ERG And VEP Assessment Machine

What are ERG and VEP Vision Testing?

Electroretinography (ERG), also referred to as electroretinogram, is a diagnostic test that measures the electrical activity of the retina in reaction to a light stimulus.

The retina is a light-sensitive layer located at the back of the eye. When light from a picture enters the eye, it is turned into electrical energy by specialized cells (rods and cones) in the retina. These cells convey electrical signals through the optic nerve to the brain where the picture is processed. The ERG documents how well the retinal cells are carrying electrical signals within the eye. Results of ERG test helps in the diagnosis and treatment planning of several vision-related conditions.

Visual Evoked Potential (VEP) is a non-invasive testing technique that furnishes objective information about the functioning of the complete vision system. VEP vision testing serves as an instrument to measure the entire visual pathway, from the lens to the visual cortex, to identify neural or mechanical abnormalities connected to vision. VEP testing helps to establish the non-functional part of the visual pathway and the quality of vision.

Indications for ERG and VEP Vision Testing

Indications for ERG Vision Test

Your physician may recommend an ERG vision testing to establish if you have an acquired or inherited retinal disorder, such as:

• Macular degeneration (vision loss due to death of cells in the macula)
• Retinitis pigmentosa (a genetic disease that causes loss of night and peripheral vision)
• Retinal separation (detachment of the retina from the rear of the eyeball)
• Retinoblastoma (cancer of the retina)
• Cone-rod dystrophy (a group of eye conditions that causes vision loss)

Indications for VEP Vision Test

A VEP test is indicated in patients with neurological disorders affecting the visual pathway including:

• Lesions or tumors compressing the optic nerve
• Glaucoma
• Ocular hypertension
• Photosensitive epilepsy
• Alterations in color vision
• Poor vision in children
• Weakness of eyes

Preparations for ERG and VEP Vision Testing

Instructions provided as part of the preparation for ERG and VEP vision testing include:

• Make sure you get ample sleep the night prior to your tests.
• If you use glasses, make sure to bring them along with you for the tests.
• Wash your hair the night prior to your tests, but without using any hair lotions, oils, or chemicals.
• Take your usual medications and normal meals before the tests, but refrain from taking medications that may cause drowsiness during the tests.
• Make sure to arrive on time so that you can relax prior to the tests.
• You need to notify your physician if you have any eye conditions such as glaucoma or cataracts as this can affect the tests.

Procedure for ERG and VEP Vision Testing

ERG Procedure

In general, the ERG procedure may involve the following:

• Your physician will ask you to either sit or lie down in a comfortable position.
• Eye drops are usually used to dilate your eyes in preparation for the test.
• Anesthetic drops are used to numb your eyes if your physician places an electrode directly on the eye.
• Your physician will use a device called a retractor to hold open the eyelids. The retractor allows your physician to carefully position a small electrode on each eye. One type of electrode is about the size of a contact lens. Another type is a fine thread positioned on the cornea.
• Your physician will connect another electrode to your skin so that it works as a ground for the weak electrical signals produced by the retina. Depending upon what your physician is trying to find out, he or she may only position electrodes on the skin around the eye rather than in the eye.
• You will then view a flashing light. Your physician will perform the test in a darkened room and in a normal light room. The electrode allows your physician to measure the electrical response of the retina to light. The recorded responses in a darkened room will mostly be from your retinal rods. The recorded responses in a normal light room will mostly be from your retinal cones.
• The information from the electrodes is transferred to a monitor, and the monitor displays and records the information.
• Your doctor will discuss the results on the same day of the procedure or during your follow-up visit.

VEP Procedure

VEP testing involves visualizing a television screen displaying alternative checkerboard and other visual patterns. The procedure involves the following steps:

• You will sit in front of a television screen. Electrodes (special wires to test electric activity) are placed at the top of your head.  
• You must carefully look into the visual patterns of the television screen including black and white checks moving side to side. Children may be encouraged to concentrate on playing rhymes.
• Each eye will be tested separately. Follow the instructions provided by your technician.
• Results are recorded and interpreted by a trained physician. 
• The wires are removed off your head after the procedure is completed.
• Your doctor will discuss the results on the same day of the procedure or during your follow-up visit.

Post-procedure Care and Instructions

You will be transferred to the observation room for monitoring. You will normally be able to return home on the same day. Your eyes may feel sensitive after the tests. You must avoid rubbing your eyes for at least an hour after the tests as this may cause corneal damage as your eyes will still be numb from the anesthetic. Based on the results, your physician may order additional tests to assess your eye. Your physician may recommend surgery if the tests indicate retinal trauma or separation. Your physician may prescribe medications to treat other retinal disorders.

Risks and Complications of ERG and VEP Vision Tests

There are no major risks involved with ERG and VEP tests. Some minor risks may include:

• Slight discomfort during the tests
• Eye soreness
• Mild eye irritation
• Corneal abrasion (rarely)