Technique of Midline Positioning of the Spinal Cord Stimulator - Tripolar Paddle
Once the 3-column paddle 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 MUAP’s are seen on EMG. The lowest stimulus intensity needed to elicit a motor response is referred to as the threshold stimulus. MUAP’s 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 MUAP’s 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 right sides indicates the SCS is lateral to the PM. However, medial/lateral repositioning of the SCS is necessary only 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 (Figure 4.4A), 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 (Figure 4.4B). This assures optimal electrode column symmetry and programmability of 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 (1, 2, 3 and 5 column) array configurations.
Figure 4.4: Lateral intraoperative views. A. Lead placed off the midline. B. Perfect alignment on the midline.
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