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Visual Electrodiagnostic Testing-8 . Mutez Gharaibeh,MD. EOG …… .. ERG …… .. VEP …… . Optic nerve and visual cortex. Retinal function. Electro-Oculogram ( EOG). Cornea of the eye is electrically positive relative to the back of the eye - PowerPoint PPT Presentation
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Visual Visual Electrodiagnostic Electrodiagnostic Testing-8 Testing-8
Mutez Gharaibeh,MDMutez Gharaibeh,MD
EOG……..EOG…….. ERG……..ERG……..
VEP ……. Optic nerve and visual VEP ……. Optic nerve and visual cortex.cortex.
Retinal function
Electro-Oculogram Electro-Oculogram ( EOG)( EOG) Cornea of the eye is electrically Cornea of the eye is electrically
positive relative to the back of the positive relative to the back of the eyeeye
Eye movements thus produce a Eye movements thus produce a moving (rotating) dipole source moving (rotating) dipole source and, accordingly, signals that are and, accordingly, signals that are a measure of the movement may a measure of the movement may be obtained. be obtained.
Electro-Oculogram Electro-Oculogram ( EOG)( EOG)
Electroretinogram Electroretinogram ( ERG)( ERG)
ERGERG Infants up to about 2 years of age can Infants up to about 2 years of age can
usually be tested without sedation by the usually be tested without sedation by the parent holding them bundled in a blanketparent holding them bundled in a blanket..
It is difficult to get a child less than 5 It is difficult to get a child less than 5 years of age to allow a contact lens or years of age to allow a contact lens or speculum recording electrode in their eye, speculum recording electrode in their eye, so skin or scleral electrodes can be used, so skin or scleral electrodes can be used, with their limitationswith their limitations..
Alternatively, the child is sedated or Alternatively, the child is sedated or anesthetizedanesthetized. .
Separating Cone/Rod Separating Cone/Rod ERGERG
Rods and cones differ in number, peak color Rods and cones differ in number, peak color sensitivity, threshold and recoverysensitivity, threshold and recovery..
There are about 120 million rods in each There are about 120 million rods in each retina and about 6-7 million cones .retina and about 6-7 million cones .
Because of sheer numbers, the ERG Because of sheer numbers, the ERG following a white flash is dominated by the following a white flash is dominated by the mass response of the rodsmass response of the rods. .
By manipulating adaptation level and By manipulating adaptation level and background illumination, flash intensity, color background illumination, flash intensity, color of the flash and rate of stimulation, rod and of the flash and rate of stimulation, rod and cone activity can be significantly isolatedcone activity can be significantly isolated..
Peak wavelength sensitivity for rods is Peak wavelength sensitivity for rods is around 510 nm and the peak sensitivity around 510 nm and the peak sensitivity of cones as a group is about 560 nm of cones as a group is about 560 nm ..
By using color filters you can By using color filters you can essentially isolate rod and cone ERGs essentially isolate rod and cone ERGs using dim flash stimuli into photopic using dim flash stimuli into photopic ((conecone))and scotopic and scotopic ((rodrod) ) signals.signals.
Dim red analyses both rod and cone. Dim red analyses both rod and cone. Rods are about three log units more Rods are about three log units more sensitive than cones. However cones sensitive than cones. However cones recover faster than rods. recover faster than rods.
Using different rates Using different rates ((flickerflicker) ) of stimulus of stimulus presentation also allows rod and cone presentation also allows rod and cone contributions to the ERG to be separatedcontributions to the ERG to be separated. .
Even under ideal conditions rods cannot Even under ideal conditions rods cannot follow a flickering light up to 20 per follow a flickering light up to 20 per second whereas cones can easily follow a second whereas cones can easily follow a 30 Hz flicker, which is the rate routinely 30 Hz flicker, which is the rate routinely used to test if a retina has good cone used to test if a retina has good cone physiology physiology
How to perform ERGHow to perform ERG
1. Dark adapt patient for a set time of 30 minutes.1. Dark adapt patient for a set time of 30 minutes. 2. Attach electrodes using dim red illumination. 2. Attach electrodes using dim red illumination. 3. Record ERG using single scotopically-balanced dim 3. Record ERG using single scotopically-balanced dim
blue and red flashes, and bright white flashes blue and red flashes, and bright white flashes 4. Turn on moderately high background illumination 4. Turn on moderately high background illumination
for about 10 minutes and record ERGs using 30 Hertz for about 10 minutes and record ERGs using 30 Hertz flicker and bright white flashes. flicker and bright white flashes.
Responses recorded using moderately high Responses recorded using moderately high background illumination accentuate the cone system background illumination accentuate the cone system by bleaching the rods and only cones can recover fast by bleaching the rods and only cones can recover fast enough between flashes to accurately follow a enough between flashes to accurately follow a flickering 30 Hertz light.flickering 30 Hertz light.
Recording scotopic ERGs Thirty minutes or more in the dark produces a state of Thirty minutes or more in the dark produces a state of
98% or more dark adaptation in most individuals.98% or more dark adaptation in most individuals. The use of 2 or more log unit filters to reduce flash The use of 2 or more log unit filters to reduce flash
intensity and dim blue filters, limits the ERG to intensity and dim blue filters, limits the ERG to reflecting rods only. reflecting rods only.
The purpose of this is to establish a standard so that The purpose of this is to establish a standard so that differences between rod and cone physiology can be differences between rod and cone physiology can be more easily detected. more easily detected.
The scotopic dim blue ERG is most sensitive not only The scotopic dim blue ERG is most sensitive not only to rod disorders but also to systemic metabolic to rod disorders but also to systemic metabolic aberrations and retinal toxicity.aberrations and retinal toxicity.
A limitation of the traditional global or A limitation of the traditional global or fullfull--field ERG is that the recording is a field ERG is that the recording is a massed potential from the whole retinamassed potential from the whole retina. .
Unless 20% or more of the retina is Unless 20% or more of the retina is affected with a diseased state the ERGs affected with a diseased state the ERGs are usually normalare usually normal..
In other words a legally blind person In other words a legally blind person with macular degeneration, enlarged with macular degeneration, enlarged blind spot or other central scotomas will blind spot or other central scotomas will have normal global ERGshave normal global ERGs..
The multifocal ERG The multifocal ERG mfERGmfERG
Visual Evoked Visual Evoked PotentialPotentialVEPVEP
Thank you Thank you
