Key fundamental point about evoked potentials is that it's the only way of
showing in a human, that there is demyelination. So I'll just break that
down a bit, so one of the key things about MS is that the attack is on the
myelin of the nerve leaving the underlying nerve fiber intact and the
result of that as far, as the nerve is concerned, is that electricity passes
much slower. Now the only way of testing whether that's happened in a human
directly is to actually put electricity into the human and see if it conducts
down a nerve slower. And there are three basic ways of doing that, the least used
one, is something called the brainstem evoked potentials. Where there's a
clicking noise outside the ear and electrical impulses pass through the ear
nerve and into the brain and the brain stem and they're recorded here. Next is
something called somatosensory evoked potentials where literally some
stimulation is applied to the arm or the leg and the speed with which that
travels all the way up the lim and into the brain is recorded. And then the most
common form which people may still be asked to do even at the time of
diagnosis of MS is the visual evoked potentials, and consists of someone
watching a black and white chess board flashing. So that's a stimulus and then
some wires put on the back of the head measuring the electric current over the
scalp. And that is a poor man's version of the electrical activity in the brain
in the visual area by flashing lots of times and by recording lots of times you
can get an idea of how long it takes for a change in signal out there
to be registered in the eye, to be transmitted all the way through to the
visual part of the brain. And in people with MS that's slowed down, because
very typically the optic nerve, which is a nerve connecting the eye to the rest
of the brain has been demyelinated and so the conduction of electricity has slowed.
So this used to be evoked potentials and particularly visual evoked potentials,
used to be a really common part of the diagnostic workup of someone with MS
particularly before MRI scans. Because you could actually say this person has
got demyelination. The reason that we still have evoke potentials and
particularly in clinical trials is because if you're doing a trial of a
drug that you think will repair demyelination such as a trial of
Bexarotene in Cambridge at the moment then you want a direct measure of
remyelination and so we use evoked potentials, particularly visual evoked
potentials to measure how fast the nerve the optic nerve is conducting before
treatment, and then after treatment. And what we would hope is an effective drug
causing remyelination would result in the visual evoked potential being
shorter or faster. And that would show us that the optic nerve has been repaired.
Well the thing that is curious, that might surprise you, is that if you take a
random group of people with MS who've never experienced optic neuritis, at
least half of them will have abnormal VEP's, visual evoked potentials suggesting
they have indeed have experienced optic neuritis in such a small way that it
wasn't obvious to them
It takes about half an hour, the electrodes are applied directly to the
skin with a bit of jelly and then some tape over them. And sometimes you have to
force the hair apart to get a bit of a good landing strip sometimes even shave
a tiny bit of hair to put the electrodes on. And depending on how the visual
evoked potential is done that might mean just three electrodes on
either side or two or or sometimes more eight, up to eight.