Having an Electroretinogram


An electroretinogram

What is it?

The electroretinogram (ERG) test is an eye test that aims to discover how well the retina at the back of your eye is working, and whether it has any kind of disease.

Time to take the test?

The ERG test is very helpful in assessing any problems with your retina that you may have inherited, such as retinitis pigmentosa, or damage to it resulting from drugs etc.

Good reasons why?

The test gives the ophthalmologist vital information to help them decide the best way to treat your eye.

When light falls on the retina it stimulates many different types of cells which in turn produce a variety of electrical responses. Studying these responses can help us detect which cells are healthy and which are not.

Your retina has cells called rods and cones which process light. During the ERG test, these cells release tiny amounts of electricity in response to flashes of light. If we know exactly how much light enters the eye and how much electricity comes out, we can figure out how the rods and cones are working.

Doing the business

To prepare you for the ERG test, drops are placed in the your eyes to dilate the pupils. These drops are the same as the ones doctors routinely use for eye examinations. You will then be asked to sit in a dark room for 30 minutes to allow your pupils to dilate. During this time your retina will also steadily adapt to the dark and become more sensitive to light, which means it will give a stronger response when tested.

After dark adaptation, anaesthetic drops are given and a contact lens is placed on your eye. Often both eyes are tested at the same time. Once the contact lens is in place, how your retina responds to as series of blue and red lights is recorded. This part of the test may take 30 to 45 minutes. The total time for the ERG test, including the period of dilation and dark adaptation, is approximately 1½ hours.


Article approved by Moorfields Children’s Information Group

Last Reviewed: 08/10/08


Last updated: 20th November 2017