The Difference Between a NCS and an SSEP

We always say that an EMG/NCS is done to help rule out MS, but an SSEP is done to help diagnose (rule in) MS.  The two tests seem similar and the difference confuses people.

Nerves may travel in only one direction.  Those that start in the brain and leave to go out into the body are those that CAUSE something to happen.  The easiest to understand are the "motor nerves" that originate in the brain and then proceed to go down into the spinal cord, leave and go to make muscles move.  The other type are the nerves that carry information, usually sensation, from out in the body to the brain.  These are called the sensory nerves.

The Central Nervous System consists of all of those nerves that are still in the spinal cord, brainstem, and the brain.  These are also both sensory nerves and motor nerves.  MS is a disease that only involves the Central Nervous System.

When the Nerve Conduction Study is done, it is to diagnose a "Peripheral" neuropathy, that is, a problem in the Peripheral Nervous System (PNS).  The PNS is all the nerves that have already exited through the spinal column.  These nerves include both motor nerves and sensory nerves.

Let's say that you have pain in your forearm.  There is no injury to account for the pain, so the doctor thinks it is nerve (neuropathic) pain.  He choses a test to see if he can determine where the damage to the nerve has occurred.  One of the first test chosen is the EMG/NCS.  The NCS of this test is a Nerve Conduction Study (also known as a Nerve Veocity Study).  This is to see if there is a peripheral neuropathy.

For example, if the pain is felt in the forearm, the NCS will stimulate a nerve below the forearm (in the hand or wrist) and measure the velocity of the signal carried by the peripheral nerve as it travels up to the spinal cord.


forearm --> elbow --> upper arm/shoulder--> entry to cord --> cervical spinal cord --> brain.

||----------------------------NCV----------------------------------------------||---------------SSEP-------------------||

                     (peripheral nervous system)                            (central nerous system)

A nerve conduction study looks at the sensory signal from the hand -->wrist --> elbow -> shoulder --> spine.  When we see a slowing of the signal or a break in the signal at the elbow, we know that the damage has already occurred.  So we can say that the nerve damage is below the elbow.  The more sites that are monitored the closer we can pinpoint the site of nerve injury.  If the sensation travels normally from the finger to the wrist, but then we see that the signal is slowed or absent at the elbow, we know the damage is in the wrist or forearm.

If the signal is fine until the entry point into the cervical spine then we can assume some type of compression as the peripheral nerve emerges from the spinal column.

An SSEP is similar to the NCV but it measures slowing within the spinal cord and finally in the brain.  So we would still see a normal signal at the point that the sensory nerve impusle entered the spinal cord.  Then we can't measure that particular, exact nerve, but we know what kind of signal to expect as the signal rises in the cord and finally as the sensation is experienced in the brain.  Over the brain the test uses electrodes like for an EEG.  In fact, an SSEP is like a very localized EEG.

The SSEP measures the velocity  of a sensory nerve signal from the point of stimulation to its arrival in the brain.  The signal is typically measured at 3 or 4 spots along the way, way from the foot to the hip to the spine to the brain.

If there has been a demyelinating lesion (like in MS) the lesion will be in the spinal cord or the brain.  So the first or second measurements will be normal because they are measuring the sensation signal out in the peripheral nerves.  If the signal shows that the spot of delayed conduction is in the cord or the brain, then the result is suggest of MS.

In MS the signal is measured both in speed and in it;s particular wave form.  If the wave form looks good, but the signal is delayed somewhere in the central nervous system, this suggests MS.  If the signal is delayed, but the signal waveform is broken up then it suggests some other kind of damage - not MS.

So the SSEP is fairly specific for MS.

[mokibear]

So if one's SSEP comes back normal, does that rule out MS?

Moki

[Sunnytoday]

So, if I haven't had either of these two tests, but Evoked potentials WHERE does that leave me?
OH, and once again, you did a very wonderful job of explaining this, as this was very confusing, I will print and save this for my files- thank you!
~Sunnytoday~

[Quixotic1]

The SSEP is fairly specific for MS, but it is not a very sensitive test.  It misses a lot of the lesions in MS.  A psoitive test is added evidence pointing "toward" MS, but does not prove it.  A negative SSEP tells us nothing about whether you do or don't have MS.

So, Moki, the answer to your question is a definite 'No".

Only 50% or so of people with MS have a +SSEP.

Quix

[Lulu54]

Q- I trust you will copy thhis over into a health page for easy reference?  Well done - Lu

PS - plus if you post it there I can find it to read it over and over until I finally have an "aha" moment. :-)

[doni54]

Thanks for this, as you know I will have the NCV test on my first neuro appt. and still had some questions.

If the NCV shows your problems to be in the peripheral nervous system, do they still do all the other tests?

This will make a great health page!!!

doni

[mokibear]

Thank you Quix.

You did do a great job on this and thank you for doing this. I too think it would be a great health page!

Moki

[Quixotic1]

To all the people who caught the error (in science Sensitive and Specific have real definitions)

The SSEP is fairly Sensitive for MS.  If it is positive, it is more "likely" that you have MS.

It is not as Specific.  If the test is negative/normal, it does little to rule out MS.

Not that many people with MS will have a +SSEP, about 50%

Quix

[Quixotic1]

If the positive NCV can explain all of your symptoms, then there is no need to run other tests for MS.  For example if your complaints were pain or numbness and weakness in both hands and the NCV was positive in both arms - you have a very reasonable answer to the problem.

If you have dizziness, cognitive difficulties, and visual loss, spasticity in your legs and pain in your feet, then a positive NCV in the legs only answers the foot pain.  It would not explain the first four symptoms, and the doc would have to look further.

Quix

[doni54]

Thanks, that makes it clearer to my muddled brain.

[shoshin]

Quix: Thanks for the clear explanation!

Also sorry, I think I was the one who introduced the non-scientific use of specific on another post by saying an abnormal SSEP wasn't specific to MS as opposed to other things.

After my abnormal SSEP, the neuro still did want to rule out some other stuff and I had various blood tests (B12, NMO/Devic's disease, etc.) and a lip biopsy for Sjogren's. He did seem to be talking an awful lot about MS, but was careful to say that we didn't know anything for sure yet.

sho

[Quixotic1]

Sho - no, no.  I didn't mean to speak about anyone else's use of the words.  REally!  I only meant to correct mine, because I know we have research PhD's who would notice my use.

I probably shouldn't have psoted my comment.  I never mean to make anyone look bad.

Q