Lecture 2c- Opthalmic electrophysiology (EOG and ERG) Flashcards
What is a transducer
It is able to convert one form of energy to another form
Is a photoreceptor a transducer
Yes as it converts light to a signal that our brains can interpret
What can the electrical signals recorded from the retina tell us
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
What is the ERG
a recording of the electrical response of the retina to flashes of light or patterns
What is the EOG
the electrical response of the retina to changes in steadystate illumination.
What is the a wave
light dependent decrease in rod and cone dark current gives a wave plus release of potassium K+
What is the B wave
Muller cells absorb extracellular K+ resulting in part of the b wave; rest of B wave comes from bipolar cells
What are the origins of ERG components
-a-wave from rods and cones
-b-wave from bipolar cells and Müller cells
-oscillatory potentials from amacrine cells
How is the ERG recorded
which electrodes and where
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
How is the type of electrode used chosen
its depending on the patient and purpose of
investigations.
What type of patients are DTL fibre electrode used on
in adult outpatients
What type of patients are infraorbutal skin electrodes used for
infants
What type of patients are CL electrodes used for
patients in theatre under GA
When is local anaesthesia necessary and how much
(with which electrode)
Yes for comfort when CL electrodes are used
(typically 0.4% g-oxybuprocaine)
Why are dilating drops needed and how much is used
To ensure that the pupils are non reactive and and therefore the same diameter for all stimuli regardless of stimulus brightness
Whay is another reason why pupils are dilated
It allows 84 times as much light in but more importantely it offers consistent and repeatble stimulation
Which type of electrode gives the smallest ERG recording
infraorbital skin electrodes
What amplitude is achieved using fibre electrodes
300 µV
Which type of ERG recording is the biggest
with which electrode
when CL electrodes are used
How many ERG responses need to be averaged for a clear recording
10 responses
What 2 types of signal can affect the quality of the recording
Small signal
Noisy signal
- more recordings may be needed at this point
What bandwidth is needed for a full response
0.3 Hz - 300 Hz
What bandwidth is needed to examine oscillary potentials
300 Hz
What are corneal electrodes also called
JET corneal electrodes
What are JET electrodes used for
record ERGs under GA in theatre at SCH -
if the patient were awake, they would need a topical anaesthetic
What are the pros of JET electrodes
D + C
Disposable and cost around £9 each
What are the cons of JET electrodes
affect on the pupil
Occlude some of the pupil
What are DTL electrodes
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)
What are the pros of DTL electrodes
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
How is the ERG stimulus given
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
How do you 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
What kind of flash is used to record scotopic ERG responses
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.
How do you record a light adapted (photopic) ERG
the rods are suppressed by a background light of 30 cd/m² for 10 minutes before testing, and during recording.
What kind of flash is used for a photopic ERG
Standard flashes are used to elicit a
‘cone response’.
What needs to be recorded from an ERG
A wave, B wave, timing, implict time
Where is the A wave amplitude measured from
baseline to first negative trough
What is the B wave amplitude
a-wave negative trough to next positive
peak
What is implict time
Time from light stimulus being applied to response peak occurring gives information about the response time
What are the possible ERG responses (5)
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
What can no ERG response indicate
R,O,T
Retinitis Pigmentosa (severe retinal degeneration)
Ophthalmic artery occlusion
Total retinal detachment
What can reduced a- and b-wave amplitudes (both light-adapted and dark-adapted) indicate
R, D, R
o Rod / cone dystrophy
o Drug toxicity
o Retinal degeneration
What can normal a wave and reduced b wave indicate
C,J,C
o Congenital stationary night-blindness
o Juvenile retinoschisis (splitting of retinal layers)
o Central retinal artery occlusion
What can a normal scotopic and an abnormal photopic response indicate
cone dysfunction
What can normal photopic responses, abnormal scotopic responses indicate
rod dysfunction
What can diminshed oscillary potentials indicate
Early retinal dysfunction in diabetes - ischaemia
What is the EOG used for
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)
What is difficult to measure
Standing potentials are difficult to measure because of uncertainty concerning
baseline position – e.g. electrode offset potentials.
How to overcome the difficulty measuring standing potentials
the signal of interest is made to vary with time by voluntary eye movements
How are patients helped to make eye movements in time
2 LEDs in the Ganzfeld, subtending an arc of 30°, are illuminated alternately for 1 second and the subject asked to track them.
How are EOGs recorded
which electrodes and where
Recording electrodes are placed on the nasal and lateral canthus
Reference
electrode placed equidistant from recording electrodes
What is the signal size and bandwidth of EOG
Signal size is approx. 1 mV ( approx 30 µV / ° )
Bandwidth of signal approx. 0.01 Hz - 30 Hz
How often is a response recorded for an EOG
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
What is the decreased light peak/ dark trough ratio sometimes called
light rise ratio
(arden ratio is old terminology)
What can the light rise ratio diagnose in regards to retinal function
5
*Best vitelliform macular dystrophy (essential)
* Retinal pigment epithelium disease
* Central retinal artery occlusion
* Quinine toxicity
* Retinitis Pigmentosa
- but results parallel the ERG
What are some other uses of the EOG
5
-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
Are some changes that are seen in the EOG also seen in the ERG
Yes
What are some requirements of the ERG
Willing and able particpants
Smooth saccades
Which diagnosis is the EOG essential for
Best disease- it is the greatest diagnostic power for this.
Where are oscillary potentials generated
Amacrine cells
What is the normal Arden ratio
1.85
What is the unit of measuring light passing through the pupil
Trolands (the product of luminance (cd/m2) and pupil area (mm2)).[2]
Normal EOG in
O, C
Optic nerve disease
Carotid occlusive disease
Drug increases standing potential
Temolol
Condition that increases standing potential
Retinal hypoxia
What does the EOG measure
measures the existing resting electrical potential between the cornea and Bruch’s membrane
What dilating drops are used
typically tropicamide 1% and phenylephrine 2.5%
