Technical Considerations

Modern stimulation electrodes are either "paddle''-type laminectomy electrodes or cylindrical percutaneously placed electrodes. The spacing between each contact varies from 1 to 10 mm, depending on the application. Spinal stimulation requires depolarization of a target neuron (making the neuron positively charged) which is done by activating a negatively charged electrode (cathode). Positively charged electrodes hyperpolarize the neuron, limiting propagation of action potentials. Therefore, the cathode is the stimulus site for active neuronal targeting and the resultant electrical field is conformed or limited by the anodal effects.17 As most electrodes are placed epidurally, the conductivity of various substances in the anatomical vicinity are important in determining the extent of electrical field propagation. Within the spinal cord itself, longitudinally arrayed white matter tracts are most conductive, while fat within the epidural space is not very conductive. Vertebral bone is least conductive of all, and cerebrospinal fluid is most conductive (see Table 20.1). The conductivity of cere-brospinal fluid can become important in those cases where accidental dural puncture has occurred, as stimulation occurs at significantly lower amplitudes, and may propagate paresthesias to larger areas. The goals of SCS are to stimulate midline sensory fibers electrically without stimulating the more lateral nerve roots, which may cause abdominal cramping or other irritating sensations. Depending on the anatomical location in the spine, the distance between the electrode(s) and the spinal cord can vary significantly. The thickness of the area between the electrode and the cord (the 8CSF) is maximal in the mid-thoracic spine, and this has implications for the occurrence of some variable postural effects of stimulation for patients. Thus, in some patients, simply flexing or extending the torso can create stimulation that is either imperceptible or painful (see Figure 20.1 ).18 Oakley and Prager17 describe the concepts of "perception threshold'' (the point at which the patient first detects the paresthesia), and "discomfort threshold'' (the point at which the paresthesia is uncomfortable). The area between these thresholds is then called the "usage range.'' Power consumption increases with increasing 8CSF, which has implications for recharging frequency and pulse generator longevity.

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