Ocular Electrophysiology

Electro-retinogram (ERG),

Visual evoked potential (VEP) and

Electro-oculogram (EOG).

These are used to locate the site of disease whether in the retina, optic nerve or brain, and to decide the extent of disease. Electro Neuro Diagnostic Centre offers you a dedicated Ocular Electrodiagnostics Service for the diagnosis of disorders related to the visual pathway.

What is the VEP Test?

VEP / VER stands for “Visual Evoked Potential/ Response”.

The VEP is the electrical response of the brain to a simple patterned stimuli. A response in form of a waveform is generated which normally takes about a tenth of a second. An example of such a result is shown in the figure below. Diseases which affect the optic nerve can cause delays.

Procedure of the test

Test is performed with the subject sitting comfortably in front of a television and three electrodes are placed on the subjects scalp area and a pattern is subsequently flashed on the TV. This generates a signal in the eye and conveyed to the vision centre in the brain. The response time and its strength is measured by the machine

What is the ERG?

ERG stands for “Electroretinogram”. The ERG is the massed electrical response of the retina to brief flashes of light. It is used in the diagnosis of retinal diseases. The ERG is recorded using a small fiber electrode under the lid. In an ERG result, there is are two parts. Initially there is a negative phase called the a-wave, which is the electrical response of the photoreceptor, which is where light gets converted to an electrical signal. The second part is where the graph rises, called the b-wave. This is the response of the post-receptor cells such as the bipolar cells and other neurons. This test make it possible to pinpoint in different diseases the site of the disease’s action.

Differentiation of different cell types affected can be judged by doing dark adapted (scotopic) and light adapted (photopic) ERG. Sample waveform extracted from an electroretinogram (ERG)

Special Notes

EOG cannot be done for children less than 5 yrs. as it involves moving the eyes left to right on command.

VEP/ERG in children less than 3yrs. requires cooperation and may require sedation, generally with trichlophos (pedichloryl) and the appointments for the same are given at 9 AM.

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