Electrodiagnostics Flashcards
full field ERG
performed using a Ganzfeld bowl which illuminates the whole retina with a full-field luminance stimulus, based on the ISCEV standard.
what is an ERG
A measurement of the retinal electrical response to a light stimulus
Electrode placed in contact with cornea and reference electrode on forehead
what is an ERG affected by
Adaptive state of the eye eg photopic (cone ERG) vs scotopic (rod ERG)
Type of stimulus
waves in ERG
o a-wave: Photoreceptors ( downstroke on ERG readout)
o b-wave: Inner retinal: muller and on-bipolar cells (upstroke on ERG readout)
o c-wave: RPE and photoreceptors
a wave
Negative A wave, arising from the photoreceptors
o A1 originates in cones
o A2 originates in rods
b wave
generated by Muller cells (which act as a sink for potassium
ions released by depolarising bipolar cells): surrogate for bipolar cell function
c wave
: slow positive wave generated by RPE but also depends on
photoreceptor integrity. Can be used to represent the function of these two
structures and their interactions
d wave
only seen when using stimulus of long duration which is then stopped
(ie. cessation of constant illumination)
implicit time
time from stimulus to peak of b wave
intensity of stimulus and restuls
in a constant subject, increasing the intensity of the stimulus will first cause
increase in the b wave amplitude followed by development of the preceding a wave
which also increases in size and both waves become faster
peak rod colour response
rod responses peak at the blue-green region of spectrum
peak cone colour respnse
cones responses vary with types of cones but average peak is at orange light (high
end).
critical fusion frequency
critical fusion frequency (CFF) represents the
maximum frequency that can be perceived as flickering
highest CTF for rod vision
rod vision is 15-28Hz (hence a higher frequency flicker will
only elicit a cone response)
highest CTF for cone vision
ighest CFF for cone vision is 50Hz
bright white flash in ERG
(standard stimulus): both a and b wave amplitudes are maximal
in scotopic (dark adapted) conditions
dim white or blue flash
response is generated only by rods
bright background or high frequency
Bright background (saturates rods) or high frequency flickers elicit a pure cone
response (since rods have poor temporal resolution): low amplitudes but very fast
kinetics (time to peak is 30-32ms)
ERG and small localised lesions
he ERG is unaffected by small localised lesions and will therefore be normal in
disease confined to the macula and will also not detect disease of the ganglion cell
layer or optic nerve
ERG and VA
does not measure VA
Different adaptations in full field ERG - dark adapted DA 0.01
this is rod specific
b-wave originates from on-bipolar cells
inner retinal response. It cannot differentiate if the problem is at the level of the photoreceptor or the inner retina
Different adaptations in full field ERG - dark adapted DA 3.0
this is a mixed rod-cone response
consists of a-wave and b-wave
Different adaptations in full field ERG - dark adapted 10.0/30.0
a-wave primarily reflects photoreceptor function-localise dysfunction to photoreceptor/ inner retina. DA 10.0/30.0 should be interpreted with DA 0.01
therefore if DA 0.01 reduced, with marked reduction of 10.0/30.0-this indicates photoreceptor dysfunction
if DA.01 reduced, with normal DA10.0/30.0-that means this is not a photoreceptor dysfunction, hence this must be an inner-retinal dysfunction
Different adaptations in full field ERG - light adapted 3.0
photopic single flash ERG
a-wave: Cone photoreceptor plus off-bipolar
b-wave: on and off-bipolar cells
Different adaptations in full field ERG - light adapted 30Hz flicker
delay indicates general cone problem
no delay but reduced amplitude: focal cone problem
results reduced b wave
Reduced b wave (with preserved a wave implying normal photoreceptors but
abnormality with bipolar cells) occurs in many conditions including CRAO,
congenital stationary night blindness and retinoschisis
results - reduced or absent phototopic response
Reduced or absent photopic response: cone dystrophies (which show a normal
or subnormal but present scotopic ERG)
o Achromatopsia
results - no response
No response (extinguished): Batten’s disease, Leber’s congenital amaurosis
results - increased a-wave
Increased a wave: albinism
results - Normal a-wave and reduced b-wave (negative ERG, with b-wave amplitude lower than a-wave
o Central retinal artery occlusion
o Central retinal vein occlusion
o Congenital stationary night blindness
o X linked juvenile retinoschisis
pattern ERG
o stimuli: Alternating contrast
o Two components 1) P50 2) N95:
o P50: Retinal Ganglion cell layer (GCL) 70% and 30% Macula photoreceptors
o N95: Retinal GCL. Measure of central retinal ganglion cell function
o amplitude of P50 is used to assess macula function
o PERG used to determine if there is primary GCL disease or optic nerve disease. For example Leber’s predominantly affects the GCL.e
example of pattern ERG
approach to looking at pattern ERGs - step 1
o look at DA 0.01
o primarily just a b-wave
o if reduced then this is a rod photoreceptor or rod inner retinal pathway problem
approach to looking at pattern ERGs - step 2
o look at DA 10.0/30.0
o if a-wave reduced: Photoreceptor problems (If DA 0.01 was reduced then this is most likely a rod photoreceptor problem)
o if a-wave normal and b-wave reduced, (electronegative): inner retinal problem
approach to looking at pattern ERGs - step 3
o look at 30Hz flicker
o if delay: general cone problem
o if no delay but reduce amplitude: Focal cone problem
mutlifoacal ERF
- The multifocal ERG is topographical map of central retinal function.
o measures the central 30 degrees/central cone photoreceptors
o has 250 focal points
o light adapted test
indications for multifocal ERG
o indications: Central vision problems or outer retinal problems
o central vision issues: Signal generally normal in optic nerve disease, so this can be useful when trying to figure out if there is macula or optic nerve problem.
o good for cone and cone-rod dystrophy
o outer retinal problems: AZOOR and MEWDS ( these are white dot syndromes)
o not as effective as full-field ERG for pathologies of the macula that do not affect the photoreceptors .
electro-oculogram
Measures the corneoretinal standing potential: the potential between the cornea and
Bruch’s membrane
the cornea is
The cornea is positively charged compared to the RPE
resting potential of the eye
The resting potential of the eye is: 60mV
how to preform electro-oulogram
Uses fixed excursion lateral eye movements under conditions of varying luminance
Electrodes are placed at the outer and inner canthi during dark adaptation and
followed by bright light
amplitude in electro-oculogram
The amplitude should increase markedly during light adaptation compared to a
trough in the dark
arden ratio
The ratio of the light peak to dark trough is known as the Arden ratio and is
normally expressed as a percentage
Arden is usually >180% in normal e
EOG reflects what
The EOG reflects retinal pigment epithelium activity
what can EOG be used to differeniate between
EOG can be used to distinguish local from diffuse retinal disease
NB: requires pupillary dilatation
visually evoked potentials
Measurement of occipital cortical electrical response to flash or pattern stimulus
(pattern is preferred here as it has better inter-subject reliability)
NB: the wave pattern of flash VEP shows great variability between people
where does the pattern VEP arise from
V1 area of the cortext
where does the flash VEP arise from
V2 area od the cortex
VEPs primarily represent what function
optic nerve function and are useful to quantify function
between the retina and the cortex
Can approximate to visual acuity: loss of amplitude approximates to reduced VA
how to perform VEP
Electrodes placed over occipital scalp areas
Pupils must not be dilated as accommodation is lost, and refractive error must be
corrected (except for flash VEPs)
Flash VEPs do not require as much cooperation from the patient and so can be
useful in children or unconscious patients
waveforms in VEPs
N75
P100
N135
P100 wave
: this is the major component and is very reliable between subjects and
generally stable from age 5 to 60 (time to peak only slows 1ms across this age
gap). The p100 is predominantly a macular response
P100 latency is the most useful measurement but amplitude is also useful
The amplitude approximates to visual acuity
monocular stimulu
evoke responses from both cortices representing the degree of
chiasmal crossover
what can affect VEP
Any abnormalities of the visual pathways or visual cortex can affect the VEP
results of VEP in MS
delayed P100 component in affected eye (presentation is
usually unilateral therefore asymmetric retrobulbar neuritis). Over time both
nerves tend to be involved and amplitudes are diminished too
results of VEP in trauma
visual pathway compression may initially lead to no VEP response
being elicited but as inflammation subsides it may return so progress can be
monitored
results of VEP in tumours
g in patients with NF1. VEPs may show prolongation and
eventually diminished amplitudes then obliteration
results of VEP in drug toxcities
slow P100 components eg ethambutol, amiodarone, methanol,
carbon monoxide, isoniazid, linezolid, sildenafil, infliximab
results of VEP in albinism
enhanced cross-over (a larger proportion of retinal ganglion cell
fibres cross at the chiasm in albino subjects)
results of VEP in ambylopia
flash VEP is normal but pattern VEP is abnormal
caution with VEPs
VEP abnormalities are symptomatic but not diagnostic. Retinal disease
alone will drastically affect VEP and so they cannot reliably distinguish optic
neuropathy from retinal disorders
