Does it matter that the contact impedance of spinal cord stimulation electrodes is variable?

Objectives. The impedance of a percutaneous SCS electrode is highly variable and affects the currents injected by the various anodes (-) and cathodes (+) when using a standard single-channel, constant-voltage stimulation device. The impedance is affected in particular by the position of the percutaneous lead in the epidural space and the presence of fibrous encapsulation and scar tissue [1]. The aim of this study is to demonstrate the influence of variable electrode impedances on the distribution of currents among the anodes and cathodes, and thus on the characteristics of the electrical field activating spinal neural pathways. Methods. A simple equivalent electrical network of SCS was considered. Contacts of the same polarity (cathodes and anodes) were connected in parallel and the connection between the anodes and cathodes was serial. A 1V pulse was applied between the two ends of the circuitry. Given the impedance of each contact, the corresponding currents were calculated using Ohm’s and Kirchoff’s law. An application program calculating these currents in a network of up to 4 contacts of the same polarity was made in Matlab. Results. As an example we used a network consisting of two cathodes having impedances of 400 Ohms each and two anodes with 350 Ohms each. Three separate changes to this network were considered: 1) When the impedance of one cathode was increased twofold its current decreased by 44% while the current of the other cathode increased by 14%. The anodal currents decreased by 15% each. 2) When one cathode was excluded (e.g. a broken cable), the current of the remaining cathode increased by 31%, while the anodal currents decreased by 35%. 3) When another cathode having a 400 Ohms impedance was added, the current of each cathode was decreased by 19%, whereas the current of each anode was increased by 22%. Conclusions. These simple calculations show that in single-channel, voltage-controlled SCS an additional contact or an accidental change in just one contact impedance results in a change of the current supplied by each cathode and anode. As a consequence, the stimulation of the neural targets and thus paresthesia coverage will change. Reference 1. Manola L, Holsheimer J, Veltink PH. Technical performance of percutaneous leads for spinal cord stimulation: a modeling study. Neuromodulation 2005;8:88-99.