ELECTRO-RETINOGRAM (ERG)

  

INTRODUCTION:

The electroretinogram (ERG) represents a record of the action potential produced by the retina on stimulation by light of adequate intensity.

 The test is useful as it provides a fast and non-invasive assessment of the electrophysiology of the retina.

The electroretinography recording is usually made by using an active electrode placed on the cornea or on the skin, just below the lower eyelid and a reference electrode positioned on the forehead of the subject.
 

COMPONENTS OF ERG:

The potential between the electrodes is amplified and displayed in the form of a graph. The normal ERG is biphasic. It consists of the following waves:

a-wave: It is the initial fast negative deflection produced by the photoreceptors, with a peak (implicit time) of 100 msec.

b-wave: It is the next slower positive deflection, having a higher amplitude. It is generated from fluxes of potassium ions, within and surrounding Muller cells. However, it is directly dependent on photoreceptor function and is a reflection of photoreceptor integrity.

The amplitude of b-wave is measured from the trough of the a-wave to the peak of b-wave. It increases with dark adaptation, as well as, increased light stimulus. 

 

The b-wave is subdivided into two sub-components:

b-1 represents rod and cone activity

b-2 represents cone activity alone

Rod and cone activity can be singled out by special techniques.

The normal ERG consists of 5 recordings. The first 3 are elicited after 30 minutes of dark adaptation (scotopic), the last 2 after 10 minutes of moderately bright diffuse illumination (photopic). In children it is difficult to dark adapt for 30 minutes, so dim light (mesopic) can be used.

SCOTOPIC ERG	Rod	Elicited with very dim flash of white light or a blue light, resulting in a large b-wave and a small or non-recordable a-wave
	Combined Rod and Cone responses	Elicited with a very bright white flash resulting in a prominent a-wave and a b-wave
	Oscillatory potentials	Elicited by using a bright flash and changing the recording parameters. The oscillatory wavelets occur on the ascending limb of the b-wave and are generated by cells within the inner plexiform layer as a consequence of inhibitory feedback loops involving primarily amacrine cells.

 

PHOTOPIC ERG	Cone responses	They are elicited with a single bright flash, resulting in an a-wave and a b-wave with small oscillations
	Cone flicker	Used to isolate cones by using a flickering light stimulus at a frequency of 30Hz to which rods cannot respond. The amplitude and implicit time of the cone b-wave are measured by this element. Cone responses can be elicited up to 50Hz in normal eyes, beyond which individual responses are no longer recordable (critical flicker fusion).

 

TYPES OF ERG:

The ERG evoked by a flash of light (flash ERG) is affected more by outer retinal elements and is not typically abnormal in glaucoma. Flash ERG is the summed electrical activity of different groups of retinal cells, in response to a flash of light. The primary waveforms in the flash ERG are the a- and b-waves. These arise predominantly from the outer and middle retinal layers respectively. However, the flash ERG is rather a gross method to diagnose or monitor glaucoma, since the condition is characterized by selective loss of retinal ganglion cells (RGCs).

ERG related studies have demonstrated no difference in a-wave, b-wave, and implicit time between POAG and normal subjects. Some studies have demonstrated that these parameters may be affected later in the disease, suggesting involvement of outer or middle retinal layers as the disease progresses.

However, other studies have shown the photopic negative response (PhNR) in primary open glaucoma glaucoma (POAG) could be affected. This is a slow negative potential that follows the b-wave in response to a photopic stimulus and originates from the inner retina.

PhNR amplitudes are found to be consistently smaller in POAG patients and correlate with the mean deviation determined by static perimetry, even with mild visual field sensitivity losses.

Another study has shown that PhNR amplitudes correlate with decrease in function and morphology of retinal neurons in eyes with OAG, indicating that inner retinal function apparently declines proportionately with neural loss in glaucomatous eyes.

The amplitude of the PhNR has been shown to decrease with an increase in visual field defects and is significantly correlated with the retinal nerve fiber layer thickness (RNFLT) and the rim area of the optic disc as well as the cup:disc ratio.

ERGs can also be evoked by providing a stimulus in the form of alternate gratings (e.g. checkerboard stimuli) at a constant mean luminance. This is called Pattern-ERG (PERG). It represents a more focal response from a specific area of the retina, being stimulated. PERG is a direct and objective measure of RGC and optic nerve functions. 

A number of studies have reported PERG to have reduced amplitudes in glaucomatous individuals. PERG is found to be abnormal in early glaucoma and those ocular hypertensives who are prone to convert to POAG. This could prove useful in discriminating between those patients with ocular hypertension who will develop visual field loss and those who will not. These abnormal results have been recorded even one year prior to the development of visual field changes suggestive of conversion.

A study has demonstrated that PERG amplitude correlates with RNFL thickness in early glaucoma but not ocular hypertension.

Decreased amplitude and increased peak latency reportedly correlate with increasing age, paralleling the estimated normal loss of ganglion cells. Reduction in PERG is also directly related to histologically defined optic nerve damage in a monkey model.

Multifocal ERG (mfERG) permits simultaneous recording of multiple spatially localized ERGs. mfERG permits topographic display of retinal function and allows for much more detailed assessment of retinal function at specific areas affected by disease.

 

It consists of the same components as a standard ERG. Preliminary studies suggest that it does not appear to correlate well with glaucomatous damage and may be able to detect abnormalities before automated achromatic visual fields can show changes.

MfERG has shown utility in assessing inner retinal function by identification of the optic nerve head component (ONHC), which is of inner retinal origin and is thought to arise in the vicinity of the optic nerve head. This component shows most relevance to the detection of early glaucoma because it has been shown that its propagation time correlates with the length of the ganglion nerve fibers and thus seems to be dependent on the nerve fiber layer.

Studies have demonstrated significant changes in the mfPERG of glaucoma patients, with components of the mfPERG significantly decreased, most distinctly in the central ring, and decreasing further with a progression of glaucoma stage.

Though abnormalities can be readily detected in mfERG recordings from glaucomatous eyes, the advantage of topographic analysis offered by the technique has not proven important for glaucoma diagnosis.

CONCLUSION:

While electrophysiological studies such as ERG showed early promise, but the cumbersome instrumentation, lack of understanding and utility of the procedures made this method go into relative obscurity in clinical practice. With newer instruments and studies there is renewing interest in these modalities.