ELECTRORETINOGRAM

What is Electroretinogram?

An Electroretinogram or ERG is an eye test used to detect abnormal function of the retina (the light-detecting portion of the eye).

Specifically, in this test, the light-sensitive cells of the eye, the rods and cones, and their connecting ganglion cells in the retina are examined.

Why Has An Electroretinogram Been Recommended?

An ERG is useful in evaluating both inherited (hereditary) and acquired disorders of the retina. An ERG can also be useful in determining if retinal surgery or other types of ocular surgery such as cataract extraction might be useful.

An Electroretinogram can provide important diagnostic information on a variety of retinal disorders including:

congenital stationary Night Blindness,
Leber congenital amaurosis, and
cancer-associated Retinopathy.

Moreover, an ERG can also be used to monitor disease progression or evaluating for retinal toxicity with various drugs or from a retained intraocular foreign body.

What Conditions Can an Electroretinogram Diagnose?

Retinitis Pigmentosa,
Retinitis Punctata Albescens,
Retinitis Pigmentosa Sine Pigmento,
Related Hereditary Retinal Degenerations,
Disorders that mimic Retinitis Pigmentosa,
Leber's Congenital Amaurosis,
Choroideremia,
Gyrate atrophy of the choroid,
Gyrate atrophy of the retina,
Goldman-Favre syndrome,
Congenital stationary night blindness,
X-linked juvenile retinoschisis,
Achromatopsia,
Cone dystrophies, and
Usher syndrome.

What Information Can an Electroretinogram Show?

The focal ERG (fERG; also known as the foveal ERG) is primarily useful in measuring the functional integrity of the fovea and is therefore useful in providing information on diseases limited to the macula.

Differing field sizes varying from 3 degrees to 18 degrees and light stimulus frequencies have been used in the various methods, however, each technique deals with the challenge of limiting the amount of light scattered outside the focal test area.

Focal ERG is useful for assessing macular function in conditions such as age-related macular degeneration, however, requires good fixation from the subject.

The full-field ERG (Ganzfeld; ffERG) measures the stimulation of the entire retina with a flashlight source under dark-adapted (scotopic) and light-adapted (photopic) types of retinal adaptation. This is useful in detecting disease with widespread generalized retinal dysfunction i.e. cancer-associated retinopathy, toxic retinopathies, and cone-rod dysfunction.

Due to the massed retinal electrical response, small retinal lesions may not be revealed in ffERG recordings

Benefits an Electroretinogram Test

It helps to identify abnormal results relating to the conditions such as

Arteriosclerosis damage to the retina
Congenital retinoschisis, which is a splitting of layers in the retina
Congenital Night Blindness
Giant Cell Arteritis
Retinal detachment
Cone-rod dystrophy
Vitamin A deficiency
And Trauma

Preparation For The Test - What To Bring, What To Wear, How Long

According to ISCEV 2015 guidelines:

Maximally dilate the pupils
Before Dark-adapted protocols- 20 min of dark adaptation
Before light adapted protocols- 10 min of light adaptation
Present low strength flashes before stronger flashes- so that the partial light adaptation due to bright light does not occur.
Insert corneal contact electrodes (when these are used) under dim red light after dark adaptation period. Avoid strong red light. Allow 5 min of extra dark adaptation after insertion of contact lens electrode
Allow at least 30 min recovery time in ordinary room illumination after use of strong light for retinal imaging (fundus photography, fluorescein angiography and others).
Request the patient to fix and not move eyes. Ocular movements can change the positions of electrodes, can cause blockage of light by eyelids or electrode and may induce electrical artifacts.

What Happens During an Electroretinogram?

Readings during electroretinography are usually taken first in normal room light. The lights are then dimmed for 20 minutes, and readings are again taken while a white light is shined into the eyes.

The final readings are taken as a bright flash is directed toward the eyes.

Diagnostic Pathway for an Electroretinogram

a-wave: initial corneal-negative deflection, derived from the cones and rods of the outer photoreceptor layers. This wave reflects the hyperpolarization of the photoreceptors due to the closure of sodium ion channels in the outer segment membrane. Absorption of light triggers the rhodopsin to activate transducing, a G-protein. This leads to the activation of cyclic guanosine monophosphate phosphodiesterase (cGMP PDE) eventually leading to a reduction in the level of cGMP within the photoreceptor. This leads to closure of the sodium ion channels resulting in a decrease of inwardly directed sodium ions, or a hyperpolarization of the cell. The a-wave amplitude is measured from the baseline to the trough of the A-wave.

b-wave: corneal-positive deflection; derived from the inner retina, predominantly Muller and ON-bipolar cells. The hyperpolarization of the photoreceptor cells results in a decrease in the amount of neurotransmitter released, which subsequently leads to a depolarization of the postsynaptic bipolar cells. The bipolar-cell depolarization increases the level of extracellular potassium, subsequently generating a trans retinal current. It is this trans retinal current that depolarizes the radially oriented Muller cells and generates the corneal-positive deflection. The b-wave amplitude is generally measured from the trough of the a-wave to the peak of the b-wave. This wave is the most common component of the ERG used in a clinical and experimental analysis of human retinal function.

c-wave: derived from the retinal pigment epithelium and photoreceptors. The c-wave is a reflection of the resulting change in the transepithelial potential due to the hyperpolarization at the apical membrane of the RPE cells and the hyperpolarization of the distal portion of the Muller cells. The c-wave generally peaks within 2 to 10 seconds following a light stimulus, depending on flash intensity and duration. Due to the c-wave response developing over several seconds, it is susceptible to influences from electrode drift, eye movements, and blinks.

d-wave: off bipolar cells.