Electrophysiology VEP, ERG, EOG

ELECTROPHYSIOLOGY
KAPIL GAUTAM
IOM, MMC
Presented on
Biratnagar Eye Hospital

Presentation layout
 VEP
 ERG
 EOG
 Introduction
 Types
 Pre- requirements
 Indications
 Procedure
 Clinical uses
 Limitations
 Summary
5/18/2019
Electrophysiology- VEP, ERG, EOG
2

Definition
 Is the branch of physiology that studies the electrical
properties of biological cells and tissues
 Measurements of voltage changes or electric
current or manipulations on a wide variety of scales
 Electrical potential recorded from a human or
animal presentation of a stimulus
5/18/2019Electrophysiology- VEP, ERG, EOG 3
Scanziani, Massimo; Häusser, Michael (2009). "Electrophysiology in the age of
light". Nature. 461 (7266): 930–39. doi:10.1038/nature08540. PMID 19829373.

Classification of Evoked potentials
Sensory Evoked
potential
Motor Evoked
potential
Event Related
Potentials
Visual Evoked
Potential
Auditory Evoked
Potential
Somatosensory
Evoked Potential

VISUAL EVOKED POTENTIAL - VEP

Visual Evoked Potential - VEP
 Small cortical Potential solicited by Visual stimuli
 On the order of 5micro-volt
 Elicited mostly by striate cortex
 Can be V2 or geniculate nucles
 Test for Optic Nerve and Macular Function
 Central 5 degrees of Visual field

Maturation of VEP
 P1 component of flash VEP can be recorded in full
term infant within 5 weeks of age with a peak
time of less than 200 ms
 Retinal development, cortical cell density,
myelination and VA are close enough to that of
an adult by age of 5 years

Flash VEP
 Responds to diffusely flashing light stimulus that
subtends a VF of 20 deg
 Only indicates that light has been perceived by
cortex
 Indications -
1. Media haze
2. Infants
3. Poor patient co-operation

Pattern on /off VEP
 The pattern is abruptly exchanged with an equilluminant
diffuse grey background
 Pattern onset duration should be 200ms separated by
400ms of diffuse background

 At least 2 pattern element size should be used checks
of 60 min and 15 min per side is required
 More intersubject variability than pattern reversal VEP
 Indications –
1.Estimate potential VA in preverbal children
2. Helpful in VEP assessment in pt’s with
malingering and nystagmus

Pattern Reversal VEP
 Stimuli consists of high contrast black and white checkboard
 checkboard is of rectangular or circular with fixation point at
the center

Pretest evaluation
 Pt co-operation
 Usual glass if any should be put on
 VA, pupillary diameter and field chart
 In pt with VF defects, lateral displacement of electrode is
necessary becoz field defects alter potential difference of p100

Techniques of recording VEP
 Electrodes placed slightly above inion ( occipital area) &
at the vertex
 Stimulus:
 intense diffuse light or stimulus (Flash VEP)
 Checker border (with black & white chess board
squres) (pattern VEP)
 Pattern VEP depends on the form sense and thus gives a
rough estimate of the visual acuity

Condt…
 Temporal half of retina leads to response in cortex of same
side & nasal half to opposite side
 Recording consists of sum of both responses recorded by
averaging M shaped wave
 Consists of tow negative & two positive peaks as N1, P1, N2,
P2
 Amplitude = 10-25microvolt
 Duration = 200-250 msec

Factors Affecting VEP
 Size of the stimulus
 Position of electrode on scalp
 Gender :male have longer latency because of longer head
 Age : Below 1 year p100 may be 160 ms and above 60 year also it gets
delayed up to 120 ms
 Eye dominance :Dominant eye have shorter latency and longer amplitudes
 VA :VA deterioration up to 20/200 doesn’t alter significantly
 Drugs :Pupillary constriction cause dec. latency and vice versa does the
luminance

Clinical consideration
 Objective method to assess contrast sensitivity & visual acuity
 VEPs cant sometimes deliver true picture of anomaly
 Over estimation of VA in infants
 Could be normal in some cortical lesions
 Establish the existence of a visual deficient, don’t localize the
causative lesion
 Any defect in visual pathway that affects central vision may
result in abnormal VEP
Eg macular, optical nerve & cortical lesion

Condt..
 Used in conjunctiva with other gross potentials (co-
investigation)
 PERG normal but VEP abnormal: Post retinal lesion
 VEP & PERG both reduced: retinal lesion
 Normal ERGs & abnormal VEPs: amblyopia
 Macular Degeneration
 Only diseases within eye that will effect VEP
 Multiple Sclerosis
 Longer latency
 Normal = 100ms, Never above 120ms

 Optic Nerve Diseases
 Optic Neuritis
 Reduced amplitude & increased latency
 Following resolution, Amplitude may be moral but
latency is almost always prolonged
 Compressive optic Nerve lesions
 Reduced amplitude
Condt..

 Lateralization of defects in the visual pathway
 Asymmetry of the amplitudes of VEP recorded over each hemisphere implicit a
hemianopia visual pattern
 Amblyopia
 Normal flash VEP but reduced amplitude in patterns VEP
 Glaucoma
 Defect central field defects
 Refraction
 Amplitude in pattern VEP depends on where the stimulus if focus On the
retina
 Greatest amplitude is generated by a pattern in exact focus on the retina
Condt..

Any question ?

Electroretinogram - ERG

INTRODUCTION
• ERG is the measure of an action potential produced by the
retina when it is stimulated by light of adequate intensity.
• ERG responses are recorded with an active extracelluar
electrode positioned on cornea, vitreous and at different
levels of retina.
• It is the composite of electrical activity from the
photoreceptors, Muller cells & RPE.
• It is on the order of 5mv.

PROCEDURES
According to ISCEV 2015 guidelines:
• Maximally dilate the pupils
• Before Dark adapted protocols- 20-30 min
of dark adaptation
• Before light adapted protocols- 10 min of
light adaptation

• Insert corneal contact electrodes (when these are used)
under dim red light after dark adaptation period.
• Avoid strong red light. Allow 5 min of extra dark
adaptation after insertion of contact lens electrode.
• Request the patient to fix and not move eyes.

ELECTRODES
• GROUND ELECTRODE – Forehead
• REFERENCE ELECTRODE – Outer
canthus
• ACTIVE ELECTRODE - Cornea (contact
lens electrode) in flash ERG
Conjunctival sac – used in pattern ERG

TYPES OF ERG
• FULL FIELD ERG
• FOCAL ERG
• MULTIFOCAL ERG
• PATTERN ERG

Full-Field ERG
• Also referred to as the standard or flash ERG
• Measures the stimulation of entire retina with flash
light source under light adapted or dark adapted
types of retinal adaptation .
• Useful in detecting diseases with widespread
generalized retinal dysfunction i.e.
• Cancer associated retinopathy,
• Toxic retinopathies,
• Cone rod dysfunction.

FOCAL ERG
• Used for detecting small focal lesions or pathologies
which are missed by standard full field ERG.
• Used to measure the functional integrity of fovea and thus
useful in providing information of diseases limited to
macula such as ARMD, macular scar etc.
• Mostly used in research setting than in clinical setting.

MULTIFOCAL ERG
 Enables ERGs to be recorded for many different locations
in a brief period of time (approx. 4min).
 Patient is presented with a stimulus array consisting of
about 240 hexagons, half of which are illuminated at only
one instance.
 Useful in the diagnosis of localized retinal abnormalities
such as
 BRAO,
 fundus flavimaculatus,
 stargardts diseases and
 earlier diagnosis of more generalized retinal
diseases, such as RP and glaucoma.5/18/2019Electrophysiology- VEP, ERG, EOG 34

Pattern ERG
• It mainly represents inner retinal activity (especially
ganglion cell activity)
• Useful in differentiating optic nerve disorders from
macular disorders.
• Unlike flash ERG, pattern ERG is a very small response.

Indications & Clinical Uses of ERG
• To evaluate visual function in infants & children.
• To determine presence or absence of retinal function.
• To evaluate progression of retinal degeneration.
• To confirm diagnosis of a particular disease (dystrophies).
• For early detection of toxic retinopathies.
• Assisting in diagnosing the retinal conditions in which
clinical findings don't match with visual complaints
(unexplained visual loss).

Components
• a-wave: Initial Corneal-negative deflection, derived from
the cones and rods of the outer photoreceptor layers
• b-wave: Corneal-positive deflection; derived from the
inner retina, predominantly Muller and On-bipolar cells
• c-wave: Derived from the retinal pigment epithelium
• d-wave: off bipolar cells.

Interpretation of wave sizes
Amplitudes
b-amp: trough of a to crest of b
a-amp: baseline to trough of a
Normal values
microvolt photopic scotopic
A-latency 25-75 150-250
B –latency 75-200 250-400

Interpretation of implicit time
Evaluate the speed of responses and their relation to
each other
Normal values
ms photopic scotopic
A- Latency 7-15 9-17
B- Latency 25-33 38-58

Factors affecting the ERG
• Physiological : Pupil, Age, Sex, Ref. Error, Dark
adaptation, Anesthesia
• Instrumental : amplification, gain, stimulus,
electrodes
• Artifacts : Blinking, tearing, eye movements, air
bubbles under electrode.

Interpretation of ERG
• ERG is abnormal only if more than 30% to 40% of
retina is affected
• A clinical correlation is necessary
• Media opacities, non-dilating pupils & nystagmus
can cause an abnormal ERG
• ERG reaches its adult value after the age of 2yrs
• ERG size is slightly larger in women than men

ERGs in RP

ERG IN Cone dystrophy

CHLOROQUINE
RETINOPATHY

Limitations of ERG
• Since the ERG measures only the mass response of the
retina, isolated lesions like a hole hemorrhage, a small
patch of chorioretinitis or localized area of retinal
detachment can not be detected by amplitude changes.
• Disorders involving ganglion cells (e.g. Tay sachs’ disease),
optic nerve or striate cortex do not produce any ERG
abnormality

Summary
Non recording ERG Leber congenitalAmaurosis,
Retinitis pigmentosa,
Total RD
RetinalAplasia
Abnormal or non recordable photopic
ERG
(often mild rod ERG abnormalities)
Cone degenerations
Achromatopsia
X-linked blue cone monochromatism
X-linked cone dystrophy
Non recordable Rod ERG (abnormal
dark adapted bright flash ERG)
Normal to near normal
Congenital stationary blindness
Early RP
Barely or Non recordable Scotopic ERG
(abnormal photopic B-Wave ERG)
Rod Cone Degenerations
Night blindness

Any question ?

Electro oculogram - EOG
5/18/2019 Electrophysiology- VEP, ERG, EOG 49

Introduction
 Eye movement dependent votltage recorded b/w
electrodes placed near eyes
 Source of voltage
 Corneofundal potential
 cornea 6 to10mV positive with respect to back of eye
 Depends upon the integrity of the RPE,
photoreceptors and possibly the inner retinal layer

 Measures the resting or standing potential
between the electrical positive cornea &
electrically negative Retina
 Light insensitive (standing potential)
 Standing potential
 Measured in dark adapted state
 Measures function of REP without having to
stimulate photoreceptors

Origins of EOG
 Corneo-fundal potential
results from the metabolic
activity of several epithelia
within the eye
 Cornea, lens and RPE
 Only RPE is photosensitive
 Dark adapted EOG = RPE
 Light adapted EOG = rod
activity with little contribution
of cones

 Pupil dilation
 Electrodes
 Placed over orbital margin near
inner and outer canthi
 Forehead electrode as ground
electrode
 Stimulus –
 Pair of fixation lights separated by
30 degrees of visual angle in
ganzfeld bowl
Techniques of EOG:

 The patient looks from right to left at an approximate
rate of 16 to 20 rotations per minutes
 When eyes move to the right the positive cornea becomes
closer to the one of the electrode which is accordingly
more positive than other electrode
 Opposite happens when eyes move to the left

 Recording procedure started with stimulus
lights on
 The initial amplitudes serve as a baseline
 After standardized period, lights
extinguished
 Dark adaptation
 Recordings for 15 mins under dark-adapted
condition
 Trough reached in approximately 8 to 12
minutes
 Light adaptation
 Stimulus lights turned on and responses
recorded for 15 mins under light-adapted
condition
 Peak reached in approximately 6 to 9
minutes
Recording EOG

 Recordings are sampled at intervals
of approximately one minute
 Arden ratio Calculated:
=
 Normal ratio: 1.85 or greater
 Subnormal ratio: 1.65 to 1.85
 Abnormal ratio: < 1.65
Light peak
Dark trough
×100
Recording EOG

Recording EOG
 Normal EOG can be more significant than
abnormal values
 Only 20% of normal retina can give normal EOG
 Reflects the response of entire retina
 Consistent tracking of targets for 30 minutes
 Not suitable for young children

 Adjunct to ERG
 EOG is normal in any condition where
ERG is abnormal
 Reverse is not always true
 Abnormal EOG with a normal ERG
 Vitelliform dystrophy or Best’s disease
 Butterfly-shaped pigment dystrophy of
the fovea
 Fundus flavimaculatus
 Advanced drusen
 Normal ERG and abnormal EOG
 Chloroquine toxicity
 Metallosis bulbi
Applications of EOG:

Expected status of gross electric potentials in various
diseases conditions
Condition EOG sERG Focal PERG VEP
Macular lesions Normal Normal Abnormal Abnormal
Retinitis pigmentosa Abnormal Abnormal Abnormal Abnormal
ON diseases Normal Normal Abnormal Abnormal
Amblyopia Normal Normal Normal Abnormal
Hysteria, malingering Normal Normal Normal Normal

Any
question ?

References:
 Retina & Vitreous – AAO
 ISCEV = international Society for clinical Electrophysiology of Vision
articles
 Principles & Practice Of Ophthalmology-Peyman
 Ebersole and Trimothy A.pedley
 Visual evoked potential by Donnell J_creel
 Guidelines on Visual Evoked Potential 2008
American Clinical Neurophysiology society (ACNS)
 Internet

Electrophysiology VEP, ERG, EOG

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