VEP

Question 1

What is a VEP?

Answer 1

Visual Evoked Potential

Electrical response of the visual cortex to alternating chequerboards or flashes of light

“The VEP is a recording of the electrical activity that occurs in the brain in response to visual stimulation by time-variant diffuse (non-structured) or structured stimuli”

Question 2

Why is VEP Flash Stimulus good?

Answer 2

• Great for testing infants, and adults with very poor vision/cooperation (those who cannot fixate on a checkerboard stimulus)
• Good for detecting misrouting
• Can rudimentarily estimate VA only

Question 3

What stimulus is used in VEPs?

Answer 3

Reversing Chequerboard similar to PERG

Question 4

What is a confounding variable when doing a VEP?

Answer 4

Nystagmus is a confounding variable as the pattern is ‘smeared’ by the movement giving a similar effect to reduced contrast

Steady fixation is necessary. Requires cooperation & focus
& patient must be refracted

Question 5

How do we get around nystagmus as an issue in VEPs? How does this help?

Answer 5

In such patients, we get around this problem by using an onset
chequerboard where a 100% contrast chequerboard pattern appears from a 50% grey background and then disappears, eliciting a response to both onset and offset. This is better in the case of nystagmus, but responses are more variable than for reversal stimuli in normals. Importantly, the mean luminance remains constant for both the reversal and onset stimuli.

Question 6

How small are the chequers for macular stimulation and for foveal stimulation?

Answer 6

• 1° chequers (macular stimulation)
• 15’ chequers (foveal stimulation) (15 minutes’)

Question 7

How often does the reversing chequerboard reverse per second in VEPs?

Answer 7

Typically 2 reversals per second

Question 8

What is the stimulus field in VEPs?

Answer 8

Stimulus field >15°

Question 9

What is the arrangement for recording VEPs?

Answer 9

• Patient
• Electrodes
• Amplifier
• Filter
• Analog to Digital Converter
• Computer
• Stroboscope or Pattern Stimulator

Question 10

How are VEP electrodes placed?

Answer 10

Any age patient.

Electrodes on ridge on back of head – occipital lobe/visual cortex. Central and then 3 lateral to pick up the 2 hemispheres.

This shows a typical electrode montage for the VECP. The array of three electrodes at the occiput allows recording from both hemispheres. Many laboratories only use the mid-occipital electrode (B), and risk missing many disorders that would result in an asymmetric response profile.

Question 11

What is a typical pattern reversal VEP?

Answer 11

• Electrodes mid occiput – forehead
• N70 is a ‘foveal’ component (at 70ms)
• P100 is a ‘macula’ component (at 100ms)
• N135 is a ‘paramacular’ component (at 135ms)

Question 12

What is an N70 in VEP?

Answer 12

‘Foveal’ component occurring at 70ms (macular-mediated component)

Question 13

What is the P100 in VEPs?

Answer 13

P100 is a ‘macula’ component (at 100ms)

The P100 is also a macular component, foveal if the chequers are small enough

Main measurement as we measure the peak to trough of this signal

Question 14

What is the N135 of VEPs?

Answer 14

N135 is a paramacular component at 135ms

Question 15

How does chequer size affect pattern reversal VEP morphology?

Answer 15

As we change the chequer size it changes the morphology. As we get smaller we’re using more foveal area than macula so we might start thinking it’s a VA thing at the fovea that’s causing the waveform change. Also changes in the early stages of infancy, particularly in the first 6 months of life.

Question 16

What happens as chequer size gets smaller in VEPs?

Answer 16

The response morphology differs markedly with chequer size. As chequers get smaller the N70 becomes much larger. As I mentioned previously, this is a macular component, as is the P100 which also gets bigger with smaller chequers. The N135 gets smaller.

Question 17

What happens as chequer size gets larger in VEPs?

Answer 17

Larger chequers preferentially stimulate the paramacular representation leading to a bigger N135,

Question 18

What can VEPs help diagnose?

Answer 18

• 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
• Functional integrity of the visual pathway (like in trauma or stroke patients)
• Objective cortical visual acuity measurement

Question 19

At what age do children’s VEP acuity reach that of adult levels?

Answer 19

By 6 months of age

Question 20

What are the two methods of measuring VEPs?

Answer 20

• Minimum VEP Acuity
• Sweep VEP

Question 21

What us the minimum VEP acuity?

Answer 21

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

I.e., if responses were recorded to 5’ chequers, minimum VEP or ‘cortical’ acuity is approx 6/(6 x 5) which is 6/30

Question 22

What is wrong with getting VA from VEPs?

Answer 22

• Likely to underestimate actual acuity
• If responses only recordable to flash, then VA likely to be rudimentary only
• Patient may not be completely blind even if no VEPs recordable at all because only one neuron is needed to see light, but thousands must be working for a recordable response!

Question 23

What is Sweep VEP?

Answer 23

Rapid presentation of different chequer sizes

Question 24

What do we plot in a Sweep VEP?

Answer 24

Amplitudes plotted.

Even at point patient can’t perceive squares, VA is at a certain acuity objectively when we see this straight line going down so can estimate VA. From the point at which detail begins to become difficult to resolve, the cortical response amplitude drops-off linearly with increasing spatial frequency as we approach the limit of visual acuity. Ends up with a chequerboard that looks grey.

Question 25

What do Sweep VEPs rely on?

Answer 25

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

The test relies on the fact that VEP amplitude reduces linearly with decreasing chequer size as the limit of visual acuity is approached.

Question 26

What’s a cruder method of Sweep VEP?

Answer 26

A cruder method, is to quote the minimum check size to which a recognisable response can be observed, but this tends to under-estimate acuity.

Question 27

What can we not rule out in Sweep VEP?

Answer 27

Whichever method is used, it does not rule-out a post cortical problem.

Question 28

What is a full-field stimulation?

Answer 28

Full-field stimulation excites both hemispheres of the brain equally. Each optic nerve decussates at the chiasma, with the temporal fibres projecting to the ipsilateral hemisphere, and the nasal fibres to the contralateral hemisphere. This fact allows us to excite just one half of the brain by using half-field stimulation. This allows localisation of lesions affecting different parts of the visual pathways.

You can see that Electrode B, referred to the mid-forehead reference electrode, will pick-up potentials from both hemispheres equally, but not so for the lateral electrodes at positions A and C. These ‘look’ across each hemisphere, with interesting results…

Question 29

What is Right Half-Field Stimulation?

Answer 29

In normal subject, stimulus OD or OS will activate the left hemisphere
- Temporal projection OS
- Nasal projection OD

Question 30

What is Paradoxical Lateralisation of P100 to half-field stimulation?

Answer 30

• Half-field stimulation activates one hemisphere only
• P100 paradoxically recorded from side of scalp ipsilateral to stimulated half-field

Question 31

Look at the diagrams on slide 94. The middle column shows what?

Answer 31

If we look at the middle column, we can see that during left half-field stimulation, when we are activating the right hemisphere, the biggest waveform showing the P100 is actually from the left electrode, A. This is known as the paradoxical lateralisation of the P100.

Question 32

Look at slide 94
What do we see from the right-hand electrode?

Answer 32

If you look at the bottom trace from the right electrode, C, you will see that the P100 is actually inverted (as is the N70, and N135). The whole waveform is turned up-side down. We get the opposite profile to right half-field stimulation as shown in the right-hand column.
Half-field stimulation effectively simulates the effect of a complete hemianopia (visual field defect where either the right or left half-field is missing) by activating only one hemisphere.

Question 33

Why does Paradoxical Lateralisation of the P100 occur?

Answer 33

Occurs in half-field stimulation

• P100 produced by dipole generators in the calcarine sulcus
• Electrode on scalp ipsilateral to stimulated half-field better placed to detect P100
• Full-field stimulation causes cancellation in lateral electrodes, but not midline

This ‘paradoxical’ lateralisation of the P100 has a straight forward explanation:
- P100 produced by dipole generators in the calcarine sulcus.

• Electrode on scalp ipsilateral to stimulated half-field better placed to detect the P100.
• The contralateral scalp electrode ‘sees’ the other side of the activity, and records an inverted waveform.
• The midline electrode is unaffected by all this, and records the same waveform regardless.

Question 34

What are cross & uncrossed RGCs used for?

Answer 34

Used to determine ocular albinism who have too much cross-over at the optic chiasm (sending signals from temporal fibres across to the other hemisphere)

Carriers of the X-linked OCA however will show normal routing

Question 35

What does the misrouting in albinism look like in VEPs?

Answer 35

Misrouting in albinism 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.

Asymmetry of opposite sense is seen in achiasmia and in compression of the crossing fibres e.g. pituitary adenoma (not so with craniopharyngioma though)

Question 36

How does ocular albinism look in Flash VEP? (See slide 99)

Answer 36

Example of a ‘crossed asymmetry’ in the flash VEP as seen in cases of ocular albinism.

• Misrouting is clearly evidenced by lateralisation of the response to the hemisphere contralateral to the stimulated eye (which is opposite to what we expect to see!)
• Note that reverse crossing seen in achiasmia where the response is lateralised to the hemisphere ipsilateral to the stimulated eye.

Question 37

What are VEPs good for?

Answer 37

Demyelinating Conditions

Question 38

What do we need to remember that’s really annoying about neurophysiology and graphs?

Answer 38

You get the drift. In neurophysiology, for some reason, waveforms may be presented ‘upside-down’ (i.e. with –ve at the top of the y-axis), we don’t know
why, and neither do they.