ELECTRO-ELECTRO-OCULOGRAPHYOCULOGRAPHY

Dr S R PatiDr S R Pati

DEFINITIONDEFINITION

The clinical electro-oculogram is an The clinical electro-oculogram is an electrophysiological test of function of the electrophysiological test of function of the outer retina and retinal pigment epithelium in outer retina and retinal pigment epithelium in which the change in the electrical potential which the change in the electrical potential between the cornea and the fundus is recorded between the cornea and the fundus is recorded during successive periods of dark and light during successive periods of dark and light adaptation.adaptation.

HISTORYHISTORY

Emil du Bois-Reymond (1848) observed that the Emil du Bois-Reymond (1848) observed that the cornea of the eye is electrically positive relative to the cornea of the eye is electrically positive relative to the back of the eye.back of the eye.

Elwin Marg named the electrooculogram in 1951 and Elwin Marg named the electrooculogram in 1951 and

Geoffrey Arden (Arden et al. 1962) developed the Geoffrey Arden (Arden et al. 1962) developed the first clinical applicationfirst clinical application

The eye has a standing electrical potential between front The eye has a standing electrical potential between front and back, sometimes called the corneo-fundal potential. and back, sometimes called the corneo-fundal potential. The potential is mainly derived from the retinal pigment The potential is mainly derived from the retinal pigment epithelium (RPE), and it changes in response to retinal epithelium (RPE), and it changes in response to retinal illuminationillumination

The potential decreases for 8–10 min in darkness. The potential decreases for 8–10 min in darkness. Subsequent retinal illumination causes an initial fall in the Subsequent retinal illumination causes an initial fall in the standing potential over 60–75 s (the fast oscillation standing potential over 60–75 s (the fast oscillation (FO)), followed by a slow rise for 7–14 min (the light (FO)), followed by a slow rise for 7–14 min (the light response). These phenomena arise from ion response). These phenomena arise from ion permeability changes across the basal RPE membrane. permeability changes across the basal RPE membrane.

The clinical electro-oculogram (EOG) The clinical electro-oculogram (EOG) makes an indirect measurement of the makes an indirect measurement of the minimum amplitude of the standing minimum amplitude of the standing potential in the dark and then again at its potential in the dark and then again at its peak after the light rise. This is usually peak after the light rise. This is usually expressed as a ratio of ‘light peak to dark expressed as a ratio of ‘light peak to dark trough’ and referred to as the trough’ and referred to as the Arden ratioArden ratio. .

Measurement of the clinical EOGMeasurement of the clinical EOG

The calibration of the signal may be achieved by The calibration of the signal may be achieved by having the patient look consecutively at two different having the patient look consecutively at two different fixation points located a known angle apart and fixation points located a known angle apart and recording the concomitant EOGs .recording the concomitant EOGs .

By attaching skin electrodes on both sides of an eye By attaching skin electrodes on both sides of an eye the potential can be measured by having the subject the potential can be measured by having the subject move his or her eyes horizontally a set distancemove his or her eyes horizontally a set distance . .

Typical signal magnitudes range from 5-20 µV/°.Typical signal magnitudes range from 5-20 µV/°.

A ground electrode is attached usually to A ground electrode is attached usually to either the forehead or earlobe.either the forehead or earlobe.

Either inside a Ganzfeld, or on a screen in Either inside a Ganzfeld, or on a screen in front of the patient, small red fixation lights front of the patient, small red fixation lights are place 30 degrees apart .are place 30 degrees apart .

The distance the lights are separated is not The distance the lights are separated is not critical for routine testing.critical for routine testing.

The patient should be light adapted such as in an The patient should be light adapted such as in an well-illuminated room, and their eyes dilatedwell-illuminated room, and their eyes dilated

The patient keeps his or her head still while moving The patient keeps his or her head still while moving the eyes back and forth alternating between the two the eyes back and forth alternating between the two red lights.red lights.

The movement of the eyes produces a voltage swing The movement of the eyes produces a voltage swing

of approximately 5 milli volts between the electrodes of approximately 5 milli volts between the electrodes on each side of the eye, which is charted on graph on each side of the eye, which is charted on graph paper or stored in the memory of a computer.paper or stored in the memory of a computer.

The standard methodThe standard method

After training the patient in the eye After training the patient in the eye movements, the lights are turned off.movements, the lights are turned off.

About every minute a sample of eye About every minute a sample of eye movement is taken as the patient is asked to movement is taken as the patient is asked to look back and forth between the two lights .look back and forth between the two lights .

After 15 minutes the lights are turned on and After 15 minutes the lights are turned on and the patient is again asked about once a minute the patient is again asked about once a minute to move his or her eyes back and forth for to move his or her eyes back and forth for about 10 seconds.about 10 seconds.

The standard methodThe standard method

Typically the voltage becomes a little smaller in the Typically the voltage becomes a little smaller in the dark reaching its lowest potential after about 8-12 dark reaching its lowest potential after about 8-12 minutes, the so-called “minutes, the so-called “dark troughdark trough”.”.

When the lights are turned on the potential rises, the When the lights are turned on the potential rises, the light rise, reaching its peak in about 10 minutes.light rise, reaching its peak in about 10 minutes.

When the size of the "When the size of the "light peaklight peak" is compared to the " is compared to the "dark trough" the relative size should be about "dark trough" the relative size should be about 2:1 or 2:1 or greater .greater .

A light/dark ratio of less than about 1.7 is A light/dark ratio of less than about 1.7 is considered abnormal.considered abnormal.

APPLICATIONSAPPLICATIONS The light response is affected in:The light response is affected in: - diffuse disorders of the RPE and the photoreceptor - diffuse disorders of the RPE and the photoreceptor

layer of the retina including some characterized by rod layer of the retina including some characterized by rod dysfunctiondysfunction

- chorio-retinal atrophic and inflammatory diseases- chorio-retinal atrophic and inflammatory diseases

In most of these there is correlation with the In most of these there is correlation with the electroretinogram (ERG), except notably in the case of electroretinogram (ERG), except notably in the case of Best’s vitelliform maculopathyBest’s vitelliform maculopathy, in which the clinical EOG , in which the clinical EOG is usually highly abnormal in the presence of a normal is usually highly abnormal in the presence of a normal ERGERG

May be an early indicator of Chloroquine toxicityMay be an early indicator of Chloroquine toxicity

BEST DiseaseBEST Disease

BEST DiseaseBEST Disease

Sight loss can be variable but, like other Sight loss can be variable but, like other macular problems, Best's disease threatens macular problems, Best's disease threatens central vision in one or both eyes.central vision in one or both eyes.

Within 5 identifiable stages, examination of Within 5 identifiable stages, examination of the eye discloses a distinct progression. At first the eye discloses a distinct progression. At first and second stages, there may be little or no and second stages, there may be little or no effect on sight.effect on sight.

BEST DiseaseBEST Disease

Initially a recording of eye movements and eye position Initially a recording of eye movements and eye position identifies abnormal electrical potential. identifies abnormal electrical potential.

At the second stage (usually between 10-25 years of age), At the second stage (usually between 10-25 years of age), typical yellow spots, sometimes accompanied by material typical yellow spots, sometimes accompanied by material leaking into a space by the retina, can be observed; an leaking into a space by the retina, can be observed; an observation called "egg-yolk" lesion.observation called "egg-yolk" lesion.

When part of the lesion becomes absorbed this is identified as When part of the lesion becomes absorbed this is identified as stage three.stage three.

At the fourth stage, when the "egg-yolk" breaks up, in a At the fourth stage, when the "egg-yolk" breaks up, in a process referred to as "scrambled-egg", sight will probably be process referred to as "scrambled-egg", sight will probably be affected.affected.

The fifth and final stage is when the condition causes the most The fifth and final stage is when the condition causes the most severe sight loss.severe sight loss.

Other diseasesOther diseases

The curves of the EOG of the The curves of the EOG of the depressed depressed patients have lower amplitude.patients have lower amplitude.

The normalised mean EOG amplitudes The normalised mean EOG amplitudes obtained from a group of obtained from a group of amblyopicamblyopic eyes were eyes were significantly lower that the normalised mean significantly lower that the normalised mean amplitudes from the fellow eyes at all time amplitudes from the fellow eyes at all time points during the EOG recordingpoints during the EOG recording

↓↓ed Amplitude of EOG seen with use of :ed Amplitude of EOG seen with use of : Mannitol,Acetazolamide,BicarbonateMannitol,Acetazolamide,Bicarbonate

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