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Functional Testing of the Eye:
Clinical Electrophysiology of Vision
Matthew L. Severns, Ph.D.
LKC Technologies, Inc. USA
Electrophysiology and Psychophysics

Electrophysiology tests record the electrical
responses generated by the eyes or visual cortex
 Psychophysical tests measure the patient’s responses
through mental process and behavior
 Both are functional tests, but electrophysiology is
objective and psychophysics is subjective
 We will focus on electrophysiology tests
Common Visual Electrodiagnostic Tests

ERG (Electroretinogram)
– Ganzfeld
– Pattern
– Multifocal


EOG (Electro-oculogram)
VEP/VER (Visual Evoked Potential/Response)
– Pattern
– Flash
ERG: Functional Testing of Retina

A flash of light will elicit an electrical response
from the retina
 The response can be recorded by placing
electrodes on the surface of eye
 The recorded response is weak and needs to be
amplified
 Recorded data can be stored and analyzed on a
computer
ERG Recording Setup
Amp.
Reference
Electrode
Recording
Electrode
Computer
Ear
Ground
Electrode
ERG Response
Ganzfeld Dome
Typical ERG Response

A-Wave: Mostly due to Photoreceptor activity (outer
retina)
 B-wave: Mostly due to On- and Off- Bipolar and Müller
cell activity (inner retina)
Elec troretinogram
300
B-Wave
200
100
1: (µV) Od
0
-100
-200
-300
-400
A-Wave
Scotopic 0 dB Flash
-500
0
25
50
75
milliseconds
100
125
150
Anatomy of the Retina
The Origin of A-Wave

The photoreceptor cells are hyper-polarized in
response to a flash stimulus, causing the negative
A-wave
 Dim flash does not elicit an A-wave early enough
to be recorded
 The early part of the A-wave is a direct measure of
function of the photoreceptor cells including the
Transducin (G-protein) cascade
The Origin of B-Wave

Photoreceptors trigger the On- and Off- Bipolar cells
 Bipolar cell depolarization causes extracellular K+
changes, which trigger Müller cell membrane potential
changes
 Most of the B-wave is caused Bipolar and Müller cells
 Because the ratio of Rods vs Cones is about 13:1,
scotopic B-wave is a measure of the response from the
Rod system, especially for dim flash
ERG: Test Procedure



Dilate the pupil with mydriatic to maximize the light
entering the eye and minimize the interference from pupil
contraction
Dark adapt > 25 minutes to maximize the rod
responsiveness
Connect the electrodes:
– Corneal electrodes on eyes
– Reference electrode on forehead
– Ground on ear
ERG: Recording Electrodes
Commonly used corneal electrodes:
ERG-Jet
Burian-Allen
DTL
ISCEV ERG Protocol: Step #1
“Rod Response”

Patient is dark adapted, and there is no background light
when ERG is recorded. The response is “Scotopic”
 A dim flash stimulus (-24 dB) activates Rod photoreceptor
cells but not Cones. Only B-wave response is recorded
 Useful for the evaluation of Rod function
Elec troretinogram
300
1: (µV) Od
200
100
0
-100
Scotopic - 24 dB Flash
-200
300
2: (µV) Os
200
100
0
-100
Scotopic - 24 dB Flash
-200
0
50
100
milliseconds
150
200
ISCEV ERG Protocol: Step #2
“Maximal Response”
Elec troretinogram
500
250
1: (µV) Od



Patient remains dark adapted, and so the response is also
Scotopic
Standard flash stimulus (0 dB) activates both Rods and Cones
The response contains both A-wave and B-waves
In normal retina, this stimulus intensity elicits the maximal
response
0
-250
Scotopic 0 dB Flash
-500
500
250
2: (µV) Os

0
-250
Scotopic 0 dB Flash
-500
0
50
100
milliseconds
150
200
ICSEV ERG Protocol: Step #3
“Oscillatory Potentials”



Same stimulus as Step #2 also elicits Oscillatory Potentials
(OPs), which ride on the ascending B-wave
OPs have frequency range of 100-160 Hz
Affected by retinal ischemia:
– Diabetics, CRVO have reduced OP Amplitude
– OP Amplitude predicts high-risk diabetic patients
Elec troretinogram
500
Unfiltered ERG
1: (µV) Od
250
0
-250
-500
100
Filtered ERG
50
2: (µV) Od
LKC software provides
automatic analysis of
Oscillatory Potentials
0
-50
-100
0
25
50
75
milliseconds
100
125
150
ICSEV ERG Protocol: Step #4
“Cone Response”
The patient is exposed to background light (30 cd/m2) and
then stimulated with a standard flash (0 dB), “Photopic”
 The Rod photoreceptors are bleached by the background
light, so response from Rods is suppressed
 The response is mainly from Cone photoreceptors

Elec troretinogram
300
2
1: (µV) Od
200
100
0
1
-100
200
Photopic
0 dB Flash
Photopic
0 dB Flash
2
2: (µV) Os
100
0
1
-100
-200
0
50
100
milliseconds
150
200
ICSEV ERG Protocol: Step #5
“Flicker Response”



Flicker stimulation (15-60 Hz) at the standard intensity (0 dB)
with background on elicits photopic response
The B-wave from Cones is recorded, primarily inner retinal
response
Applications: Retinal Ischemia; cone and rod-cone disorders
Elec troretinogram
100
Ampl.: 132.7
µV, Latency: 32.8 ms
2
0
-50
1
Photopic 0 dB 30 Hz
-100
Ampl.: 135.2 µV, Latency: 32 ms
100
50
2: (µV) Os
LKC software provides
automatic analysis of
Flicker ERG
1: (µV) Od
50
0
-50
Photopic 0 dB 30 Hz
-100
0
50
100
150
milliseconds
200
250
ERG: Clinical Applications



Helps Diagnose:
– Retinitis Pigmentosa and other inherited retinal degenerations
– Congenital and acquired night blindness
– Inflammatory conditions (AZOOR, MEWDS)
– Vitamin A deficiency
Helps Manage:
– Diabetic Retinopathy
– Central and Branch Vein or Artery Occlusion
– Monitor retinal toxicity of drugs such as Plaquenil, Quinine,
Cisplatin, Vigabatrin
Helps Prognosis:
– Ocular trauma
– Detached Retina
ERG: Additional Tests

Pattern ERG
– Important point: Patient need to be refracted using tri-lenses.
Use temporal fossa for reference electrode, and forehead for
ground electrode.
– Recording electrode: DTL or Gold Foil Electrode (no lens
electrode)
– Generated by retinal ganglion cells
– Glaucoma evaluation
– Macular dysfunction

Very bright flash (+25dB) test for pre-operative
evaluation
– Dense cataract
– Vitreous hemorrhage
ERG: Additional Tests

Photopic Negative Response ERG
–
–
–
–

On/Off Response ERG
–
–
–
–

Test condition: Dilated, photopic test
Stimulus: Red Flash on Blue Background
Generated by retinal ganglion cells
Early glaucoma evaluation
Test condition: Dilated, photopic test
Stimulus: Red Flash on Blue Background
Looking at On and Off Bipolar Cells responses
Inner retina dysfunction
S-Cone ERG
–
–
–
–
Test condition: Dilated, photopic test
Stimulus: Blue Flash on Amber background
Generated by S-Cone Photoreceptors
Enhanced S-Cone Syndrome
ERG: Additional Tests - Research

Scotopic Threshold response ERG
– Test condition: Dilated, scotopic test
– Stimulus: Series of flash of increasing intensity
starting from below threshold (starting intensity
is species dependent)

Double Flash ERG
– Stimulus: Bright Flash followed by medium
flash
EOG: The Electro-Oculogram




Records the standing potential between the front and back
of eye
Also called “Corneo-Fundal Potential”
Measures function of Retinal Pigment Epithelium (RPE)
Amplitude of potential changes with retinal illumination
over a period of minutes
– Dark: smaller potential
– Light: larger potential
EOG Testing: First Steps

Pupil dilation and dark adaptation are not required for EOG
test

Connect electrodes to inner and outer canthii:
EOG electrodes
1-
1+
2-
2+
 Patient looks side to side at alternating lights

LKC system automatically measures the potentials, and
analyzes EOG data
EOG: Recording Phases





Three phases are typically recorded in EOG
The pre-adapt light phase is to standardize the standing
potential, taking 1-5 min.
The dark-adapt phase is to “discharge” the standing
potential, taking 10 - 20 min.
The light phase is to “recharge” the standing potential,
taking 4 - 10 min.
The test takes about 30 - 40 min in total. Recording of
both eyes are recommended to save time
EOG: A Normal Recording
Arden Ratio: Light / Dark > 2.0 is OK
EOG: Clinical Applications


Most commonly used in Best’s Disease (Best’s Vitelliform
Macular Dystrophy)
– ERG Normal, EOG Abnormal is CONFIRMING
diagnosis
– Abnormal EOG even in patients with no symptoms of
the disorder
Abnormal EOG also found in:
– Retinal pigmentary degenerations
– Chorioretinal dystrophies (e.g. choroideremia)
VEP: Visual Evoked Potential

Measures function of visual
pathway: fovea, optic nerve,
primary visual cortex
 Pattern or Flash Stimulus
 Normally use pattern stimulus
(less variability)
– Alternating grating, sinusoid, or
checkerboard pattern
– Stimulus may be full field or
hemi-field

Record signals at visual cortex
VEP: Electrode Placement
Ground
Electrode
Recording
Electrode
Reference
Electrode
Computer
Amp.
VEP: Recording Procedure




VEP response is very small, about 20V or less, and
spontaneous brain activity and EMG may dominate the
individual responses
Need to average 50-100 responses to remove noise and
reveal the underlying response
Artifacts caused by head movements may distort the
recording, and so the sweeps contaminated with artifact
should be rejected. LKC software automatically does this.
For Pattern VEP
– Patient should be properly refracted (near correction)

For Flash VEP
– Must patch contralateral eye to avoid artifacts
Pattern VEP: A Normal Recording
Pattern VER
25
20
P100
( 100 ms)
15
10
2
1: (µV) Oz R
5
0
-5
1
-10
-15
-20
32x32 100% Contrast Checks 2 Hz
-25
0
50
100
150
milliseconds
200
250
Pattern VEP: Applications



Optic Nerve Disorders:
– Optic neuropathy (compressive, ischemic)
– Optic nerve atrophy
– Compressive tumors
– Demyelinating disease (e.g., Multiple Sclerosis)
– Toxic optic neuropathies (ethambutol, cisplatin)
Malingering, hysterical blindness
Can use hemifield stimulation to distinguish pre-chiasmal
from post-chiasmal effects
Flash VEP: Applications


Assessing visual function behind media opacities
Surgical monitoring
– Intraorbital surgery with risk for optic nerve damage
– Endoscopic sinus surgery
Multifocal ERG (MFERG):
Mapping of Retinal Function





MFERG tests individual retinal areas in central 30° area
Stimulation is provided by video display
Sophisticated algorithms extract the response of each retinal
area from the overall recording
Photopic test (cone function)
Response amplitude related to cone density. Typically,
stimulus areas are scaled to provide equal response.
MFERG: The Concept
Amp.
Reference
Electrode
Recording
Electrode
Computer
Ear
Ground
Electrode
Stimulus on highquality video monitor
MFERG: The Individual Response
and 3-Dimensional Display
Blind spot
Focal ERG from each retina area
Foveal peak
3D display of ERG response density
MFERG: Map And Focal


Analyze summarized ERG responses from different regions
Analyze the overall response from the central retina area of
50 - 600 view angle
MFERG: Recording Procedure






Dilate patient’s pupil with a mydratic. No dark adaptation is
necessary.
Refractive correction is recommended but not required.
Recording using Burian-Allen or DTL electrode on the eye, a
reference electrode (only for DTL), and a ground electrode
The test is composed of several segments, 10 - 30 seconds
each, and total recording time is 5 - 10 minutes per eye
It is critical that patient is staring at the fixation during
recording; the eye can be monitored using a fixation camera
Eye or body movement will distort the recording, and the
segments should be repeated if there is too much noise
MFERG: Applications

Diagnosing macular disease:
– ARMD, others
 Retinal toxicity (Plaquenil and other drugs)
 AZOOR (Acute Zonal Occult Outer Retinopathy)
 Macula vs optic nerve in unexplained visual loss
 Early diagnosis of retinal disease: Many retinal disorders
affect small areas in early stages
– Diabetic retinopathy
– Retinitis pigmentosa
Conclusions
• Visual Electrodiagnostic testing provides a way to measure the
function of the retina and the visual pathway.
• The functional examination is at the cellular level, and the
recordings can be further studied with morphological data.
• Clinical applications of visual electrophysiology are broad,
and researches are being carried out for more applications.
• LKC Technologies, the leader in diagnostic electrophysiology
of vision, has been providing high quality techniques and
products for nearly 30 years.
Thank You!
LKC Technologies, Inc.
2 Professional Drive
Gaithersburg, MD 20879
USA
Tel.: +1-301-840-1992
Website: www.lkc.com