Investigations > Electrodiagnostic tests > Flashcards

Electrodiagnostic tests Flashcards

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1
Q

What is the aim of electrodiagnostic tests (EDTs)?

A

provide objective evaluation of visual pathway function

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2
Q

What are the 6 basic tests used in the electrophysiology lab?

A
  1. Full-field ERG
  2. Pattern ERG (PERG)
  3. multifocal ERG (mfERG)
  4. EOG
  5. VEP
  6. dark adaptometry
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3
Q

How is each EDT interpreted?

A

by the polarity and amplitude of the electrophysical deflections and their latency (implicit time)

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4
Q

What is the society that sets the standardisation for EDTs?

A

International Society for Clinical Electrophysiology of Vision (ISCEV)

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5
Q

What is recorded by ERG?

A

the mass electrical actiity from the retina when stimulated by a flash of light

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6
Q

What are 4 indications for full field ERG?

A
  1. clinical presentation doesn’t always correlate with severity of visual symptoms
  2. confirm or eclude specific diagnosis
  3. prognostication
  4. assessment of retinal function in specific cases
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7
Q

What are 7 conditions that could be confirmed/excluded with full-field ERG?

A
  1. RP
  2. Leber’s congenital amaurosis (LCA)
  3. choroideraemia
  4. gyrate atrophy
  5. achromatopsia
  6. congenital stationary night blindness (CSNB)
  7. conde dystrophies
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8
Q

What are 5 examples of situations when assessment of retinal function with ffERG is required in specific cases?

A
  1. investigating family members for known hereditary retinal dystrophies
    2.** carrier states** of retinal dystrophies
  2. evaluation of suspected functional visual loss
  3. evaluation of retinal function in the context of opaque media
  4. evaluation of retinal function in uncooperative cases (e.g. paeds and learning difficulties)
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9
Q

What type of stimulation is used to perform full field ERG?

A

full-field (ganzfeld) stimulation

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10
Q

What eletrodes are used to achieve full-field (ganzfeld) stimulation?

A

electrodes that contact the cornea or nearby bulbar conjunctiva (CL electrodes, conductive fibres and foilds, conjunctival loop electrodes, corneal wicks)

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11
Q

What are 4 examples of electrodes that can be used for full-field (ganzfeld) ERG?

A
  1. Contact lens electrodes
  2. conductive fibres and foils
  3. conjunctival loop electrodes
  4. corneal wicks
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12
Q

What provides the result from a full-field ERG?

A

rod-response recorded in dark-adapted eyes (after 30 min in the dark)

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12
Q

In what state are the eyes to perform full-field ERG and how is this achieved?

A

dark-adapted - after 30 min in the dark

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13
Q

How is the maximal ERG response in ffERG obtained and which photoreceptors produce this response?

A

using a bright white flash - mixed rod and cones response

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14
Q

How are photopic repsonses from ffERG obtained (in bright light, chiefly cones)?

A

acquired with a background that suppresses rod activity;
* photopic single-flash cone response obtained in light-adapted eyes (after 10 min in the light)
* cone-derived flicker response is acquired using a 40Hz white light flicker stimulus

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14
Q

Why are rods unable to respond to the cone-derived flicker response with a 30Hz white light flicker stimulus?

A

due to their temporal resolution

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15
Q

What are the 3 key consistuents of the result from an ERG?

A
  1. negaive ‘a wave’
  2. positive ‘b wave’
  3. superimposed oscillatory potentials (OPs)
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16
Q

How long does the ERG response to a bright single-flash stimulus last?

A

<250ms

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17
Q

What are the 2 parameters relevant to the a wave and b wave on the ERG result?

A
  1. amplitude (microvolts)
  2. implicit time (milliseconds)
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18
Q

What does the negative a-wave of the ERG arise from?

A

photoreceptors

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19
Q

What does the positive b-wave of the ERG arise from (2 things)?

A
  1. bipolar cells
  2. Müller cells
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20
Q

What do OPs in the ERG recording arise from?

A

amacrine cells

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21
Q

What 2 parameters of the stimulus for ERG that can be varied and what additional factor can be varied, to selectively stimulate different parts of the eye?

A
  1. stimulus parameter: intensity
  2. stimulus parameter: frequency
  3. adaptive state of the eye
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22
Q

What can ERG be useful for in CRVO?

A

distinguishing between non-ischaemic and ischaemic CRVO

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23
What is seen in large areas of ischaemia in the ERG in CRVO?
b-wave has reduced amplitude in large areas of ischaemia, causing reduced b:a wave ration, and/or prolonged b-wave implicit time
24
What does a normal full-field ERG look like?
25
What are 7 possible differentials for ERGs showing reduced a- and b-waves?
1. Rod-cone dystrophies (including RP) 2. Total retinal detachment 3. Metallosis 4. Drug toxicity (e.g. phenothiazines) 5. Autoimmue retinopathy 6. Cancer-associated retinopathy (CAR) 7. Ophthalmic artery occlusion
26
What are 7 differntials for full-field ERG showing normal a-wave with reduced scotopic b-wave?
1. CSNB (congenital stationary night blindness) 2. X-linked retinoschisis (XLRS) 3. CRAO or CRVO 4. Myotonic dystrophy 5. Oguchi's disease 6. Quinine toxicity 7. Melanoma-associated retinopathy (MAR)
27
What are 2 differentials for ffERG showing abnormal photopic and normal scotopic ERGs?
1. Achromatopsia 2. Cone dystrophy
28
What are 2 differentials for ffERG with reduced OPs?
1. Diabetic patients 2. Drug toxicity e.g. vigabatrin
29
What can reduced OPs in diabetic patients' ffERG correlate with?
increased risk of developing severe proliferative diabetic retinopathy (PDR)
30
What is the indication for PERG (pattern ERG)?
objective assessment of macular function
31
What is the method used to achieve PERG?
**reversing chequerboard** evokes small potentials that arise from the **inner** retina
32
What does a normal PERG comprise of?
prominent positive components at ~50ms (P50) and larger negative component at 95mg (N95)
33
What does P50 of a PERG represent?
photoreceptor driven, key to assessing macular **cone** function
34
What does P95 of a PERG represent?
originates from macular ganglion cells
35
What 3 parameters are evaluated when interpreting the PERG?
1. amplitudes 2. peak times 3. N95/P50 ratio (typically >1.1)
36
How does mfERG (multifocal) differ from standard ERG?
ERG sums the electrical potentials from the whole retina, but mfERG creates a **topographical functional map** of the stimulated retina
37
What are the indications for multifocal ERG (mfERG)?
almost any retinal disorder, especially where retinal dsfunction is localised or patchy e.g. early hydroxychloroquine toxicity
38
What is the method for multifocal ERG?
multiple small areas of the retina are stimulated with appropriately scaled stimuli; fourier transformation of the responses results in topographical localisation of retinal function as it varies across the stimualted retina
39
In what form are the results generated from mfERG?
2D map demonstrating the topographical variation in responses across the retina; can be transformed into a 3D map of retinal function that resembles the hill of vision
40
How can loss of function be highlighted using mfERG?
showing the differences between the mfERG and a reference mfERG (from normal subjets)
41
What does EOG demonstrate?
reflects activity of the RPE and photoreceptors of the entire retina; measures the **standing potential at the RPE-photoreceptor interface**
42
What causes the potential at the RPE-photoreceptor interface (measured by EOG) to vary normally?
whether the eye is dark-adapted (low potential) or light-adapted (high potential)
43
What are 3 indications for EOG?
1. diagnosis of certain macular dystrophies e.g. Best's disease 2. earlydetection/screening of individuals at risk e.g. Best's disease 3. Aids diagnosis of certain inherited or acquired retinopathies/maculopathies
44
What are 3 examples of inherited and acquired retinopathies/maculopathies that EOG can aid in the diagnosis of?
1. central serous chorioretinopathy (CSC) 2. acute zonal occult outer retinopathy (AZOOR) 3. drug toxicity
45
Where are electrodes placed to perform EOG?
Medial and lateral canthi
46
Where are electrodes placed to perform EOG?
Medial and lateral canthi
47
How is EOG performed?
electrodes placed at medial and lateral canthi, patients intermittently follow targets that move from right to left over a 30 degree horizontal plane
48
How are results measured from EOG?
the cornea makes the nearest electrode more positive, compared to the other, and the difference between the two electrodes is measured
49
In what dark/light adapted state is EOG performed?
both dark- and light-adapted states
50
How are results from EOG processed?
results are based on the Arden index [(light peak/dark trough) x 100] and inform regarding RPE function
51
What is a normal vs abnormal EOG result?
the potential normally doubles from the dark-adapted to the light-adapted eye; >180% is considered to be normal, subnormal is 140-180%, and anormal <140%
52
What is the visual evoked potential (VEP)?
gross electrical response recorded from the **visual cortex** in response to a changing visual stimulus such as multiple **flash or pattern stimuli**
53
What is required for VEP to be a reliable test of visual pathway function?
relatively normal retinal/macular function
54
What type of VEP gives the most clinical information?
pattern reversal VEP (PR-VEP)
55
Which type of VEP is useful in poorly cooperative patients?
flash VEP
56
What are 3 indications for VEP?
1. optic nerve disease (e.g. subclinical demyelination) 2. chiasmal and retrochiasmal dysfunction 3. suspicion of non-organic visual loss
57
How is PR-VEP carried out?
activity over the visual cortex is measured following a reversing high-contrast black-and-white chequerboard occipital cortex voltage changes over time are plotted as waveforms
58
What does the nature of the stimulus and the size of the VF stimulated (central 15 degrees) imply?
imply the PR-VEP predominantly reflects macular cone acitivity
59
What are the normal results of PR-VEP?
positive deflection occurs at about 100ms (P100), negative deflections occur at 75ms (N5) and 135ms (N135)
60
What are 2 situations that can cause reduced amplitude and increased peak time of P100 in VEP?
1. optic neuropathies/optic neuritis 2. maculopathies
61
Why should PR-VEP typically be interptered in conjunction with retinal functino tests (PERG, ERG)?
as delayed/reduced PR-VEP is not pathognomonic of optic nerve disease (also occurs in maculopathies)
62
What is meant by dark adaptometry (DA)?
measures the absolute **threshold of photoreceptor activity** with time in the **dark-adapted eye**
63
What tests is DA performed in conjunction with?
EOG and ERG
64
What is a key example of dark-adaptometry?
Goldmann-Weekers adaptometry
65
What are 3 indications for Goldmann-Weekers adaptometry?
1. retinal disorders causing night blindness e.g. vit A deficiency 2. cone dysfunction 3. evaluation of drugs affecting dark adaptation e.g. vit A analogues, isotretinoin
66
What are 4 stages to the method of Goldmann-Weekers adaoptometry?
1. Photoreceptors are totally bleached by a bright background light 2. Light is then extinguished 3. Subjects presented with series of dim flashes of increasing intensity int he dark 4. the threshold at whihc the light is perceived is plotted over time
67
What is the typical result generated from Goldmann-Weekers adaptometry?
biphasic curve - first curve represents cone threshold (reached at 5-10min), second curve rod threshold, reached at 30min
68
At what time point in dark adaptometry is the cone threshold of the biphasic wave reached?
5-10min
69
At what time point in dark adaptometry is the rod threshold of the biphasic wave reached?
30 minutes
70
What happens at 30 minutes into dark adaptometry leading to the rod threshold being reaching?
rhodopsin has fully regenerated and retinal sensitiity has reached its peak
71
What change will be seen in dark adaptometry (GWA) in defects in rhodopsin metabolism?
produce high thresholds, with an abnormal DA curve
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