the rods alone? Because the rods are very sensitive to light at the blue end of the spectrum, a weak blue-light stimulus produces an essentially pure rod response (Fig. 103.5).

Finally, a red stimulus under scotopic conditions results in a biphasic response in which the initial wave represents the more rapidly responding cones and the second response the slower responding rods (Fig. 103.6). This biphasic response occurs because the rods are relatively insensitive to light at this longer wavelength.

To allow more reliable comparisons of ERGs between labs, a standardization of the clinical full-field ERG was established by an International Standardization Committee.⁵ The committee proposed standards for the following five commonly obtained responses:

(For the details of the basic technology and clinical protocol see reference 5.)

The ERG is helpful in diagnosing a number of disorders. It can be used:

Among the multitude of generalized degenerations of the retina, retinitis pigmentosa is the best known. Although this term has been used generically to describe any generalized retinal degeneration, attention to a family history and evaluation of other family members; assessment of complaints that may indicate systemic disease, long-standing uveitis, or drug use; and careful evaluation of the ERG will help to clarify the diagnosis of disorders in this group and place them into better-defined entities (Table 103.1). It will also help to avoid a mistaken diagnosis of a generalized retinal degeneration.

Any patient with a generalized heredoretinal degeneration, of which retinitis pigmentosa may be considered the prototype, has an abnormal ERG. In most cases, the ERG is extinguished or markedly reduced in amplitude and, in most instances, it has prolonged implicit times.⁶ In a few cases, usually early in the course of the disease, the ERG is affected only slightly in terms of amplitude (usually reduction of the b-wave), but the prolonged photopic implicit time directs the examiner to the appropriate diagnosis (Fig. 103.7).⁷