The Electrooculogram (EOG)
EOG
The clinical electrooculogram is an electrophisiological
test of function of the outer retina and retinal pigment
epithelium in which the change in the electrical potential
between the cornea and the ocular fundus is recorded
during successive periods of dark and light adaptation.
Today the recording of the EOG is a routinely applied
diagnostic method in investigating the human oculomotor
system.
The application of digital computers has considerably
increased the diagnostic power of this method .
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Electrophisiology of RPE in dark and
light adaptation
Emil du Bois-Reymond (1848) observed that the
cornea of the eye is electrically positive relative to the
back of the eye.
Elwin Marg named the electrooculogram in 1951 and
Geoffrey Arden (Arden et al. 1962) developed the first
clinical application.
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EOG
This positive potential behaves as if it were a single dipole
oriented from the retina to the cornea.
Such corneoretinal potentials are well established and are
in the range of 0.4 - 1.0 mV .
Eye movements thus produce a moving (rotating) dipole
source and, accordingly, signals that are a measure of the
movement may be obtained .
The chief application of the EOG is in the measurement of
eye movement.
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EOG
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Measuremant of the clinical EOG
The calibration of the signal may be achieved by
having the patient look consecutively at two different
fixation points located a known angle apart and
recording the associated EOGs .
By attaching skin electrodes on both sides of an eye
the potential can be measured by having the subject
move his or her eyes horizontally a set distance .
Typical signal magnitudes range from 5-20 µV/°.
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Measuremant of the clinical EOG
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Measuremant of the clinical EOG
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Measuremant of the clinical EOG
A ground electrode is attached usually to either the
forehead or earlobe.
Either inside a Ganzfeld, or on a screen in front of the
patient, small red fixation lights are place 30 degrees
apart .
The distance the lights are separated is not critical for
routine testing.
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Measuremant of the clinical EOG
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Saccadic Response
Saccadic movements describe quick jumps of the eye
from one fixation point to another.
The speed may be 20 - 700°/s.
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Saccadic Response
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Saccadic Response
The trajectory and velocity of saccades cannot voluntarily be altered.
Typical values of these parameters are 400°/s for the maximum velocity,
20° for the amplitude, 80 ms for the duration, and 200 ms for the
latency .
When making large saccades (>25°), the eyes reach the maximum
velocity earlier, and then have a prolonged deceleration.
Normally the duration and amplitude are approximately linearly
correlated to each other.
Several factors such as fatigue, diseases, drugs, and alcohol influence
saccades as well as other eye movements.
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The standard mehtod
The patient should be light adapted such as in an well-
illuminated room, and their eyes dilated.
After the electrodes are attached the procedure is
explained and the patient asked to practice several
times while baseline data are recorded.
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The standard mehtod
The procedure is simply that the patient keeps his or
her head still while moving the eyes back and forth
alternating between the two red lights.
The movement of the eyes produces a voltage swing
of approximately 5 millivolts between the electrodes
on each side of the eye, which is charted on graph
paper or stored in the memory of a computer.
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The standard mehtod
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The standard mehtod
After training the patient in the eye movements, the lights
are turned off.
About every minute a sample of eye movement is taken as
the patient is asked to look back and forth between the
two lights .
Some laboratories have the patients move their eyes the
entire testing period.
After 15 minutes the lights are turned on and the patient is
again asked about once a minute to move his or her eyes
back and forth for about 10 seconds.
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The standard mehtod
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The standard mehtod
Typically the voltage becomes a little smaller in the dark
reaching its lowest potential after about 8-12 minutes, the
so-called "dark trough.“
When the lights are turned on the potential rises, the light
rise, reaching its peak in about 10 minutes.
When the size of the "light peak" is compared to the "dark
trough" the relative size should be about 2:1 or greater.
A light/dark ratio of less than about 1.7 is considered
abnormal.
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The standard mehtod
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BEST Disease
Sight loss can be variable but, like other macular
problems, Best's disease threatens central vision in
one or both eyes.
Within 5 identifiable stages, examination of the eye
discloses a distinct progression. At first and second
stages, there may be little or no effect on sight.
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BEST Disease
• Initially a recording of eye movements and eye position identifies
abnormal electrical potential.
• At the second stage (usually between 10-25 years of age), typical yellow
spots, sometimes accompanied by material leaking into a space by the
retina, can be observed; an observation called "egg-yolk" lesion.
• When part of the lesion becomes absorbed this is identified as stage
three.
• At the fourth stage, when the "egg-yolk" breaks up, in a process referred
to as "scrambled-egg", sight will probably be affected.
• The fifth and final stage is when the condition causes the most severe
sight loss.
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BEST Disease
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Diseases
• The curves of the EOG of the depressed patients have
lower amplitude.
• The normalised mean EOG amplitudes obtained from
a group of amblyopic eyes were significantly lower
that the normalised mean amplitudes from the fellow
eyes at all time points during the EOG recording.
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Practical notes, instruments and
definitions
• Amplifiers: for the lowpass filter, 30 Hz is sufficient.
• Amplifier saturation: EOG potentails measured
during saccadic eye movements can vary by about 5:1
in amplitude between subjects, which, with the light
rise, may mean a total amplitude range of up to 15:1.
thus, the operator must be able to see the recordings
of the saccades to ensure saturation dose not occur,
and to adjust the amplifier gain setting accordingly.
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Practical notes, instruments and
definitions
• Arden ratio: the Arden ratio is the peak EOG
amplitude occurring in the light phase, divided by the
minimum amplitude during the dark phase.
• Compliance of the patient: some patient suffer
claustrophobia or fear of the dark, and so the testing
must be perform in such a way as to minimise these
fears. In most cases, coaching under observation can
remedy poor co-operation.
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Practical notes, instruments and
definitions
• Electrodes: recording the EOG is relatively undemanding
as regards the electrodes. These shoud be relatively non-
polarisable such as standard medical EEG or ECG
electrodes, of a size appropriate for attachment to the side
of the nose.
• Full field (Ganzfield) stimulator: this should be as large
as practicable to allow adequate distance from eye to
fixation lights. It should have a chin rest and forehead bar
to ensure stable head position.
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Practical notes, instruments and
definitions
• Plotting: the average EOG amplitude calculated from
each 10 second trial shoud be potted. It is helpful if
any uncertain values have been identified and marked
at the time of recording, so that they can be ignored
when identifying the underlying curve.
• Pupil dilation: having dilated pupils means less
variability in the light entering the eye. If pupils are
not artificially dilated, then the report should state
this.
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Light
• Luminance: the calibration of the ganzfield stimulator
shoud be carried out periodically, once ayear, and
corrective action applied.
• Colour: there are several possible sources of adapting
light such as tungsten, halogen, LED and fluorescent.
For a commercial recording system, the type will be
stated in manufacturer’s literature.
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Reporting
• Basic factual report: this should include the Arden
ratio, the first dark trough amplitude, the time from
the start of the light phase to the light peak, the pupil
size at the end of the test, and the type of the adapting
light source.
• Saccade measurement: use a scale to measure the
change in EOG potential resulting from each saccade,
and calculate an average for each 10 second trial. the
average should include only those measurements
judged to be reiable.
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Reporting
• Standing potentials: reporting of the minimum standing
potential, taken from the underlying response curve, not
the minimum recorded value. This value is not often used
in diagnosis at present, but if the value is abnormally low
it may indicate an inactive retina. and the calculated
Arden ratio may be unreliable beacause of the low value
of the divisor in the ratio.
• warning of start of each trial: there shoud be a warning,
verbal or automatic, of the impeding start of each trail, to
ensure readiness of both test subject and operator.
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EOG
The EOG is redundant with the ERG in most retinal
disorders.
Retinal diseases producing an abnormal EOG will
usually have an abnormal ERG which is the better test
for analysis of scotopic and photopic measures.
The most common use of the EOG is to confirm Best's
disease.
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EOG
The most important disadvantages relate to the fact
that the corneoretinal potential is not fixed but has
been found to vary diurnally, and to be affected by
light, fatigue, and other qualities. Consequently, there
is a need for frequent calibration and recalibration.
Additional difficulties arise owing to muscle artifacts
and the basic nonlinearity of the method.
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EOG
The advantages of this technique include recording
with minimal interference with subject activities and
minimal discomfort .
The EOG had advantages over the ERG in that
electrodes did not touch the surface of the eye.
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