Lecture 3 Flashcards
what do the mferg , perg and vep have in common
they multifocal , pattern and vep use structured stimuli rather than diffuse stimuli
what is the mferg
mferg= multifocal electroretinogram
the mferg is a recording of responses from multiple discrete areas of retina
- primarily used to assess spatial variations in cone system function function such as discrete retinal lesions involving an area too small to affect the erg (e.g. enlarged blind spot)
what conditions can be picked up using an mferg
useful for detecting small retinal lesions that involve too small an are to affect the erg
enlarged blind spot syndrome
Maculopathy (stargardt and amd)
acute zonal occult outer retinopathy
what stimulus is used for there mferg
mferg employs a structured stimulus consisting of multiple elements which stimulate many areas of the retina - lots of individual hexagons which flash on and off in a particular pattern
each hexagon (element) flashes following a pattern of ons and offs determined by a maximum length or m sequence - this may be described as pseudorandom
individual responses from the retina are deconvolve - from the mass response to give a miniature erg for each area- i..e each area gives a response at more or less the same time - picking up the whole set of responses up using an electrode but you can deconvolve responses to look at the separately - the m sequence ensures that for each element all other responses are noise and average out along with random noise
how do the hexagon sizes vary in a mferg
the further you are away from the centre of fixation the larger the pattern elements are reflecting the density of cones in the normal retina - i.e. highest density of cones in the fovea - no rods in the fovea
useful for the detecting of small retinal lesions
why is there spatial distortion in the stimulus pattern
scaling (spatial distortion) of the stimulus pattern is needed to account for the spatial variation in con e density throughout the retina
l and m cones are at max density at the fovea , thinning out towards the periphery
there are no rods or s cones in the fovea
elements increase in size increasing eccentiricy to give approximately equal sized responses
how are mferg’s recorded
merges are recorded using dtl thread electrodes to avoid interfering with vision
dilated pupils are required- so amount of light entering eye is constant- for consistent and repeatbale retinal liimuninace
focus/ contrast is less critical than for erg
alignment and steady fixation = critical
the amplitudes of main positive peaks are typically represented as a surface plot to aid visualisation, although the trace arrays contain much more information
there will be a small response at the blind spot - this is because the blind spot is overlapped with several hexagons with a 61hexagon stimulus
what does a normal mferg look like
61 elements (hexagons are shown)
you would see a blind spot 20 degreee from fovea in some patients the blind spot is not seen at all
what would a m erg displaying evidence of Maculopathy look like
in Maculopathy e.g. stargdt disease you would see a reduction in the response amplitude in the centre 10 - 15 degrees - smaller responses in the centre - displaying evidence of Maculopathy
what is the pattern erg
the erg is recorded using a counterpoising reversing (chequerboard) stimulus - the mean luminance remains constant at all times , typically 50cd/-2 - for the erg contrast is a important parameter which is highly dependent on focus and reactive - i.e. not dilated pupils are required and contrast reducing optic capacities
e.g. cataracts should be characterised- they would have reduced contrast and that would therefore effect the results
recording a réponse from the eye from the macular region patient fixates on the red spot in the centre of the chequerboard -
a patient is complaining of a visual field defect and on examination they have a normal fundus and oct has been conducted but results suggest it is normal how would you prove that it is a real visual defect and that it has a real retinal origin
a multifocalelectroretinogram is conducted - and you would see smaller response amplitudes in the area of reduced visual field
important to conduct as funds and oct may be normal
compare the perg to the mferg
the perg stimulus covers more or less the whole macular region- response is small and helps to differentiate between macular and optic nerve disease so it is not as specially selective as the mferg - it is quicker and easier to record
the perf is a tiny retinal response of -5uv in amplitude - it differentiates macular/ optic nerve disease
describe components of perg waveform
p50= macular function
n95= retinal ganglion cell function
if you have a Maculoptahy the p50 will be affected if you have a optic neuropathy will affect n95
how would you differentiate a Maculopathy and a optic neuropathy using a perg
normal - n95 is larger than the p50
Maculopathy p50 will be delayed
n95 concomitantly reduced with p50
p50 may be delayed
optic neuropathy - n95 smaller than p50- e.g. glaucoma
if you suspect that a visual field defect is not caused by a retinal problem but a optic nerve problem what test would you conduct
a visual evoked potential
what types of patients is the perg unsuitable for
patients under 6 and those with nystagmus
how is the waveform affected in Maculopathy and optic neuropathy
the n95 amplitude depends on the p50 amplitude , thus both tend to be reduced in Maculopathy but typically only the n95 is attenuated in optic neuropathy
what is a vep
vep - visual evoked potential
the vep is a recording of the electrical activity that occurs in the brain n response to visual stimulation by time variant diffuse or structured stimuli
diffuse = flash stimulus
what are the pros and cons of veps
p- good for testing infants / people with poor coop
good for detecting misrouting - that occurs in patients with ocular albinism
cannot estimate cortical acuity better than rudimentary as patients can still see a bright flash even when almost blind
typically counterpoising or reverse chequerboard is used
this elicits most consistent and readable veps but is confounded by motion blur from nystagmus which reduces contrast
in such patients we reduce this problem by using an onset chequerboard where a 100% contrast chequerboard appears from a 50% grey background and then dissapears exciting a response to both onset and offset - this is better in the case of nystagmus but responses are more variable than fro reversal stimuli in normals
important mean luminance remains constant for both the reversal and onset stimuli
what does chequerboard stimuli consist of
chequerboard stimuli consist of 1 degree cheqeuers for macula stimulation or 15 chequers for foveal stimulation -typicaly 2 reversals per second are presented with a stimulus filed more than 15 degrees
steady fixation is necessary
the test requires cooperation so that patient mustn’t be dilated and must be adaquetley refracted
describe the system for recording a flash pattern/ pattern veps
patient - electrodes - amplifier - filter - analog to digital converter - computer - stroboscope or pattern stimulator
where are electrodes placed in veps (vep electrode montage)
electrodes are placed on the back of the head
one in the middle (mid occipital electrode) - placed on the calcimine sulcus
and one on either hemisphere
what are the three components of a waveform produce by a vep- typical pattern reversal
n70= foveal component
p100 = macula component
n135= paramacular component
if you use a range of chequer sizes you can selectively stimulate the fovea , macular or paramacular region that causes the shape of the chequerboard to change
how would you label a wave produced by a vep
n70 = negative peak occurring around 70 ms
p100= positive peak occurring around 100 milliseconds and
n135 - negative peak occurring at 150ms
how does the shape of the waveform vary
the shape of the waveform varies markedly according to the chequer size, with small chequers preferentially stimulating the foveal representation in the visual cortex leading to a bigger n70, larger chequers preferentially stimulate the paramacular representation leading to a bigger n135
how does conduction velocity vary with age
myelination of optic nerves in infancy results in
a considerable increase in conduction velocity in the first 6 months - cortical maturation results in greater response amplitude and complexity - the erg matures simoliary too as the photoreceptors develop
what are the applications of the vep
demyelination
large majority of patients with ms show an incereased peak time even in the absence of symptoms
powerful at detecting sub clinical optic neuritis
compression of of the optic nerve from space occupying lesions - results in slow and attenuated responses
function v structure advantage
optic neuropathy - results in attenuated responses
functional integrtity of the visual pathway - from the photoreceptors to area v1 of the visual cortex
objective cortical visual acuity measurement - very useful with preverbal and malingering patients
how can you use a vep to measure visual acutity
visual acuity estimation - reps are recorded using pattern stimuli with many different elements down to the limit of visual acuity - infants found to approach adult levels of vep acuity by 6months of age
what are the two methods of estimating va with vep
sweep vep and minimum vep acuity
how would you calculate minimum vep acuity
6/6x spatial element size (in minutes of arc) thus if responses recorded to 5’ chequers the minimum
e..g if responses were recored to 5 chequers then it would be 6/ 6 x spatial element size (in minutes of arc) - minimum vep or cortical acuity
likely to underestimate vep if responses are only recordable to flash - then va is likely to be rudimentary only
a patient may not be completely blind even if no veps are recordable at all as only a small amount of neurones are required to see a flash , but we need thousands to fire for a recordable vep
what is a sweep vep
this employs the rapid presentation of dfferent chequer sizes - some variants of this test sweep through a large range of different size
test relies on the fact that vep amplitude reduces linearly with decreasing chequer size as the limit of va is approached in its simplest form amplitudes are plotted in a straight line least squares fit is extrapolated to 0uv
how is the vep used to assess the geniculostriate pathway
veps can be used to locate lesions on either side of the optic nerve , to the chasm and to either side of the chasmal pathway
electrode b - placed in the middle- is looking between the calcimine sulcus and the middle
good for looking at both hemispheres - good at recording information from both sides of the brain
what is half field stimuli
if the chequer is on your right
= right visual field left hemisphere
in normal subjects the stimulus presented to either eye will activate the left hemisphere
temporal projection for the left eye and nasal projection for the right
if the chequers are on your left it will activate the right hemisphere nasal projection for the left eye and temporal projection for the right eye
what is full field stimulation
fulll field stimulation should activate both hemispheres equally weather monocular or binocular
half field stimulation activates 1 hemisphere only
half filed stimulation along with three occipital recording electrodes allows us tp locate lesions to R/L side of the R/L optic nerves , chasm and R/L optic radiation/heemisphere and to locate misrouting as seen in ocular Albinism (excessive decussation at the chasm) and achiasmia no decussation is very useful
the vep p100 is paradoxically recorded from the side of scalp which is the same side as the stimulated half field
describe paradox lateralisation of the p100 to half field stimulation
p100 is produced by dipole generators in the calcimine sulcus
- when you stimulate the left visual field you are activating the right hemisphere - you therefore have a big response coming from the electrode on the left and a inverted response coming from the right - so if you stimulate the other visual field you see the opposite happening
electrode on scalp ipsilateral to stimulated half field better placed to detect p100
half field stimulation. activates one hemisphere same hemisphere for either eye in normal eyes
what does full field stimulation cause in the lateral electrodes
full field stimulation causes cancellation in lateral electrodes but not midline
activates contralateral hemisphere to the stimulated eye in albnism
activates ipsilateral side to the the stimulated eye in achismia
what are crossed and uncrossed rgcs
all abinos display misrouting - excessive decussation at the chimes
oca
ac
cheiuak hibachi syndrome
hermanday pudklak sundrome
warden burg syndrome
albodism
normal routing in carriers of x linked oct though
what would you see in a flash vep with a patient with albnism
you would see crossed assymmery
