SSEP

Dear Dr:
I recently had a SSEP as part of a work-up to differeniate between Sjogren's and MS. My questions are about what occured during the test since I won't be seeing my neuro for another month for results.

1) When they hooked up the electrodes to my wrist as well as my ankle, my thumb and big toe twitched after some adjustments to the voltage, etc. I could not feel my thumb and big toe twitch. I could feel the pulses in my fingers and soles of my feet but not the other digits that were twitching. Why is that?
2) On my right foot, where I was previously diagnosed with RSD many years ago but no longer have issues, I could feel and see the big toe move. However, the tech could not get an F wave. What is an F wave and how does it factor into the equation?

I have chronic  pain which is assumed to be neuropathic in nature. Can't seem to find the right balance of lyrica. Hoever, recently, past 4-6 days I have had what I can best describe as intense aching bone pain, unrelieved by massage or warmth or another  Klonopin  for muscle cramps. any suggestions? Thoughts?

Thank you for your time,
Rendean

[J T Junig, MD PhD]

SSEP-- Somatosensory evoked potentials-- are used to test the function of sensory nerves-- as opposed to an EMG, which measures the function of motor nerves.  The nerves carry signals in opposite directions;  sometimes both types of nerves travel together, but in other areas, including the spinal cord, they travel separately.  

Whether you felt the twitches or not is not that important;  likewise the moving of the toe is only because of 'spillover' of current to other nerves-- motor nerves-- that don't have anything to do with the test.  The test measures the electrical response in the brain;  one nerve stimulation will not make enough of a response to show up above the normal 'static' of brain electrical activity, but by stimulating the nerve at the skin many times, the 'static' averages out, and the electrical wave from the sensory impulse will show up.  the 'k' wave is one component of a waveform that has several peaks, which appear as the impulse passes through each area of processing.  

It is hard to know exactly what is going on when pain takes on different 'characters' over short periods of time; pain will often feel different-- like 'bone' or 'muscle' pain, for example-- based on complex changes in nerve function that can't really be sorted out that specifically.