Title: Functional Testing of the Eye: Clinical Electrophysiology of Vision
1Functional Testing of the Eye Clinical
Electrophysiology of Vision
- Matthew L. Severns, Ph.D.
- LKC Technologies, Inc. USA
2Electrophysiology and Psychophysics
- Electrophysiology tests record the electrical
responses generated by the eyes or visual cortex - Psychophysical tests measure the patients
responses through mental process and behavior - Both are functional tests, but electrophysiology
is objective and psychophysics is subjective - We will focus on electrophysiology tests
3Common Visual Electrodiagnostic Tests
- ERG (Electroretinogram)
- Ganzfeld
- Pattern
- Multifocal
- EOG (Electro-oculogram)
- VEP/VER (Visual Evoked Potential/Response)
- Pattern
- Flash
4ERG 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
5ERG Recording Setup
6Typical 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)
7Anatomy of the Retina
8The 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
9The 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 131,
scotopic B-wave is a measure of the response from
the Rod system, especially for dim flash
10ERG Test Procedure
- Dilate the pupil with mydriatic to maximize the
light entering the eye and minimize the
interference from pupil contraction - Dark adapt gt 25 minutes to maximize the rod
responsiveness - Connect the electrodes
- Corneal electrodes on eyes
- Reference electrode on forehead
- Ground on ear
11ERG Recording Electrodes
Commonly used corneal electrodes
12ISCEV ERG Protocol Step 1Rod 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
13ISCEV ERG Protocol Step 2Maximal Response
- 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
14ICSEV ERG Protocol Step 3Oscillatory
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
15ICSEV ERG Protocol Step 4Cone 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
16ICSEV ERG Protocol Step 5Flicker 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
LKC software provides automatic analysis of
Flicker ERG
17ERG 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
18ERG 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
19ERG Additional Tests
- Photopic Negative Response ERG
- Test condition Dilated, photopic test
- Stimulus Red Flash on Blue Background
- Generated by retinal ganglion cells
- Early glaucoma evaluation
- On/Off Response ERG
- 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
20ERG 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
21EOG 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
22EOG Testing First Steps
- Pupil dilation and dark adaptation are not
required for EOG test
- Connect electrodes to inner and outer canthii
EOG electrodes
- Patient looks side to side at alternating lights
- LKC system automatically measures the potentials,
and analyzes EOG data
23EOG 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
24EOG A Normal Recording
Arden Ratio Light / Dark gt 2.0 is OK
25EOG Clinical Applications
- Most commonly used in Bests Disease (Bests
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)
26VEP 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
27VEP Electrode Placement
28VEP 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
29Pattern VEP A Normal Recording
30Pattern 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
31Flash VEP Applications
- Assessing visual function behind media opacities
- Surgical monitoring
- Intraorbital surgery with risk for optic nerve
damage - Endoscopic sinus surgery
32Sweep VEP Objective Measurement of Visual Acuity
- Useful for patients who are unable to respond
- Pre-verbal infants (assessment of amblyopia)
- Traumatic brain injury
- Mental retardation
- Detection of malingering
33Sweep VEP Recording and Analysis Procedure
- The electrode placement is same as for regular
VEP - There are several steps of recording, each of
which is made with a different grating width - The response amplitudesare automatically
meas-ured, and the estimatedacuity is
determined by - the user selection of valid
- data points
34Sweep VEP Technique
- The subject must be looking at the screen for the
entire sweep (approx 10 seconds). This can be
difficult with babies - Remove other stimuli (especially faces)
- Use movies on screen to attract attention before
sweep - Laser pointer or other device to direct babys
attention - Average several sweeps for best results.
- 5 to 10 sweeps may be needed for a good result
- Select proper points for analysis
35Sweep VEP A Typical Recording
Larger Stripes
Smaller Stripes
36Multifocal 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.
37MFERG The Concept
Stimulus on high-quality video monitor
38MFERG The Individual Response and 3-Dimensional
Display
Blind spot Foveal peak
3D display of ERG response density
Focal ERG from each retina area
39MFERG Map And Focal
- Analyze summarized ERG responses from different
regions - Analyze the overall response from the central
retina area of 50 - 600 view angle
40MFERG Recording Procedure
- Dilate patients 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
41MFERG 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
42Conclusions
- 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.
43Thank You!
LKC Technologies, Inc. 2 Professional
Drive Gaithersburg, MD 20879 USA Tel.
1-301-840-1992 Website www.lkc.com