Department of Neurology
Electro-retinography and Electro-oculography
Neurology and Ophthalmology Departments, Women's and Children's Hospital
Electroretinography (ERG) is a test of function of the retina; the light receptor structure at the back of the eye. ERG recordings can provide valuable diagnostic information in cases of visual disturbance. ERG can be performed alone or as a useful adjunct to the cortical visually evoked potential (VEP). VEPs are usually performed to test the function of the optic nerve, which carries information from the eye to the brain. (See VEP protocol)
Electrical activity generated within the retina in response to a flashing light is detected with an electrode placed on or near the eye and processed using an computerised system.
The recording conditions may be varied so as to test the overall health of the retina and the function of specific light receptors (different retinal cells); ie the cones and rods responsible for day and night vision respectively.
Optimal responses are recorded using contact lens type electrodes. Other types of electrodes are suitable for clinical recordings. These include filament, fibre and gold foil type electrodes. Useful results can also be achieved with surface electrodes placed on lower eye-lids. All except the latter require a local anaesthetic eye-drop to allow the electrodes to rest comfortably. Dilation of the pupil is also required to obtain standardised responses.
International standards for visual electrophysiology have been published by ISCEV (International Society for Clinical Electrophysiology of Vision) http://www.iscev.org/standards/ and guidelines have also been published by OSET (International Organisation of Societies for Electrophysiological Technology).
Some technical information on the ERG technique as performed in our laboratory is detailed below for those interested.
When scleral electrodes are used for recordings, a local anaesthetic will be applied to the eye and to obtain standardised responses a pupil dilation agent will be used also.
Skin electrode sites are prepared with a commercially available "skin prep" so as to reduce impedances to similar low (less than 5 kohms) values.
Electrodes are placed as in figure 1.
Ground on forehead. Active on the sclera, or lower eye-lid as close to lower eye lashes as practicable. Reference is placed ipsilateral to the active electrode and 2 cms lateral to the outer canthus of the eye.
When using the Nicolet Ganzfeld as stimulator, photopic flash intensity is .75 Log with background illumination of 10ftL. Scotopic testing with blue light is with a flash intensity of .50 Log and no background illumination.
When a strobe flash is used, the light is placed 15 cms from the eyes.
Recordings are performed binocularly. Clinical experience indicates minimal leakage of response to the contralateral electrodes however if there is any doubt, careful monocular occlusion may be applied.
Initial recordings are obtained with the patient watching the white Ganzfeld light or un-filtered strobe flashing at a rate of approximately 2 flashes/second. Signals are amplified and averaged using an Evoked Potential Recording System to extract the response to the stimulus from extraneous electrical "noise". The number of responses to be averaged will depend on the quality of the recordings. When skin (lower eye-lid) electrodes are used, generally 100 responses will be averaged depending on the technical quality of the recordings. Each trial will be repeated 2-3 times to verify reliability and measurements will be made from the grand averages of the individual trials.
Photopic (cone) responses are then assessed using a flicker rate of 30 flashes/second. Other slower (and faster) rates of flicker may be used to verify any delays in latency of the responses.
Scotopic (rod) responses are recorded following dark-adaptation in total darkness for 20 minutes. Ganzfeld testing is done with a low intensity blue flash and also a bright white flash. When using a strobe, a (Kodak Wratten) blue filter needs to be placed over the light. For lower eye-lid electrodes, up to 10 flashes are presented for each trial at a rate of 1 flash / 10 seconds. This is repeated with the blue filter removed.
Our current equipment for ERG and EOG testing consists of a Nicolet Bravo system with 16 channel and 2channel amplifiers. The stimulator is usually a Nicolet Ganzfeld unit. For uncooperative patients a stroboscopic flash unit may be used.
Alkane ® drops are used for local anaesthetic of the eye (1 drop).
Dilating drops used are Mydriacyl ® 0.5%, 2 drops over 5 minutes prior to dark adaptation.
In addition to increased sensitivity to sunlight, these drops also cause blurred near vision which will gradually return to normal over several hours following the testing.
The time taken to perform each component of these tests varies slightly and is influenced by patient cooperation, time taken to obtain acceptable electrode impedances and in case of slight variations, the test may need to be repeated for comparisons.
Introduction and method
An electro-oculogram (EOG) is often performed following the ERG. The EOG records the voltage difference between the back of the eye (posterior pole) and the cornea. The main source of this potential is thought to be the retinal pigment epithelium (RPE). The EOG is of limited clinical utility and is of primary value in the differential diagnosis of Best's disease when a macular dystrophy is suspected. (Kolder H.E. 1991)
EOG recordings are performed with the same lateral canthus electrodes as for the ERG and with additional electrodes placed on the medial canthus of the eyes on the nose. The common/ground electrode remains on the forehead. Again electrode sites are prepared with "skin prep" to ensure similar low impedances.
Electrodes placement is shown in figure 2.
Recordings are performed binocularly with the patient instructed to follow the alternating LEDs inside the Ganzfeld Stimulator. The changing fixation produces a moving dipole which is recorded as biphasic square wave. The computer automatically records the peak to peak voltage of the responses and plots the voltage changes over time. Responses initially decrease in voltage in darkness and increase again when the background light is turned on. There is an initial practice phase followed by a period of testing during dark adaptation. Once the bottom of the dark trough has been passed, the Ganzfeld background light switches on and light adapted recordings commence and continue until a peak amplitude value has been determined.
The ratio of EOG trough value during dark adaption and the peak value after light adaption is then used to calculate the "Arden Index".
Our current equipment for EOG testing consists of a Nicolet Bravo system with a 2 channel amplifier.
The time taken to perform the test varies slightly and is influenced by patient cooperation, time taken to obtain acceptable electrode impedances and the time taken to reach minium and maximum amplitudes.
Kolder HE (1991) Electro-oculography. from Principles and Practice of Clinical Electrophysiology of Vision edited by Heckenlively JR and Arden GB Publ Mosby St Louis.
Patient Information and Guidelines
Electroretinograms, Electro-oculograms and Visual Evoked Potentials are recorded in the Neurology Department of the Women's and Children's Hospital, 72 King William Rd North Adelaide 5006.
The Neurology Department is located on the 2nd floor of the Clarence Reiger Building which is accessed through the Main Out-Patient's Entrance on Kermode St.
The Hospital Car Park is located in Kermode St and there is limited metered street parking available. Patients should allow approx 75 minutes testing time for ERG and 60 minutes for EOG plus extra time for Patient Registration. Additional time will be required if Visual Evoked Potential testing is performed as well.
Patients should ask if eye-drops will be used to determine whether they will be able to drive themselves home.
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