Subnormal Photoreceptor Functions

Inflammatory diseases

Birdshot chorioretinopathy

Normal to reduced

Normal to reduced

Normal to reduced

Multiple evanescent white dot syndrome (MEWDS)

Normal to reduced

Normal to reduced

Normal to reduced

Luetic chorioretinitis

Normal to subnormal

Normal to subnormal

Normal

Rubella retinopathy

Normal

Normal

Normal

Retinal vasculitis

Normal to depressed

Normal to depressed

Normal

Circulatory disorders

Carotid occlusion

Normal to depressed

Normal to depressed

Normal to depressed

Central retinal artery occlusion

Normal or reduced b-wave

Normal to depressed

Normal to depressed

Diabetic retinopathy

Normal or reduced b-wave and oscillatory potentials

Normal to depressed

Normal to depressed

Other causes

Retinal or choroidal detachment

Normal to extinguished

Normal to extinguished

Normal to extinguished

Media opacities

Normal to reduced

Normal to reduced

No data

The EOG is the testing method of choice when the primary damage is thought to lie in the retinal pigment epithelium. The most important indications for the EOG are a suspicion of Best's vitelliform macular dystrophy and the suspicion of a pigment epitheliopathy, especially when assessing visual function during chronic chloroquine therapy.

Flash ERG Definition

The phototransduction process in the photoreceptor outer segments, i.e., the transformation of a light stimulus into an electrical signal, and the further transmission of this signal elicits field potentials in various layers of the retina. In the flash ERG the potentials are excited by short flashes of light and detected by recording electrodes contacting the anterior surface of the eye.

Contact lenses, foil, or fiber electrodes are used for detecting the electrical potentials of the flash ERG. The fiber electrodes are most readily tolerated by children. As in EOG testing, recording is done with fully dilated pupils. A uniform illumination of the retina is obtained by having the patient positioned at a Ganzfeld sphere (a nearly spherical device with a neutral white interior finish much like that of a perimeter, and with a small port left open for access to the patient's eye) into which the light is flashed.

Depending on the level of light adaptation, the luminance of the surround, and the stimulus strength, a variety of time-based potential curves can be recorded. The Ganzfeld stimulus covers the entire retina, and the recorded responses are a summation of the electrical potentials generated by the entire retina - a mass response of multiple contributing potentials that are layered one on top of the other, and which are emitted by the various layers and cell types of the retinal neural network.

Note

Circumscribed, small area retinal lesions do not lead to measurable changes in the flash ERG.

A complete flash ERG according to the ISCEV standards basically includes records at two different levels of light adaptation:

1. During dark adaptation, i.e., with scotopic conditions, all records consist mainly of responses of the rod system, which is maximally sensitive in dim light or darkness (rod ERG and rod-cone ERG, ■ Fig. 7.2 a-c).

2. Subsequent recordings, done at fully light-adapted, i.e., photopic levels of illumination, comprise signals of the photopic cone system responding at daylight luminance levels (cone ERG, ■ Fig. 7.2 d, e).

In the ERG wave elicited by a single flash, one can identify typical components of the electrical potentials corresponding to various neuroanatomic structures. The two primary components, the a- and b-waves, are particularly clear and easily recognizable in an ERG response (■ Fig. 7.2 b) that was evoked by a strong flash of light in a fully dark-adapted eye.

■ A-wave. A negative component arising at the beginning of the recorded response, produced primarily by the hyperpolarization of the photoreceptor outer segments. This response appears immediately after a strong light flash stimulus. The a-wave is only the leading edge of the negative potential contributed by the photorecep-tors. Its further course is masked by the onset of the b-wave.

■ B-wave. The b-wave arises from electrical activity in the inner layers of the retina, and is primarily generated by potassium currents that are liberated from the depolarizing bipolar cells during the neural processing of the photoreceptor input signals. Additional contributions to the positive b-wave currents arise from Muller's glial cells, which are oriented mostly radial to the vitreous body. The b-wave tests on one hand the integrity of the second neuron of the afferent path (the bipolar cell) and of Muller's cells, and on the other hand, it indirectly reflects photoreceptor function, since the activity of the bipolar cells is determined by the strength of the signal from the rods and cones. This explains why a reduction of the ERG a-wave because of photoreceptor disease also causes a reduction of the b-wave response. Conversely, there are retinal diseases that do not impair photoreceptor function or the a-wave of the ERG, but which selectively depress the b-wave because of damage to the inner layers of the retina, such as in retinal arterial occlusions (see below).

Electroretinogram

Fig. 7.2. Flash electroretinogram (ERG) a Dark-adapted isolated rod response to a flash of low intensity. b Dark-adapted rod-cone mixed response to a light flash of higher intensity (maximal response). c Dark-adapted oscillatory potentials. d Light-adapted cone response to a single flash. e Light-adapted cone response to a 30-Hz flickering stimulus. A- and b-waves as well as the second oscillatory potentials (P2) that are usually recorded are marked on the tracings

Fig. 7.2. Flash electroretinogram (ERG) a Dark-adapted isolated rod response to a flash of low intensity. b Dark-adapted rod-cone mixed response to a light flash of higher intensity (maximal response). c Dark-adapted oscillatory potentials. d Light-adapted cone response to a single flash. e Light-adapted cone response to a 30-Hz flickering stimulus. A- and b-waves as well as the second oscillatory potentials (P2) that are usually recorded are marked on the tracings

Rod ERG

If the dark-adapted eye is stimulated with a dim flash of light that lies below the threshold for cone responses, only rods with their maximally elevated sensitivity (a consequence of their dark-adapted state) will respond. This produces a pure rod ERG. At these weak stimulus intensities, the ERG response curves typically show no recognizable a-wave, since it is completely masked by the b-wave activity (■ Fig. 7.2 a).

I Pearl

The density of rod photoreceptors is maximal at 15 to 20° of eccentricity, and is zero at the foveal center, which contains only red and green cones. The rod ERG response reflects (almost entirely) the function of the peripheral retina.

Rod-Cone ERG

Dark-adapted responses to higher stimulus intensities, as shown in ■ Fig. 7.2 b, are indeed rod dominated, but also contain a small contribution from the cone system. These are therefore called rod-cone responses, mixed responses, or maximal responses. Moreover, comparison of the tracings in ■ Fig. 7.2 a and b illustrate that with increasing stimulus intensity the amplitude of the b-wave also typically rises and its implicit time shortens. (The implicit time is similar to a latency; it is the time from stimulus onset to the peak of a response).

Oscillatory Potentials

A number of high-frequency voltage oscillations are typically found on the rising slope of the positive b-wave (see ■ Fig. 7.2 b), and they have been named oscillatory potentials. They are thought to arise from delayed responses coming of amacrine cells and horizontal crossed connections between the cells of the inner plexiform layer. These potentials are sensitive detectors of damage to the inner layers of the retina, whether in ischemic, toxic, or hereditary disorders. They can be isolated and recorded by special filtering of the ERG responses (■ Fig. 7.2 c).

Retinal ganglion cells make no detectable contribution to the flash ERG. A test that may be able to isolate ganglion cell function is the pattern ERG (see below).

Cone ERG

An isolated test of the cone system can be recorded when the eye is fully light adapted, i.e., under photopic conditions. The rod system is completely saturated and bleached at higher levels of light adaptation, and it does not contribute to the responses of the cone ERG. Isolated cone responses to bright single flashes of light resemble those of the dark-adapted responses to bright flashes in the rod ERG in that they include an a-wave, a b-wave, and a number of oscillatory potentials (■ Fig. 7.2 d). They are, however, different from the dark-adapted tracings in that they have smaller amplitudes and shorter implicit times.

Cone-specific responses can also be elicited with rapidly flickering light stimuli at a frequency of 30 Hz, since the rod system is not capable of responding to this high frequency of flicker stimulus. The ERG responses consist of a series of periodic, positive deflections of the tracings (■ Fig. 7.2 e).

The density of cone photoreceptors peaks to a maximal level within the macula, but the macula contributes little to the collective signals recorded by the cone ERG, since it has such a small area.

The flash ERG of cones shows changes only when caused by widespread, generalized photoreceptor degenerations, such as cone dystrophies, cone-rod dystrophies, and rod-cone dystrophies. With purely macular disease, it often shows no changes at all.

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  • Luisella
    What does subnormal ERG?
    6 years ago

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