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Electrodiagnosis > Lecture 2c- Opthalmic electrophysiology (EOG and ERG) > Flashcards

Lecture 2c- Opthalmic electrophysiology (EOG and ERG) Flashcards

1
Q

What is a transducer

A

It is able to convert one form of energy to another form

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2
Q

Is a photoreceptor a transducer

A

Yes as it converts light to a signal that our brains can interpret

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3
Q

What can the electrical signals recorded from the retina tell us

A

About normal and, crucially, abnormal
physiology. There are many methods and recording techniques optimised for observing the working of different components on the visual system

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4
Q

What is the ERG

A

a recording of the electrical response of the retina to flashes of light or patterns

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5
Q

What is the EOG

A

the electrical response of the retina to changes in steadystate illumination.

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6
Q

What is the a wave

A

light dependent decrease in rod and cone dark current gives a wave plus release of potassium K+

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7
Q

What is the B wave

A

Muller cells absorb extracellular K+ resulting in part of the b wave; rest of B wave comes from bipolar cells

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8
Q

What are the origins of ERG components

A

-a-wave from rods and cones
-b-wave from bipolar cells and Müller cells
-oscillatory potentials from amacrine cells

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9
Q

How is the ERG recorded

which electrodes and where

A

DTL fibre electrodes, Infraorbital skin electrodes and contact lens electrpdes amd a ground electrode on the forehead and a reference electore on the ipsialteral temple

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10
Q

How is the type of electrode used chosen

A

its depending on the patient and purpose of
investigations.

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11
Q

What type of patients are DTL fibre electrode used on

A

in adult outpatients

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12
Q

What type of patients are infraorbutal skin electrodes used for

A

infants

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13
Q

What type of patients are CL electrodes used for

A

patients in theatre under GA

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14
Q

When is local anaesthesia necessary and how much

(with which electrode)

A

Yes for comfort when CL electrodes are used
(typically 0.4% g-oxybuprocaine)

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15
Q

Why are dilating drops needed and how much is used

A

To ensure that the pupils are non reactive and and therefore the same diameter for all stimuli regardless of stimulus brightness

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16
Q

Whay is another reason why pupils are dilated

A

It allows 84 times as much light in but more importantely it offers consistent and repeatble stimulation

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17
Q

Which type of electrode gives the smallest ERG recording

A

infraorbital skin electrodes

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18
Q

What amplitude is achieved using fibre electrodes

A

300 µV

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18
Q

Which type of ERG recording is the biggest

with which electrode

A

when CL electrodes are used

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19
Q

How many ERG responses need to be averaged for a clear recording

A

10 responses

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20
Q

What 2 types of signal can affect the quality of the recording

A

Small signal
Noisy signal
- more recordings may be needed at this point

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21
Q

What bandwidth is needed for a full response

A

0.3 Hz - 300 Hz

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22
Q

What bandwidth is needed to examine oscillary potentials

A

300 Hz

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23
Q

What are corneal electrodes also called

A

JET corneal electrodes

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24
Q

What are JET electrodes used for

A

record ERGs under GA in theatre at SCH -
if the patient were awake, they would need a topical anaesthetic

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25
Q

What are the pros of JET electrodes

D + C

A

Disposable and cost around £9 each

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26
Q

What are the cons of JET electrodes

affect on the pupil

A

Occlude some of the pupil

27
Q

What are DTL electrodes

A

invented by Dawson, Trick and Litzkow in 1978 and are used routinely in clinic at
RHH, these are great; no anaesthetic is required, though drops may be given for comfort (e.g. if the patient/subject also suffers with dry eyes)

28
Q

What are the pros of DTL electrodes

A

Worn all day
Doesn’t interfere with vision
Suitable for all diffuse and structures stimuli
Disposable
Cost £8 each
Easier to get in than eye drops
Patients cant feel it as much only the sticker feeling

29
Q

How is the ERG stimulus given

A

Stimuli are delivered via a Ganzfeld (“full-field”)
stimulator. ‘Standard’ flash luminance defined as 3 cd·s·m-² which is quite bright, especially if you have dilated pupils

30
Q

How do you record a dark adapted (scotopic) response

A

eyes are dark adapted for approximately 20 minutes, usually under dim red lights (red is a very poor stimulus for rods!) prior to recording

31
Q

What kind of flash is used to record scotopic ERG responses

A

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.

32
Q

How do you record a light adapted (photopic) ERG

A

the rods are suppressed by a background light of 30 cd/m² for 10 minutes before testing, and during recording.

33
Q

What kind of flash is used for a photopic ERG

A

Standard flashes are used to elicit a
‘cone response’.

34
Q

What needs to be recorded from an ERG

A

A wave, B wave, timing, implict time

35
Q

Where is the A wave amplitude measured from

A

baseline to first negative trough

36
Q

What is the B wave amplitude

A

a-wave negative trough to next positive
peak

37
Q

What is implict time

A

Time from light stimulus being applied to response peak occurring gives information about the response time

38
Q

What are the possible ERG responses (5)

A

No ERG response
Reduced A and B wave amplitudes
Normal A wave and reduced B wave
Normal scotopic and abnormal photopic response
Normal photopic and abnormal scotopic

39
Q

What can no ERG response indicate

R,O,T

A

Retinitis Pigmentosa (severe retinal degeneration)
Ophthalmic artery occlusion
Total retinal detachment

40
Q

What can reduced a- and b-wave amplitudes (both light-adapted and dark-adapted) indicate

R, D, R

A

o Rod / cone dystrophy
o Drug toxicity
o Retinal degeneration

41
Q

What can normal a wave and reduced b wave indicate

C,J,C

A

o Congenital stationary night-blindness
o Juvenile retinoschisis (splitting of retinal layers)
o Central retinal artery occlusion

42
Q

What can a normal scotopic and an abnormal photopic response indicate

A

cone dysfunction

43
Q

What can normal photopic responses, abnormal scotopic responses indicate

A

rod dysfunction

44
Q

What can diminshed oscillary potentials indicate

A

Early retinal dysfunction in diabetes - ischaemia

45
Q

What is the EOG used for

A

to record electrical responses in the retina which result from changes in steady-state illumination and can inform us about the health of the retinal pigment
epithelium (RPE)

46
Q

What is difficult to measure

A

Standing potentials are difficult to measure because of uncertainty concerning
baseline position – e.g. electrode offset potentials.

47
Q

How to overcome the difficulty measuring standing potentials

A

the signal of interest is made to vary with time by voluntary eye movements

48
Q

How are patients helped to make eye movements in time

A

2 LEDs in the Ganzfeld, subtending an arc of 30°, are illuminated alternately for 1 second and the subject asked to track them.

49
Q

How are EOGs recorded

which electrodes and where

A

Recording electrodes are placed on the nasal and lateral canthus

Reference
electrode placed equidistant from recording electrodes

50
Q

What is the signal size and bandwidth of EOG

A

Signal size is approx. 1 mV ( approx 30 µV / ° )
Bandwidth of signal approx. 0.01 Hz - 30 Hz

51
Q

How often is a response recorded for an EOG

A

Record response for 10 secs every 1 min (to avoid fatigue) for 16 mins under dark adaption
conditions and 16 mins during light adaptation

i.e. recording 10 seconds of alternating saccades every minute for 16 minutes in
the dark, following 16 minutes with a background light switched on

52
Q

What is the decreased light peak/ dark trough ratio sometimes called

A

light rise ratio
(arden ratio is old terminology)

53
Q

What can the light rise ratio diagnose in regards to retinal function

5

A

*Best vitelliform macular dystrophy (essential)
* Retinal pigment epithelium disease
* Central retinal artery occlusion
* Quinine toxicity
* Retinitis Pigmentosa
- but results parallel the ERG

54
Q

What are some other uses of the EOG

5

A

-Electronystagmography (ENG)
-Saccadic velocity
-Horizontal angle of gaze- with electrodes either side of the eye
-Vertical angle of gaze- with electrodes above and below the eye
-Position of gaze- a vector derived from vertical and horizontal angles

55
Q

Are some changes that are seen in the EOG also seen in the ERG

A

Yes

56
Q

What are some requirements of the ERG

A

Willing and able particpants
Smooth saccades

57
Q

Which diagnosis is the EOG essential for

A

Best disease- it is the greatest diagnostic power for this.

58
Q

Where are oscillary potentials generated

A

Amacrine cells

59
Q

What is the normal Arden ratio

A

1.85

60
Q

What is the unit of measuring light passing through the pupil

A

Trolands (the product of luminance (cd/m2) and pupil area (mm2)).[2]

61
Q

Normal EOG in

O, C

A

Optic nerve disease
Carotid occlusive disease

62
Q

Drug increases standing potential

A

Temolol

63
Q

Condition that increases standing potential

A

Retinal hypoxia

64
Q

What does the EOG measure

A

measures the existing resting electrical potential between the cornea and Bruch’s membrane

65
Q

What dilating drops are used

A

typically tropicamide 1% and phenylephrine 2.5%

Electrodiagnosis (9 decks)

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