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.