When a somatosensory evoked potential is performed, an electrical stimulus is applied to a nerve in the arm or leg. Electrodes are placed in various regions, and the machine measures how this electrical stimulus is transmitted to the brain. When the stimulus is applied to the nerve it may cause the muscle to contract, the recordings in the spine and brain are independant of the twitching. The only time a good twitch needs to be documented is if no signals are being detected. Then the presence of a twitch is used to confirm that the electrical stimulus is being applied correctly. Therefore, if the technician was able to record waveforms, the absence of the twitch is not clinically important.
Without reviewing your MRI, I cannot tell you if is consistent with MS. However, based on the description it seems appropriate to consider it. I hope this helps. Good luck.