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Electrophysiology > ERG > Flashcards

ERG Flashcards

1
Q

What is an ERG?

A

Electroretinogram

Electrical response of the retina to flashes of light or patterns

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2
Q

What stimulator do we use in ERGs?

A

A Ganzfeld (whole/full field) stimulator delivers diffuse flashes that evenly illuminate the maximal area of retina (do in dark and light).

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3
Q

How does a Ganzfeld (whole/full field) stimulator work?

A

A Ganzfeld (whole/full field) stimulator delivers diffuse flashes that evenly illuminate the maximal area of retina (do in dark and light).

The full-field ERG assesses generalised retinal function under dark-adapted (DA) and light-adapted (LA) conditions.

Responses to the flashes are recorded with electrodes in contact with the cornea or with infra-orbital skin electrodes.

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4
Q

How do we record dark-adapted (scotopic) responses in Ganzfeld full-field stimulators in ERGs?

A

To record a dark-adapted (scotopic) response, eyes
are dark adapted for approximately 20 minutes,
usually under dim red lights (red is a very poor
stimulus for rods!) prior to recording. A dim flash
(0.01 cd·s·m-²) is used to produce a ‘rod response’
(~2.5 log units below, or 1/316 of, standard flash
luminance), and then standard flashes are used to produce a mixed response from rods and cones.

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5
Q

How do we record light-adapted (photopic) responses in Ganzfeld full-field stimulators in ERGs?

A

To record a light-adapted (photopic) response, the rods are suppressed by a background light of 30
cd/m² for 10 minutes before testing, and during recording. Then standard flashes are used to elicit a
‘cone response’.

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6
Q

What is the source of an ERG?

A

A-wave:
Light-dependant decrease in rod and cone dark current gives ‘a’ wave plus release of K+ (potassium)

B-wave:
Müller cells absorb extracellular K+, resulting in part of the ‘b’ wave; rest of ‘b’ wave comes from bipolar cells

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7
Q

What are A-waves in ERGs?

A

In darkness a steady current flows inward through the plasma membrane of the rod outer segments.

It is balanced by equal outward current distributed along the remainder of each rod.

Flashes of light produce a photocurrent which transiently reduces the dark current resulting in the a-wave of the electroretinogram.

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8
Q

What are B-waves in ERGs?

A

Bipolar Cells (mainly) and Muller Cells

K+ leakage increases, with consequent absorption of K+ by the Muller cells, resulting in part of the b-wave of the electroretinogram.

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9
Q

Where do a-wave, b-wave and oscillatory potentials come from?

A

A-wave from rods and cones

B-wave mainly from bipolar (and also Müller) cells

Oscillatory potentials from amacrine cells

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10
Q

Where do a-wave and b-waves originate from in ERGs?

A

Generally it is agreed that the a-wave is the ‘receptor’ component and basically reflects the reduction in the ‘dark’ currents due to light absorption in the photoreceptor outer segments.
It is widely agreed that the b-wave originates in retinal cells that are post-synaptic to the photoreceptors.

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11
Q

How do we place electrodes in ERGs?

A

Electrodes: contact lens or fibre electrodes (infra-orbital skin for children), with Ag/AgCl ground electrode on forehead and ‘reference’ electrode on ipsilateral temple

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12
Q

How do we measure between electrodes in ERGs?

A

Thread/fibre electrode from inner to outer canthi. References to temporal electrode. Measure difference between cornea and temporal electrode.

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13
Q

Why must our eyes be dilated in ERGs?

A

Must be dilated for this test to ensure the pupil is the same diameter for all flashes of light so not contracting to the light = standardised responses

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14
Q

What do we use to dilate the eyes in ERGs?

A

Dilating drops (typically tropicamide 1% and phenylephrine 2.5%) ensure pupils are non-reactive and
therefore the same diameter for all stimuli regardless of stimulus brightness.

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15
Q

What are the smallest and largest electrodes for recording ERGs?

A

ERGs are smallest with infra-orbital skin electrodes and largest with contact lens electrodes

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16
Q

What signal size do we use in fibre ERGs?

A

Approximately 300 µV with fibre

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17
Q

How many responses do we need to average in ERGs?

A

Much bigger size of the signal at 300uV so don’t need to record that many so do between 10 – 30 flashes of light (more depending on compliance and extraneous variable)

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18
Q

What bandwidth so we use in ERGs for the full response?

A

0.3 Hz - 300 Hz

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19
Q

What bandwidth do we use specifically for measuring oscillatory potentials alone in ERGs?

A

75 Hz - 300 Hz for oscillatory potentials alone

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20
Q

What is a DTL Fibre Electrode?

A

DTL is the standard fibre electrode.
- Easy to tolerate,
- Medial and lateral canthi electrodes with fibre below lower lid touching the cornea
- Can get into children as young as 6 years old compared to having eye-drops which are more difficult
- Can we worn all day
- No effect on VA
- Disposable
- No anaesthetic required (required in contact lens, typically 0.4% g-oxybuprocaine)

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21
Q

What is a JET Corneal Electrode?

A
  • Used for ERGs under GA in theatre at SCH
    Would need topical anaesthetic otherwise
  • Holds eyes open
  • Large signals – fewer averages required
  • Relatively expensive
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22
Q

What electrodes do we use in ERG for adults and children?

A

Fibre electrodes (DTLs) are used for adult outpatients, infra-orbital skin (gel skin-patch) electrodes for infants, and contact lens electrodes for patients in theatre (under general anaesthetic).

23
Q

What are Infraorbital Skin Electrodes?

A
  • No contact with the eye, easier to fit to children.
  • Smaller signals than other types, requiring more averages.
  • Requires gaze to be slightly below primary position.
  • Patients have to be awake
24
Q

What are some issues with Infraorbital Skin Electrodes?

A

Placed as close to lower eyelid as possible. Not as good in terms of signal size (smaller) compared to the other two so need to do far more signal averaging to remove the noise which means 100-200 flashes needed. Hard because they need to be looking slightly in downgaze.

25
Q

How is the light stimuli delivered in ERG?

A

Stimuli ideally delivered via Ganzfeld (German for whole field) stimulator

26
Q

How does a Ganzfeld stimulator work?

A

Generates flashes and background light for diffuse retinal stimulation

Fixation target and camera to monitor fixation/pupil size. Has infrared camera at the back to check on the patient’s pupils during testing.

27
Q

What is the ‘standard’ flash luminance defined as in ERGs?

A

3 cd·s·m-²

28
Q

How do we get dark-adapted responses in ERGs?

A
  • Eyes are dark adapted for approx. 20 mins
  • ‘Dim’ flash (0.01 cd·s·m-² candelas squared something???) used to produce ‘rod system response’ (~2.5 log units below, or 1/316 of, standard flash luminance)
  • Then standard flash used to produce a mixed response from both rod and cone systems (3 candelas per meter squared)
29
Q

How do we get light-adapted responses in ERGs?

A
  • Rods suppressed by 30 cd/m² for 10 mins, then standard flash used to produce ‘cone system response’
30
Q

What are the 3 responses in a normal flash ERG?

A
  • Scotopic = mainly a b-wave = rod response
  • Maximal response/ DA 3 – also a dark adapted response but with the standard flash = has a larger A wave (from the photoreceptors; 75% rods, 25% cones) and B wave (post-photoreceptor function).
  • Photopic = LA 3 response = Cone response = Smaller response from the A-wave and then smaller B-wave from post-photoreceptor
31
Q

What is the DA 0.01 in an ERG?

A

After the period of dark adaptation, a dim flash, below the threshold for the DA cone system, is used to elicit the DA 0.01 ERG, arising in the rod bipolar cells but dependent on input from functional rod photoreceptors.

32
Q

What is a DA 3 response in ERGs?

A

The DA 3 and DA 10 ERGs are mixed rod and cone system responses but in a healthy retina the rod system contribution dominates.

Both have cornea-negative a-waves but the DA 10 ERG a-wave is larger and of shorter peak time, consistent with a greater rod photoreceptor contribution. Both have cornea-positive b-waves that arise largely in the rod-driven On-bipolar cells.

33
Q

What is a LA 3 response in ERGs?

A

Light-adapted testing includes the single flash LA 3 ERG. This has an a- and b- wave. The LA 3 ERG a-wave is dominated by the activity of the Off- bipolar cells; the b-wave is dominated by a combination of On- and Off- bipolar cell activity, with contributions mediated by L-, M- and S-cone mechanisms.

34
Q

What flash, amplitude and waves are in DA 0.01 ERGs?

A

Dim flash, dark-adapted
– lower amplitude and long b-wave implicit time, small a-wave, just rods

35
Q

What flash, amplitude and waves in DA3 ERGs?

A

Standard flash, dark-adapted
- larger amplitude, larger a-wave, medium b-wave implicit time, 75% rods and 25% cones

36
Q

What flash, amplitude and waves in LA3 ERGs?

A

Standard flash, light-adapted
- lower amplitude, short b-wave implicit time, just cones

37
Q

Where does the most clinical information come from in ERGs?

A

Amplitudes of the responses

38
Q

How is the A-wave amplitude measured?

A

A-wave amplitude normally measured from baseline to first trough

39
Q

How is the B-wave amplitude measured?

A

B-wave amplitude normally measured from a-wave trough to next +ve peak

40
Q

What is the ‘implicit time’ in ERGs?

A

Period from light stimulus being applied to response peak occurring gives information about the response time and is known as the ‘implicit time’.

41
Q

What are ‘Reference Ranges’?

A

Physiological measurements are interpreted with the aid of ‘reference ranges’. Established in research.

42
Q

What is a ‘normal’ reference range in an ERG?

A

When a measurement lies within the reference range box then it’s likely ‘normal’ (95% of normals lie within this)

43
Q

What is an ‘abnormal’ reference range in an ERG?

A

If a measurement lies outside the reference range box, then it is probably not ‘normal’ (5% lie outside of this box with 2.5% above and 2.5% below)

44
Q

What does ERG amplitude increase with?

A

ERG amplitude increases with increasing flash luminance

Waveform morphology (i.e. shape) changes due to the successive emergence of non-linear, saturating responses from the bipolar cells, Müller cells and photoreceptors

The first responses originate in the rod system. As the flashes get brighter the cone system contributes more. (approx 20% of the total amplitude to bright flashes)

45
Q

What disorders have NO ERG response?

A
  • Advanced Retinitis Pigmentosa (rod-cone dystrophy)
  • Ophthalmic artery occlusion
  • To confirm total retinal detachment when imaging is impossible
46
Q

What disorders have REDUCED ERG responses (both light and dark-adapted)?

A
  • Early rod / cone dystrophy
  • Drug toxicity (e.g. HCQ)
  • Choroideremia
47
Q

What are full-field ERG’s useful for establishing?

A

Full-field ERG recording enables the distinction between generalised outer and inner retinal dysfunction and between predominantly rod or cone system dysfunction. Symptoms and/or clinical signs may suggest a retinopathy or retinal dystrophy, but the presence, severity and nature of retinal dysfunction cannot always be inferred.

48
Q

What disorders might be indicated in NORMAL A-wave but REDUCED B-wave?

A
  • Complete congenital stationary night-blindness (cCSNB)
  • Juvenile retinoschisis (splitting of retinal layers)
  • Central retinal artery occlusion
  • Melanoma Associated Retinopathy (MAR)
  • Batten Disease (Neuronal Ceroid Lipofuscinosis)
49
Q

What disorders might be indicated in NORMAL dark-adapted responses and ABNORMAL light-adapted responses in ERGs?

A

Cone Dystrophy

50
Q

What disorders might be indicated in ABNORMAL dark-adapted responses and NORMAL light-adapted responses in ERGs?

A

Rod Dystrophy

51
Q

What do Diminished Oscillatory Potentials indicate in ERGs?

A

Early retinal dysfunction in diabetes - ischaemia

52
Q

SEE CLINICAL EXAMPLES ON SLIDES 64, 65, 66 & 67

A
53
Q
A

Electrophysiology (7 decks)

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