Revision lecture Flashcards
how are electroretinograms recorded
jet corneal electrodes are used for eegs under ga in theatre
would need topical anesthetic otherwise
disposable
9 pound each
blephrostats used to hold eyelid open
when patients are awake what electrodes will be fitted
DTL fibre electrode
dawson , trick and litzkoq
used in clinic
no anaesthetic required
can be worn all day
no effect on visual acuity
suitable for all diffuse and structured stimuli
disposable
3.50 each
often easier to get in than drops
patients don’t remember them
what type of diffuse stimuli are used for erg’s
light stimuli = diffuse stimuli
stimuli Is delivered via the ganzfeld stimulator
gurantees that the light goes over then entire retina evenly
standard flash luminance defined as 3cd.s.m-2 - (3 candlea flash)
which is quite bright especially if you have dilated pupils
deliver this in the dark adpated state and the light adapted state
what are the two responses from the erg
rods = low light (scotopic conditon)
cones
cones = photopic - colour - light
light adapted (photooptic response)
rods suppressed by 30cd/m2 for 10 mins
then standard flash used to produce ‘‘cone response’’
dark adapted (scotopic response)-
eyes are adapted for approx 20mins
dim flash (0.01 cod’s-2) used to produce rod response (-2.5 log units below or 1/316 of standard flash luminance)
then standard flash used to produce a mixed response from both rods and cones
so 3 conditions are looked at - the rods , the cones , and then the rods and cones
what would you expect in a normal dark adapted response 0.01
dim flash
dark adapted
Lower amplitude and the long b wave implicit times , small a wave , just rods
what would you expect in a dark adapted state 3.0
standard flash
dark adapted
larger amplitude
larger a wave
medium b wave
implicit time
rods and cones
what would you expect from a light adapted response 3.0
standard flash
light adapted
lower amplitude
short b wave implicit time
just cones- suppress the rods
how are measurements made from the erg
most clinical imformation comes from the amplitudes of responses
a wave amplitude normally measured from baseline to first rough
b wave amplitude normally measured from a wave trough to next +ve peak
however timing is an important factor
period from light stimulus being applied to response peak occurring gives information about the response time and is known as the implicit time
what happens if you vary the flash intensity/luminance in the dark adapted state
the waveform will evolve and it will change shape
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, muller cells and photoreceptors
the first response originates In the rod system and as the flashes get brighter the cone system contributes more
in what conditions would you expect a abnormal erg
in a normal erg you would see a normal a wave and a normal b wave
there Is a condition called congenital stationary night blindness - same as melanoma associated retinopathy
in both conditions the on bipolar cells are defective / not there so you don’t get a b wave to a dim flash - that is because it is largely from the on bipolar cells
could be the photoreceptors that are faulty
what would a normal a wave indicate
a normal a wave would indicate the rods are okay and the cones
if the b wave is abnormal what does that tell you
that something is wrong after phototransduction
what does no a wave indicate
cone dystrophy
no cones present
if there is a flat line what does that indicate
no rod response and no cone response
what would you expect to see on a erg with a patient with retinoschisis
- post phototransduction dysfunction
photoreceptors are okay but signals are not getting through the bipolar cells because the retinal layers have split
there is an a wave but not much of an b wave
what is the source of the electrooculargram
dependent on the rest of the retina working - photoreceptors e.g.
i.e. if the photoreceptors are not working than the eog will be flat
what are the recording methods for the eog
difficulty recording standing potentials because of the uncertainty concerning baseline position - e.g. electrode offset potentials - caused by the patient sweating
therefore signal of interest made to vary with time by voluntary eye movements
two lids in ganzifeld , subtending an arc of 30 degrees, illuminated alternately for 1 second and subject asked to track them
where are the electrodes placed when conducting an eog
recording electrodes are placed on the naison and lateral canthus with a reference electrode on the ear lobe
get patient to look left and right
as the eye looks left and right it rotates between the two electrodes
signal size approx 1mv (about 30uv)
bandwidth of signal approx 0.01hz- 30hz
how is an eog recorded
you record in the dark
record response for 10 seconds every minute
to avoid fatigue for 15mins during dark adaptation
amplitude ‘‘dark tough’’ occurs after typically 12 mins
then a bright light is put on 500cd/m2 steady illumination switched on
recording continued until light peak amplitude occurs typically after 10 mins
what is the Arden ratio
arden ratio= ‘‘light peak/ dark trough’’- a value of greater than 1.85 is regarded as normal
what are the clinical uses of the eog
best vitelliform macular dystrophy- v subnormal
pretty much essential for the diagnosis
acute quinine toxicity
retinitis pigments
(rod/ cone dystrophy)
but results parallel the erg
azoor - can have subtle changes in the electroretinogram - - eoog can therefore help to confirm the diagnosis - supernormal
adult vitelliform macular dystrophy
can be normal, but tends to be low-normal to slightly subnormal
central retinal artery occlusion - flat but erg much more informative
how is the electrooculargram important for measuring position of gaze
electronystamography (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
can derive a vector from vertical and horizontal angles if testing both eyes
can put electrodes either Side of the eye and above and below
you can determine the position of the eye from the recordings
however there can be potential changes due to light - for it to be reliable you need the patient to be exposed to the same level of light for some time so that you can have stable conditions - however it varies so there is a need to frequently recalibrate the equipment because of that - once that patient looks up and down between a known angle the equipment is calibrated - you can determine where they are looking
in what type of condition would you see a completely flat eog
in a retinitis pigmentosa you would expect to see a completely flat eog
what do nerves conduct
nerves conduct digital electrical impulses around the body which can be electrically stimulated and the responses can be recorded for diagnostic purposes
what is conduction velocity increasedbby
conduction velocity is increased by the presence of myelin
information is coded by pulse frequency modulation
what does the erg record
the erg records responses to brief visual stimuli
how can rod and cone responses be selected
adaptation and stimulus luminance can be used to select rod and cone reponses
what does the eog record
the eog records slow changes in the retinal trans epithelial potential in responses to general illumination levels
what is the multifocal erg (mferg) used for
responses from multiple discrete areas of retina
primarily used to assess spatial variations in cone function
discrete retinal lesions - involving too small an area to affect the erg
enlarged blind spot syndrome
abs
Maculopathy starboard and
azzor
acute zonal occult outer retinopathy
what is the mferg useful for
the mferg is useful for identifying zonal dysfunction in the retina
what stimus is used for the mferg
black and white circle - the spacing o fit elements get. bigger as you move away from the centre
structured stimulus - multiple elements may stimulate many areas of the retina simultaneously
each element flashes following a pattern of ons and offs determined by a maximum length or m sequence e.g. 00010010001011
individual responses deconvolve from mass responses to give miniature erg’s for each area
the concentration of cones in the retina varies with eccentricity - you have the greatest density of cones in the fovea
what is mferg stimulus scaling
scaling (spatial distortion) of the stimulus patten is needed too account for the spatial variation in cone density throughout the retina
elements increase in size increasing in eccentricity to give approximately equal sized responses
stimulating the central 50 degrees
produces a tracer ray
what type of electrodes are used for mferg recordings
recorded using dtl thread electrodes to avoid interfering with vision
and dilated pupils for consistent and repeatable retinal illmuninance
focus/contrast is less important
luminance related response not co toast related
what would you expect to see on a normal mfERG
you would see traces of approximately equal amplitude you would see a smaller response which would correspond to the blind spot
what would you expect to see in a mfERG in a patient with Maculopathy
you would expect to see a loss of responses from the macular region
what is the pattern erg
the pattern eg is recording using a counterpoising (reversing) chequerboard stimulus
don’t dilate for this- undilated pupils are required because contrast is the most important parameter - highly dependent on focus
mean luminance remains constant at all times typically 50cd/m-2
white squares at 100cd/m2 and black squares at 0cd/m2 so the mean luminance is 50
how are erg recordings interpreted
gives information about the macula and the optic nerve
p50 is measured from n35
n35 means negative trough from 35 milliseconds
p50 represents macular function
n95 represents retinal ganglion cell function
the test is unsuitable for patients with nystagmus and patients under 6 years old (generally)
5uv= a tiny retinal response
how would you differentiate between Maculopathy and optic neuropathy with perg
in a normal response you would expect n95 to be larger than p50
in Maculopathy you would expect n95 to be concomitantly reduced with p50 , p50 may also be delayed
in optic neuropathy you would expect n95 to be smaller than p50
to look at optic nerve function further what would you do
you would look at visual evoked potential (vep)
the vep is a recording of the electrical activity that occurs in the brain in response to visual stimulation by time variant diffuse or structured stimuli
when is the vep flash stimulus indicated for use
testing infants and people with poor vision / cooperation
cannot estimate visual acuity to better than rudimentary though - if a response is given doesn’t indicate good vision just indicates that the visual pathway is intact
good for detecting misrouting - e.g. in ocular albinism
what is a reversing chequerboard
similar to perg
confounded by nystagmus
pattern is smeared by the movement - is liar effect to reduced contrast
what chequer boards are usually used
typically 1 degree chequers for macular stimulation and 15 degree chequers for foveal stimulation
differentiate between foveal dysfunction and macular dysfunction by using two different chequer sizes
typically 2 reversals per second
stimulus field > 15 degrees
steady fixation is necessary
requires cooperation and focus
patient must be refracted
what is the typical arrangement for recording vep’s
patient - electrodes - amplifier - filter - analog to digital converter - computer - stroboscope or pattern stimulator
where are vep electrodes placed
one on the midline
and one on either side of the hemisphere - allows us to tell which side of the brain the response is coming from
what is the typical pattern reversal vep
electrodes mocc- mf
n70= foveal component - if someone has good va they usually have good visual acuity
p100 is macular component = measured from the n70
n135 is a particular component
what are reps typically used for
demyelination
large majority of patients with ms show increased peak time even in the absence of symptoms
powerful at detecting sub clinical optic neuritis
compression of the optic nerve from space occupying lesions
function v structure advantage
optic neuropathy - amplitude is attenuated
functional integrity of visual pathway
objective visual cortical acuity measurements
what is the geniculostriate pathway
you have 3 electrodes and all 3 are referenced towards the forehead
looking at activity occurring from the back of the head to the forehead
you have. further channel that you connect across the lateral electrodes i.e. a and c
if both half of the brains are doing the same thing you are ether going to see the same thing in the two lateral electrodes or they will cancel out
if there is a difference between what the two hemispheres are doing that is either because they are stimulating 1 hemisphere selectively or because there is a visual field defect
what is the geniculostriate pathway
you have 3 electrodes and all 3 are referenced towards the forehead
looking at activity occurring from the back of the head to the forehead
you have. further channel that you connect across the lateral electrodes i.e. a and c
if both half of the brains are doing the same thing you are ether going to see the same thing in the two lateral electrodes or they will cancel out
if there is a difference between what the two hemispheres are doing that is either because they are stimulating 1 hemisphere selectively or because there is a visual field defect
how would you deliberately stimulate one half of the hemipshere
you would use half field stimulation
in normal subject stimulus od or os will activate the left hemisphere
the checkerboards are on the right side - this is right half field stimulation and this will activate the left hemisphere
how do the responses vary under full field stimulation and half filed stimulation conditions
with full field stimulation you get a big response from the mid line electrode - which is between both hemispheres the lateral electrodes show a smaller signal and
if you deliberately stimulate one hemisphere with a half field stimulus then you would get a response from just 1 hemisphere so the electrode placed over the left hemisphere records a positive waveform - electrode over the left records a negative waveform
half field stimulation activates 1 hemisphere only
p100 paradoxically recorded from side of scalp ipislateral to stimulated half field
explain paradox lateralisation
the electrode placed open the left side of the brain picks up signals from the right side better
so you would see positivity on the opposite side to the electrodes placed- because it is picking up positivity from the opposite hemisphere and negativity on the aide that the electrodes are placed
p100 produced by dipole generators in the calcimine sulcus
electrode on scalp ipsilateral to stimulated heal field better placed to detect p100
full field stimulation causes cancellation in lateral electrodes but not midline
what is misrouting and what conditions is it associated with
crossed and uncrossed retinal ganglion cells
in normal patients you would see decussation
you would see macular fibres projecting to the other hemisphere
in patients with albinism you have excessive decussation - i.e. more fibres cross than they should
associated with ocular albinism - all abinos display misrouting
oca
oa
cordial - hibachi syndrome
warden burg syndrome
albinoidism
normal routing in carriers of x linked oca though
i.e. In normal eye you have a 50- 45% of fibres crossing and In a albino you have 80- 20% of fibres crossing- so only 1 half of the hemisphere is activated
what does vep asymmetry cause
mistouring in albnism results in occipital lateralisation of the vep- this is seen in all modalities but the degree to which each displays this best varies according to the age of
the patient
in excessive decussation you have marked asymmetry
assymetrey of an opposite sense is seen in achiasmia and in compression of the crossing fingers e.g. pituitary adenoma ) not so with cranipharyngignioma though)
you can see via the pattern of crossing weather it is achismia or albinism
