Spinal Cord Stimulation Techniques

Abstract

Spinal and cardiovascular surgeries impart mechanical [1] or ischemic [2] stress to the spinal cord. To prevent spinal cord injury due to surgical stress, the intraoperative monitoring of spinal cord function is important. Until the 1960s, there were no available spinal cord function monitoring methods. In the late 1960s, a novel tool for monitoring spinal cord function became available; it recorded somatosensory-evoked potentials (SEPs) from the scalp. In this technique, the electrical potentials evoked by peripheral nerve stimulation are recorded on an electroencephalogram (EEG). The short-latency component of SEPs may indicate cervical spinal cord function in waveforms (latencies and amplitudes) [3].

Appendix: Techniques and Physiology

Introduction

The development of the catheter electrode has made it possible to stimulate the spinal cord epidurally and to record epidurally human spinal cord potentials for monitoring spinal cord function during an operation [35].

The Catheter Electrodes

Human spinal cord potentials (SCPs) can be recorded from the ­epidural space using the same catheter used in continuous epidural block. The epidural catheter electrode can be made simply by insertion of a stainless steel wire through the epidural catheter approximately 5 mm beyond its tip (Appendix Fig. 6.9a). This simple catheter has been used for the recording of spinal cord potentials in patients during surgical operations or in patients with spinal cord diseases and are also used for simulation of spinal cord. An epidural catheter with three orifices on the side and three platinum wire electrodes was developed in our laboratory for multiple applications (Appendix Fig. 6.9b, c), including monitoring of spinal cord potentials, measurement of epidural pressure and epidural tissue blood flow, epidural spinal cord stimulation, and epidural injection of drugs [36].

Appendix Fig. 6.9

Three types of epidural catheter electrodes are shown. A stainless steel wire can be placed through an epidural catheter approximately 5 mm beyond its tip (a). Three platinum wire electrodes can be placed on the catheter (b) that may also have three orifices on the side to measure epidural pressure and ­epidural tissue blood flow, epidural spinal cord stimulation, and epidural injection of drugs (c)

Accurate insertion of the catheter electrode at the required site in the epidural space is critical for these applications. We have been using three methods to determine the proper placement of the catheter electrodes in the posterior epidural space: (1) epidural electrical stimulation test, (2) recording of the spinal cord potentials evoked by stimulation of the segmental, heterosegmental nerves, or dorsal cord [11, 37, 38], and (3) image examination such as X-ray, MRI, or CT scan.

When the catheter electrodes are situated in the posterior epidural space on the midline, stimulation through the catheter electrodes produces the bilateral twitches of the segmental muscles. When it produces unilateral muscle twitches in the same spinal segment, the electrodes might be situated laterally in the epidural space. By this stimulation test, you can verify the spinal segment of the position and laterality of the catheter electrode in the epidural space. When the catheter electrode is situated in the anterior epidural space, polarity of the segmental SCPs is reversed as expected. Laterality of the catheter electrodes in the epidural space can also be judged by the waveform characteristics of the SCPs. When the catheter electrodes are situated ipsilateral to the stimulated peripheral nerves and close to the roots, the initial positive spikes and P₂ wave are recorded larger than those recorded contralateral to the nerves.

The procedure used to introduce the catheter electrodes into the epidural space is the same as methods used to place catheters for continuous epidural anesthesia [35]. The patients are placed in the lateral position and flexed to open the interspaces of the vertebral column. After making a skin wheal aseptically and injecting 0.5–1.0% lidocaine (5 mL), a 16–18 gauge Tuohy needle is inserted into the epidural space using the paramedian approach, with the bevel parallel to the sagittal plane and targeting the predicted segment. When the tip of the Tuohy needle is located in the epidural space, the direction of the bevel is adjusted, and the catheter electrode is inserted approximately 5 cm into the epidural space. The tip of the catheter electrode and the skin surface electrode are connected, respectively, to the negative and positive outlets of an electrical nerve stimulator [38]. Using these electrodes, recordings can be done at several spinal levels (Appendix Fig. 6.10).

Appendix Fig. 6.10

Recording of the human SCPs. The recording electrodes are placed at various levels of the spine (a) into the epidural space (b)

Origins of Each Component of the Segmental SCPs

The initially positive spike, P₁, of the segmental SCPs is believed to be a reflection of the extracellular events associated with the propagation of action potentials through the roots into the spinal cord [11, 39, 40]. The generation of an action potential at a node of Ranvier creates a positive capacitive current, which is conducted electrotonically down the axon. This current is responsible for the initial positivity of the triphasic spike (Appendix Fig. 6.11a). The capacitative current is also responsible for depolarizing the cell membrane at the next node to threshold, thereby initiating the production of an action potential here. The rising phase of the action potential is generated by an influx of Na⁺ into the axon from the extracellular space. The loss of Na⁺ causes the extracellular space to become negatively charged; this event is recorded as the negative component of the triphasic spike (Appendix Fig. 6.11b). The falling phase of an action potential is due to a K⁺ efflux from the axon into the extracellular space. The additional positivity in the extracellular space is recorded from the cord dorsum as the second positive component of the triphasic spike (Appendix Fig. 6.11b).

Appendix Fig. 6.11

Origins of each component of the segmental SCPs. The initial positivity of the triphasic spike is believed to be a reflection of the extracellular events associated with the propagation of action potentials through the roots down the axon into the spinal cord (a). The rising phase of the action potential is generated by an influx of Na⁺ into the axon from the extracellular space resulting in the extracellular space becoming negatively charged; this event is recorded as the negative component of the triphasic spike (b). The falling phase of an action potential is due to a K⁺ efflux from the axon into the extracellular space which is recorded from the cord dorsum as the second positive component of the triphasic spike (b). The negative waves, N1, (c) are thought to be reflections of changes in the extracellular environment produced by activity of dorsal horn interneurons. The slow positive wave, P₂, of the segmental SCPs (d) has been demonstrated as the extracellular manifestation of the process of primary afferent depolarization. Positive ionic current leaves the extracellular space at excited axo-axonal synapses (sinks) resulting in the dorsal most portion of the spinal cord becoming positively charged (d)

The negative waves, N1, of the segmental SCPs (Appendix Fig. 6.11c) are thought to be reflections of changes in the extracellular environment produced by activity of dorsal horn interneurons (see also Appendix Fig. 6.12). When the interneurons are synaptically activated, positive ionic current leaves the extracellular space at the synapses (sinks) and reappears along the ventrally projecting axons of the cells (sources). Thus, the dorsal horn takes on a negative charge and the ventral horn takes on a positive charge [39].

Appendix Fig. 6.12

Origins of each component of the segmental SCPs. The negative waves, N1, are thought to be reflections of changes in the extracellular environment produced by activity of dorsal horn interneurons. The positive wave, P₂, of the segmental SCPs has been demonstrated as the extracellular manifestation of the process of primary afferent depolarization

The slow positive wave, P₂, of the segmental SCPs (Appendix Fig. 6.12d) has been demonstrated as the extracellular manifestation of the process of primary afferent depolarization (PAD) just as observed in the spinal animals (Appendix Fig. 6.12) [11, 37, 39–49]. Positive ionic current leaves the extracellular space at excited axo-axonal synapses (sinks) and reappears along the primary afferents (sources). Thus, the dorsal most portion of the spinal cord becomes positively charged (Appendix Fig. 6.11d) [11, 39–42, 49, 50].