Common Tests and Procedures | Overview
Care for eye conditions and visual impairment begins with a precise diagnosis. Boston Children's Hospital is one of the few in the country that uses child-focused, high-tech diagnostic exams:
An Electroretinogram (ERG) is an eye test that evaluates the function of the film (retina) that lines the inside of the eye.
The processes that start vision take place in the retina. The retina converts light to small electrical signals that can be sent to the brain. The retina is the most metabolically active tissue in the body. The ERG test helps diagnose diseases limited to the retina, and also diseases that affect parts of the body other than the eye.
How the test works
The idea of the ERG test is that when flashes of light come to the eye, the rods and the cones and the other cells in the retina make tiny amounts of electricity. If we know exactly how much light comes to the eye, and how much electricity comes out, we can figure out how the cells, including the rods and the cones, are working. To pick up the electricity that the retina makes, contact lens like electrodes are placed on the surface of the eye.
What happens during the test
It may not be possible for a child to hold still for the contact lenses and flashes of light. For these children, the ERG may be best done under anesthesia. The anesthesia doctors determine the appropriate anesthesia. Depending on your child's medical condition, it may be important to have a pre-anesthesia consultation with the anesthesia doctors.
To prepare for the ERG test, drops will be placed on your child's eye to dilate the pupils (enlarge the black spot in the middle of the eye). These drops are the same drops that eye doctors routinely use for examination of the eyes. Another important preparation is to have the eyes well adjusted to darkness. This is accomplished by covering the eyes with eye patches.
As with the use of any contact lens, there is a risk of scraping the surface of the eye; this is called a corneal abrasion. We take a number of precautions to minimize the risk of abrasion and it is unlikely that an abrasion will occur. In the event that an abrasion occurs it will be treated promptly. In the recovery room, it is important that your child not rub his/her eyes as this too will help minimize the risk of abrasion.
How long it takes
The time in the operating room is approximately one hour. This includes time to establish the anesthesia, the ERG test, the examination of the eyes and reversing the anesthesia. The actual ERG test time is 20-30 minutes.
Results
Immediately after the test, some information about the results is available. We will convey this to you. However, the final report depends on measurement and analysis of the ERG records. We will send a copy of the ERG report to you and also to any physicians designated by you.
For more information
We are happy to discuss the reports with you and your child's other physicians. We are glad to discuss the ERG test with you and answer questions. Please contact us at (617) 355-5685, if you have any questions prior to the visit.
Sweep Visual Evoked Potential Test
The Sweep Visual Evoked Potential (SVEP) test is used to measure visual acuity, that is, vision for detail and patterns.
In adults, visual acuity is measured with lines of letters. The SVEP measures acuity by assessing the response of brain waves to moving black and white stripes. The SVEP acuity test is, perhaps, most helpful in those youngsters for whom behavioral measures (Preferential Looking) give ambiguous or incomplete information about visual acuity.
How the test is done
To do this test, your child will be positioned to view a monitor. Moving ("sweeping") black and white stripes will be shown on the monitor. Your child may sit on your lap or in a chair.
The vision of the stripes is passed from the eye and along the visual pathways to the visual part of the brain. The SVEP is designed to find the finest black and white stripes that produce a reliable response.
For the test, small metal discs, about the diameter of the tip of the little finger, are placed on the head. The discs are connected to a computer by fine cables. The discs are held in place by a paste like toothpaste. Three discs are placed at the back of the head, one disc is placed on top of the head and one is placed on the forehead. A band is wrapped around the child's head to hold the electrodes in place during the test.
How long it takes
Although the test doesn't always take the full amount time, we allow an hour for the test to be done. This includes the time it takes to place the metal discs and to accommodate for wiggly or intermittently inattentive children. Often, because of the child's eye condition, it is important to test the vision of each eye separately. To do this, we place a patch over one eye while the other is being tested.
Results
We know the visual acuity portion of the results at the time of the visit. After the recordings are completed, the records are analyzed and the final results are included in the written report of the visit. The report will be forwarded to you, and also to any of your child's doctors as designated by you. We are more than happy to discuss the report with you and your child's other physicians.
For more information
When you are here for the SVEP, we will show you the equipment and answer any questions you may have about the nature and purpose of the procedure. Please contact us at 617-355-5685, if you have any questions prior to the visit.