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The mfERG, PERG and VEP
ORT211 Electrodiagnosis
Lecture 3 2017
Dr. Lawrence Brown
Lead Clinical Scientist
Ophthalmic Electrophysiology
The Multifocal ERG (mfERG)
Responses from multiple discrete areas of retina
Primarily used to assess spatial variations in cone function
discrete retinal lesions
involving too small an area to affect the ERG
enlarged blind spot etc.
mfERG Stimulus
Structured stimulus: Multiple elements stimulate
many areas of the retina simultaneously
Each element flashes following a pattern of ons and offs determined by a maximum-length or ‘M’-sequence e.g. 000100110101111
Individual responses deconvolved from mass response to give miniature ERGs for each area
1st & higher order responses possible
mfERG Stimulus Scaling
Scaling (spatial distortion) of the stimulus pattern is needed to account for the spatial variation in cone density throughout the retina
Elements increase in size with increasing eccentricity to give approximately equal sized responses
TEMPORAL NASAL
Osterberg 1935 m
m-2 x
10
3
180 -
mfERG Recording
Recorded using DTL thread electrodes to avoid interfering with vision
Dilated pupils for consistent and
repeatable retinal illuminance
focus/contrast less important
A relatively normal mfERG
Blind spot
Fovea
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A mfERG in Maculopathy
Loss of responses
from macular region
The Pattern ERG (PERG)
Recorded using a counterphasing (reversing) chequerboard stimulus
mean luminance remains constant at all times typically 50 cd/m-2
Undilated pupils are required because contrast is the most important parameter
highly dependent on focus
PERG Recording
A tiny retinal response ~5µV
Differentiates macular / optic nerve disease: P50 = macula function N95 = retinal ganglion
cell function
Unsuitable for Patients with
nystagmus < 6 years (generally)
Differentiating maculopathy & optic neuropathy
Maculopathy
Optic neuropathy
The Visual Evoked Potential (VEP)
“The VEP is a recording of the electrical activity that occurs in the brain in response to visual stimulation by time-variant diffuse or structured stimuli.”
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VEP Flash Stimulus
Great for testing infants, and adults with very poor vision/cooperation
Cannot estimate VA to better than
‘rudimentary’ though
Good for detecting misrouting
Reversing Chequerboard
Similar to PERG
Confounded by nystagmus though
pattern is ‘smeared’ by the movement
similar effect to reduced contrast
Onset / Offset Chequerboard
Mean luminance remains constant
100% contrast chequerboard pattern appears from a 50% grey background
Better in the case of nystagmus, but responses more variable than for reversal
Chequerboards
Typically 1° chequers (macular stimulation) 15’ chequers (foveal stimulation)
Typically 2 RPS (reversals per second) i.e. 1Hz
Stimulus field >15° Steady fixation is necessary
requires cooperation & focus patient must be refracted
System for Recording Flash/Pattern VEPs
VEP Electrode Montage
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A Typical Pattern Reversal VEP
Electrodes MOcc – MF
N70 is a ‘foveal’ component
P100 is a ‘macula’ component
N135 is a ‘paramacular’ component
Pattern Reversal VEP Morphology Vs. Chequer Size
VEP Applications
Demyelination
large majority of patients with MS show increased peak time even in the absence of symptoms
powerful at detecting sub-clinical optic neuritis
Compression of the optic nerve from space-occupying lesions
Optic neuropathy
Functional integrity of the visual pathway
Objective cortical visual acuity measurement
Visual Acuity Estimation
VEPs recorded using pattern stimuli with different element sizes to the limit of visual acuity
Infants found to approach adult levels of VEP acuity by 6 months of age
TWO METHODS…
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“Minimum VEP Acuity”
= 6/(6 x spatial element size in minutes of arc)
Thus, if responses recorded to 5’ square-wave gratings, the minimum VEP or ‘cortical’ acuity is
6/(6 x 5) = 6/30
Likely to underestimate actual acuity
If responses only recordable to flash, then VA likely to be rudimentary only
Patient may not be completely blind even if no VEPs recordable at all
Sweep VEP
Rapid presentation of different chequer sizes
Good paradigms ensure robust and objective measurements in as little as 10 seconds! (should be repeated
though!)
Sweep VEP cont.
Amplitudes plotted Straight-line, least-
squares fit, extrapolated to 0µV from peak of function
Intercept (~14 cpd) gives a Snellen equivalent acuity of approximately 6/24 at the level of the striate cortex
Geniculostriate Pathway
Modified from Regan 1998
Right Half-Field Stimulation
In normal subject, stimulus OD or OS will activate the left hemisphere
Temporal projection OS
Nasal projection OD
Left Half-Field Stimulation
In normal subject, stimulus OD or OS will activate the right hemisphere
Nasal projection OS
Temporal projection OD
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Paradoxical Lateralisation of the P100 to half-field stimulation
Half-field stimulation activates one hemisphere only
P100 paradoxically recorded from side of scalp ipsilateral to stimulated half-field
Modified from Regan 1998
The Paradox Explained!
P100 produced by dipole generators in the calcarine sulcus
Electrode on scalp ipsilateral to stimulated half-field better placed to detect P100
Full-field stimulation causes cancellation in lateral electrodes, but not midline
Modified from Regan 1998
Flash VEP Crossed Asymmetry in Albinism
Suggested Reading
Principles & Practice of Clinical Electrophysiology of Vision (2nd Edition) J R Heckenlively & G B Arden
http://www.iscev.org/books.html
Human Brain Electrophysiology – Evoked Potentials & Evoked Magnetic Fields in Science & Medicine D. Regan
Electrophysiologic Testing in Disorders of the Retina, Optic Nerve, and Visual Pathway G A Fishman
Suggested Reading
Electrodiagnosis of Retinal
Disease Y Miyake
www.iscev.org/standards http://www.iscev.org/standards/pdfs/ISCEV-mfERG-Standard-2012.pdf
http://www.iscev.org/standards/pdfs/ISCEV-ERG-standard-2009.pdf
http://www.iscev.org/standards/pdfs/ISCEV-EOG-Standard-2011.pdf
http://www.iscev.org/standards/pdfs/ISCEV-PERG-Standard-2013.pdf
http://www.iscev.org/standards/pdfs/ISCEV-VEP-Standard-2010.pdf
Helpline: [email protected]
