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What is an Electro-retinogram (ERG)?

This is a test of the responsiveness of the retina which is part of the eye.   It is used to determine how healthy the retina is.


If you wear glasses please bring them with you.

How is it performed?

A very fine thread is placed between the lower eyelid and the eyeball.  The thread is placed quickly and you may feel a little discomfort for a few seconds but this will not last.   Leads will also be placed on your face at the side of your eyes and one lead on your forehead.  Once the leads are in place you will be asked to sit in the dark for 20 minutes.  This is to allow certain cells in your retina to be at their most active.  You will then be asked to look at some flashing lights at different intensities.  This is followed by sitting in the light for 10 minutes then being shown some more flashing lights.  Towards the end of the test you may be asked to look at a screen with a chequerboard pattern.  It may be necessary to modify the test procedure for a small number of patients including young children/babies.  The fine thread maybe replaced by a small disc on the skin, near to the eye.  The physiologist will discuss this with you before the test if it is required.

Eye drops - to read 

How do I prepare for the test?

If you are taking medication please continue to take it and bring a list of your medication with you.

Continue to eat meals as normal.

If you suffer from epilepsy you should let the Clinical Physiologist know at the start of your test.

How long will I have to wait?

You have been given a specific appointment time and patients are usually seen on time.  It is important that you arrive on time.  Please allow plenty of time for parking.  This test takes up to 90 minutes to complete.  An ERG test is sometimes done together with a Visual Evoked Potential (VEP).  This will lengthen the test time up to 2 hours.  A separate information leaflet is available for the VEP.

When will I get the results?

The results are sent to the doctor that referred you for the test and are usually available within seven days.


Please telephone the Neurophysiology Department   01422 222976 if you have any concerns or questions.