Transcript ppt

The Electrooculogram EOG
Faezeh heydari
86133102
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 .
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.
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.
EOG
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 concomitant 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/°.
Measuremant of the clinical EOG
Measuremant of the clinical EOG
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.
Measuremant of the clinical EOG
Saccadic Response
Saccadic movements describe quick jumps of the
eye from one fixation point to another.
The speed may be 20 - 700°/s.
Saccadic Response
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.
Nystagmography
Nystagmography refers to the behavior of the visual
control system when both vestibular (balance) and visual
stimuli exist.
Nystagmoid movement is applied to a general class of
unstable eye movements, and includes both smooth and
saccadic contributions.
Based on the origin of the nystagmoid movement, it is
possible to separate it into vestibular and optokinetic
nystagmus.
Despite their different physiological origin, these signals
do not differ largely from each other.
Vestibular Nystagmus
Nystagmography is a useful tool in the clinical
investigation of the vestibular system .
The vestibular system senses head motion from the signals
generated by receptors located in the labyrinths of the
inner ear.
Inappropriate compensatory eye movements can easily be
recognized by the trained clinician.
Such an examination must be made in the absence of
visual fixation (since the latter suppresses vestibular eye
movements) and is usually carried out in darkness or with
the patient's eye closed. Consequently, monitoring eye
movement by EOG is the method of choice.
Optokinetic Nystagmus
Another example of nystagmoid movement is
where the subject is stationary but the target is in
rapid motion.
The oculomotor system endeavors to keep the
image of the target focused at the retinal fovea.
Nystagmography
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.
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.
The standard mehtod
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.
The standard mehtod
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.
The standard mehtod
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.
BEST Disease
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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.
BEST Disease
diseases
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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.
Practical notes, instruments and
definitions
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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 rocordings of the saccades to ensure saturation
dose not occur, and to adjust the amplifier gain
setting accordingly.
Practical notes, instruments and
definitions
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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.
Practical notes, instruments and
definitions
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Electrodes: recording the EOG is relatively undemanding
as regards the electrodes. These shoud be relatively nonpolarisable 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.
Practical notes, instruments and
definitions
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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.
Light
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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.
Reporting
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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.
Reporting
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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.
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.
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.
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.
The electroretinogram ERG
The global or full-field electroretinogram (ERG)
is a mass electrical response of the retina to photic
stimulation.
The intense flash of light elicits a biphasic
waveform recordable at the cornea.
ERG
Two principal measures of the ERG waveform are
taken: 1) The amplitude (a) from the baseline to
the negative trough of the a-wave, and the
amplitude of the b-wave measured from the
trough of the a-wave to the following peak of the
b-wave; and 2) the time (t) from flash onset to the
trough of the a-wave and the time (t) from flash
onset to the peak of the b-wave .
ERG
ERG recording electrodes
There are a number of corneal ERG electrodes
that are in common use.
Some are speculum structures that hold the eye
open and have a contact lens with a wire ring that
"floats" on the cornea supported by a small
spring .
Some versions use carbon, wire or gold foil to
record electrical activity.
ERG recording electrodes
ERG recording electrodes