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
