Article on Electroretinography

What is electroretinography?

Electroretinography (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. During the test, an electrode is placed on the cornea (at the front of the eye) to measure the electrical responses to light of the cells that sense light in the retina at the back of the eye. These cells are called the rods and cones.

How is an ERG done?

The patient assumes a comfortable position (lying down or sitting up). Usually the patient's eyes are dilated beforehand with standard dilating eye drops. Anesthetic drops are then placed in the eyes, causing them to become numb. The eyelids are then propped open with a speculum, and an electrode is gently placed on each eye with a device very similar to a contact lens. An additional electrode is placed on the skin to provide a ground for the very faint electrical signals produced by the retina.

During an ERG recording session, the patient watches a standardized light stimulus, and the resulting signal is interpreted in terms of its amplitude (voltage) and time course. This test can even be performed in cooperative children, as well as sedated or anesthetized infants. The visual stimuli include flashes, called a flash ERG, and reversing checkerboard patterns, known as a pattern ERG.

What do the electrodes do?

The electrodes measure the electrical activity of the retina in response to light. The information that comes from each electrode is transmitted to a monitor where it is displayed as two types of waves, labeled the A waves and B waves.

How are eletroretinography readings made?

Readings during eletroretinography 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.

Why is an ERG done?

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.

What diseases is my doctor/Optometrist looking for with an ERG?

There are a number of conditions, mostly ocular in nature, in which the ERG may provide useful information. The diagnoses most commonly suspected when ordering an ERG are predominantly conditions of the retina, including:

• 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 is a multifocal ERG?

The multifocal ERG focuses on different areas of the retina, looking for localized areas of abnormality. This tests will takes about half an hour to complete.

What is a normal outcome for an ERG?

A normal ERG shows a normal A- and B-wave pattern with appropriate increases in electrical activity with increased light intensities.

What does an abnormal ERG mean?

An abnormal ERG result suggests abnormal function of the retina due to diseases of the retina or abnormal function of the retina as occurs with other conditions such asarteriosclerosis (hardening of the arteries) involving the eye vessels, giant cell arteritiswith eye involvement, metabolic diseases called mucopolysaccharidoses, detachment of the retina, siderosis (excess iron deposits), and vitamin A deficiency.

Does the test hurt?

The test is painless. However, the electrode that rests on the eye may feel a little like an eyelash has lodged in the eye. This sensation may persist up to several hours following completion of the ERG.

What are the risks of an ERG?

There are no risks specifically associated with an ERG. Some patients experience mild ocular discomfort during or after the procedure. Rarely, a corneal abrasion may occur, which is readily treated with early detection. If you believe you have irritation or a corneal abrasion following an ERG, you should call your eye doctor or the doctor who ordered your ERG.

How long does the ERG take?

The ERG takes about an hour or less.

How about after the test?

One should not rub the eyes for an hour after an ERG (or any test in which the cornea has been anesthetized), lest one injure the cornea.

(From http://www.medicinenet.com)

Electroretinography

From Wikipedia, the free encyclopedia

Electroretinography
Intervention
Maximal response ERG waveform from a dark adapted eye.
ICD-9-CM	95.21
MeSH	D004596

Electroretinogram in Advanced Electrodiagnostic Laboratory at Universiti Teknologi MARA (UiTM), Puncak Alam Campus.

Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells (bipolar and amacrine cells), and the ganglion cells. Electrodes are usually placed on the cornea and the skin near theeye, although it is possible to record the ERG from skin electrodes. During a recording, the patient's eyes are exposed to standardizedstimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions (flash or pattern stimulus, whether a background light is present, and the colors of the stimulus and background) can elicit stronger response from certain components.

If a flash ERG is performed on a dark-adapted eye, the response is primarily from the rod system. Flash ERGs performed on a light adapted eye will reflect the activity of the cone system. Sufficiently bright flashes will elicit ERGs containing an a-wave (initial negative deflection) followed by a b-wave (positive deflection). The leading edge of the a-wave is produced by the photoreceptors, while the remainder of the wave is produced by a mixture of cells including photoreceptors, bipolar, amacrine, and Muller cells or Muller glia.^[1] The pattern ERG, evoked by an alternating checkerboard stimulus, primarily reflects activity of retinal ganglion cells.

Clinically used mainly by ophthalmologists, the electroretinogram (ERG) is used for the diagnosis of various retinal diseases.^[2]

Inherited retinal degenerations in which the ERG can be useful include:

• Retinitis pigmentosa and related hereditary degenerations
• Retinitis punctata albescens
• Leber's congenital amaurosis
• Choroideremia
• Gyrate atrophy of the retina and choroid
• Goldman-Favre syndrome
• Congenital stationary night blindness - normal a-wave indicates normal photoreceptors; absent b-wave indicates abnormality in the bipolar cell region.
• X-linked juvenile retinoschisis
• Achromatopsia
• Cone dystrophy
• Disorders mimicking retinitis pigmentosa
• Usher Syndrome

Other ocular disorders in which the standard ERG provides useful information include:

• Diabetic retinopathy
• Other ischemic retinopathies including central retinal vein occlusion (CRVO), branch vein occlusion (BVO), and sickle cell retinopathy
• Toxic retinopathies, including those caused by Plaquenil and Vigabatrin. The ERG is also used to monitor retinal toxicity in many drug trials.
• Autoimmune retinopathies such as Cancer Associated Retinopathy (CAR), Melanoma Associated Retinopathy (MAR), and Acute Zonal Occult Outer Retinopathy (AZOOR)
• Retinal detachment
• Assessment of retinal function after trauma, especially in vitreous hemorrhage and other conditions where the fundus cannot be visualized.

The ERG is also used extensively in eye research, as it provides information about the function of the retina that is not otherwise available.

Other ERG tests, such as the Photopic Negative Response (PhNR) and pattern ERG (PERG) may be useful in assessing retinal ganglion cell function in diseases like glaucoma.

The multifocal ERG is used to record separate responses for different retinal locations.

The international body concerned with the clinical use and standardization of the ERG, EOG, and VEP is the International Society for the Clinical Electrophysiology of Vision ^[3]

[edit]References

1. ^ Webvision at University of Utah
2. ^ Electroretinography, U.S. National Library of Medicine, 11 april 2005 (accessed 19 January 2007)
3. ^ ISCEV Website