Abnormalities in veps.

Hi, Just wondered, I’m undiagnosed at the moment, the attack I had could have been a one off, it’s now a wait & see situation. This was a year ago. My doctor only recently told me that there were abnormalities with the vep test I had done. Now I didn’t have any problems with my vision (not that I was aware of) & I remember when the neurologist looked at the back of my eye, that was fine. So what could have been abnormal about the veps? What sort of things are there that could be classed as abnormal?

Hi Anon, The VEPs don’t actually test for a lot of different things. It’s not like a standard eye test, which could pick up all kinds of conditions. Basically, it’s measuring how long signals take to travel from your eyes, to the visual cortex, at the back of the brain, where the processing is done. They know how long these signals typically ought to take. They can also measure the difference between the two eyes. Any delay, or retardation, could indicate inflammation of the optic nerve - optic neuritis. Sometimes there is evidence that the signals are getting delayed, even if the person has not been consciously aware of any sight problems. So I’d say this must be what has happened. Tina

VEPs test how the signals from the eyes reach the areas of the brain that process them. Visual signals are pretty consistent in the normal population, really only varying with what the person is looking at. The P in VEPs means potentials - the voltage of the signal. When we’re not looking at anything, the potential bobs about a wee bit, not doing much until it’s called upon to act. At that point, it dips briefly and then rises to a peak and falls in an arc shape, falling to lower than it started before recovering and returning to its bobbing around, “resting”, level. This shape is called a “waveform”. How many peaks and troughs are expected as well as where they occur depends on the images that have been shown. The expected response is very well documented for normal people of different ages and all sorts of patient groups.

There’s actually quite a lot of information in VEPs, but the main things are how long it takes to go from rest to the peak (peak latency), how big the peak is (amplitude) and the shape of the waveform (basically does it have the expected peaks and troughs and are they in the right places). Because they put electrodes in various places, they can measure the signal as it travels.

Which now (finally!) brings me to what you are probably more interested in: it is possible to have apparently normal vision and still have abnormal VEPs. Looking into our eyes only checks the retina - this is where visual signals originate, but they then travel all the way from there to the very back of the brain with one stop en route and a few tricky manoeuvres too - and the signal can go wrong anywhere along this path.

The typical finding in MS is a longer latency - basically it takes our signals longer to get to the peak than people who don’t have MS. This is because demyelination slows signals down.

Just like all tests, it is possible to get false negatives and false positives - in other words, an abnormal VEP does not mean anything on its own and there are lots of things that can cause it anyway (e.g. people with dyslexia have abnormal VEPs to certain stimuli).

On a more personal note, I had an abnormal VEP when I was about 22 (I wasn’t being tested for MS). I wasn’t diagnosed with MS for another 10 years and have never had ON. I didn’t start wearing (reading) glasses until I was in my 40s - like an awful lot of people, MS or not! So your abnormal VEP does not necessarily mean that your vision is vulnerable - ON is common in MS, but a lot of us never get it, no matter the VEP results.


Karen x

I had an abnormal VEP but my vision is absolutely fine apart from short-sightedness which I have had for 30 years! Teresa xx

Thanks for your replies, I feel a bit more assured now & understand what they were actually about in the first place x