Elimination of EMG interference during recording of somatosensory evoked potentials elicited by posterior tibial nerve stimulation in patients with cervical spinal cord injury.
Recording SEPs to PTN stimulation in some patients with acute injuries of the cervical spine can be difficult due to the presence of involuntary activity of the neck and jaw muscles. The electrical potentials derived from this myogenic activity have a distribution along the sagittal line of the head which is uniform in amplitude while the amplitude of the SEPs recorded from points along this sagittal line is maximal over a point close to the vertex and less in front and behind this point. This fact has been utilized to devise a 'bipolar' derivation with one electrode on the vertex and two electrodes, one in front and one behind it on the sagittal line tied together as a reference. This effectively eliminates potentials of muscle origin from the SEP record. Other 'bipolar' combinations are not as effective in obtaining evoked potentials free of this muscle interference.