Visual And Ocular Electrodiagnosis Flashcards
What are electrodiagnostics for
- confirm normal functional status
- testing infants and unresponsive patients
- monitor for development of drug toxicity
- detect a sub-clinical disease
- detect early functional loss ins a progressive disease
- detect carrier state disease
- discriminate level of deficit
- monitor progression/reolsution of condition
Larger potentials you can measure in the human body
Electrooculogram
6miliVolts
ERG potential
Larger potential but less than EOG and ECG
What kind of potentials do pERG, VEP, and mfERG
VERY small
Voltage of eye
10-30mV from front to back of the eye
In an EOG, when the eye rotates between a set of electrodes
It’s like the rotation of a generator that produces a sinusoidal change in voltage that can be measured
The voltage measured in the EOG
The voltage produced by the eye movement is though to emanate from the rod system but is abnormal primarily in RPE disease
Early form of visual recording
EOG
Which is more common EOG or VEP/ERG
VEP/ERG
Measuring the EOG
- place two electrodes, one on either side of the eye
- patient moves the eyes on command usually inside of a ganzfeld under both dark adapted and light adapted conditions
- there is a lower dipole voltage under dark adapted vs light adapted conditions
- called the “light rise” of EOG
- the ratio between the light and dark adapted voltages is called the Arden ratio which is normal between 1.65-1.80
Arden ratio
The ratio between the light and dark adapted voltages in an EOG
What is considered a good Arden ratio
1.65-1.80
Reason the ganzfeld is bigger than the FOV
To adapt the rest of the retina
EOG in Best’s doses (autosomal dominant vitiliform maculopathy)
Will have a normal ERG but not a normal EOG.
- referred to as “fried egg” macular appearance
- do not get a light rise in light adaptation
Diseases in which the EOG can be abnormal
Bests Stargardt Pattern dystrophies RP Rod/cone dystrophies Oguchi disease Fundus albipunctatus Choroidemia Gyrate atrophy
How the ERG works
- arises from a flash light stimulus that cases a dynamic change in the measured voltage (1Mv) from the eye
- unlike the standing potential of the EOG, the ERG helps discriminate the functionality of more layers than just the RPE
A wave in ERG
Arises from the photoreceptors
B wave in ERG
Arises from bipolar layer and the depolarization of the mueller cells
Oscillatory potentials in ERG
Represent “ringing” in the neural transmission at the amacrine cell layer
C wave for ERG
RPE
Mueller cells
Glial cells
Absorb the depolarization of the neurons
Full field ERG
- full field ERG technique produced a “robust” measure of retinal function because it seeks to maximally stimulate almost all photoreceptor simultaneously
- uses a ganzfeld to integrate responses over entire retina
- patients frequency dilated
- not as useful for maculopathies because of small area of retina affected
- better for wide area diseases
Full field ERG depends on stimulus conditions
By controlling the stimulus parameters, and the patients state of adaption, the ffERG allows the clinical to see the responses of the rod vs cone system
Normal sequence of ERG stimuli
Things get brighter and as they get brighter, things get faster
FfERG in RP
Not much activity
Scotopic ERG
- dark adapted state
- ganzfeld
- patient dilated
- dim blue light stimulus limits response to rods only
- dark adapted recording of dim red stimulus includes some cone as well as rod response
- gradually increasing levels of white light give stronger and stronger ERGtraces
- note that with increasing stimulus intensity, the a-wave, and b-wave amps increase and their peaks move to earlier time course (latency of implicit time)
When going from 30hz-60hz
Actually goes down, instead of going up like you think
-this is the flicker fusion frequency cut off
Recap of ffERG
-changing patient adaptation and stimulus parameters leads to differential ERG traces that give us the ability to tease apart the functionality of the various component cells in the retina
FfERG importance
An integrated look at the entire function of the retina layers and therefore looks best at problems like RP that affect entire classes of cells
- not ideal for focal lesions such as maculopathy
- can be helpful in the differentiation of and progress monitoring of a wide variety of retina diseases and hereditary conditions
