Once the three-column paddle (Fig. 20.2) is placed in the dorsal epidural space, the superior midline contact is stimulated at minimal settings and the EMG trace recording associated with the dermatomal level of stimulation is monitored. The stimulus intensity is gradually increased until MUAPs are seen on EMG. The lowest stimulus intensity needed to elicit a motor response is referred to as the threshold stimulus. MUAPs will be seen bilaterally at the threshold stimulus if the midline contact of the SCS is in line with the PM of the spinal cord. If MUAPs are seen unilaterally, the threshold intensity for that side is recorded and the stimulus is further increased to elicit a response on the other side. A difference in threshold stimulus intensity between the left and the right sides indicates that the SCS is lateral to the PM. Medial/lateral repositioning of the SCS is only necessary, however, if the difference in threshold intensity between the two sides is greater than 2 mA. In this case the paddle may not be perfectly flat on the lateral x-ray (Fig. 20.3a), thus dissection of the lateral recesses or proximal/superior lamina is further performed until the electrode is perfectly aligned with vertebrae on a lateral view (Fig. 20.3b). This ensures optimal electrode column symmetry and programmability relative to the PM. Of course, as a last resort, the laminotomy can be extended to a full laminectomy to allow perfect alignment of the paddle on the PM. In this case tissue must typically be identified to suture the electrode in place to prevent migration. These tenants hold true for all paddle (one-, two-, three- and five-column) array configurations.

There appears to be a correlation between the muscles with objective EMG activation during intraoperative monitoring and the subjective paresthesia obtained postoperatively as described in Table 13.​1. Thus this may be explored to generate a precise model of paresthesia coverage and create a functional dermatomal mapping of perceived stimulation threshold after the surgery. Furthermore the EMG activation threshold may be a reliable predictor of the patient’s perceived paresthesia threshold. It has been the author’s experience that EMG activation correlated with pain control at amplitudes lower than the paresthesia threshold (i.e., subthreshold stimulation) and that occasionally persistent EMG activation intra- and postoperatively may be seen lasting as long as 15 min after the stimulation is discontinued. These patients typically respond extremely well to the stimulation therapy. It seems likely that these patients are the occasional patients who use their stimulation only intermittently, often having effective long-term pain relief while using their system only for a portion of each day.

EMG and SSEP monitoring are well-accepted modalities of neurophysiological testing in traditional spine surgery, but their application relative to implantation of neuromodulatory systems, in particular epidural paddle electrode systems, is comparatively new. Given the limitations associated with other methods of implantation, however, we expect that the techniques described in this chapter will continue to gain acceptance and improve outcomes by objectively localizing neurophysiological stimulation targets.