Spinal cord stimulation (SCS) is effective in treating chronic neuropathic pain, yet, very little is known about the underlying mechanisms of this intervention. Improved understanding of SCS would not only help better tailor treatments for better results in the clinic. This can be achieved through enhanced communication between clinicians and basic researchers to design more effective treatments.
Current paradigms of SCS are based on the principle of gating mechanism for pain transmission, in which pain signal is mediated by large diameter fibers. However, a number of studies have shown SCS not only produces neuronal excitation, but also to neuronal inhibition. In addition, many neurotransmitter systems are affected, and the range of excitability is wide.1 Descending serotonergic pathways may also be activated by SCS, secondary to dorsal column activation.
There has been a trend for parameters of SCS (amplitude, frequency, pulse width) to be considered independently of one another, for their effect on modulating pain pathways. In an article recently published in the journal Neuromodulation, authors sought to review currently used parameters, and determine how their combination affects charge delivery to alleviate pain.2
Threshold of action potential generation follows a strength-duration curve of hyperbolic shape, in which amplitude x pulse width is constant. Axon size and myelination, as well as distance from site of stimulation, all influence the strength-duration curve. This concept was tested on patients to determine optimal parameters for paresthesia coverage.3 Higher stimulation amplitude leads to increased number of fibers recruited, and increased paresthesia sensation. Pulse width may also impact fiber recruitment, although to a lesser extent.
Increasing pulse width during SCS will help determine optimal amplitude for perception of paresthesia, as well as threshold of discomfort, thus establishing a strength-duration curve specific to individual patients to achieve optimal pain relief.
SCS stimulation frequency needs to be considered alongside pulse amplitude and width. Authors propose a new model for SCS stimulation, based on ‘charge per second’, to more accurately define stimulation paradigms. Scientists compare use of the charge per second parameter to medication dosage, and stress the importance of understanding mechanisms of charge delivery in order to optimally use SCS for pain relief.
1. Foreman RD, Linderoth B. Neural mechanisms of spinal cord stimulation. Int Rev Neurobiol. 2012;107:87-119.
2. Miller JP, Eldabe S, Buchser E, Johanek LM, Guan Y, Linderoth B. Parameters of Spinal Cord Stimulation and Their Role in Electrical Charge Delivery: A Review. Neuromodulation. 2016;19(4):373-84.
3. Abejón D, Rueda P, Del saz J, Arango S, Monzón E, Gilsanz F. Is the introduction of another variable to the strength-duration curve necessary in neurostimulation?. Neuromodulation. 2015;18(3):182-90.