Programming—SCS

Current-controlled versus voltage-controlled systems

The first SCS system was voltage controlled, meaning that stimulation output voltage is held constant during stimulation pulse delivery. The first current-controlled system, which holds current constant during pulse delivery, was introduced in 2004, and today most SCS systems are current controlled. With respect to programming SCS devices, voltage-controlled systems may require more programming adjustments over time if electrode impedances change (e.g., due to scar tissue development) compared to current-controlled systems. The reason is that neuronal activation depends on the electric field along the neuron, which is proportional to current flow, not voltage [ ]. When impedances change in a voltage-controlled system, voltage remains constant and current changes according to Ohm's law ( I = V / R ), which changes the electric field, and therefore, neuronal activation. Reprogramming may be needed to adjust the electric field to maintain therapy. For a single-source (one current source) current-controlled system using a single electrode, when the impedance of the electrode changes, the current remains constant, and therefore neuronal activation also remains constant. SCS systems are commonly programmed to use multiple electrodes. In this case, although the total current remains constant in a single-source current controlled system, the current flowing through individual electrodes may change due to impedance changes, which may affect neuronal activation.

Multiple current sources versus single current source

To maintain the amount of current flowing through each active electrode, some SCS systems have been designed to have multiple current sources. In addition to being able to maintain a stable electric field even with impedance changes, multiple-source current controlled systems may also offer more precise control of the stimulation field. In a single-source current-controlled system, each electrode can be set as either active or not, and the current through each electrode depends on its impedance relative to the impedances of the other active electrodes. More current will flow through electrodes with lower impedances. In a multiple-source current-controlled system, the amount of current through each electrode can be set independently of the other electrodes regardless of their impedances. With respect to programming SCS devices, a multiple-source current-controlled system may provide more precise control over the electric field than a single-source current-controlled system because a precise amount of current can be specified for each electrode. A consequence of this precise control is that for a given set of electrodes, more stimulation locations may be accessible to a multiple-source current-controlled system than a single-source current-controlled system [ , ].

Lead options

Percutaneous leads and paddle leads are available for most SCS systems. Currently available percutaneous leads have 4 to 16 cylindrical electrodes with spacing (edge-to-edge distance between adjacent electrodes) typically ranging from 1 to 6 mm. Paddle leads typically have 2 to 5 columns of planar electrodes with the total number of electrodes typically ranging from 8 to 32 and are insulated on the posterior side. Rostrocaudal electrode spacing typically ranges from 1 to 4.5 mm. The total number of electrodes that can be utilized varies from 4 to 32 depending on which SCS system is used. With respect to programming SCS systems, the number of electrodes and their spacing determine the coverage, meaning the locations in the spinal cord that are accessible to stimulation. The “footprint” of the electrodes determines the rostrocaudal and mediolateral span that can be stimulated. Two percutaneous leads are often placed side by side to improve mediolateral coverage and stimulation control. The electrode spacing determines the resolution of stimulation. More closely spaced electrodes may allow more complete coverage of the area between adjacent electrodes [ ]. Increasing electrode spacing to increase total span of coverage may result in locations between electrodes that are difficult to stimulate (i.e., loss of stimulation resolution) [ ]. As mentioned in the previous section, whether a system has multiple current sources or a single current source is also a factor in determining coverage.

Because paddle leads are insulated on the posterior side and typically situated closer to the spinal cord, they may require lower stimulation amplitudes. A randomized controlled trial (RCT) comparing percutaneous leads and paddle leads with the same number of electrodes found that paddle leads provide better coverage of pain areas with paresthesia and require lower stimulation amplitudes compared to percutaneous leads [ ]. These results are consistent with computational modeling showing less recruitment of dorsal root fibers when electrodes are closer to the spinal cord [ ]. The same RCT followed patients for 3 years and found that the responder rate of patients with paddle leads is higher than that of patients with percutaneous leads at 2 years but this difference disappears at 3 years [ ]. Kinfe et al. showed similar pain relief with percutaneous and paddle leads [ ] while Villavivencio et al. showed better outcomes with paddle leads [ ]. Differences in programming options may account for the differences in results.