VEP Flashcards
What is a VEP?
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”
Why is VEP Flash Stimulus good?
- 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
What stimulus is used in VEPs?
Reversing Chequerboard similar to PERG
What is a confounding variable when doing a VEP?
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
How do we get around nystagmus as an issue in VEPs? How does this help?
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.
How small are the chequers for macular stimulation and for foveal stimulation?
- 1° chequers (macular stimulation)
- 15’ chequers (foveal stimulation) (15 minutes’)
How often does the reversing chequerboard reverse per second in VEPs?
Typically 2 reversals per second
What is the stimulus field in VEPs?
Stimulus field >15°
What is the arrangement for recording VEPs?
- Patient
- Electrodes
- Amplifier
- Filter
- Analog to Digital Converter
- Computer
- Stroboscope or Pattern Stimulator
How are VEP electrodes placed?
Any age patient.
Electrodes on ridge on back of head – occipital lobe/visual cortex. One central to the front of the head and 3 at the back; 1 central and 1 on either hemisphere.
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.
What is a typical pattern reversal VEP?
- 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)
What is an N70 in VEP?
‘Foveal’ component occurring at 70ms (macular-mediated component)
What is the P100 in VEPs?
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
What is the N135 of VEPs?
N135 is a paramacular component at 135ms
How does chequer size affect pattern reversal VEP morphology?
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.
What happens as chequer size gets smaller in VEPs?
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.
What happens as chequer size gets larger in VEPs?
Larger chequers preferentially stimulate the paramacular representation leading to a bigger N135
What can VEPs help diagnose?
- 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
- Albinism
At what age do children’s VEP acuity reach that of adult levels?
By 6 months of age
What are the two methods of estimating VA in VEPs?
- Minimum VEP Acuity
- Sweep VEP
What is the minimum VEP acuity?
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
What is wrong with getting VA from VEPs?
- 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!
What is Sweep VEP?
Rapid presentation of different chequer sizes
What do we plot in a Sweep VEP?
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.
What do Sweep VEPs rely on?
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.
What’s a cruder method of Sweep VEP?
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.
What can we not rule out in Sweep VEP?
Whichever method is used, it does not rule-out a post cortical problem.
What is a full-field stimulation?
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…
What is Right Half-Field Stimulation?
In normal subject, stimulus OD or OS will activate the left hemisphere
- Temporal projection OS
- Nasal projection OD
What is Paradoxical Lateralisation of P100 to half-field stimulation?
- Half-field stimulation activates one hemisphere only
- P100 paradoxically recorded from side of scalp ipsilateral to stimulated half-field
Look at the diagrams on slide 94. The middle column shows what?
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.
Look at slide 94
What do we see from the right-hand electrode?
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.
Why does Paradoxical Lateralisation of the P100 occur?
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.
What are cross & uncrossed RGCs used for?
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
What does the misrouting in albinism look like in VEPs?
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)
How does ocular albinism look in Flash VEP? (See slide 99)
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.
What are VEPs good for?
Demyelinating Conditions
What do we need to remember that’s really annoying about neurophysiology and graphs?
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.
What can look like Ocular Albinism on a VEP?
Remember by:
Ocular Albinism Also Feels Fucking Icky
Ocular - Optic Nerve Decussation
Albinism - Angelmann Syndrome
Also - Anterior Segment Dysgenesis
Feels - FHONDA
Fucking - Foveal Hypoplasia
Icky - iCSNB
iCSNB and FHONDA (foveal hypoplasia, optic nerve decussation defects and anterior segment dysgenesis syndrome). (Also Angelman syndrome where the maculae can appear normal!)
