What to Expect During an ERG Electroretinogram Eye Test?

An electroretinogram (ERG) is a noninvasive test that records how well the cells in your retina (the light-sensitive tissue at the back of the eye) respond to light ^[1]. Rather than taking a picture of the eye’s anatomy, it measures the electrical activity of your retinal cells—such as rods (used in dim light) and cones (used for color and fine detail)—when they are exposed to brief flashes of light ^[2][3]. Because the name sounds clinical, many patients worry about the test, but the procedure is generally well-tolerated and straightforward ^[4].

Preparing for the Test

Before the test begins, the technician will prepare your eyes to ensure the measurements are accurate and that you are comfortable. The preparation typically involves:

• Dark Adaptation: You may be asked to sit in a dark room for a period (often 20 to 30 minutes) before the test begins. This allows your eyes to fully adjust to the dark, which is necessary for testing how your rod cells function (dark-adapted testing) ^[5].
• Light Adaptation: Other parts of the test evaluate cone cells in normal room light (light-adapted testing) ^[5].
• Eye Drops: Depending on the specific protocol, you will likely receive dilating drops to widen your pupils, allowing more light to reach the retina. If an ocular sensor is used, the technician will also administer topical anesthetic (numbing eye drops) so you do not feel sharp pain from the sensors ^[2].

Placing the Sensors

To measure the electrical signals your retina produces, tiny sensors (called electrodes) must be placed near your eyes ^[6]. Depending on whether you are having a full-field ERG, a multifocal ERG, or a pattern ERG, the sensors may be:

• Corneal contact lenses: Small, specialized contact lenses that rest directly on the surface of your eye ^[6].
• Thread electrodes: A very fine thread placed gently just inside your lower eyelid ^[7].
• Skin electrodes: Small sticky patches placed on the skin right beneath your eyes or on your forehead ^[7].

The Light Stimulation

Once the sensors are in place, the measurement begins. You will look at a light source, often inside a bowl-shaped machine, or at a screen. The machine will produce various brief flashes of light or patterned stimuli ^[8][2]. You do not need perfect central vision to do this test; the technician will guide you on where to look, though some specialized ERGs may ask you to focus on a target ^[9].

This test helps doctors assess how different parts of the retina respond to light, supporting the evaluation of disorders like Stargardt disease ^[5]. For Stargardt disease, a standard full-field ERG might be normal in the early stages, while a multifocal ERG (which measures localized responses in the macula) can provide more specific information about your central vision ^[9][2].

Does It Hurt and What Happens After?

ERG testing is considered noninvasive ^[4]. The numbing drops may sting briefly when applied. While you should not feel sharp pain, contact lenses or thread sensors can cause mild pressure, scratchiness, tearing, or a tickling sensation ^[3][2]. If you feel significant pain during the test, tell the technician immediately. The bright flashes can be dazzling, and you may see temporary afterimages once the test is complete, but these are harmless ^[2].

After the procedure, the sensors are removed. It is important to follow standard safety guidelines:

• Protect Numb Eyes: Do not rub or touch your eyes for at least an hour after the test. The numbing drops temporarily reduce your corneal sensation, meaning you might accidentally scratch your eye without feeling it ^[2].
• Driving Safety: If you received dilating drops, your vision may be blurry and highly sensitive to bright light for several hours. Do not drive until your vision is completely clear and you feel safe. It is highly recommended to arrange a driver for your appointment ^[10]. Wear sunglasses outside.
• When to Call: Contact your clinic if you experience persistent pain, marked redness, worsening vision, or unusual light sensitivity later in the day ^[10].