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Electro-retinography & Electro-oculography
Electro-retinography protocol
Neurology & Ophthalmology Departments WCH Aug 2002
Introduction
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).
Method
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.
Equipment
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.
Settings
ERG / 16channel amplifier
Photopic
Amplifier Sensitivity 20uv, Filters LFF 1Hz, HFF 2.5kHz, Sweep Dur. 200ms
Ganzfeld Stimulator 1.9 flash / second, Background Lum 10FtL, Strobe Intens
.75Log
Flicker
As for Photopic but 30 flash / second
Dark Adapted
Blue Filter
Amplifier Sensitivity 500uv, Filters LFF 1Hz, HFF 2.5kHz, Sweep Dur. 500ms
Ganzfeld Stimulator 0.1 flash / second, Background Lum OFF, Strobe Intens 0.5Log
White Filter
Amplifier Sensitivity as for Blue
Ganzfeld Stimulator 0.1 flash / second, Background Lum OFF, Strobe Intens 0.75Log
For ERGs we use a 16channel Nicolet Bravo Amplifier.
Ch 11 = Right Eye Ref,
Ch 12 = Right Eye Active,
Ch 13 = Left Eye Ref,
Ch 14 = Left Eye Active.
Common = Forehead,
Reference linked to Ch 11.
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.
Electro-oculography protocol
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.
The stimulus for EOG consists of two small red LEDs approximately 20cm horizontally
separated on the posterior surface of the Ganzfeld stimulator. The LEDs flash
alternately five times each with a 1 minute rest period between each series.
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".
Equipment
Our current equipment for EOG testing consists of a Nicolet Bravo system with
a 2 channel amplifier.
Settings
EOG / 2 channel amplifier
Amplifier Sensitivity 2mV, Filters LFF .01Hz, HFF 70Hz, Sweep Dur. 1000ms
Stimuli: ISI 60 seconds.
Ganzfeld Stimulator Background Lum 20FtL
Test: Auto.
For EOGs we use a 2channel Nicolet Bravo Amplifier.
Ch1 = Left Outer Canthus,
Ch2 = Left Nasion,
Ch3 = Right Nasion,
Ch4 = Right Outer Canthus.
Common = Forehead,
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.
References
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 & 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.
This page is maintained by Paul F. Weston BSc Hons Email westonp@wch.sa.gov.au
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