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Electrodiagnosis > Lecture 3c- VEP (Visual Evoked Potential) > Flashcards

Lecture 3c- VEP (Visual Evoked Potential) Flashcards

1
Q

What is the VEP

A

a recording of the electrical activity that occurs in the brain in response to visual stimulation
by time-variant diffuse or structured stimuli

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2
Q

What type of stimulus is used for VEP

A

diffuse or structured stimuli

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3
Q

What types of patient is the flash stimulus best for

A

infants or adults with very poor co op or vision

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4
Q

What is the flash stimulus good for and what can’t it estimate

A

great for detecting misrouting but cannot estimate cortical acuity to better than ‘rudimentary’ as patients can still see a
bright flash even when almost totally blind!

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5
Q

What can the VEP not do

A

estimate cortical acuity

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6
Q

What type of chequerboard is used

A

a counter-phasing or ‘reversing’ chequerboard. (Similar to that used for the PERG.)

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7
Q

What is the benefit of using a reversing chequerboard

A

it elicits the most consistent and repeatable VEPs

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8
Q

What type of patients is the VEP unsuitable for

A

motion blur from nystagmus, which reduces contrast

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9
Q

When is an onset chequerboard presented

A

When patients have nystagmus and motion blur

an onset chequerboard where a 100% contrast chequerboard pattern appears from a 50% grey background and then disappear

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10
Q

What can the onset chequerboard do

A

elicit a response to both onset and offset

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11
Q

What type of patients is the onset chequerboard best for

A

this is better in the case of
nystagmus, but responses are more variable than for reversal stimuli in normals.

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12
Q

What remains constant for both the reversal and onset stimulus

A

mean luminance

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13
Q

What does the chequerboard stimulus consist of

A

1° chequers for macular stimulation
OR
15′ chequers for foveal stimulation

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14
Q

How many reversals per second are presented with a stimulus field >15 degrees

A

2 reversals per second

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15
Q

What is necessary for VEP

S, C, F

A

steady fixation, co operation and focus

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16
Q

Should the patient be dilated and refracted

A

no dilation and adequate refraction is needed

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17
Q

What is the order of the set up for VEP

A

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

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18
Q

Where are the electrodes placed for VEP (A, B, C)

A

3 behind the head and a reference electrode on the forehead

sometimes only the mid occipital electrode is recorded from B

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19
Q
A
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20
Q

What are the components for the VEP (3)

A

N70, P100, N135

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21
Q

What is the N70

A

Foveal component
= e.g. negative peak at 70 milliseconds

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22
Q

What is the P100

A

Macular component
= e.g. positive peak at 100 milliseconds

23
Q

What is the N135

A

Paramacular component

24
Q

What convention is followed in VEP recording graph

A

Cartesian convention with +ve at y- axis

25
Q

Does the shape of the waveform vary

A

yes according to chequer size

26
Q

What do smaller chequers stimulate

A

the foveal representation in the visual cortex, leading to a bigger N70

27
Q

What do larger chequers stimulate

A

the paramacular representation leading to a bigger N135

28
Q

What does myelinaton of optic nerves in infancy result in

A

a considerable increase in conduction velocity
over the first 6 months. Cortical maturation results in greater response amplitude and complexity.

29
Q

At what age do infants have adult size ERG responses and morphology

A

the ERG matures and all responses approximate adult size and morphology by 6 months of age

30
Q

What are some applications of the VEP (5- detail in notes)

DCOFO

A

Demyelination
Compression of optic nerve
Optic neuropathy
Functional integrity of the visual pathway
Objective cortical VA measurement

31
Q

How is the VEP recorded

A

using pattern stimuli with many different element sizes down to the limit of visual acuity

32
Q

When do infants approach adult levels of VEP acuity

A

at 6 months of age

33
Q

What are the 2 methods of VEP recording

A

Minimum VEP Acuity
Sweep VEP

34
Q

What is the minimum VEP likely to do

A

underestimate actual acuity. if responses are only recordable to flash then VA is rudimentary.

35
Q

Blind patients and VEP

A

A patient may not be completely blind even if no VEPs recordable at all as only a small number of neurones are required for a patient to see a ‘flash’ but we need thousands to fire for a recordable VEP.

36
Q

What is the calculation of minimum VEP acuity

A

6/6 x spatial element size (in mins of arc)

37
Q

How does the sweep VEP work

A

Theres a rapid presentation of different chequer sizes. Some variants of this test sweep through a large range of different sizes regardless of what is recorded
Some determine the next size of chequer based on the response to the previous chequers, in real-time

38
Q

What do good paradigms ensure

A

robust and objective measurements in as little as 10 seconds

39
Q

How does the VEP amplitude reduce

A

it reduces linearly with decreasing chequer size as the limit of visual acuity is approached.

40
Q

Basic how sweep VEP works

A

In its simplest form, amplitudes are
plotted and a straight-line, leastsquares fit, is extrapolated to 0 µV
from peak of function.

41
Q

Whats on the sweep VEP x and y axis

A and S

A

Amplitude and Spatial frequency

42
Q

What is half field stimulation like in normal patients

A

this stimulus presented to either eye will activate the left hemisphere

43
Q

What is the projection like with half field stimuli

A

Temporal projection for the left eye & Nasal
projection for the right

Nasal projection for the left eye & Temporal
projection for the right

44
Q

What does full field stimulation activate

A

both hemispheres equally whether monocular or binocular

45
Q

How many hemispheres does half field stimulation activate

A

one hemisphere only

46
Q

What does half field stimulation along with three occipital recording electrodes allow for

Locating x and x

A
  • locating lesions to R/L side of the R/L optic nerves, chiasm and R/L optic radiation/hemisphere
  • locating misrouting as seen in ocular albinism (excessive decussation at the chiasm) and achiasmia (no decussation)
47
Q

Where is the VEP P100 paradoxically recorded from

A

the side of scalp ipsilateral to stimulated half-field

48
Q

Where is the P100 produced

A

by dipole generations in the calcarine sulcus

49
Q

What is the best electrode placemenet to detect P100

A

Electrode on scalp ipsilateral to stimulated half-field

50
Q

What does half field stimulation activate

A

one hemisphere which is the sane for either eye in normals

51
Q

What does full field stimulation in normals cause

A

cancellation in lateral electrodes, but not at the midline

52
Q

Which hemisphere is activated in albinism

A

the contralateral hemisphere to the stimulated eye

53
Q

What is activated in achiasmia

A

the ipsilateral side to the stimulated eye

54
Q

What does crossed asymmetry in flash VEPs suggest

A

excessive decussation in ocular albinism

Electrodiagnosis (9 decks)

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