Lecture 3

Question 1

what do the mferg , perg and vep have in common

Answer 1

they multifocal , pattern and vep use structured stimuli rather than diffuse stimuli

Question 2

what is the mferg

Answer 2

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)

Question 3

what conditions can be picked up using an mferg

Answer 3

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

Question 4

what stimulus is used for there mferg

Answer 4

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

Question 5

how do the hexagon sizes vary in a mferg

Answer 5

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

Question 6

why is there spatial distortion in the stimulus pattern

Answer 6

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

Question 7

how are mferg’s recorded

Answer 7

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

Question 8

what does a normal mferg look like

Answer 8

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

Question 9

what would a m erg displaying evidence of Maculopathy look like

Answer 9

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

Question 10

what is the pattern erg

Answer 10

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 -

Question 11

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

Answer 11

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

Question 12

compare the perg to the mferg

Answer 12

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

Question 13

describe components of perg waveform

Answer 13

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

Question 14

how would you differentiate a Maculopathy and a optic neuropathy using a perg

Answer 14

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

Question 15

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

Answer 15

a visual evoked potential

Question 16

what types of patients is the perg unsuitable for

Answer 16

patients under 6 and those with nystagmus

Question 17

how is the waveform affected in Maculopathy and optic neuropathy

Answer 17

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

Question 18

what is a vep

Answer 18

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

Question 19

what are the pros and cons of veps

Answer 19

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

Question 20

what does chequerboard stimuli consist of

Answer 20

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

Question 21

describe the system for recording a flash pattern/ pattern veps

Answer 21

patient - electrodes - amplifier - filter - analog to digital converter - computer - stroboscope or pattern stimulator

Question 22

where are electrodes placed in veps (vep electrode montage)

Answer 22

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

Question 23

what are the three components of a waveform produce by a vep- typical pattern reversal

Answer 23

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

Question 24

how would you label a wave produced by a vep

Answer 24

n70 = negative peak occurring around 70 ms

p100= positive peak occurring around 100 milliseconds and

n135 - negative peak occurring at 150ms

Question 25

how does the shape of the waveform vary

Answer 25

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

Question 26

how does conduction velocity vary with age

Answer 26

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

Question 27

what are the applications of the vep

Answer 27

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

Question 28

how can you use a vep to measure visual acutity

Answer 28

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

Question 29

what are the two methods of estimating va with vep

Answer 29

sweep vep and minimum vep acuity

Question 30

how would you calculate minimum vep acuity

Answer 30

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

Question 31

what is a sweep vep

Answer 31

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

Question 32

how is the vep used to assess the geniculostriate pathway

Answer 32

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

Question 33

what is half field stimuli

Answer 33

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

Question 34

what is full field stimulation

Answer 34

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

Question 35

describe paradox lateralisation of the p100 to half field stimulation

Answer 35

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

Question 36

what does full field stimulation cause in the lateral electrodes

Answer 36

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

Question 37

what are crossed and uncrossed rgcs

Answer 37

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

Question 38

what would you see in a flash vep with a patient with albnism

Answer 38

you would see crossed assymmery