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Investigations > Electrodiagnostics > Flashcards

Electrodiagnostics Flashcards

1
Q

full field ERG

A

performed using a Ganzfeld bowl which illuminates the whole retina with a full-field luminance stimulus, based on the ISCEV standard.

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2
Q

what is an ERG

A

A measurement of the retinal electrical response to a light stimulus
 Electrode placed in contact with cornea and reference electrode on forehead

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3
Q

what is an ERG affected by

A

 Adaptive state of the eye eg photopic (cone ERG) vs scotopic (rod ERG)
 Type of stimulus

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4
Q

waves in ERG

A

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

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5
Q

a wave

A

 Negative A wave, arising from the photoreceptors
o A1 originates in cones
o A2 originates in rods

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6
Q

b wave

A

generated by Muller cells (which act as a sink for potassium
ions released by depolarising bipolar cells): surrogate for bipolar cell function

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7
Q

c wave

A

: 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

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8
Q

d wave

A

only seen when using stimulus of long duration which is then stopped
(ie. cessation of constant illumination)

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9
Q

implicit time

A

time from stimulus to peak of b wave

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10
Q

intensity of stimulus and restuls

A

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

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11
Q

peak rod colour response

A

rod responses peak at the blue-green region of spectrum

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12
Q

peak cone colour respnse

A

cones responses vary with types of cones but average peak is at orange light (high
end).

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13
Q

critical fusion frequency

A

critical fusion frequency (CFF) represents the
maximum frequency that can be perceived as flickering

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14
Q

highest CTF for rod vision

A

rod vision is 15-28Hz (hence a higher frequency flicker will
only elicit a cone response)

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15
Q

highest CTF for cone vision

A

ighest CFF for cone vision is 50Hz

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16
Q

bright white flash in ERG

A

(standard stimulus): both a and b wave amplitudes are maximal
in scotopic (dark adapted) conditions

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17
Q

dim white or blue flash

A

response is generated only by rods

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18
Q

bright background or high frequency

A

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)

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19
Q

ERG and small localised lesions

A

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

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20
Q

ERG and VA

A

does not measure VA

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21
Q

Different adaptations in full field ERG - dark adapted DA 0.01

A

 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

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22
Q

Different adaptations in full field ERG - dark adapted DA 3.0

A

 this is a mixed rod-cone response
 consists of a-wave and b-wave

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23
Q

Different adaptations in full field ERG - dark adapted 10.0/30.0

A

 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

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24
Q

Different adaptations in full field ERG - light adapted 3.0

A

 photopic single flash ERG
 a-wave: Cone photoreceptor plus off-bipolar
 b-wave: on and off-bipolar cells

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25
Q

Different adaptations in full field ERG - light adapted 30Hz flicker

A

 delay indicates general cone problem
 no delay but reduced amplitude: focal cone problem

26
Q

results reduced b wave

A

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

27
Q

results - reduced or absent phototopic response

A

Reduced or absent photopic response: cone dystrophies (which show a normal
or subnormal but present scotopic ERG)
o Achromatopsia

28
Q

results - no response

A

No response (extinguished): Batten’s disease, Leber’s congenital amaurosis

29
Q

results - increased a-wave

A

Increased a wave: albinism

30
Q

results - Normal a-wave and reduced b-wave (negative ERG, with b-wave amplitude lower than a-wave

A

o Central retinal artery occlusion
o Central retinal vein occlusion
o Congenital stationary night blindness
o X linked juvenile retinoschisis

31
Q

pattern ERG

A

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

32
Q

example of pattern ERG

A
33
Q

approach to looking at pattern ERGs - step 1

A

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

34
Q

approach to looking at pattern ERGs - step 2

A

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

35
Q

approach to looking at pattern ERGs - step 3

A

o look at 30Hz flicker
o if delay: general cone problem
o if no delay but reduce amplitude: Focal cone problem

36
Q

mutlifoacal ERF

A
  • 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
37
Q

indications for multifocal ERG

A

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 .

38
Q

electro-oculogram

A

 Measures the corneoretinal standing potential: the potential between the cornea and
Bruch’s membrane

39
Q

the cornea is

A

The cornea is positively charged compared to the RPE

40
Q

resting potential of the eye

A

The resting potential of the eye is: 60mV

41
Q

how to preform electro-oulogram

A

 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

42
Q

amplitude in electro-oculogram

A

The amplitude should increase markedly during light adaptation compared to a
trough in the dark

43
Q

arden ratio

A

 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

44
Q

EOG reflects what

A

 The EOG reflects retinal pigment epithelium activity

45
Q

what can EOG be used to differeniate between

A

EOG can be used to distinguish local from diffuse retinal disease
 NB: requires pupillary dilatation

46
Q

visually evoked potentials

A

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

47
Q

where does the pattern VEP arise from

A

V1 area of the cortext

48
Q

where does the flash VEP arise from

A

V2 area od the cortex

49
Q

VEPs primarily represent what function

A

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

50
Q

how to perform VEP

A

 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

51
Q

waveforms in VEPs

A

 N75
 P100
 N135

52
Q

P100 wave

A

: 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

53
Q

monocular stimulu

A

evoke responses from both cortices representing the degree of
chiasmal crossover

54
Q

what can affect VEP

A

Any abnormalities of the visual pathways or visual cortex can affect the VEP

55
Q

results of VEP in MS

A

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

56
Q

results of VEP in trauma

A

visual pathway compression may initially lead to no VEP response
being elicited but as inflammation subsides it may return so progress can be
monitored

57
Q

results of VEP in tumours

A

g in patients with NF1. VEPs may show prolongation and
eventually diminished amplitudes then obliteration

58
Q

results of VEP in drug toxcities

A

slow P100 components eg ethambutol, amiodarone, methanol,
carbon monoxide, isoniazid, linezolid, sildenafil, infliximab

59
Q

results of VEP in albinism

A

enhanced cross-over (a larger proportion of retinal ganglion cell
fibres cross at the chiasm in albino subjects)

60
Q

results of VEP in ambylopia

A

flash VEP is normal but pattern VEP is abnormal

61
Q

caution with VEPs

A

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

Investigations (9 decks)

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