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      Predicted effects of pulse width programming in spinal cord stimulation: a mathematical modeling study

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          Abstract

          To understand the theoretical effects of pulse width (PW) programming in spinal cord stimulation (SCS), we implemented a mathematical model of electrical fields and neural activation in SCS to gain insight into the effects of PW programming. The computational model was composed of a finite element model for structure and electrical properties, coupled with a nonlinear double-cable axon model to predict nerve excitation for different myelinated fiber sizes. Mathematical modeling suggested that mediolateral lead position may affect chronaxie and rheobase values, as well as predict greater activation of medial dorsal column fibers with increased PW. These modeling results were validated by a companion clinical study. Thus, variable PW programming in SCS appears to have theoretical value, demonstrated by the ability to increase and even ‘steer’ spatial selectivity of dorsal column fiber recruitment. It is concluded that the computational SCS model is a valuable tool to understand basic mechanisms of nerve fiber excitation modulated by stimulation parameters such as PW and electric fields.

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          Which elements are excited in electrical stimulation of mammalian central nervous system: a review.

          J Ranck (1975)
          (1) There are data on the amount of current necessary to stimulate a myelinated fiber or cell body and/or its axon a given distance away from a monopolar electrode over the entire range of practical interest for intracranial stimulation. Data do not exist for other electrode configurations. (2) Currents from a monopolar cathode of more than 8 times threshold may block action potentials in axons. Therefore, only axons lying in a shell around the electrode are stimulated. Elements very close to the electrode may not be stimulated. Close to an electrode small diameter axons may be stimulated and larger ones may not be. (3) Most, and perhaps all, CNS myelinated fibers have chronaxies of 50-100 musec. When gray matter is stimulated, the chronaxie is often 200-700 musec. It is not clear what is being stimulated in this case. Current-duration relations should be determined for many more responses. (4) There are no current-distance or current-duration data for central finely myelinated or unmyelinated fibers. (5) It takes less cathodal current than anodal to stimulate a myelinated fiber passing by a monopolar electrode. When a monopolar electrode is near a cell body, on the opposite side from the axon, often the lowest threshold is anodal, but sometimes cathodal. Stimulation of a neuron near its cell body is not well understood, but in many cases the axon is probably stimulated. (6) Orientation of cell body and axons with respect to current flow is important. For an axon it is the component of the voltage gradient parallel to the fiber that is important. (7) The pia has a significant resistance and capacitance. Gray matter, white matter, and cerebrospinal fluid have different resistivities, which affect patterns of current flow. (8) More is known about stimulation of mammalian CNS than most workers are aware of. Much of what is unknown seems solvable with current methods.
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            Electrical inhibition of pain by stimulation of the dorsal columns: preliminary clinical report.

            C Shealy, J T Mortimer, J B Reswick (2015)
            Article 0 comments Cited 123 times – based on 0 reviews     Review now
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              Extracellular stimulation of central neurons: influence of stimulus waveform and frequency on neuronal output.

              Jeremy McIntyre, Warren Grill (2002)
              The objective of this project was to examine the influence of stimulus waveform and frequency on extracellular stimulation of neurons with their cell bodies near the electrode (local cells) and fibers of passage in the CNS. Detailed computer-based models of CNS cells and axons were developed that accurately reproduced the dynamic firing properties of mammalian motoneurons including afterpotential shape, spike-frequency adaptation, and firing frequency as a function of stimulus amplitude. The neuron models were coupled to a three-dimensional finite element model of the spinal cord that solved for the potentials generated in the tissue medium by an extracellular electrode. Extracellular stimulation of the CNS with symmetrical charge balanced biphasic stimuli resulted in activation of fibers of passage, axon terminals, and local cells around the electrode at similar thresholds. While high stimulus frequencies enhanced activation of fibers of passage, a much more robust technique to achieve selective activation of targeted neuronal populations was via alterations in the stimulus waveform. Asymmetrical charge-balanced biphasic stimuli, consisting of a long-duration low-amplitude cathodic prepulse phase followed by a short-duration high-amplitude anodic stimulus phase, enabled selective activation of local cells. Conversely, an anodic prepulse phase followed by a cathodic stimulus phase enabled selective activation of fibers of passage. The threshold for activation of axon terminals in the vicinity of the electrode was lower than the threshold for direct activation of local cells, independent of the stimulus waveform. As a result, stimulation induced trans-synaptic influences (indirect depolarization/hyperpolarization) on local cells altered their neural output, and this indirect effect was dependent on stimulus frequency. If the indirect activation of local cells was inhibitory, there was little effect on the stimulation induced neural output of the local cells. However, if the indirect activation of the local cells was excitatory, attempts to activate selectively fibers of passage over local cells was limited. These outcomes provide a biophysical basis for understanding frequency-dependent outputs during CNS stimulation and provide useful tools for selective stimulation of the CNS.
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                Author and article information

                Contributors
                : Dongchul.Lee@bsci.com
                Journal
                Journal ID (nlm-ta): Med Biol Eng Comput
                Title: Medical & Biological Engineering & Computing
                Publisher: Springer-Verlag (Berlin/Heidelberg )
                ISSN (Print): 0140-0118
                ISSN (Electronic): 1741-0444
                Publication date (Electronic): 29 April 2011
                Publication date PMC-release: 29 April 2011
                Publication date (Print): July 2011
                Volume: 49
                Issue: 7
                Pages: 765-774
                Affiliations
                [1 ]Boston Scientific Neuromodulation, 25155 Rye Canyon Blvd, Valencia, CA 91355 USA
                [2 ]Comprehensive Pain & Rehabilitation, 28260 Highway 98, Daphne, AL 36526 USA
                Article
                Publisher ID: 780
                DOI: 10.1007/s11517-011-0780-9
                PMC ID: 3121943
                PubMed ID: 21528381
                SO-VID: fd92a187-ac9f-4131-bb95-0b85d4e09dc5
                Copyright © © The Author(s) 2011
                History
                Date received : 2 September 2009
                Date accepted : 6 April 2011
                Categories
                Subject: Original Article
                Custom metadata
                issue-copyright-statement © International Federation for Medical and Biological Engineering 2011

                ScienceOpen disciplines: Biomedical engineering
                Keywords: spinal cord stimulation,mathematical nerve model,neurostimulation,non-linear nerve model,pulse width,implantable pulse generator,paresthesia,dermatome
                Data availability:
                ScienceOpen disciplines: Biomedical engineering
                Keywords: spinal cord stimulation, mathematical nerve model, neurostimulation, non-linear nerve model, pulse width, implantable pulse generator, paresthesia, dermatome

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