Evoked Potential

Has many people found the Evoked Potential test a good conclusive diagnosis for MS. I have been living with pretty much all the symptoms except all my MRI scans are clear, which Is very annoying, been ill for 9 months, can’t work or drive.

Evoked Potential (EP) tests

Evoked Potential tests are procedures for measuring the speed of impulses along neurons. Responses can be measured using EEG readings from electrodes attached to the scalp and occasionally other areas of the skin. Although this may sound like something from Frankenstein, they are in fact completely painless and entirely harmless. Based on input signals to the particular sense being measured, the time taken for that response to register can be accurately measured and compared to normal readings. The results are then analysed on a computer and average speeds recorded.

Demyelinated neurons transmit nerve signals slower than non-demyelinated ones and this can be detected with EP tests. Although they may appear to function perfectly, even remyelinated neurons are slower than normal nerves and so historical lesions can be detected in this way.

There are three main types of evoked potential test:

Visually Evoked Potential (VEP)

This test measures the speed of the optic nerve. The patient has to focus on the centre of a “TV” screen on which there is a black and white chequered pattern. Each square in the pattern alternates between black and white at measured intervals. The patient wears a patch on one eye for a while and then on the other, so that the speed of both optic nerves can be measured.

85-90% of people with definite MS and 58% of people with probable MS will have abnormal VEP test results.

Follow this link for more information on Visually Evoked Potential.

Brainstem Auditory Evoked Response (BAER)

The BAER test measures the speed of impulses along the auditory portion of Cranial Nerve VIII. This nerve arises in the Pons area of the Brainstem and therefore this test may be indicative of lesions in that area. The patient lies down in a darkened room to prevent visual signals from interfering with measurements. A series of clicks and beeps are played back to the patient.

67% of people with definite MS and 41% of people with probable MS will have abnormal BAER test results.

Follow this link for more information on Brainstem Auditory Evoked Response.

SomatoSensory Evoked Potential (SSEP)

The SSEP test involves strapping an electrical stimulus around an arm or leg. The current is switched on for 5 seconds and electrodes on the back and skull measure the response at particular junctions. The current is very low indeed and completely painless. The speed of various nerves can be measured in this way and the points of slow-down (i.e. demyelinated lesions) approximated to because of the sampling at several places.

77% of people with definite MS and 67% of people with probable MS will have abnormal SSEP test results.

Follow this link for more information on SomatoSensory Evoked Potential.

Slow nerve responses in any of these tests are not necessarily indicative of MS but can be used in conjunction with a neurological examination, medical history, an MRI and a spinal tap to deduce some kind of diagnosis.

Lots to say on your MRI; on my way out will reply tomorrow


Thank you ggood, some very useful information thank you for taking the time


Hiya Chris,

Reference your clear MRI first of all was it of your brain and spine? If was just brain; your lesion/s may be in your spine or vica versa.

The brain is more or less a wide open space and lesion/s are usually picked out. The spine on the other hand is just a thin; bony; flexible tube with millions of nerves running through it and a lesion/s sometimes cannot be detected.

Then we have the MRI machine itself. For this please look at Rizzo’s excellent explanation https://community.mssociety.org.uk/forum/new-diagnosis-and-diagnosis/brief-beginners-guide-brain-and-mri

Below I have copied an email she sent to me explaining how lesion/s can be missed. I must apologise for bombarding you with some technical information but I feel it’s the only way to give you possible reasons why your MRIs are clear.

The last T2 FLAIR scan I did of my brain used 70 slices (on a 3T scanner). The one I had done today - 13 (on a 1.5T scanner). THIRTEEN. THIRTEEN!!!

Thankfully I wrote it out the voxels/T2 reply in Word before posting - to avoid the dreaded time-out! So here it is…

A MRI image typically consists of voxels (3D pixels). Slice thickness is one dimension (on the z-axis if you think of maths). The images you see on the CD show you the other two dimensions (on the x- and y-axes). You can set the voxel size to anything you want, in any dimension; all that happens is that it changes the time the scan takes to run (and therefore, of course, how much it costs). The smallest voxel size used in everyday MRI is typically 1mm x 1mm x 1mm. The “off the shelf” scan that I used to use for this size of voxel had 176 slices. The voxels (and slices) cover the whole brain irrespective of what the voxel size is – nothing is missed out (but see later).

If a standard T2 sequence is used for the scan, white matter gives off a poor signal and shows up as dark whereas lesions (which are full of fluid) give a strong signal and show up as bright.

However, the brightness of a voxel depends on the average of the response from the matter represented by that voxel. So a voxel that is 1mm x 1mm x 4mm will show the signal generated by all matter located in that 4mm3 cube. That is, if the voxel only contains white matter it will be dark in the image, if it only contains fluid it will be bright, but if it contains a mix of white matter and fluid it will look somewhere between dark and bright, depending on the proportion of the different matter types.

So if you have a large voxel (say 4x4x4) and a small lesion (say 1x1x1), the overall signal in the voxel will only be slightly higher than one without a lesion (and therefore look only slightly brighter, and therefore may be overlooked). [NB Small lesions would also not always be completely contained within one large voxel – it is more likely that it would be partially in at least two. So this makes it worse.]

But if you have small voxels and a large lesion, then you will get several very bright voxels (where the matter is all fluid), some intermediate voxels (where there is a mix of fluid and white matter), and some vaguely brighter voxels (that contain predominantly white matter).

In other words, small voxels are much better for detecting lesions.

So, can lesions be missed if you use thick slices? Basically, yes. It is entirely feasible. However, they would have to be much smaller than the slice thickness because if they are closer in size, they would contribute sufficient signal to make the voxels significantly brighter than the surrounding voxels and would (should!) be picked up by a decent radiologist. Saying that, it is possible that it might be missed if a small lesion, by chance, spans lots of voxels (e.g. if it is centred on where four voxels meet on that slice) and the signal is lost by the averaging with the white matter signal in those voxels.

However, there are new “pulse sequences” (the settings that programme the scanner) that are particularly sensitive to fluid. If you use one of these rather than a standard T2 sequence, you will be able to use bigger voxels and still be able to detect lesions relatively easily. And the power of the scanner makes a big difference too. A 3T scanner is much better than a 1.5T scanner.

So, if a hospital has a 1.5T scanner and a neuro is ordering a standard T2 scan, then he should be asking for a high resolution (i.e. small voxel size).

If the hospital has a 3T scanner and the neuro is ordering a FLAIR or another new type of pulse sequence that’s good for fluid, then he can get away with a lower resolution.

[NB A related point: it is possible to set gaps between slices. For example, the MRI may capture signal from 0-4mm, 8-12mm, 16-20mm etc rather than 0-4mm, 4-8mm, 8-12mm etc. This would DEFINITELY miss lesions!]

So the trick to not missing lesions is not so much about the number of slices, but about the voxel size, whether or not the slices cover the whole brain without any gaps, the power of the scanner and the choice of pulse sequence.

Lastly when your able to go back to work contact Access to Work; lots of help available. Taxis to and from; you only pay equivalent bus fare; furniture; infrastructure; even a helper to do parts of your job you find difficult. See Get support in work if you have a disability or health condition (Access to Work) - GOV.UK

Good luck


Thanks George, I have had 3 brain MRI one with contrast and a spinal one which unfortunately all came back unremarkable. Thanks for information I will give it a good read when I get a quiet minute